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05. NEC Handbook 2005 Ch 5 Special Occupancies

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					                                                                                                                                 5
                                                                                     Special Occupancies
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                                                                           Article 500     Hazardous (Classified) Locations,     646              Article 518            Assembly Occupancies                          800
                                                                                           Classes I, II, and III, Divisions 1                    Article 520            Theaters, Audience Areas of                   805
                                                                                           and 2                                                                         Motion Picture and Television
                                                                           Article 501     Class I Locations                     671                                     Studios, Performance Areas, and
                                                                           Article 502     Class II Locations                    698                                     Similar Locations
                                                                           Article 503     Class III Locations                   708              Article 525            Carnivals, Circuses, Fairs, and               823
                                                                           Article 504     Intrinsically Safe Locations          713                                     Similar Events
                                                                           Article 505     Class I, Zone 0, 1, and 2 Locations   718              Article 530            Motion Picture and Television                 826
                                                                           Article 506     Zone 20, 21, and 22 Locations for     735                                     Studios and Similar Locations
                                                                                           Combustible Dusts, Fibers, and                         Article      540       Motion Picture Projection Rooms               831
                                                                                           Flyings                                                Article      545       Manufactured Buildings                        834
                                                                           Article 510     Hazardous (Classified)                741              Article      547       Agricultural Buildings                        835
                                                                                           Locations—Specific                                     Article      550       Mobile Homes, Manufactured                    841
                                                                           Article 511     Commercial Garages, Repair and        741                                     Homes, and Mobile Home Parks
                                                                                           Storage                                                Article 551            Recreational Vehicles and                     853
                                                                           Article   513   Aircraft Hangars                      748                                     Recreational Vehicle Parks
                                                                           Article   514   Motor Fuel Dispensing Facilities      752              Article      552       Park Trailers                                 869
                                                                           Article   515   Bulk Storage Plants                   759              Article      553       Floating Buildings                            880
                                                                           Article   516   Spray Application, Dipping, and       766              Article      555       Marinas and Boatyards                         882
                                                                                           Coating Processes                                      Article      590       Temporary Installations                       888
                                                                           Article 517     Health Care Facilities                774




                                                                                                                                                                                                                       645

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                 500.1                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                              ARTICLE 500                                                                   (D) Threading
                     Hazardous (Classified) Locations,                                                      (E) Fiber Optic Cable Assembly
                                                                                                          500.9 Specific Occupancies
                      Classes I, II, and III, Divisions
                                   1 and 2
                                                                                                               FPN: Rules that are followed by a reference in brackets
                                                                                                               contain text that has been extracted from NFPA 497,
                    Summary of Changes                                                                         Recommended Practice for the Classification of Flamma-
                                                                                                               ble Liquids, Gases, or Vapors and of Hazardous (Classi-
                    • 500.2: Revised definition of purged and pressurized for                                  fied) Locations for Electrical Installations in Chemical
                      correlation with NFPA 496 and also to clarify that pressur-                              Process Areas, 2004 edition, and NFPA 499, Recom-
                      ization is an acceptable protection technique for Class II                               mended Practice for the Classification of Combustible
                      and Class III locations.                                                                 Dusts and of Hazardous (Classified) Locations for Elec-
                                                                                                               trical Installation in Chemical Process Areas, 2004 edi-
                    • 500.7(K): Added requirement that the type of detection                                   tion. Only editorial changes were made to the extracted
                      equipment, its listing, installation location(s), alarm and                              text to make it consistent with this Code.
                      shutdown criteria, and calibration frequency be docu-
                      mented when combustible gas detectors are used as a                               500.1 Scope — Articles 500 Through 504
                      protection technique.                                                             Articles 500 through 504 cover the requirements for electri-
                                                                                                        cal and electronic equipment and wiring for all voltages in
                                                                                                        Class I, Divisions 1 and 2; Class II, Divisions 1 and 2; and
                                                                                                        Class III, Divisions 1 and 2 locations where fire or explosion
                    Contents                                                                            hazards may exist due to flammable gases or vapors, flamma-
                     500.1 Scope — Articles 500 Through 504                                             ble liquids, combustible dust, or ignitible fibers or flyings.
                     500.2 Definitions
                     500.3 Other Articles                                                               Article 500 is limited to locations classified as Class I, II,
                     500.4 General                                                                      or III, Division 1 or 2. Article 505 contains the requirements
                       (A) Documentation                                                                for using the Class I, Zone 0, Zone 1, and Zone 2 method
                       (B) Reference Standards                                                          of area classification in accordance with the International
                     500.5 Classifications of Locations                                                 Electrotechnical Commission standard IEC 60079-10, Elec-
                       (A) Classifications of Locations                                                 trical Apparatus for Explosive Gas Atmospheres.
                       (B) Class I Locations                                                                 See the commentary following 505.1. New in the 2005
                       (C) Class II Locations                                                           Code, Article 506 provides the requirements for using the
                       (D) Class III Locations                                                          Class II or Class III, Zone 20, Zone 21, and Zone 22 method
                     500.6 Material Groups                                                              of area classification in accordance with IEC 61241-10,
                       (A) Class I Group Classifications                                                Electrical Apparatus for Use in the Presence of Combustible
                       (B) Class II Group Classifications                                               Dust — Part 10: Classification of Areas Where Combustible
                     500.7 Protection Techniques                                                        Dusts Are or May Be Present.
                       (A) Explosionproof Apparatus                                                          The Code does not classify areas where the manufacture,
                       (B) Dust Ignitionproof                                                           transportation, storage, or use of explosive materials, such as
                       (C) Dusttight                                                                    ammunition, dynamite, and blasting powder, occurs. Areas
                       (D) Purged and Pressurized                                                       where such materials are present are not considered hazard-
                       (E) Intrinsic Safety                                                             ous (classified) locations in accordance with Article 500.
                        (F) Nonincendive Circuit                                                        However, many organizations responsible for the safety of
                       (G) Nonincendive Equipment                                                       such areas require equipment and wiring methods suitable
                       (H) Nonincendive Component                                                       for hazardous locations as part of many safety precautions,
                         (I) Oil Immersion                                                              even though the equipment and wiring have not been investi-
                        (J) Hermetically Sealed                                                         gated for such locations. The logic is that hazardous location
                       (K) Combustible Gas Detection System                                             equipment and wiring methods are safer than ordinary loca-
                       (L) Other Protection Techniques                                                  tion equipment and wiring methods. Further information
                     500.8 Equipment                                                                    on these locations can be found in NFPA 495, Explosive
                       (A) Approval for Class and Properties                                            Materials Code.
                       (B) Marking                                                                           The NEC does not classify specific Class I, Class II,
                       (C) Temperature                                                                  and Class III locations. NFPA technical committees with the


                 646                                                                                                                       2005 National Electrical Code Handbook
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                                                     Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                    500.2



                           experience and expertise of working with flammable liquids,                     Combustible metallic dusts are not covered by Article 506.
                           gases, vapors, dusts, and flyings that are inherent to a process                This is similar to Article 505, which covers zone classifica-
                           or that may be present under abnormal conditions of opera-                      tion as an alternative to the division classification system
                           tion determine the parameters, distances, and degrees of                        covered in Article 500.
                           hazard associated with classified locations. Some of this
                           information has been extracted from other NFPA documents
                           and is included in Articles 511 through 517 of the Code.                        500.2 Definitions
                                Articles 500 through 506 cover the requirements for                        For purposes of Articles 500 through 504 and Articles 510
                           electrical installations in locations that are classified as haz-               through 516, the following definitions apply.
                           ardous locations due to the materials handled, processed, or
                           stored in those locations. Some of the most common materi-                      Associated Nonincendive Field Wiring Apparatus. Ap-
                           als encountered in hazardous (classified) locations are flam-                   paratus in which the circuits are not necessarily nonincendive
                           mable liquids. A flammable liquid is one that has a flash                       themselves but that affect the energy in nonincendive field
                           point below 100 F. A flammable or combustible liquid must                       wiring circuits and are relied upon to maintain nonincendive
                           be at its flash point for an explosion to occur. No. 1-D diesel                 energy levels. Associated nonincendive field wiring appara-
                           fuel oil and kerosene have flash points higher than 100 F                       tus may be either of the following:
                           and therefore do not emit flammable vapors unless heated
                                                                                                           (1) Electrical apparatus that has an alternative type of pro-
                           above their flash points. They are considered combustible
                                                                                                               tection for use in the appropriate hazardous (classified)
                           liquids.
                                                                                                               location
                                For information on hazardous locations in general, in-
                                                                                                           (2) Electrical apparatus not so protected that shall not be
                           cluding background on the classification of areas, equipment
                                                                                                               used in a hazardous (classified) location
                           protection systems, ignition sources, static electricity and
                           lightning, and requirements that apply outside the United                              FPN: Associated nonincendive field wiring apparatus
                           States, see Electrical Installations in Hazardous Locations,                           has designated associated nonincendive field wiring ap-
                           by Peter J. Schram and Mark W. Earley. This book is avail-                             paratus connections for nonincendive field wiring appara-
                           able from NFPA.                                                                        tus and may also have connections for other electrical
                                                                                                                  apparatus.

                                   FPN No. 1: The unique hazards associated with explo-                    Combustible Gas Detection System. A protection tech-
                                   sives, pyrotechnics, and blasting agents are not addressed              nique utilizing stationary gas detectors in industrial estab-
                                   in this article.                                                        lishments.
                                                                                                           Control Drawing. A drawing or other document provided
                           FPN No. 1 was added to the 2005 Code to alert users that
                                                                                                           by the manufacturer of the intrinsically safe or associated
                           explosives, pyrotechnics, and blasting agents are not covered
                                                                                                           apparatus, or of the nonincendive field wiring apparatus or
                           by the Code. These materials are unique in that they can be
                                                                                                           associated nonincendive field wiring apparatus, that details
                           set off by bumping, jarring, and static charges from various
                                                                                                           the allowed interconnections between the intrinsically safe
                           sources, in addition to electrical arcs, sparks, or heat.
                                                                                                           and associated apparatus or between the nonincendive field
                                                                                                           wiring apparatus or associated nonincendive field wiring




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                                   FPN No. 2: For the requirements for electrical and elec-                apparatus.
                                   tronic equipment and wiring for all voltages in Class I,
                                   Zone 0, Zone 1, and Zone 2 hazardous (classified) loca-                 Dust-Ignitionproof. Equipment enclosed in a manner that
                                   tions where fire or explosion hazards may exist due to                  excludes dusts and does not permit arcs, sparks, or heat
                                   flammable gases or vapors or flammable liquids, refer
                                   to Article 505.                                                         otherwise generated or liberated inside of the enclosure to
                                   FPN No. 3: For the requirements for electrical and elec-                cause ignition of exterior accumulations or atmospheric sus-
                                   tronic equipment and wiring for all voltages in Zone 20,                pensions of a specified dust on or in the vicinity of the
                                   Zone 21, and Zone 22 hazardous (classified) locations                   enclosure.
                                   where fire or explosion hazards may exist due to combus-
                                   tible dusts or ignitible fibers or flyings, refer to Article                   FPN: For further information on dust-ignitionproof en-
                                   506.                                                                           closures, see Type 9 enclosure in ANSI/NEMA 250-
                                                                                                                  1991, Enclosures for Electrical Equipment, and ANSI/
                                                                                                                  UL 1203-1994, Explosionproof and Dust-Ignitionproof
                           FPN No. 3 was added to the 2005 Code to alert users that                               Electrical Equipment for Hazardous (Classified) Loca-
                           a new article, Article 506, was developed to cover the re-                             tions.
                           quirements for zone classification in locations where com-
                           bustible dusts, fibers, and flyings may be present.                             Dusttight. Enclosures constructed so that dust will not enter
                                                                                                           under specified test conditions.

                           National Electrical Code Handbook 2005                                                                                                                  647

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                 500.2                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                            FPN: See ANSI/ISA 12.12.01-2000, Nonincendive Elec-                  Nonincendive Equipment. Equipment having electrical/
                            trical Equipment for Use in Class I and II, Division 2,              electronic circuitry that is incapable, under normal operating
                            and Class III, Divisions 1 and 2 Hazardous (Classified)
                            Locations, and UL 1604-1994, Electrical Equipment for                conditions, of causing ignition of a specified flammable
                            Use in Class I and II, Division 2 and Class III Hazardous            gas–air, vapor–air, or dust–air mixture due to arcing or ther-
                            (Classified) Locations.                                              mal means.
                    Electrical and Electronic Equipment. Materials, fittings,                           FPN: For further information, see ANSI/ISA 12.12.01-




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                    devices, appliances, and the like that are part of, or in connec-                   2000, Nonincendive Electrical Equipment for Use in
                    tion with, an electrical installation.                                              Class I and II, Division 2, and Class III, Divisions 1 and
                                                                                                        2 Hazardous (Classified) Locations.
                            FPN: Portable or transportable equipment having self-
                            contained power supplies, such as battery-operated                   Nonincendive Field Wiring. Wiring that enters or leaves
                            equipment, could potentially become an ignition source               an equipment enclosure and, under normal operating condi-
                            in hazardous (classified) locations. See ISA-RP12.12.03-
                            2002, Portable Electronic Products Suitable for Use in
                                                                                                 tions of the equipment, is not capable, due to arcing or
                            Class I and II, Division 2, Class I Zone 2 and Class III,            thermal effects, of igniting the flammable gas–air, vapor–air,
                            Division 1 and 2 Hazardous (Classified) Locations.                   or dust–air mixture. Normal operation includes opening,
                                                                                                 shorting, or grounding the field wiring.
                    Explosionproof Apparatus. Apparatus enclosed in a case
                    that is capable of withstanding an explosion of a specified
                    gas or vapor that may occur within it and of preventing the                  A nonincendive circuit employs a protection technique that
                    ignition of a specified gas or vapor surrounding the enclosure               prevents electrical circuits from causing a fire or explosion
                    by sparks, flashes, or explosion of the gas or vapor within,                 in a hazardous (classified) location. Section 500.7(F) points
                    and that operates at such an external temperature that a                     out that this protection technique is permitted only for
                    surrounding flammable atmosphere will not be ignited                         Division 2 areas of Class I or Class II locations and for
                    thereby.                                                                     Division 1 or 2 areas of Class III locations. Nonincendive
                                                                                                 circuits are not permitted in Division 1 areas of Class I or
                            FPN: For further information, see ANSI/UL 1203-1994,
                            Explosion-Proof and Dust-Ignition-Proof Electrical                   Class II locations. Nonincendive circuits and equipment are
                            Equipment for Use in Hazardous (Classified) Locations.               tested or evaluated essentially in the same way that intrinsi-
                                                                                                 cally safe circuits and equipment are tested and evaluated,
                    Hermetically Sealed. Equipment sealed against the en-
                                                                                                 except that abnormal conditions are not considered. Because
                    trance of an external atmosphere where the seal is made by                   of its definition, a nonincendive circuit is a low-energy
                    fusion, for example, soldering, brazing, welding, or the fu-
                                                                                                 circuit.
                    sion of glass to metal.                                                           The definition of nonincendive field wiring was added
                            FPN: For further information, see ANSI/ISA 12.12.01-                 to Article 100 of the 1999 Code to alert users that, although
                            2000, Nonincendive Electrical Equipment for Use in                   the circuits and equipment may have been evaluated and
                            Class I and II, Division 2, and Class III, Division 1 and
                                                                                                 approved as nonincendive, field wiring is not generally ap-
                            2 Hazardous (Classified) Locations.
                                                                                                 proved by a testing laboratory. The definition was moved
                    Nonincendive Circuit. A circuit, other than field wiring,                    from Article 100 and now appears in 500.2. Field wiring
                    in which any arc or thermal effect produced under intended                   meeting this definition would require limitations of energy
                    operating conditions of the equipment is not capable, under                  on the wiring under conditions such as opening, shorting,
                    specified test conditions, of igniting the flammable gas–air,                or grounding. For example, stored energy in the form of
                    vapor–air, or dust–air mixture.                                              mutual inductance or capacitance could be released during
                            FPN: Conditions are described in ANSI/ISA 12.12.01-                  an opening, shorting, or grounding of nonincendive field
                            2000, Nonincendive Electrical Equipment for Use in                   wiring, thus defeating the purpose of this protection tech-
                            Class I and II, Division 2, and Class III, Divisions 1 and           nique. Further information regarding nonincendive protec-
                            2 Hazardous (Classified) Locations.                                  tion techniques is found in the 500.2 definitions of
                    Nonincendive Component. A component having contacts                          nonincendive circuit, nonincendive component, nonincen-
                    for making or breaking an incendive circuit and the con-                     dive equipment, and nonincendive field wiring apparatus.
                    tacting mechanism is constructed so that the component is
                    incapable of igniting the specified flammable gas–air or
                    vapor–air mixture. The housing of a nonincendive compo-                      Nonincendive Field Wiring Apparatus. Apparatus in-
                    nent is not intended to exclude the flammable atmosphere                     tended to be connected to nonincendive field wiring.
                    or contain an explosion.
                                                                                                        FPN: For further information see ANSI/ISA 12.12.01-
                            FPN: For further information, see UL 1604-1994, Elec-                       2000, Nonincendive Electrical Equipment for Use in
                            trical Equipment for Use in Class I and II, Division 2,                     Class I and II, Division 2, and Class III, Divisions 1 and
                            and Class III Hazardous (Classified) Locations.                             2 Hazardous (Classified) Locations.


                 648                                                                                                                2005 National Electrical Code Handbook

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                                                   Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                               500.4



                           Oil Immersion. Electrical equipment immersed in a protec-                                   of the electrical equipment being an ignition source in the
                           tive liquid in such a way that an explosive atmosphere that                                 volatile or potentially volatile environments covered in these
                           may be above the liquid or outside the enclosure cannot be                                  articles. An example of how the hazardous (classified) loca-
                           ignited.                                                                                    tion articles of Chapter 5 modify the general requirements
                                                                                                                       is found in the wiring method requirements of 501.10,
                                   FPN: For further information, see ANSI/UL 698-1995,
                                   Industrial Control Equipment for Use in Hazardous                                   502.10, and 503.10. These sections of the Code limit the
                                   (Classified) Locations.                                                             wiring methods that can be used in Class I, Class II, and
                                                                                                                       Class III locations in order to provide the highest degree of
                           Purged and Pressurized. The process of (1) purging, sup-                                    protection against physical damage. The installation of any
                           plying an enclosure with a protective gas at a sufficient flow                              wiring methods allowed by Articles 500 through 506 must
                           and positive pressure to reduce the concentration of any                                    be in accordance with the Chapter 3 article that governs
                           flammable gas or vapor initially present to an acceptable                                   that particular wiring method and with any modifications in
                           level; and (2) pressurization, supplying an enclosure with a                                Articles 500 through 517.
                           protective gas with or without continuous flow at sufficient
                           pressure to prevent the entrance of a flammable gas or vapor,
                           a combustible dust, or an ignitible fiber.                                                  500.4 General
                                                                                                                       (A) Documentation All areas designated as hazardous
                           The definition of purged and pressurized has been revised                                   (classified) locations shall be properly documented. This
                           to correlate with the definitions of these terms as specified                               documentation shall be available to those authorized to de-
                           in NFPA 496, Standard for Purged and Pressurized Enclo-                                     sign, install, inspect, maintain, or operate electrical equip-
                           sures for Electrical Equipment. The revised definition makes                                ment at the location.
                           it clear that an enclosure can be purged continuously by a
                           supply of clean air or inert gas or it can be pressurized with                              One type of documentation consists of area classification
                           a supply of clean air or inert gas sufficient to prevent the                                drawings. This type of documentation provides the necessary
                           entrance of flammable gas or vapors, combustible dust, or                                   information for installers, service personnel, and authorities
                           flyings into the enclosure.                                                                 having jurisdiction to ensure that electrical equipment in-
                                                                                                                       stalled or maintained in classified areas is of the proper type.
                                   FPN: For further information, see ANSI/NFPA 496-
                                                                                                                       See the fine print note to 505.4(A).
                                   2003, Purged and Pressurized Enclosures for Electrical
                                   Equipment.
                                                                                                                       (B) Reference Standards Important information relating
                           Unclassified Locations. Locations determined to be neither                                  to topics covered in Chapter 5 may be found in other publica-
                           Class I, Division 1; Class I, Division 2; Class I, Zone 0;                                  tions.
                           Class I, Zone 1; Class I, Zone 2; Class II, Division 1; Class
                           II, Division 2; Class III, Division 1; Class III, Division 2;
                                                                                                                       The NFPA and ANSI standards referenced in Articles 500
                           or any combination thereof.
                                                                                                                       through 517 are essential for proper application of those
                                                                                                                       articles. The following NFPA codes, standards, and recom-
                           500.3 Other Articles
                                                                                                                       mended practices include information on hazardous (classi-
                           Except as modified in Articles 500 through 504, all other                                   fied) locations and the extent of hazardous (classified)
                           applicable rules contained in this Code shall apply to electri-                             locations in specific occupancies or industries.
                           cal equipment and wiring installed in hazardous (classified)
                           locations.                                                                                            NFPA 30, Flammable and Combustible Liquids Code
                                                                                                                                 NFPA 30A, Code for Motor Fuel Dispensing Facilities
                           The first four chapters of the Code cover general installation                                        and Repair Garages
                           requirements for all electrical equipment and wiring (see                                             NFPA 32, Standard for Drycleaning Plants
                           90.3). This means that materials and equipment must be                                                NFPA 33, Standard for Spray Application Using Flam-
                           suitable for environmental conditions such as rain, snow,                                             mable or Combustible Materials
                           ice, altitude, and heat; deteriorating effects on the conductors
                                                                                                                                 NFPA 34, Standard for Dipping and Coating Processes
                           and equipment; and interrupting rating sufficient for the
                                                                                                                                 Using Flammable or Combustible Liquids
                           nominal circuit voltage and available fault current, just to
                           name a few. The requirements in Articles 500 through 506                                              NFPA 35, Standard for the Manufacture of Organic
                           amend or modify the general rules to ensure the integrity                                             Coatings
                           of the electrical installation and to minimize the possibility                                        NFPA 36, Standard for Solvent Extraction Plants


                           National Electrical Code Handbook 2005                  --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                                                          649

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                 500.5                                                                 Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                                                                                                                                              Institute (API), and the Instrumentation, Systems, and
                                                                      NFPA 45, Standard on Fire Protection for Laboratories                   Automation Society (ISA) that may be of use in the
                                                                      Using Chemicals                                                         classification of various locations, the determination of
                                                                                                                                              adequate ventilation, and the protection against static
                                                                      NFPA 50A, Standard for Gaseous Hydrogen Systems                         electricity and lightning hazards.
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                      at Consumer Sites                                                       FPN No. 2: For further information on the classification
                                                                      NFPA 50B, Standard for Liquefied Hydrogen Systems                       of locations, see NFPA 30-2003, Flammable and Com-
                                                                      at Consumer Sites                                                       bustible Liquids Code; NFPA 32-2004, Standard for Dry-
                                                                                                                                              cleaning Plants; NFPA 33-2003, Standard for Spray
                                                                      NFPA 51, Standard for the Design and Installation of                    Application Using Flammable or Combustible Materials;
                                                                      Oxygen–Fuel Gas Systems for Welding, Cutting, and                       NFPA 34-2003, Standard for Dipping and Coating Pro-
                                                                      Allied Processes                                                        cesses Using Flammable or Combustible Liquids; NFPA
                                                                                                                                              35-1999, Standard for the Manufacture of Organic Coat-
                                                                      NFPA 51A, Standard for Acetylene Cylinder Charging                      ings; NFPA 36-2004, Standard for Solvent Extraction
                                                                      Plants                                                                  Plants; NFPA 45-2004, Standard on Fire Protection for
                                                                      NFPA 52, Compressed Natural Gas (CNG) Vehicular                         Laboratories Using Chemicals; NFPA 50A-1999, Stan-
                                                                                                                                              dard for Gaseous Hydrogen Systems at Consumer Sites;
                                                                      Fuel Systems Code
                                                                                                                                              NFPA 50B-1999, Standard for Liquefied Hydrogen Sys-
                                                                      NFPA 54, Natural Fuel Gas Code                                          tems at Consumer Sites; NFPA 58-2004, Liquefied Petro-
                                                                                                                                              leum Gas Code; NFPA 59-2004, Utility LP-Gas Plant
                                                                      NFPA 58, Liquefied Petroleum Gas Code                                   Code; NFPA 497-2004, Recommended Practice for the
                                                                      NFPA 59, Utility LP-Gas Plant Code                                      Classification of Flammable Liquids, Gases, or Vapors
                                                                                                                                              and of Hazardous (Classified) Locations for Electrical
                                                                      NFPA 59A, Standard for the Production, Storage, and                     Installations in Chemical Process Areas; NFPA 499-
                                                                      Handling of Liquefied Natural Gas (LNG)                                 2004, Recommended Practice for the Classification of
                                                                      NFPA 61, Standard for the Prevention of Fires and Dust                  Combustible Dusts and of Hazardous (Classified) Loca-
                                                                      Explosions in Agricultural and Food Products Facilities                 tions for Electrical Installations in Chemical Process
                                                                                                                                              Areas; NFPA 820-2003, Standard for Fire Protection in
                                                                      NFPA 85, Boiler and Combustion Systems Hazards                          Wastewater Treatment and Collection Facilities; ANSI/
                                                                      Code                                                                    API RP 500-1997, Recommended Practice for Classifica-
                                                                                                                                              tion of Locations of Electrical Installations at Petroleum
                                                                      NFPA 88A, Standard for Parking Structures                               Facilities Classified as Class I, Division 1 and Division
                                                                      NFPA 99, Standard for Health Care Facilities                            2; ISA 12.10-1988, Area Classification in Hazardous
                                                                                                                                              (Classified) Dust Locations.
                                                                      NFPA 407, Standard for Aircraft Fuel Servicing
                                                                                                                                              FPN No. 3: For further information on protection against
                                                                      NFPA 409, Standard on Aircraft Hangars                                  static electricity and lightning hazards in hazardous (clas-
                                                                      NFPA 495, Explosive Materials Code                                      sified) locations, see NFPA 77-2000, Recommended
                                                                                                                                              Practice on Static Electricity; NFPA 780-2004, Standard
                                                                      NFPA 496, Standard for Purged and Pressurized Enclo-                    for the Installation of Lightning Protection Systems; and
                                                                      sures for Electrical Equipment                                          API RP 2003-1998, Protection Against Ignitions Arising
                                                                                                                                              Out of Static Lightning and Stray Currents.
                                                                      NFPA 497, Recommended Practice for the Classifica-
                                                                                                                                              FPN No. 4: For further information on ventilation, see
                                                                      tion of Flammable Liquids, Gases, or Vapors and of                      NFPA 30-2003, Flammable and Combustible Liquids
                                                                      Hazardous (Classified) Locations for Electrical Instal-                 Code; and API RP 500-1997, Recommended Practice for
                                                                      lations in Chemical Process Areas                                       Classification of Locations for Electrical Installations at
                                                                                                                                              Petroleum Facilities Classified as Class I, Division 1
                                                                      NFPA 499, Recommended Practice for the Classifica-                      and Division 2.
                                                                      tion of Combustible Dusts and of Hazardous (Classified)                 FPN No. 5: For further information on electrical systems
                                                                      Locations for Electrical Installations in Chemical Pro-                 for hazardous (classified) locations on offshore oil- and
                                                                      cess Areas                                                              gas-producing platforms, see ANSI/API RP 14F-1999,
                                                                      NFPA 654, Standard for the Prevention of Fire and                       Recommended Practice for Design and Installation of
                                                                                                                                              Electrical Systems for Fixed and Floating Offshore Petro-
                                                                      Dust Explosions from the Manufacturing, Processing,                     leum Facilities for Unclassified and Class I, Division 1
                                                                      and Handling of Combustible Particulate Solids                          and Division 2 Locations.
                                                                      NFPA 655, Standard for Prevention of Sulfur Fires and
                                                                      Explosions                                                       500.5 Classifications of Locations
                                                                                                                                       (A) Classifications of Locations Locations shall be classi-
                                                                                                                                       fied depending on the properties of the flammable vapors,
                                                                      FPN No. 1: It is important that the authority having
                                                                      jurisdiction be familiar with recorded industrial experi-        liquids, or gases, or combustible dusts or fibers that may be
                                                                      ence as well as with the standards of the National Fire          present, and the likelihood that a flammable or combustible
                                                                      Protection Association (NFPA), the American Petroleum            concentration or quantity is present. Where pyrophoric mate-



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                                                    Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                  500.5



                           rials are the only materials used or handled, these locations                        (2) Interiors of spray booths and areas in the vicinity
                           shall not be classified. Each room, section, or area shall be                            of spraying and painting operations where volatile
                                                                                                                    flammable solvents are used
                           considered individually in determining its classification.                           (3) Locations containing open tanks or vats of volatile
                                                                                                                    flammable liquids
                           Pyrophoric materials ignite spontaneously upon contact with                          (4) Drying rooms or compartments for the evaporation
                                                                                                                    of flammable solvents
                           air. The use of electrical equipment that is suitable for a                          (5) Locations containing fat- and oil-extraction equip-
                           hazardous (classified) location will not prevent ignition of                             ment using volatile flammable solvents
                           pyrophoric materials. The process containment system                                 (6) Portions of cleaning and dyeing plants where flam-
                           should be designed to prevent contact between pyrophoric                                 mable liquids are used
                           material and air.                                                                    (7) Gas generator rooms and other portions of gas man-
                                                                                                                    ufacturing plants where flammable gas may escape
                                                                                                                (8) Inadequately ventilated pump rooms for flammable
                                                                                                                    gas or for volatile flammable liquids
                                   FPN: Through the exercise of ingenuity in the layout of                      (9) The interiors of refrigerators and freezers in which
                                   electrical installations for hazardous (classified) loca-                        volatile flammable materials are stored in open,
                                   tions, it is frequently possible to locate much of the                           lightly stoppered, or easily ruptured containers
                                   equipment in a reduced level of classification or in an                     (10) All other locations where ignitible concentrations
                                   unclassified location and, thus, to reduce the amount of                         of flammable vapors or gases are likely to occur in
                                   special equipment required.                                                      the course of normal operations

                               Rooms and areas containing ammonia refrigeration sys-                           FPN No. 2: In some Division 1 locations, ignitible con-
                           tems that are equipped with adequate mechanical ventilation                         centrations of flammable gases or vapors may be present
                                                                                                               continuously or for long periods of time. Examples in-
                           may be classified as ‘‘unclassified’’ locations.
                                                                                                               clude the following:
                                   FPN: For further information regarding classification                       (1) The inside of inadequately vented enclosures con-
                                   and ventilation of areas involving ammonia, see ANSI/                           taining instruments normally venting flammable
                                   ASHRAE 15-1994, Safety Code for Mechanical Re-                                  gases or vapors to the interior of the enclosure
                                   frigeration, and ANSI/CGA G2.1-1989, Safety Require-                        (2) The inside of vented tanks containing volatile flam-
                                   ments for the Storage and Handling of Anhydrous                                 mable liquids
                                   Ammonia.                                                                    (3) The area between the inner and outer roof sections
                                                                                                                   of a floating roof tank containing volatile flammable




                                                                                                                                                                                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           (B) Class I Locations Class I locations are those in which                              fluids
                           flammable gases or vapors are or may be present in the                              (4) Inadequately ventilated areas within spraying or coat-
                           air in quantities sufficient to produce explosive or ignitible                          ing operations using volatile flammable fluids
                           mixtures. Class I locations shall include those specified in                        (5) The interior of an exhaust duct that is used to vent
                                                                                                                   ignitible concentrations of gases or vapors
                           500.5(B)(1) and (B)(2).
                                                                                                                     Experience has demonstrated the prudence of
                           (1) Class I, Division 1 A Class I, Division 1 location is a                         avoiding the installation of instrumentation or other elec-
                           location                                                                            tric equipment in these particular areas altogether or
                                                                                                               where it cannot be avoided because it is essential to the
                           (1) In which ignitible concentrations of flammable gases                            process and other locations are not feasible [see 500.5(A),
                               or vapors can exist under normal operating conditions,                          FPN] using electric equipment or instrumentation ap-
                               or                                                                              proved for the specific application or consisting of intrin-
                                                                                                               sically safe systems as described in Article 504.
                           (2) In which ignitible concentrations of such gases or vapors
                               may exist frequently because of repair or maintenance
                               operations or because of leakage, or                                     Fine print note No. 2 describes locations that are defined in
                           (3) In which breakdown or faulty operation of equipment                      500.5(B)(1) as Class I, Division 1 locations. These locations
                               or processes might release ignitible concentrations of                   are Class I, Zone 0 locations in accordance with 505.5(B)(1).
                               flammable gases or vapors and might also cause simulta-                  Where a location is classified as a Zone 0 location, only
                               neous failure of electrical equipment in such a way as                   intrinsically safe equipment and wiring suitable for Zone 0
                               to directly cause the electrical equipment to become a                   locations are permitted to be used.
                               source of ignition.
                                   FPN No. 1: This classification usually includes the fol-             (2) Class I, Division 2 A Class I, Division 2 location is a
                                   lowing locations:                                                    location
                                   (1) Where volatile flammable liquids or liquefied flam-
                                       mable gases are transferred from one container to                (1) In which volatile flammable liquids or flammable gases
                                       another                                                              are handled, processed, or used, but in which the liquids,



                           National Electrical Code Handbook 2005                                                                                                               651

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                 500.5                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                        vapors, or gases will normally be confined within closed                        FPN: Dusts containing magnesium or aluminum are par-
                        containers or closed systems from which they can escape                         ticularly hazardous, and the use of extreme precaution
                                                                                                        is necessary to avoid ignition and explosion.
                        only in case of accidental rupture or breakdown of such
                        containers or systems or in case of abnormal operation
                        of equipment, or                                                         Section 500.5(C)(1) was revised for the 2005 Code to clarify
                    (2) In which ignitible concentrations of gases or vapors are                 what conditions would require a space to be classified as
                        normally prevented by positive mechanical ventilation                    Class II, Division 1. One condition is dust in suspension
                        and which might become hazardous through failure or                      in the air under normal operating conditions in sufficient
                        abnormal operation of the ventilating equipment, or                      quantities to produce an explosive or ignitible mixture. An-
                    (3) That is adjacent to a Class I, Division 1 location, and                  other condition is the malfunction of processing equipment
                        to which ignitible concentrations of gases or vapors                     that causes combustible dust to be released and, at the same
                        might occasionally be communicated unless such com-                      time, an ignition source is created by the malfunction or
                        munication is prevented by adequate positive-pressure                    some other ignition source occurs.
                        ventilation from a source of clean air and effective safe-
                        guards against ventilation failure are provided.                         (2) Class II, Division 2 A Class II, Division 2 location is
                                                                                                 a location
                            FPN No. 1: This classification usually includes locations
                            where volatile flammable liquids or flammable gases or               (1) In which combustible dust due to abnormal operations
                            vapors are used but that, in the judgment of the authority               may be present in the air in quantities sufficient to
                            having jurisdiction, would become hazardous only in                      produce explosive or ignitible mixtures; or
                            case of an accident or of some unusual operating condi-              (2) Where combustible dust accumulations are present but
                            tion. The quantity of flammable material that might es-
                            cape in case of accident, the adequacy of ventilating                    are normally insufficient to interfere with the normal
                            equipment, the total area involved, and the record of the                operation of electrical equipment or other apparatus,
                            industry or business with respect to explosions or fires                 but could as a result of infrequent malfunctioning of
                            are all factors that merit consideration in determining the              handling or processing equipment become suspended
                            classification and extent of each location.                              in the air; or
                            FPN No. 2: Piping without valves, checks, meters, and                (3) In which combustible dust accumulations on, in, or in
                            similar devices would not ordinarily introduce a hazard-
                            ous condition even though used for flammable liquids                     the vicinity of the electrical equipment could be suffi-
                            or gases. Depending on factors such as the quantity and                  cient to interfere with the safe dissipation of heat from
                            size of the containers and ventilation, locations used for               electrical equipment, or could be ignitible by abnormal
                            the storage of flammable liquids or liquefied or com-                    operation or failure of electrical equipment.
                            pressed gases in sealed containers may be considered
                            either hazardous (classified) or unclassified locations.                    FPN No. 1: The quantity of combustible dust that may
                            See NFPA 30-2003, Flammable and Combustible Liquids                         be present and the adequacy of dust removal systems
                            Code, and NFPA 58-2004, Liquefied Petroleum Gas                             are factors that merit consideration in determining the
                            Code.                                                                       classification and may result in an unclassified area.
                                                                                                        FPN No. 2: Where products such as seed are handled
                    (C) Class II Locations Class II locations are those that are                        in a manner that produces low quantities of dust, the
                                                                                                        amount of dust deposited may not warrant classification.
                    hazardous because of the presence of combustible dust. Class
                    II locations shall include those specified in 500.5(C)(1) and                (D) Class III Locations Class III locations are those that
                    (C)(2).                                                                      are hazardous because of the presence of easily ignitible
                                                                                                 fibers or flyings, but in which such fibers or flyings are not
                    (1) Class II, Division 1 A Class II, Division 1 location is                  likely to be in suspension in the air in quantities sufficient to
                    a location                                                                   produce ignitible mixtures. Class III locations shall include
                                                                                                 those specified in 500.5(D)(1) and (D)(2).
                    (1) In which combustible dust is in the air under normal                                                                                                   --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---



                        operating conditions in quantities sufficient to produce                 (1) Class III, Division 1 A Class III, Division 1 location
                        explosive or ignitible mixtures, or                                      is a location in which easily ignitible fibers or materials
                    (2) Where mechanical failure or abnormal operation of ma-                    producing combustible flyings are handled, manufactured,
                        chinery or equipment might cause such explosive or                       or used.
                        ignitible mixtures to be produced, and might also pro-                          FPN No. 1: Such locations usually include some parts
                        vide a source of ignition through simultaneous failure                          of rayon, cotton, and other textile mills; combustible fiber
                        of electric equipment, through operation of protection                          manufacturing and processing plants; cotton gins and
                                                                                                        cotton-seed mills; flax-processing plants; clothing manu-
                        devices, or from other causes, or                                               facturing plants; woodworking plants; and establishments
                    (3) In which Group E combustible dusts may be present in                            and industries involving similar hazardous processes or
                        quantities sufficient to be hazardous.                                          conditions.


                 652                                                                                                                2005 National Electrical Code Handbook

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                                                                                                  Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                     500.6



                                                                                FPN No. 2: Easily ignitible fibers and flyings include                          tifying equipment for use in the various atmospheric
                                                                                rayon, cotton (including cotton linters and cotton waste),                      groups.
                                                                                sisal or henequen, istle, jute, hemp, tow, cocoa fiber,
                                                                                oakum, baled waste kapok, Spanish moss, excelsior, and
                                                                                other materials of similar nature.
                                                                                                                                                         Determining the proper group classification for flammable
                                                                                                                                                         gases and vapors involves evaluating explosion pressures
                                                                            (2) Class III, Division 2 A Class III, Division 2 location                   and maximum safe clearances between parts of a clamped
                                                                            is a location in which easily ignitible fibers are stored or                 joint under several conditions and comparison of the values
                                                                            handled other than in the process of manufacture.                            obtained with those obtained for classified materials under
                                                                                                                                                         the same test conditions. Although some work has been done
                                                                            Sections 500.5(B), 500.5(C), and 500.5(D) describe three                     on the classification of flammable materials on the basis of
                                                                            classes of hazardous (classified) locations based on the type                chemical structure, this method is not sufficiently refined
                                                                            of material involved. Within each class there are varying                    or accurate enough to ensure proper classification of all
                                                                            degrees of hazard, so each class is subdivided into two                      flammable materials.
                                                                            divisions. The classification by division is based on the                         For additional information on the rationale for classifi-
                                                                            likelihood the material will be present. The requirements for                cation, reference may be made to the following:
                                                                            Division 1 of each class are more stringent than those for                    1. Rationale for Classification of Combustible Gases, Va-
                                                                            Division 2.                                                                      pors, and Dusts with Reference to the National Electri-
                                                                                 The materials in the three classes are defined as follows:                  cal Code, Publication NMAB 353-6, 1982, a report of
                                                                            Class I, flammable gases or vapors; Class II, combustible                        the Committee on Evaluation of Industrial Hazards, The
                                                                            dust; and Class III, combustible fibers or flyings.                              National Materials Advisory Board, Commission on En-
                                                                                 When a given location is classified as hazardous, it                        gineering and Technical Systems, National Research
                                                                            should be easy to determine to which of the three classes it                     Council. This publication is available from the National
                                                                            belongs (it may belong in more than one class). Common                           Technical Information Service (NTIS), Springfield, VA
                                                                            sense and good judgment must prevail in classifying an area                      22151.
                                                                            that is likely to become hazardous and in determining those                   2. An Investigation of Fifteen Flammable Gases or Vapors
                                                                            portions of the premises to be classified Division 1 or Divi-                    with Respect to Explosion-Proof Electrical Equipment,
                                                                            sion 2. However, if different types of material exist in a                       Bulletin of Research No. 58 by Underwriters Labora-
                                                                            process, such as flammable liquids, gases, or vapors and                         tories Inc., August 1969 (also Bulletin of Research Nos.
                                                                            combustible dust, the area must be classified as both a Class                    58A and 58B, which supplement No. 58). These publi-
                                                                            I and a Class II location. If a location is classified due to                    cations are available from Underwriters Laboratories
                                                                            two different hazards, such as a dust hazard and a flammable                     Inc., Publications Stock, Northbrook, IL 60062.
                                                                            liquids hazard, protection must be provided for both hazards.                 3. Electrical Installations in Hazardous Locations, Chap-
                                                                            Equipment that is approved for a Class I location may not                        ter 2, Peter J. Schram and Mark W. Earley. This book
                                                                            be suitable for a Class II location and vice versa.                              is available from NFPA.


                                                                            500.6 Material Groups                                                        (A) Class I Group Classifications Class I groups shall be
                                                                            For purposes of testing, approval, and area classification,                  according to 500.6(A)(1) through (A)(4).
                                                                            various air mixtures (not oxygen-enriched) shall be grouped                         FPN No. 1: FPN Nos. 2 and 3 apply to 500.6(A).
                                                                            in accordance with 500.6(A) and 500.6(B).                                           FPN No. 2: The explosion characteristics of air mixtures
                                                                                                                                                                of gases or vapors vary with the specific material in-
                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                            Oxygen enrichment can drastically change the explosion                              volved. For Class I locations, Groups A, B, C, and D,
                                                                                                                                                                the classification involves determinations of maximum
                                                                            characteristics of materials. It lowers the minimum ignition                        explosion pressure and maximum safe clearance between
                                                                            energies, increases explosion pressures, and can reduce the                         parts of a clamped joint in an enclosure. It is necessary,
                                                                            maximum experimental safe gap, rendering both intrinsically                         therefore, that equipment be identified not only for class
                                                                            safe and explosionproof equipment unsafe unless the equip-                          but also for the specific group of the gas or vapor that
                                                                            ment has been tested for the conditions involved.                                   will be present.
                                                                                                                                                                FPN No. 3: Certain chemical atmospheres may have
                                                                                                                                                                characteristics that require safeguards beyond those re-
                                                                                                                                                                quired for any of the Class I groups. Carbon disulfide
                                                                            Exception: Equipment identified for a specific gas, vapor,                          is one of these chemicals because of its low ignition
                                                                            or dust.                                                                            temperature [100 C (212 F)] and the small joint clearance
                                                                                                                                                                permitted to arrest its flame.
                                                                                FPN: This grouping is based on the characteristics of
                                                                                the materials. Facilities are available for testing and iden-            (1) Group A Acetylene. [NFPA 497:3.3]

                                                                            National Electrical Code Handbook 2005                                                                                                               653

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                 500.6                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                    (2) Group B Flammable gas, flammable liquid–produced                         for Class I locations do not vary for different kinds of gas
                    vapor, or combustible liquid–produced vapor mixed with                       or vapor contained in the group, except in those cases where
                    air that may burn or explode, having either a maximum                        seals may be used in all conduits to change the group classifi-
                    experimental safe gap (MESG) value less than or equal to                     cation. See Exception No. 1 and Exception No. 2 to
                    0.45 mm or a minimum igniting current ratio (MIC ratio)                      500.6(A)(2) for materials such as butadiene and ethylene
                    less than or equal to 0.40. [NFPA 497:3.3]                                   oxide. It is necessary to select equipment designed for use
                            FPN: A typical Class I, Group B material is hydrogen.                in the particular group involved. The reason for designating
                    Exception No. 1: Group D equipment shall be permitted to                     the groups this way is that explosive mixtures have different
                    be used for atmospheres containing butadiene, provided all                   igniting current ratios and maximum safe clearances between
                    conduit runs into explosionproof equipment are provided                      parts of a joint in an enclosure.
                    with explosionproof seals installed within 450 mm (18 in.)                        Underwriters Laboratories Inc. and Factory Mutual Re-
                    of the enclosure.                                                            search Corp. list or approve electrical equipment suitable
                    Exception No. 2: Group C equipment shall be permitted to                     for use in all groups of Class I locations. Further information
                    be used for atmospheres containing allyl glycidyl ether, n-                  is available from the UL Hazardous Locations Equipment
                    butyl glycidyl ether, ethylene oxide, propylene oxide, and                   Directory and the FM Approval Guide. It should be noted
                    acrolein, provided all conduit runs into explosionproof                      (from the UL Directory) that ‘‘only those products bearing
                    equipment are provided with explosionproof seals installed                   the appropriate listing mark and the company’s name, trade
                    within 450 mm (18 in.) of the enclosure.                                     name, trademark, or other recognized identification should
                                                                                                 be considered as covered by UL’s Listing and Follow-Up
                                                                                                 Service.’’ Other testing laboratories may also list equipment
                    The specific materials identified in Exception No. 1 and                     for hazardous locations for use in the United States. The
                    Exception No. 2 to 500.6(A)(2) produce high pressures be-                    acceptance of the listing agency is the responsibility of the
                    cause of pressure piling in unsealed conduits. If all conduits               authority having jurisdiction.
                    are sealed, the volume of air and gas between the sealing                         Several testing laboratories outside the United States
                    fittings and the arcing contacts is limited, thereby minimizing              also provide listing, approval, or certification of equipment
                    the pressure that can build within the enclosure and the                     for use in hazardous locations. However, they may not be
                    raceway.                                                                     testing and investigating the equipment for use in hazardous
                                                                                                 locations, as defined in Article 500. Some international labo-
                    (3) Group C Flammable gas, flammable liquid–produced                         ratories certify equipment for installation where the classifi-
                    vapor, or combustible liquid–produced vapor mixed with                       cation is according to the IEC classification scheme covered
                    air that may burn or explode, having either a maximum                        in Articles 505 and 506. Commentary Table 5.1 is an alpha-
                    experimental safe gap (MESG) value greater than 0.45 mm                      betical listing of selected combustible materials with their
                    and less than or equal to 0.75 mm, or a minimum igniting                     group classification and relevant physical properties. All the
                    current ratio (MIC ratio) greater than 0.40 and less than or                 materials included in this table have been evaluated for the
                    equal to 0.80. [NFPA 497:3.3]                                                purpose of designating the appropriate gas group.
                            FPN: A typical Class I, Group C material is ethylene.                     This information is used to properly select electrical
                    (4) Group D Flammable gas, flammable liquid–produced                         equipment for use in Class I locations. The combustible
                    vapor, or combustible liquid–produced vapor mixed with                       materials in Commentary Table 5.1 are classified into four
                    air that may burn or explode, having either a maximum                        Class I, division groups—A, B, C, and D—or three Class
                    experimental safe gap (MESG) value greater than 0.75 mm                      I, zone groups—IIC, IIB, and IIA—depending on their prop-
                    or a minimum igniting current ratio (MIC ratio) greater than                 erties. Commentary Table 5.1 is extracted from NFPA 497,
                    0.80. [NFPA 497:3.3]                                                         Recommended Practice for the Classification of Flammable
                                                                                                 Liquids, Gases, or Vapors and of Hazardous (Classified)
                            FPN No. 1: A typical Class I, Group D material is pro-
                            pane.
                                                                                                 Locations for Electrical Installations in Chemical Process
                            FPN No. 2: For classification of areas involving ammo-
                                                                                                 Areas. For the definitions of terms used in this table, refer
                                                                                                 to NFPA 497.
                                                                                                                                                                               --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                            nia atmospheres, see ANSI/ASHRAE 15-1994, Safety
                            Code for Mechanical Refrigeration, and ANSI/CGA                           Publication NMAB 353-4, Classifications of Dusts Rel-
                            G2.1-1989, Safety Requirements for the Storage and                   ative to Electrical Equipment in Class II Hazardous Loca-
                            Handling of Anhydrous Ammonia.                                       tions, published in 1982 by the Committee on Evaluation
                                                                                                 of Industrial Hazards, National Materials Advisory Board,
                    Class I flammable gases or vapors are separated into four                    Commission on Engineering and Technical Systems, Na-
                    different groups—A, B, C, and D. The Code requirements                       tional Research Council, and National Academy of Sciences,




                 654                                                                                                                2005 National Electrical Code Handbook

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                                                    Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                                         500.6



                           Commentary Table 5.1 Selected Chemicals

                                                                   Class I      Flash                 Vapor                                 Vapor                 Class I
                                                                   Division     Point AIT            Density                              Pressureb                Zone               MIE      MIC MESG
                                       Chemical         CAS No.     Group Typea ( C) ( C) %LFL %UFL (Air   1)                             (mm Hg)                 Groupc              (mJ)     Ratio (mm)

                           Acetaldehyde                 75-07-0     Cd          I    38   175      4.0      60.0            1.5                874.9                   IIA             0.37    0.98   0.92
                           Acetic Acid                  64-19-7     Dd         II    43   464      4.0      19.9            2.1                 15.6                   IIA                     2.67   1.76
                           Acetic Acid-tert-Butyl       540-88-5    D          II                  1.7       9.8            4.0                 40.6
                             Ester
                           Acetic Anhydride             108-24-7    D           II   54   316      2.7      10.3            3.5              4.9
                           Acetone                      67-64-1     Dd           I        465      2.5      12.8            2.0            230.7                       IIA             1.15    1.00   1.02
                           Acetone Cyanohydrin          75-86-5     D         IIIA   74   688      2.2      12.0            2.9              0.3
                           Acetonitrile                 75-05-8     D            I    6   524      3.0      16.0            1.4             91.1                       IIA                            1.50
                           Acetylene                    74-86-2     Ad        GAS         305      2.5      99.9            0.9         36,600.0                       IIC             0.017   0.28   0.25
                           Acrolein (Inhibited)         107-02-8    B(C)d        I        235      2.8      31.0            1.9            274.1                       IIB             0.13
                           Acrylic Acid                 79-10-7     D           II   54   438      2.4       8.0            2.5              4.3
                           Acrylonitrile              107-13-1      D   d
                                                                                 I   26   481      3.0      17.0            1.8                108.5                   IIB             0.16    0.78   0.87
                           Adiponitrile                111-69-3     D         IIIA   93   550                               1.0                  0.002
                           Allyl Alcohol               107-18-6     Cd           I   22   378      2.5      18.0            2.0                 25.4                                                  0.84
                           Allyl Chloride              107-05-1     D            I   32   485      2.9      11.1            2.6                366.0                                           1.33   1.17
                           Allyl Glycidyl Ether        106-92-3     B(C)e       II         57                               3.9
                           Alpha-Methyl Styrene         98-83-9     D           II        574      0.8      11.0            4.1                2.7
                           n-Amyl Acetate              628-63-7     D            I   25   360      1.1       7.5            4.5                4.2                                                    1.02
                           sec-Amyl Acetate            626-38-0     D            I         23                1.1            7.5                4.5                     IIA
                           Ammonia                    7664-41-7     Dd,f         I        498    15.0       28.0            0.6             7498.0                     IIA           680.0     6.85   3.17
                           Aniline                      62-53-3     D         IIIA   70   615     1.3       11.0            3.2                0.7                     IIA
                           Benzene                    71-43-2       Dd          I    11   498      1.2        7.8           2.8                  94.8                  IIA             0.20    1.00   0.99
                           Benzyl Chloride              98-87-3     D         IIIA        585      1.1                      4.4                   0.5
                           Bromopropyne                106-96-7     D           I    10   324      3.0
                           n-Butane                   3583-47-9     Dd,g      GAS         288      1.9       8.5            2.0                                                        0.25    0.94   1.07
                           1,3-Butadiene               106-99-0     B(D)d,e   GAS    76   420      2.0      12.0            1.9                                        IIB             0.13    0.76   0.79
                           1-Butanol                    71-36-3     Dd          I    36   343      1.4      11.2            2.6                     7.0                IIA                            0.91
                           2-Butanol                    78-92-2     Dd          I    36   405      1.7       9.8            2.6                                        IIA
                           Butylamine                  109-73-9     D         GAS    12   312      1.7       9.8            2.5               92.9                                             1.13
                           Butylene                   25167-67-3    D           I         385      1.6      10.0            1.9             2214.6
                           n-Butyraldehyde             123-72-8     Cd          I    12   218      1.9      12.5            2.5              112.2                                                    0.92
                           n-Butyl Acetate            123-86-4      D d
                                                                                I    22   421      1.7        7.6           4.0                  11.5                  IIA                     1.08   1.04
                           sec-Butyl Acetate           105-46-4     D          II     8            1.7        9.8           4.0                  22.2
                           tert-Butyl Acetate          540-88-5     D          II                  1.7        9.8           4.0                  40.6
                           n-Butyl Acrylate            141-32-2     D          II    49   293      1.7        9.9           4.4                   5.5
                             (Inhibited)
                           n-Butyl Glycidyl Ether      2426-08-6    B(C)e       II
                           n-Butyl Formal               110-62-3    C         IIIA                                                               34.3
                           Butyl Mercaptan              109-79-5    C            I    2                                     3.1                  46.4
                           Butyl-2-Propenoate           141-32-2    D           II   49            1.7        9.9           4.4                   5.5
                           para tert-Butyl Toluene      98-51-1     D         IIIA
                           n-Butyric Acid               107-92-6    Dd        IIIA   72   443      2.0      10.0            3.0                     0.8
                           Carbon Disulfide           75-15-0       d,h
                                                                                I    30    90     1.3       50.0            2.6                358.8                   IIC             0.009   0.39   0.20
                           Carbon Monoxide             630-08-0     Cd        GAS         609    12.5       74.0            0.97                                       IIA                            0.84
                           Chloroacetaldehyde          107-20-0     C         IIIA   88                                                          63.1
                           Chlorobenzene               108-90-7     D           I    29   593      1.3        9.6           3.9                  11.9
                           1-Chloro-1-                2425-66-3     C         IIIA
                             Nitropropane
                           Chloroprene                  126-99-8    D         GAS    20            4.0      20.0            3.0
                           Cresol                      1319-77-3    D         IIIA   81   559      1.1                      3.7
                           Crotonaldehyde              4170-30-3    Cd          I    13   232      2.1      15.5            2.4                  33.1                  IIB                            0.81
                           Cumene                       98-82-8     D           I    36   424      0.9       6.5            4.1                   4.6                  IIA
                           Cyclohexane                  110-82-7    D           I    17   245      1.3       8.0            2.9                  98.8                  IIA             0.22    1.0    0.94

                                                                                                                                                                                                 (continues)




                           National Electrical Code Handbook 2005                                                                  --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                       655

Copyright National Fire Protection Association                                                  Document provided by IHS Licensee=ExxonMobil/1890500101, 05/05/2005 19:58:07
Provided by IHS under license with NFPA                                                         MDT Questions or comments about this message: please call the Document Policy
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                 500.6                                                                Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                    Commentary Table 5.1 Continued

                                                                                                     Class I            Flash                    Vapor    Vapor   Class I
                                                                                                     Division           Point   AIT             Density Pressureb Zone                                    MIE          MIC MESG
                                                                           Chemical      CAS No.      Group     Typea    ( C)   ( C) %LFL %UFL (Air   1) (mm Hg) Groupc                                   (mJ)         Ratio (mm)

                    Cyclohexanol                                                         108-93-0      D         IIIA     68    300                           3.5             0.7             IIA
                    Cyclohexanone                                                        108-94-1      D           II     44    245   1.1         9.4         3.4             4.3             IIA                             0.98
                    Cyclohexene                                                          110-83-8      D            I      6    244   1.2                     2.8            89.4                                      0.97
                    Cyclopropane                                                         75-19-4       Dd           I           503   2.4       10.4          1.5          5430.0             IIB       0.17           0.84   0.91
                    p-Cymene                                                             99-87-6       D           II     47    436   0.7        5.6          4.6             1.5             IIA
                    Decene                                                               872-05-9      D           II           235                           4.8             1.7
                    n-Decaldehyde                                                        112-31-2      C         IIIA                                                         0.09
                    n-Decanol                                                            112-30-1      D         IIIA     82    288                           5.3             0.008
                    Decyl Alcohol                                                        112-30-1      D         IIIA     82    288                           5.3             0.008
                    Diacetone Alcohol                                                    123-42-2      D         IIIA     64    603   1.8         6.9         4.0             1.4

                    Di-Isobutylene                                                      25167-70-8     D   d
                                                                                                                    I      2    391   0.8         4.8         3.8                                       0.96
                    Di-Isobutyl Ketone                                                   108-83-8      D           II     60    396   0.8         7.1         4.9               1.7
                    o-Dichlorobenzene                                                    955-50-1      D         IIIA     66    647   2.2         9.2         5.1                             IIA
                    1,4-Dichloro-2,3                                                    3583-47-9      Dd           I                 1.9         8.5         2.0                                       0.25           0.98   1.07
                      Epoxybutane
                    1,1-Dichloroethane                                                   1300-21-6     D           I            438   6.2       16.0          3.4           227.0                                             1.82
                    1,2-Dichloroethylene                                                  156-59-2     D           I      97    460   5.6       12.8          3.4           204.0             IIA
                    1,1-Dichloro-1-                                                       594-72-9     C         IIIA     76                                  5.0
                      Nitroethane
                    1,3-Dichloropropene                                                 10061-02-6     D          I       35          5.3       14.5          3.8
                    Dicyclopentadiene                                                     77-73-6      C          I       32    503                                             2.8                                           0.91
                    Diethylamine                                                         109-87-9      Cd         I       28    312   1.8       10.1          2.5                             IIA                             1.15

                    Diethylaminoethanol                                                 100-37-8       C         IIIA     60    320                           4.0               1.6           IIA
                    Diethyl Benzene                                                     25340-17-4     D           II     57    395                           4.6
                    Diethyl Ether (Ethyl                                                  60-29-7      Cd           I     45    160   1.7       48.0          2.6           538.0             IIB       0.19           0.88   0.83
                      Ether)
                    Diethylene Glycol                                                    112-34-5      C         IIIA     78    228   0.9       24.6          5.6               0.02
                      Monobutyl Ether
                    Diethylene Glycol                                                    111-77-3      C         IIIA     93    241                                             0.2
                      Monomethyl Ether
                    n-n-Dimethyl Aniline                                                 121-69-7      C         IIIA     63    371   1.0                     4.2               0.7
                    Dimethyl Formamide                                                   68-12-2       D           II     58    455   2.2       15.2          2.5               4.1                                           1.08
                    Dimethyl Sulfate                                                     77-78-1       D         IIIA     83    188                           4.4               0.7

                    Dimethylamine                                                       124-40-3       C        GAS             400   2.8       14.4          1.6                             IIA
                    2,2-Dimethylbutane                                                    75-83-2      Dg          I      48    405                                         319.3
                    2,3-Dimethylbutane                                                    78-29-8      Dg          I            396
                    3,3-Dimethylheptane                                                 1071-26-7      Dg          I            325                                           10.8
                    2,3-Dimethylhexane                                                  31394-54-4     Dg          I            438
                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    2,3-Dimethylpentane                                                  107-83-5      Dg          I            335                                          211.7
                    Di-N-Propylamine                                                     142-84-7      C           I      17    299                                           27.1
                    1,4-Dioxane                                                          123-91-1      Cd          I      12    180   2.0       22.0          3.0             38.2            IIB       0.19                  0.70
                    Dipentene                                                            138-86-3      D          II      45    237   0.7        6.1          4.7                                                             1.18
                    Dipropylene Glycol                                                  34590-94-8     C        IIIA      85          1.1        3.0          5.1               0.5
                      Methyl Ether

                    Diisopropylamine                                                    108-18-9       C        GAS       6     316   1.1         7.1         3.5
                    Dodecene                                                            6842-15-5      D        IIIA    100     255
                    Epichlorohydrin                                                     3132-64-7      Cd         I      33     411   3.8       21.0          3.2             13.0
                    Ethane                                                                74-84-0      Dd       GAS      29     472   3.0       12.5          1.0                             IIA       0.24           0.82   0.91
                    Ethanol                                                               64-17-5      Dd         I      13     363   3.3       19.0          1.6            59.5             IIA                      0.88   0.89
                    Ethylamine                                                            75-04-7      Dd         I      18     385   3.5       14.0          1.6          1048.0                       2.4
                    Ethylene                                                              74-85-1      Cd       GAS       0     450   2.7       36.0          1.0                             IIB       0.070          0.53   0.65
                    Ethylenediamine                                                      107-15-3      Dd         I      33     385   2.5       12.0          2.1             12.5




                 656                                                                                                                                                        2005 National Electrical Code Handbook

Copyright National Fire Protection Association                                                                                         Document provided by IHS Licensee=ExxonMobil/1890500101, 05/05/2005 19:58:07
Provided by IHS under license with NFPA                                                                                                MDT Questions or comments about this message: please call the Document Policy
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                                                   Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                                              500.6



                           Commentary Table 5.1 Continued

                                                                     Class I                Flash                            Vapor                             Vapor          Class I
                                                                     Division               Point           AIT             Density                          Pressureb         Zone         MIE    MIC MESG
                                        Chemical         CAS No.      Group     Typea        ( C)           ( C) %LFL %UFL (Air   1)                         (mm Hg)          Groupc        (mJ)   Ratio (mm)

                           Ethylenimine                  151-56-4    Cd            I             11         320            3.3            54.8   1.5            211.0                      0.48
                           Ethylene Chlorohydrin         107-07-3    D           IIIA            59         425            4.9            15.9   2.8              7.2

                           Ethylene Dichloride           107-06-2    Dd            I             13         413            6.2            16.0   3.4             79.7
                           Ethylene Glycol               111-15-9    C            II             47         379            1.7                   4.7              2.3                              0.53   0.97
                             Monoethyl Ether
                             Acetate
                           Ethylene Glycol               112-07-2    C           IIIA                       340            0.9             8.5                     0.9
                             Monobutyl Ether
                             Acetate
                           Ethylene Glycol               111-76-2    C           IIIA                       238            1.1            12.7   4.1               1.0
                             Monobutyl Ether
                           Ethylene Glycol               110-80-5    C            II                        235            1.7            15.6   3.0               5.4          0.84
                             Monoethyl Ether
                           Ethylene Glycol               109-86-4    D            II                        285            1.8            14.0   2.6               9.2                                    0.85
                             Monomethyl Ether
                           Ethylene Oxide                75-21-8     B(C)d,e      I              20         429            3.0            99.9   1.5          1314.0             IIB       0.065   0.47   0.59

                           2-Ethylhexaldehyde            123-05-7    C             II            52         191            0.8             7.2   4.4              1.9
                           2-Ethylhexanol                104-76-7    D           IIIA            81                        0.9             9.7   4.5              0.2
                           2-Ethylhexyl Acrylate         103-09-3    D           IIIA            88         252                                                   0.3
                           Ethyl Acetate                 141-78-6    Dd             I             4         427            2.0            11.5   3.0             93.2                      0.46           0.99
                           Ethyl Acrylate (Inhibited)    140-88-5    Dd             I             9         372            1.4            14.0   3.5             37.5            IIA                      0.86
                           Ethyl Alcohol                 64-17-5     Dd             I            13         363            3.3            19.0   1.6             59.5                                     0.89
                           Ethyl Sec-Amyl Ketone         541-85-5    D             II            59
                           Ethyl Benzene                 100-41-4    D              I            21         432            0.8             6.7   3.7               9.6
                           Ethyl Butanol                 97-95-0     D             II            57                        1.2             7.7   3.5               1.5
                           Ethyl Butyl Ketone            106-35-4    D             II            46                                              4.0               3.6


                           Ethyl Chloride                75-00-3     D          GAS              50         519            3.8            15.4   2.2
                           Ethyl Formate                 109-94-4    D          GAS              20         455            2.8            16.0   2.6                             IIA                      0.94
                           Ethyl Mercaptan               75-08-1     Cd            I             18         300            2.8            18.0   2.1            527.4                              0.90   0.90
                           n-Ethyl Morpholine            100-74-3    C             I             32                                              4.0
                           2-Ethyl-3-Propyl Acrolein     645-62-5    C          IIIA             68                                              4.4
                           Ethyl Silicate                78-10-4     D            II                                                             7.2
                           Formaldehyde (Gas)            50-00-0     B          GAS              60         429          7.0              73.0   1.0              0.57
                           Formic Acid                   64-18-6     D            II             50         434         18.0              57.0   1.6             42.7                                     1.86
                           Fuel Oil 1                   8008-20-6    D            II             72         210          0.7               5.0
                           Furfural                      98-01-1     C          IIIA             60         316          2.1              19.3   3.3               2.3                                    0.94

                           Furfuryl Alcohol               98-00-0    C           IIIA            75         490            1.8            16.3   3.4               0.6
                           Gasoline                     8006-61-9    Dd             I            46         280            1.4             7.6   3.0
                           n-Heptane                     142-82-5    Dd             I             4         204            1.0             6.7   3.5             45.5            IIA       0.24    0.88   0.91
                           n-Heptene                    81624-04-6   Dg             I             1         204                                  3.4                                                      0.97
                           n-Hexane                      110-54-3    Dd,g           I            23         225            1.1             7.5   3.0            152.0            IIA       0.24    0.88   0.93
                           Hexanol                       111-27-3    D           IIIA            63                                              3.5              0.8            IIA                      0.98
                           2-Hexanone                    591-78-6    D              I            35         424            1.2             8.0   3.5             10.6
                           Hexene                        592-41-6    D              I            26         245            1.2             6.9                  186.0
                           sec-Hexyl Acetate             108-84-9    D             II            45                                              5.0
                           Hydrazine                     302-01-2    C             II            38            23                         98.0   1.1             14.4

                           Hydrogen                     1333-74-0    Bd         GAS                         520            4.0            75.0   0.1                             IIC       0.019   0.25   0.28
                           Hydrogen Cyanide              74-90-8     Cd         GAS              18         538            5.6            40.0   0.9                             IIB                      0.80

                                                                                                                                                                                                     (continues)




                           National Electrical Code Handbook 2005                                                                                                                                            657
                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---



Copyright National Fire Protection Association                                                                  Document provided by IHS Licensee=ExxonMobil/1890500101, 05/05/2005 19:58:07
Provided by IHS under license with NFPA                                                                         MDT Questions or comments about this message: please call the Document Policy
                                                                                                                Group at 303-397-2295.
                 500.6                                                             Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                    Commentary Table 5.1 Continued

                                                                                                    Class I            Flash                    Vapor                        Vapor           Class I
                                                                                                    Division           Point   AIT             Density                     Pressureb          Zone        MIE        MIC MESG
                                                                        Chemical        CAS No.      Group     Typea    ( C)   ( C) %LFL %UFL (Air   1)                    (mm Hg)           Groupc       (mJ)       Ratio (mm)

                    Hydrogen Selenide                                                   7783-07-5     C          I                                                          7793.0
                    Hydrogen Sulfide                                                    7783-06-4     Cd       GAS             260    4.0       44.0           1.2                                       0.068              0.90
                    Isoamyl Acetate                                                      123-92-2     D          I        25   360    1.0        7.5           4.5                6.1
                    Isoamyl Alcohol                                                      123-51-3     D         II        43   350    1.2        9.0           3.0                3.2                                       1.02
                    Isobutane                                                            75-28-5      Dg       GAS             460    1.8        8.4           2.0
                    Isobutyl Acetate                                                     110-19-0     Dd         I        18   421    2.4       10.5           4.0              17.8
                    Isobutyl Acrylate                                                    106-63-8     D          I             427                             4.4               7.1
                    Isobutyl Alcohol                                                     78-83-1      Dd         I        40   416    1.2       10.9           2.5              10.5                                 0.92   0.98

                    Isobutyraldehyde                                                     78-84-2      C        GAS        40 196      1.6       10.6           2.5
                    Isodecaldehyde                                                       112-31-2     C        IIIA                                            5.4              0.09
                    Isohexane                                                            107-83-5     Dg                     264                                              211.7                                  1.00
                    Isopentane                                                           78-78-4      Dg                     420                                              688.6
                    Isooctyl Aldehyde                                                    123-05-7     C         II           197                                                1.9
                    Isophorone                                                           78-59-1      D                   84 460      0.8         3.8          4.8              0.4
                    Isoprene                                                             78-79-5      Dd         I        54 220      1.5         8.9          2.4            550.6
                    Isopropyl Acetate                                                    108-21-4     D          I           460      1.8         8.0          3.5             60.4
                    Isopropyl Ether                                                      108-20-3     Dd         I        28 443      1.4         7.9          3.5            148.7                      1.14               0.94
                    Isopropyl Glycidyl Ether                                            4016-14-2     C          I

                    Isopropylamine                                                       75-31-0      D        GAS        26 402      2.3       10.4           2.0                                       2.0
                    Kerosene                                                           8008-20-6      D         II        72 210      0.7        5.0                                            IIA
                    Liquefied Petroleum Gas                                            68476-85-7     D          I           405
                    Mesityl Oxide                                                       141-97-9      Dd         I        31 344      1.4        7.2           3.4              47.6
                    Methane                                                              74-82-8      Dd       GAS       223 630      5.0       15.0           0.6                              IIA      0.28        1.00   1.12
                    Methanol                                                             67-56-1      Dd         I        12 385      6.0       36.0           1.1            126.3             IIA      0.14        0.82   0.92
                    Methyl Acetate                                                       79-20-9      D        GAS        10 454      3.1       16.0           2.6                              IIB                  1.08   0.99
                    Methyl Acrylate                                                      96-33-3      D        GAS         3 468      2.8       25.0           3.0                                                   0.98   0.85
                    Methyl Alcohol                                                       67-56-1      Dd         I           385      6.0       36.0           1.1            126.3                                         0.91
                    Methyl Amyl Alcohol                                                 108-11-2      D         II        41          1.0        5.5           3.5              5.3                                         1.01

                    Methyl Chloride                                                      74-87-3      D        GAS        46 632      8.1       17.4           1.7                                                          1.00
                    Methyl Ether                                                        115-10-6      Cd       GAS        41 350      3.4       27.0           1.6                                                   0.85   0.84
                    Methyl Ethyl Ketone                                                  78-93-3      Dd        I          6 404      1.4       11.4           2.5              92.4                     0.53        0.92   0.84
                    Methyl Formal                                                       534-15-6      Cd        I          1 238                               3.1
                    Methyl Formate                                                      107-31-3      D        GAS        19 449      4.5       23.0           2.1                                                          0.94
                    2-Methylhexane                                                     31394-54-4     Dg        I            280
                    Methyl Isobutyl Ketone                                              141-79-7      Dd        I         31 440      1.2        8.0           3.5              11.0
                    Methyl Isocyanate                                                   624-83-9      D        GAS        15 534      5.3       26.0           2.0
                    Methyl Mercaptan                                                     74-93-1      C        GAS        18          3.9       21.8           1.7
                      --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    Methyl Methacrylate                                                  80-62-6      D         I         10 422      1.7        8.2           3.6              37.2            IIA                         0.95

                    Methyl N-Amyl Ketone         110-43-0                                             D         II        49 393      1.1         7.9          3.9             3.8
                    Methyl Tertiary Butyl Ether 1634-04-4                                             D          I        80 435      1.6         8.4          0.2           250.1
                    2-Methyloctane              3221-61-2                                                                    220                                               6.3
                    2-Methylpropane               75-28-5                                             Dg         I           460                                            2639.0
                    Methyl-1-Propanol             78-83-1                                             Dd         I        40 416      1.2       10.9           2.5            10.1                                          0.98
                    Methyl-2-Propanol             75-65-0                                             Dd         I        10 360      2.4        8.0           2.6            42.2
                    2-Methyl-5-Ethyl Pyridine    104-90-5                                             D                   74          1.1        6.6           4.2
                    Methylacetylene               74-99-7                                             Cd         I                    1.7                      1.4          4306.0                       0.11
                    Methylacetylene-            27846-30-6                                            C          I                                                                                                          0.74
                      Propadiene
                    Methylal                     109-87-5                                             C          I        18 237      1.6       17.6           2.6            398.0




                 658                                                                                                                                                      2005 National Electrical Code Handbook

Copyright National Fire Protection Association                                                                                       Document provided by IHS Licensee=ExxonMobil/1890500101, 05/05/2005 19:58:07
Provided by IHS under license with NFPA                                                                                              MDT Questions or comments about this message: please call the Document Policy
                                                                                                                                     Group at 303-397-2295.
                                                   Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                                      500.6



                           Commentary Table 5.1 Continued

                                                                    Class I            Flash                    Vapor                             Vapor                 Class I
                                                                    Division           Point   AIT             Density                          Pressureb                Zone      MIE     MIC MESG
                                        Chemical        CAS No.      Group     Typea    ( C)   ( C) %LFL %UFL (Air   1)                         (mm Hg)                 Groupc     (mJ)    Ratio (mm)

                           Methylamine                   74-89-5      D        GAS           430        4.9      20.7            1.0                                        IIA                   1.10
                           2-Methylbutane                78-78-4      Dg                  56 420        1.4       8.3            2.6                688.6
                           Methylcyclohexane            208-87-2      D          I         4 250        1.2       6.7            3.4                                               0.27
                           Methylcyclohexanol          25630-42-3     D                   68 296                                 3.9
                           2-Methycyclohexanone         583-60-8      D         II                                               3.9
                           2-Methylheptane                            Dg                     420
                           3-Methylhexane               589-34-4      Dg                     280                                                      61.5
                           3-Methylpentane              94-14-0       Dg                     278
                           2-Methylpropane              75-28-5       Dg         I           460                                                  2639.0
                           2-Methyl-1-Propanol          78-83-1       Dd         I        40 223        1.2      10.9            2.5                10.5

                           2-Methyl-2-Propanol           75-65-0      Dd         I             478      2.4        8.0           2.6                  42.2
                           2-Methyloctane               2216-32-2     Dg                       220
                           3-Methyloctane               2216-33-3     Dg                       220                                                       6.3
                           4-Methyloctane               2216-34-4     Dg                       225                                                       6.8
                           Monoethanolamine              141-43-5     D                   85   410                               2.1                     0.4                IIA
                           Monoisopropanolamine          78-96-6      D                   77   374                               2.6                     1.1
                           Monomethyl Aniline            100-61-8     C                        482                                                       0.5
                           Monomethyl Hydrazine          60-34-4      C          I        23   194      2.5      92.0            1.6
                           Morpholine                    110-91-8     Cd        II        35   310      1.4      11.2            3.0                  10.1                                        0.95
                           Naphtha (Coal Tar)           8030-30-6     D         II        42   277                                                                          IIA

                           Naphtha (Petroleum)         8030-30-6      Dd,i       I        42   288      1.1        5.9           2.5                                        IIA
                           Neopentane                   463-82-1      Dg                  65   450      1.4        8.3           2.6              1286.0
                           Nitrobenzene                  98-95-3      D                   88   482      1.8                      4.3                 0.3                                          0.94
                           Nitroethane                   79-24-3      C          I        28   414      3.4                      2.6                20.7                    IIA                   0.87
                           Nitromethane                  75-52-5      C          I        35   418      7.3                      2.1                36.1                    IIA            0.92   1.17
                           1-Nitropropane               108-03-2      C          I        34   421      2.2                      3.1                10.1                                          0.84
                           2-Nitropropane                79-46-9      Cd         I        28   428      2.6      11.0            3.1                17.1
                           n-Nonane                     111-84-2      Dg         I        31   205      0.8       2.9            4.4                 4.4                    IIA
                           Nonene                      27214-95-8     D          I                      0.8                      4.4
                           Nonyl Alcohol                143-08-8      D                                 0.8        6.1           5.0                     0.02               IIA

                           n-Octane                     111-65-9      D d,g
                                                                                 I        13 206        1.0        6.5           3.9                  14.0                  IIA                   0.94
                           Octene                      25377-83-7     D          I         8 230        0.9                      3.9
                           n-Octyl Alcohol              111-87-5      D                                                          4.5                  0.08                  IIA                   1.05
                           n-Pentane                    109-66-0      Dd,g       I        40   243      1.5       7.8            2.5                513.0                          0.28    0.97   0.93
                           1-Pentanol                    71-41-0      Dd         I        33   300      1.2      10.0            3.0                  2.5                   IIA
                           2-Pentanone                  107-87-9      D          I         7   452      1.5       8.2            3.0                 35.6                                         0.99
                           1-Pentene                    109-67-1      D          I        18   275      1.5       8.7            2.4                639.7
                           2-Pentene                    109-68-2      D          I        18                                     2.4
                           2-Pentyl Acetate             626-38-0      D          I        23            1.1        7.5           4.5
                           Phenylhydrazine              100-63-0      D                   89                                     3.7                     0.03

                           Process Gas > 30% H2         1333-74-0     Bj       GAS           520        4.0      75.0            0.1                                               0.019   0.45
                           Propane                       74-98-6      Dd       GAS       104 450        2.1       9.5            1.6                                        IIA    0.25    0.82   0.97
                           1-Propanol                    71-23-8      Dd        I         15 413        2.2      13.7            2.1                 20.7                   IIA                   0.89
                           2-Propanol                    67-63-0      Dd        I         12 399        2.0      12.7            2.1                 45.4                          0.65           1.00
                           Propiolactone                 57-57-8      D                                 2.9                      2.5                  2.2
                           Propionaldehyde               123-38-6     C          I         9   207      2.6      17.0            2.0                318.5
                           Propionic Acid                79-09-4      D         II        54   466      2.9      12.1            2.5                  3.7
                           Propionic Anhydride           123-62-6     D                   74   285      1.3       9.5            4.5                  1.4
                           n-Propyl Acetate              109-60-4     D          I        14   450      1.7       8.0            3.5                 33.4                                         1.05
                           n-Propyl Ether                111-43-3     Cd         I        21   215      1.3       7.0            3.5                 62.3

                                                                                                                                                                                             (continues)




                           National Electrical Code Handbook 2005                                                                                                                                    659
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                 500.6                                                              Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                    Commentary Table 5.1 Continued

                                                                                                     Class I            Flash                    Vapor                        Vapor           Class I
                                                                                                     Division           Point   AIT             Density                     Pressureb          Zone        MIE        MIC MESG
                                                                         Chemical        CAS No.      Group     Typea    ( C)   ( C) %LFL %UFL (Air   1)                    (mm Hg)           Groupc       (mJ)       Ratio (mm)
                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    Propyl Nitrate                                                       627-13-4    Bd            I       20   175    2.0       100.0
                    Propylene                                                            115-07-1    Dd         GAS       108   455    2.0        11.1          1.5                                       0.28               0.91
                    Propylene Dichloride                                                 78-87-5     D             I       16   557    3.4        14.5          3.9             51.7                                         1.32
                    Propylene Oxide                                                      75-56-9     B(C)d,e       I       37   449    2.3        36.0          2.0            534.4                      0.13               0.70
                    Pyridine                                                             110-86-1    Dd            I       20   482    1.8        12.4          2.7             20.8             IIA
                    Styrene                                                              100-42-5    Dd            I       31   490    0.9         6.8          3.6              6.1             IIA                  1.21
                    Tetrahydrofuran                                                      109-99-9    Cd            I       14   321    2.0        11.8          2.5            161.6             IIB      0.54               0.87
                    Tetrahydronaphthalene                                                119-64-2    D          IIIA            385    0.8         5.0          4.6              0.4
                    Tetramethyl Lead                                                     75-74-1     C            II       38                                   9.2
                    Toluene                                                              108-88-3    Dd            I        4 480      1.1          7.1         3.1              28.53           IIA      0.24

                    n-Tridecene                                                         2437-56-1    D           IIIA                  0.6                      6.4            593.4
                    Triethylamine                                                        121-44-8    Cd            I        9 249      1.2         8.0          3.5             68.5             IIA      0.75
                    Triethylbenzene                                                     25340-18-5   D                     83                     56.0          5.6
                    2,2,3-Trimethylbutane                                                            Dg                       442
                    2,2,4-Trimethylbutane                                                            Dg                       407
                    2,2,3-Trimethylpentane                                                           Dg                       396
                    2,2,4-Trimethylpentane                                                           Dg                       415
                    2,3,3-Trimethylpentane                                                           Dg                       425
                    Tripropylamine                                                        102-69-2   D            II       41                                   4.9                1.5                                       1.13
                    Turpentine                                                           8006-64-2   D             I       35 253      0.8                                         4.8

                    n-Undecene                                                          28761-27-5   D           IIIA                  0.7                      5.5
                    Unsymmetrical Dimethyl                                                57-14-7    Cd            I       15 249      2.0        95.0          1.9                                                          0.85
                      Hydrazine
                    Valeraldehyde                                                        110-62-3    C           I        280 222                               3.0             34.3
                    Vinyl Acetate                                                        108-05-4    Dd          I          6 402      2.6        13.4          3.0            113.4             IIA      0.70               0.94
                    Vinyl Chloride                                                        75-01-4    Dd         GAS        78 472      3.6        33.0          2.2                                                          0.96
                    Vinyl Toluene                                                       25013-15-4   D                     52 494      0.8        11.0          4.1
                    Vinylidene Chloride                                                   75-35-4    D             I          570      6.5        15.5          3.4            599.4                                         3.91
                    Xylene                                                              1330-20-7    Dd            I       25 464      0.9         7.0          3.7                              IIA      0.2
                    Xylidine                                                             121-69-7    C           IIIA      63 371      1.0                      4.2                0.7

                    Notes:
                    a
                      Type is used to designate if the material is a gas, flammable liquid, or combustible liquid. (See 4.2.6 and
                    4.2.7.)
                    b
                      Vapor pressure reflected in units of mm Hg at 25 C (77 F) unless stated otherwise.
                    c
                      Class I, Zone Groups are based on 1996 IEC TR3 60079-20, Electrical apparatus for explosive gas
                    atmospheres — Part 20: Data for flammable gases and vapors, relating to the use of electrical apparatus,
                    which contains additional data on MESG and group classifications.
                    d
                      Material has been classified by test.
                    e
                      Where all conduit runs into explosionproof equipment are provided with explosionproof seals installed within
                    450 mm (18 in.) of the enclosure, equipment for the group classification shown in parentheses is permitted.
                    f
                      For classification of areas involving ammonia, see ASHRAE 15, Safety Code for Mechanical Refrigeration,
                    and ANSI/CGA G2.1, Safety Requirements for the Storage and Handling of Anhydrous Ammonia.
                    g
                      Commercial grades of aliphatic hydrocarbon solvents are mixtures of several isomers of the same chemical
                    formula (or molecular weight). The autoignition temperatures of the individual isomers are significantly different.
                    The electrical equipment should be suitable for the AIT of the solvent mixture. (See A.4.4.2.)
                    h
                      Certain chemicals have characteristics that require safeguards beyond those required for any of the above
                    groups. Carbon disulfide is one of these chemicals because of its low autoignition temperature and the
                    small joint clearance necessary to arrest its flame propagation.
                    i
                     Petroleum naphtha is a saturated hydrocarbon mixture whose boiling range is 20 C to 135 C (68 F to
                    275 F). It is also known as benzine, ligroin, petroleum ether, and naphtha.
                    j
                     Fuel and process gas mixtures found by test not to present hazards similar to those of hydrogen may be
                    grouped based on the test results.
                    Source: Table 4.4.2 in NFPA 497, Recommended Practice for the Classification of Flammable Liquids,
                    Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process
                    Areas, 2004 edition.




                 660                                                                                                                                                       2005 National Electrical Code Handbook

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                                                                    Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                 500.7



                           includes a description of the test methods used to determine                                Commentary Table 5.2 is an alphabetical listing of selected
                           the ignition temperatures and electrical resistivity of com-                                combustible materials with their group classification and
                           bustible dusts. The publication is available from the National                              relevant physical properties. All the materials included in
                           Technical Information Service (NTIS), Springfield, VA                                       this table have been evaluated for the purpose of designating
                           22151.                                                                                      the appropriate dust group. This information is used to prop-
                                                                                                                       erly select electrical equipment for use in Class II locations.
                                                                                                                       Combustible dusts are classified into three Class II, division
                           (B) Class II Group Classifications Class II groups shall                                    groups — E, F, and G — depending on their properties.
                           be in accordance with 500.6(B)(1) through (B)(3).                                           Commentary Table 5.2 is extracted from NFPA 499, Recom-
                                                                                                                       mended Practice for the Classification of Combustible Dusts
                           (1) Group E Atmospheres containing combustible metal                                        and of Hazardous (Classified) Locations for Electrical In-
                           dusts, including aluminum, magnesium, and their commer-                                     stallations in Chemical Process Areas. For the definitions
                           cial alloys, or other combustible dusts whose particle size,                                of terms used in the table, refer to NFPA 499.
                           abrasiveness, and conductivity present similar hazards in the                                    As in Class I locations, equipment must be approved
                           use of electrical equipment. [NFPA 499:3.3]                                                 not only for the class but also for the specific group. It is
                                   FPN: Certain metal dusts may have characteristics that                              important that, in addition to the proper selection of equip-
                                   require safeguards beyond those required for atmospheres                            ment, high standards of installation be maintained for subse-
                                   containing the dusts of aluminum, magnesium, and their                              quent additions or alterations.
                                   commercial alloys. For example, zirconium, thorium, and
                                   uranium dusts have extremely low ignition temperatures
                                   [as low as 20 C (68 F)] and minimum ignition energies                               500.7 Protection Techniques
                                   lower than any material classified in any of the Class I
                                   or Class II groups.
                                                                                                                       Section 500.7(A) through 500.7(L) shall be acceptable pro-
                                                                                                                       tection techniques for electrical and electronic equipment in
                           (2) Group F Atmospheres containing combustible car-                                         hazardous (classified) locations.
                           bonaceous dusts that have more than 8 percent total en-                                     (A) Explosionproof Apparatus This protection technique
                           trapped volatiles (see ASTM D 3175-89, Standard Test                                        shall be permitted for equipment in Class I, Division 1 or
                           Method for Volatile Material in the Analysis Sample for                                     2 locations.
                           Coal and Coke, for coal and coke dusts) or that have been
                           sensitized by other materials so that they present an explo-                                (B) Dust Ignitionproof This protection technique shall be
                           sion hazard. Coal, carbon black, charcoal, and coke dusts                                   permitted for equipment in Class II, Division 1 or 2 locations.
                           are examples of carbonaceous dusts. [NFPA 499:3.3]                                          (C) Dusttight This protection technique shall be permitted
                                                                                                                       for equipment in Class II, Division 2 or Class III, Division
                           (3) Group G Atmospheres containing combustible dusts                                        1 or 2 locations.
                           not included in Group E or F, including flour, grain, wood,
                           plastic, and chemicals.                                                                     (D) Purged and Pressurized This protection technique
                                                                                                                       shall be permitted for equipment in any hazardous (classi-
                                   FPN No. 1: For additional information on group classifi-                            fied) location for which it is identified.
                                   cation of Class II materials, see NFPA 499-2004, Recom-
                                   mended Practice for the Classification of Combustible
                                   Dusts and of Hazardous (Classified) Locations for Elec-                             NFPA 496, Standard for Purged and Pressurized Enclosures
                                   trical Installations in Chemical Process Areas.                                     for Electrical Equipment, covers purged and pressurized
                                   FPN No. 2: The explosion characteristics of air mixtures                            enclosures for electrical equipment in Class I and Class II
                                   of dust vary with the materials involved. For Class II                              hazardous (classified) locations.
                                   locations, Groups E, F, and G, the classification involves
                                   the tightness of the joints of assembly and shaft openings
                                                                                                                            In Class I locations, purged and pressurized enclosures
                                   to prevent the entrance of dust in the dust-ignitionproof                           are used to eliminate or reduce, within the enclosure, a Class
                                   enclosure, the blanketing effect of layers of dust on the                           I hazardous (classified) location classification, as defined in
                                   equipment that may cause overheating, and the ignition                              Article 500 of the Code. Purged and pressurized enclosures
                                   temperature of the dust. It is necessary, therefore, that                           make it possible for equipment that is not otherwise accept-
                                   equipment be identified not only for the class, but also
                                   for the specific group of dust that will be present.
                                                                                                                       able for hazardous (classified) locations to be used in these
                                                                                                                       locations, in accordance with the Code.
                                   FPN No. 3: Certain dusts may require additional precau-
                                   tions due to chemical phenomena that can result in the                                   Purging is the process of supplying an enclosure with
                                   generation of ignitible gases. See ANSI C2-2002, Na-                                a protective gas at a sufficient flow and positive pressure to
                                   tional Electrical Safety Code, Section 127A, Coal Han-                              reduce the concentration of any flammable gas or vapor
                                   dling Areas.



                           National Electrical Code Handbook 2005                                                                                                                              661
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                 500.7                                                              Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                                                  Commentary Table 5.2 Selected Combustible Materials

                                                                                                                                                                                                               Layer or Cloud
                                                                                              Chemical Name                                      CAS No.                NEC Group              Code          Ignition Temp. ( C)
                   --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                     Acetal, Linear                                                                                            G                 NL                  440
                                                                     Acetoacet-p-phenetidide                                                     122-82-7                      G                 NL                  560
                                                                     Acetoacetanilide                                                            102-01-2                      G                 M                   440
                                                                     Acetylamino-t-nitrothiazole                                                                               G                                     450
                                                                     Acrylamide Polymer                                                                                        G                                     240
                                                                     Acrylonitrile Polymer                                                                                     G                                     460
                                                                     Acrylonitrile-Vinyl Chloride-Vinylidenechloride copolymer (70-20-10)                                      G                                     210
                                                                     Acrylonitrile-Vinyl Pyridine Copolymer                                                                    G                                     240
                                                                     Adipic Acid                                                                 124-04-9                      G                  M                  550
                                                                     Alfalfa Meal                                                                                              G                                     200
                                                                     Alkyl Ketone Dimer Sizing Compound                                                                        G                                     160
                                                                     Allyl Alcohol Derivative (CR-39)                                                                          G                 NL                  500
                                                                     Almond Shell                                                                                              G                                     200
                                                                     Aluminum, A422 Flake                                                       7429-90-5                      E                                     320
                                                                     Aluminum, Atomized Collector Fines                                                                        E                 CL                  550
                                                                     Aluminum—cobalt alloy (60-40)                                                                             E                                     570
                                                                     Aluminum—copper alloy (50-50)                                                                             E                                     830
                                                                     Aluminum—lithium alloy (15% Li)                                                                           E                                     400
                                                                     Aluminum—magnesium alloy (Dowmetal)                                                                       E                 CL                  430
                                                                     Aluminum—nickel alloy (58-42)                                                                             E                                     540
                                                                     Aluminum—silicon alloy (12% Si)                                                                           E                 NL                  670
                                                                     Amino-5-nitrothiazole                                                       121-66-4                      G                                     460
                                                                     Anthranilic Acid                                                            118-92-3                      G                  M                  580
                                                                     Apricot Pit                                                                                               G                                     230
                                                                     Aryl-nitrosomethylamide                                                                                   G                 NL                  490
                                                                     Asphalt                                                                    8052-42-4                      F                                     510
                                                                     Aspirin [acetol (2)]                                                        50-78-2                       G                  M                  660
                                                                     Azelaic Acid                                                                109-31-9                      G                  M                  610
                                                                     Azo-bis-butyronitrile                                                       78-67-1                       G                                     350
                                                                     Benzethonium Chloride                                                                                     G                 CL                  380
                                                                     Benzoic Acid                                                                 65-85-0                      G                 M                   440
                                                                     Benzotriazole                                                                95-14-7                      G                 M                   440
                                                                     Beta-naphthalene-axo-dimethylaniline                                                                      G                                     175
                                                                     Bis(2-hydroxy-5-chlorophenyl) Methane                                       97-23-4                       G                 NL                  570
                                                                     Bisphenol-A                                                                 80-05-7                       G                 M                   570
                                                                     Boron, Commercial Amorphous (85% B)                                        7440-42-8                      E                                     400
                                                                     Calcium Silicide                                                                                          E                                     540
                                                                     Carbon Black (More Than 8% Total Entrapped Volatiles)                                                     F
                                                                     Carboxymethyl Cellulose                                                     9000-11-7                     G                                     290
                                                                     Carboxypolymethylene                                                                                      G                 NL                  520
                                                                     Cashew Oil, Phenolic, Hard                                                                                G                                     180
                                                                     Cellulose                                                                                                 G                                     260
                                                                     Cellulose Acetate                                                                                         G                                     340
                                                                     Cellulose Acetate Butyrate                                                                                G                 NL                  370
                                                                     Cellulose Triacetate                                                                                      G                 NL                  430
                                                                     Charcoal (Activated)                                                       64365-11-3                     F                                     180
                                                                     Charcoal (More Than 8% Total Entrapped Volatiles)                                                         F
                                                                     Cherry Pit                                                                                                G                                     220
                                                                     Chlorinated Phenol                                                                                        G                 NL                  570
                                                                     Chlorinated Polyether Alcohol                                                                             G                                     460
                                                                     Chloroacetoacetanilide                                                      101-92-8                      G                  M                  640




                 662                                                                                                                                                      2005 National Electrical Code Handbook

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                                                                                          Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                             500.7



                                                                       Commentary Table 5.2 Continued

                                                                                                                                                                                                           Layer or Cloud
                                                                                             Chemical Name                                    CAS No.               NEC Group               Code         Ignition Temp. ( C)

                                                                         Chromium (97%) Electrolytic, Milled                                 7440-47-3                      E                                       400
                                                                         Cinnamon                                                                                           G                                       230
                                                                         Citrus Peel                                                                                        G                                       270
                                                                         Coal, Kentucky Bituminous                                                                          F                                       180
                                                                         Coal, Pittsburgh Experimental                                                                      F                                       170
                                                                         Coal, Wyoming                                                                                      F
                                                                         Cocoa Bean Shell                                                                                   G                                       370
                                                                         Cocoa, Natural, 19% Fat                                                                            G                                       240
                                                                         Coconut Shell                                                                                      G                                       220
                                                                         Coke (More Than 8% Total Entrapped Volatiles)                                                      F
                                                                         Cork                                                                                               G                                       210
                                                                         Corn                                                                                               G                                       250
                                                                         Corn Dextrine                                                                                      G                                       370
                                                                         Corncob Grit                                                                                       G                                       240
                                                                         Cornstarch, Commercial                                                                             G                                       330
                                                                         Cornstarch, Modified                                                                               G                                       200
                                                                         Cottonseed Meal                                                                                    G                                       200
                                                                         Coumarone-Indene, Hard                                                                             G                 NL                    520
                                                                         Crag No. 974                                                         533-74-4                      G                 CL                    310
                                                                         Cube Root, South America                                             83-79-4                       G                                       230
                                                                         Di-alphacumyl Peroxide, 40-60 on CA                                  80-43-3                       G                                       180
                                                                         Diallyl Phthalate                                                    131-17-9                      G                 M                     480
                                                                         Dicyclopentadiene Dioxide                                                                          G                 NL                    420
                                                                         Dieldrin (20%)                                                        60-57-1                      G                 NL                    550
                                                                         Dihydroacetic Acid                                                                                 G                 NL                    430
                                                                         Dimethyl Isophthalate                                               1459-93-4                      G                 M                     580
                                                                         Dimethyl Terephthalate                                               120-61-6                      G                 M                     570
                                                                         Dinitro-o-toluamide                                                  148-01-6                      G                 NL                    500
                                                                         Dinitrobenzoic Acid                                                                                G                 NL                    460
                                                                         Diphenyl                                                             92-52-4                       G                 M                     630
                                                                         Ditertiary-butyl-paracresol                                          128-37-0                      G                 NL                    420
                                                                         Dithane m-45                                                        8018-01-7                      G                                       180
                                                                         Epoxy                                                                                              G                 NL                    540
                                                                         Epoxy-bisphenol A                                                                                  G                 NL                    510
                                                                         Ethyl Cellulose                                                                                    G                 CL                    320
                                                                         Ethyl Hydroxyethyl Cellulose                                                                       G                 NL                    390
                                                                         Ethylene Oxide Polymer                                                                             G                 NL                    350
                                                                         Ethylene-maleic Anhydride Copolymer                                                                G                 NL                    540
                                                                         Ferbam                                                             14484-64-1                      G                                       150
                                                                         Ferromanganese, Medium Carbon                                      12604-53-4                      E                                       290
                                                                         Ferrosilicon (88% Si, 9% Fe)                                        8049-17-0                      E                                       800
                                                                         Ferrotitanium (19% Ti, 74.1% Fe, 0.06% C)                                                          E                 CL                    380
                                                                         Flax Shive                                                                                         G                                       230
                                                                         Fumaric Acid                                                         110-17-8                      G                 M                     520
                                                                         Garlic, Dehydrated                                                                                 G                 NL                    360
                                                                         Gilsonite                                                          12002-43-6                      F                                       500
                                                                         Green Base Harmon Dye                                                                              G                                       175
                                                                         Guar Seed                                                                                          G                 NL                    500
                                                                         Gulasonic Acid, Diacetone                                                                          G                 NL                    420
                                                                         Gum, Arabic                                                                                        G                                       260

                                                                                                                                                                                                                          (continues)




                                                                       National Electrical Code Handbook 2005                                                                                                                     663

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                 500.7                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                    Commentary Table 5.2 Continued

                                                                                                                                                                       Layer or Cloud
                                                   Chemical Name                                   CAS No.               NEC Group                 Code              Ignition Temp. ( C)

                        Gum, Karaya                                                                                             G                                                   240
                        Gum, Manila                                                                                             G                     CL                            360
                        Gum, Tragacanth                                                           9000-65-1                     G                                                   260
                        Hemp Hurd                                                                                               G                                                   220
                        Hexamethylene Tetramine                                                   100-97-0                      G                     S                             410
                        Hydroxyethyl Cellulose                                                                                  G                     NL                            410
                        Iron, 98% H2 Reduced                                                                                    E                                                   290
                        Iron, 99% Carbonyl                                                       13463-40-6                     E                                                   310
                        Isotoic Anhydride                                                                                       G                     NL                            700
                        L-sorbose                                                                                               G                     M                             370
                        Lignin, Hydrolized, Wood-type, Fine                                                                     G                     NL                            450
                        Lignite, California                                                                                     F                                                   180
                        Lycopodium                                                                                              G                                                   190
                        Malt Barley                                                                                             G                                                   250
                        Manganese                                                                 7439-96-5                     E                                                   240
                        Magnesium, Grade B, Milled                                                                              E                                                   430
                        Manganese Vancide                                                                                       G                                                   120
                        Mannitol                                                                   69-65-8                      G                      M                            460
                        Methacrylic Acid Polymer                                                                                G                                                   290
                        Methionine (l-methionine)                                                  63-68-3                      G                                                   360
                        Methyl Cellulose                                                                                        G                                                   340
                        Methyl Methacrylate Polymer                                               9011-14-7                     G                     NL                            440
                        Methyl Methacrylate-ethyl Acrylate                                                                      G                     NL                            440
                        Methyl Methacrylate-styrene-butadiene                                                                   G                     NL                            480
                        Milk, Skimmed                                                                                           G                                                   200
                        N,N-Dimethylthio-formamide                                                                              G                                                   230
                        Nitropyridone                                                           100703-82-0                     G                     M                             430
                        Nitrosamine                                                                                             G                     NL                            270
                        Nylon Polymer                                                            63428-84-2                     G                                                   430
                        Para-oxy-benzaldehyde                                                     123-08-0                      G                     CL                            380
                        Paraphenylene Diamine                                                     106-50-3                      G                     M                             620
                        Paratertiary Butyl Benzoic Acid                                            98-73-7                      G                     M                             560
                        Pea Flour                                                                                               G                                                   260
                        Peach Pit Shell                                                                                         G                                                   210
                        Peanut Hull                                                                                             G                                                   210
                        Peat, Sphagnum                                                           94114-14-4                     G                                                   240
                        Pecan Nut Shell                                                          8002-03-7                      G                                                   210
                        Pectin                                                                   5328-37-0                      G                                                   200
                        Pentaerythritol                                                            115-77-5                     G                     M                             400
                        Petrin Acrylate Monomer                                                   7659-34-9                     G                     NL                            220
                        Petroleum Coke (More Than 8% Total Entrapped Volatiles)                                                 F
                        Petroleum Resin                                                          64742-16-1                     G                                                   500
                        Phenol Formaldehyde                                                       9003-35-4                     G                     NL                            580
                        Phenol Formaldehyde, Polyalkylene-p                                       9003-35-4                     G                                                   290
                        Phenol Furfural                                                          26338-61-4                     G                                                   310
                        Phenylbetanaphthylamine                                                    135-88-6                     G                     NL                            680
                        Phthalic Anhydride                                                         85-44-9                      G                     M                             650
                        Phthalimide                                                                85-41-6                      G                     M                             630
                        Pitch, Coal Tar                                                          65996-93-2                     F                     NL                            710
                        Pitch, Petroleum                                                         68187-58-6                     F                     NL                            630
                        Polycarbonate                                                                                           G                     NL                            710
                        Polyethylene, High Pressure Process                                       9002-88-4                     G                                                   380



                 664                                                                                                                2005 National Electrical Code Handbook
                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---

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                                                 Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                                                 500.7



                           Commentary Table 5.2 Continued

                                                                                                                                                                                      Layer or Cloud
                                                   Chemical Name                                                             CAS No.           NEC Group               Code         Ignition Temp. ( C)

                               Polyethylene, Low Pressure Process                                                            9002-88-4                 G                 NL                    420
                               Polyethylene Terephthalate                                                                   25038-59-9                 G                 NL                    500
                               Polyethylene Wax                                                                             68441-04-8                 G                 NL                    400
                               Polypropylene (no antioxidant)                                                                9003-07-0                 G                 NL                    420
                               Polystyrene Latex                                                                             9003-53-6                 G                                       500
                               Polystyrene Molding Compound                                                                  9003-53-6                 G                 NL                    560
                               Polyurethane Foam, Fire Retardant                                                             9009-54-5                 G                                       390
                               Polyurethane Foam, No Fire Retardant                                                          9009-54-5                 G                                       440
                               Polyvinyl Acetate                                                                             9003-20-7                 G                 NL                    550
                               Polyvinyl Acetate/Alcohol                                                                     9002-89-5                 G                                       440
                               Polyvinyl Butyral                                                                            63148-65-2                 G                                       390
                               Polyvinyl Chloride-dioctyl Phthalate                                                                                    G                 NL                    320
                               Potato Starch, Dextrinated                                                                    9005-25-8                 G                 NL                    440
                               Pyrethrum                                                                                     8003-34-7                 G                                       210
                               Rayon (Viscose) Flock                                                                        61788-77-0                 G                                       250
                               Red Dye Intermediate                                                                                                    G                                       175
                               Rice                                                                                                                    G                                       220
                               Rice Bran                                                                                                               G                 NL                    490
                               Rice Hull                                                                                                               G                                       220
                               Rosin, DK                                                                                     8050-09-7                 G                 NL                    390
                               Rubber, Crude, Hard                                                                           9006-04-6                 G                 NL                    350
                               Rubber, Synthetic, Hard (33% S)                                                              64706-29-2                 G                 NL                    320
                               Safflower Meal                                                                                                          G                                       210
                               Salicylanilide                                                                                87-17-2                   G                  M                    610
                               Sevin                                                                                         63-25-2                   G                                       140
                               Shale, Oil                                                                                   68308-34-9                 F
                               Shellac                                                                                       9000-59-3                 G                 NL                    400
                               Sodium Resinate                                                                              61790-51-0                 G                                       220
                               Sorbic Acid (Copper Sorbate or Potash)                                                         110-44-1                 G                                       460
                               Soy Flour                                                                                    68513-95-1                 G                                       190
                               Soy Protein                                                                                   9010-10-0                 G                                       260
                               Stearic Acid, Aluminum Salt                                                                    637-12-7                 G                                       300
                               Stearic Acid, Zinc Salt                                                                        557-05-1                 G                  M                    510
                               Styrene Modified Polyester-Glass Fiber                                                         100-42-5                 G                                       360
                               Styrene-acrylonitrile (70-30)                                                                 9003-54-7                 G                 NL                    500
                               Styrene-butadiene Latex (>75% styrene)                                                        903-55-8                  G                 NL                    440
                               Styrene-maleic Anhydride Copolymer                                                           9011-13-6                  G                 CL                    470
                               Sucrose                                                                                       57-50-1                   G                 CL                    350
                               Sugar, Powdered                                                                               57-50-1                   G                 CL                    370
                               Sulfur                                                                                       7704-34-9                  G                                       220
                               Tantalum                                                                                     7440-25-7                  E                                       300
                               Terephthalic Acid                                                                             100-21-0                  G                 NL                    680
                               Thorium, 1.2% O2                                                                             7440-29-1                  E                 CL                    280
                               Tin, 96%, Atomized (2% Pb)                                                                   7440-31-5                  E                                       430
                               Titanium, 99% Ti                                                                             7440-32-6                  E                 CL                    330
                               Titanium Hydride (95% Ti, 3.8% H2)                                                           7704-98-5                  E                 CL                    480
                               Trithiobisdimethylthio-formamide                                                                                        G                                       230
                               Tung, Kernels, Oil-free                                                                       8001-20-5                 G                                       240
                               Urea Formaldehyde Molding Compound                                                            9011-05-6                 G                 NL                    460
                               Urea Formaldehyde-phenol Formaldehyde                                                        25104-55-6                 G                                       240

                                                                                                                                                                                                     (continues)




                           National Electrical Code Handbook 2005         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                                                                                    665

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                 500.7                                                                     Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2




                                                                   Commentary Table 5.2 Continued

                                                                                              Chemical Name                                            CAS No.               NEC Group              Code            Layer or Cloud
                                                                                                                                                                                                                  Ignition Temp. ( C)

                                                                      Vanadium, 86.4%                                                                 7440-62-2                     E                                        490
                                                                      Vinyl Chloride-acrylonitrile Copolymer                                          9003-00-3                     G                                        470
                                                                      Vinyl Toluene-acrylonitrile Butadiene                                          76404-69-8                     G                 NL                     530
                                                                      Violet 200 Dye                                                                                                G                                        175
                                                                      Vitamin B1, Mononitrate                                                          59-43-8                      G                 NL                     360
                                                                      Vitamin C                                                                        50-81-7                      G                                        280
                                                                      Walnut Shell, Black                                                                                           G                                        220
                                                                      Wheat                                                                                                         G                                        220
                                                                      Wheat Flour                                                                   130498-22-5                     G                                        360
                                                                      Wheat Gluten, Gum                                                             100684-25-1                     G                 NL                     520
                 --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                      Wheat Starch                                                                                                  G                 NL                     380
                                                                      Wheat Straw                                                                                                   G                                        220
                                                                      Wood Flour                                                                                                    G                                        260
                                                                      Woodbark, Ground                                                                                              G                                        250
                                                                      Yeast, Torula                                                                  68602-94-8                     G                                        260
                                                                      Zirconium Hydride                                                               7704-99-6                     E                                        270
                                                                      Zirconium                                                                                                     E                 CL                     330

                                                                   Notes:
                                                                   1. Normally, the minimum ignition temperature of a layer of a specific dust is lower than the minimum ignition
                                                                   temperature of a cloud of that dust. Since this is not universally true, the lower of the two minimum ignition
                                                                   temperatures is listed. If no symbol appears between the two temperature columns, then the layer ignition
                                                                   temperature is shown. ‘‘CL’’ means the cloud ignition temperature is shown. ‘‘NL’’ means that no layer ignition
                                                                   temperature is available, and the cloud ignition temperature is shown. ‘‘M’’ signifies that the dust layer melts
                                                                   before it ignites; the cloud ignition temperature is shown. ‘‘S’’ signifies that the dust layer sublimes before it
                                                                   ignites; the cloud ignition temperature is shown.
                                                                   2. Certain metal dusts may have characteristics that require safeguards beyond those required for atmospheres
                                                                   containing the dusts of aluminum, magnesium, and their commercial alloys. For example, zirconium, thorium,
                                                                   and uranium dusts have extremely low ignition temperatures [as low as 20 C (68 F)] and minimum ignition
                                                                   energies lower than any material classified in any of the Class I or Class II groups.
                                                                   Source: Table 4.5.2 in NFPA 499, Recommended Practice for the Classification of Combustible Dusts and
                                                                   of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, 2004 edition.




                                                                   initially present to an acceptable level. The types of pressur-                   Code. Pressurization, for the purposes of NFPA 496, is the
                                                                   izing are as follows:                                                             process of supplying an enclosure with a protective gas,
                                                                                                                                                     with or without continuous flow, at sufficient pressure to
                                                                    1. Type X pressurizing reduces the classification within                         prevent the entrance of a flammable gas or vapor, a combusti-
                                                                       a protected enclosure from Division 1 or Zone 1 to                            ble dust, or an ignitible fiber.
                                                                       unclassified.                                                                      It should be noted that an atmosphere that is made
                                                                    2. Type Y pressurizing reduces the classification within a                       hazardous by combustible dust inside an enclosure cannot
                                                                       protected enclosure from Division 1 to Division 2 or                          be reduced to a safe level by supplying a flow of protective
                                                                       from Zone 1 to Zone 2.                                                        gas in the same manner as with gases or vapors. Supplying
                                                                    3. Type Z pressurizing reduces the classification within                         a flow of air into the enclosure could stir up the dust that
                                                                       a protected enclosure from Division 2 or Zone 2 to                            has accumulated at the bottom of the enclosure and create
                                                                       unclassified.                                                                 a dust cloud within the enclosure that could explode if an
                                                                                                                                                     ignition source occurs. The enclosure must be opened, and
                                                                   In Class II hazardous (classified) locations, pressurized en-                     the dust must be removed. Visual inspection can determine
                                                                   closures prevent the entrance of dusts into an enclosure.                         if the dust has been removed. Positive pressure then prevents
                                                                   Pressurized enclosures make it possible for equipment that                        dust from entering a clean enclosure.
                                                                   is not otherwise acceptable for hazardous (classified) loca-                           Field-installed devices and equipment, such as push-
                                                                   tions to be used in these locations, in accordance with the                       button controls and pilot lights, are permitted in purged and


                 666                                                                                                                                                                    2005 National Electrical Code Handbook

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                                                 Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                                     500.8



                           pressurized enclosures, provided the conductor terminals are                                 type of equipment, its listing, the location where it is in-
                           within the purged or pressurized atmosphere. Section 7.3.4                                   stalled, the type of alarm or signal, and the shutdown proce-
                           of NFPA 30, Flammable and Combustible Liquids Code,                                          dure. A schedule for calibration of the system is also required
                           and Section 4.6 of ANSI/API RP 500, Recommended Prac-                                        to be documented.
                           tice for Classification of Locations for Electrical Installa-
                           tions at Petroleum Facilities Classified as Class I, Division
                           1 and Division 2, provide guidelines on what is considered                                          FPN No. 1: For further information, see ANSI/ISA
                                                                                                                               12.13.01, Performance Requirements, Combustible Gas
                           adequate ventilation.
                                                                                                                               Detectors.
                                                                                                                               FPN No. 2: For further information, see ANSI/API RP
                                                                                                                               500, Recommended Practice for Classification of Loca-
                           (E) Intrinsic Safety This protection technique shall be per-                                        tions for Electrical Installations at Petroleum Facilities
                           mitted for equipment in Class I, Division 1 or 2; or Class                                          Classified as Class I, Division I or Division 2.
                           II, Division 1 or 2; or Class III, Division 1 or 2 locations.                                       FPN No. 3: For further information, see ISA-
                           The provisions of Articles 501 through 503 and Articles                                             RP12.13.02, Installation, Operation, and Maintenance
                           510 through 516 shall not be considered applicable to such                                          of Combustible Gas Detection Instruments.
                           installations, except as required by Article 504, and installa-
                                                                                                                        (1) Inadequate Ventilation In a Class I, Division 1 loca-
                           tion of intrinsically safe apparatus and wiring shall be in
                                                                                                                        tion that is so classified due to inadequate ventilation, electri-
                           accordance with the requirements of Article 504.
                                                                                                                        cal equipment suitable for Class I, Division 2 locations shall
                           (F) Nonincendive Circuit This protection technique shall                                     be permitted.
                           be permitted for equipment in Class I, Division 2; Class II,                                 (2) Interior of a Building In a building located in, or with
                           Division 2; or Class III, Division 1 or 2 locations.                                         an opening into, a Class I, Division 2 location where the
                           (G) Nonincendive Equipment This protection technique                                         interior does not contain a source of flammable gas or vapor,
                           shall be permitted for equipment in Class I, Division 2; Class                               electrical equipment for unclassified locations shall be per-
                           II, Division 2; or Class III, Division 1 or 2 locations.                                     mitted.

                           (H) Nonincendive Component This protection technique                                         (3) Interior of a Control Panel In the interior of a control
                           shall be permitted for equipment in Class I, Division 2; Class                               panel containing instrumentation utilizing or measuring
                           II, Division 2; or Class III, Division 1 or 2 locations.                                     flammable liquids, gases, or vapors, electrical equipment
                                                                                                                        suitable for Class I, Division 2 locations shall be permitted.
                           (I) Oil Immersion This protection technique shall be per-
                           mitted for current-interrupting contacts in Class I, Division                                (L) Other Protection Techniques Other protection tech-
                           2 locations as described in 501.115(B)(1)(2).                                                niques used in equipment identified for use in hazardous
                                                                                                                        (classified) locations.
                           (J) Hermetically Sealed This protection technique shall
                           be permitted for equipment in Class I, Division 2; Class II,                                 Some listed equipment employs unique protection tech-
                           Division 2; or Class III, Division 1 or 2 locations.                                         niques or a combination of protection techniques, such as
                                                                                                                        listed attachment plugs for use in hazardous locations and
                           (K) Combustible Gas Detection System A combustible
                                                                                                                        listed battery-operated two-way radios, cell phones, flash-
                           gas detection system shall be permitted as a means of protec-
                                                                                                                        lights, and lanterns.
                           tion in industrial establishments with restricted public access
                           and where the conditions of maintenance and supervision
                           ensure that only qualified persons service the installation.                                 500.8 Equipment
                           Gas detection equipment shall be listed for detection of the
                                                                                                                        Articles 500 through 504 require equipment construction and
                           specific gas or vapor to be encountered. Where such a system
                                                                                                                        installation that ensure safe performance under conditions of
                           is installed, equipment specified in 500.7(K)(1), (K)(2), or
                                                                                                                        proper use and maintenance.
                           (K)(3) shall be permitted.
                                The type of detection equipment, its listing, installation                                     FPN No. 1: It is important that inspection authorities
                           location(s), alarm and shutdown criteria, and calibration fre-                                      and users exercise more than ordinary care with regard
                           quency shall be documented when combustible gas detectors                                           to installation and maintenance.
                           are used as a protection technique.                                                                 FPN No. 2: Since there is no consistent relationship
                                                                                                                               between explosion properties and ignition temperature,
                                                                                                                               the two are independent requirements.
                           Where combustible gas detection systems are utilized for                                            FPN No. 3: Low ambient conditions require special con-
                           protection, documentation must be provided indicating the                                           sideration. Explosionproof or dust-ignitionproof equip-
                                                                                                                               ment may not be suitable for use at temperatures lower


                           National Electrical Code Handbook 2005                                                                                                                               667
                                                             --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




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                 500.8                                                                  Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                                                                       than 25 C ( 13 F) unless they are identified for low-                 (a) Intrinsically safe apparatus having a control drawing
                                                                       temperature service. However, at low ambient tempera-            requiring the installation of associated apparatus for a Divi-
                                                                       tures, flammable concentrations of vapors may not exist
                                                                       in a location classified as Class I, Division 1 at normal        sion 1 installation shall be permitted to be installed in a
                                                                       ambient temperature.                                             Division 2 location if the same associated apparatus is used
                                                                                                                                        for the Division 2 installation.
                    At low ambient temperatures, such as those encountered in                                                                (b) Equipment that is required to be explosionproof
                    the Arctic, explosion pressures increase at very low tempera-                                                       shall incorporate seals per 501.15(A) or 501.15(D) when the
                    tures. The strengths of materials change, and the explosion                                                         wiring methods of 501.10(B) are employed.
                    pressure in explosionproof equipment may increase beyond
                    the safe operating strength of the material. In addition, some                                                      (3) Where specifically permitted in Articles 501 through
                    sealing materials for sealing fittings may become brittle.                                                          503, general-purpose equipment or equipment in general-
                    However, the extent of the hazardous (classified) location                                                          purpose enclosures shall be permitted to be installed in Divi-
                    may also change under low ambient conditions. The material                                                          sion 2 locations if the equipment does not constitute a source
                    may be used in a location where the temperature range is                                                            of ignition under normal operating conditions.
                    so low that no vapors are produced based on the flash point
                    of the material involved.                                                                                           (4) Equipment that depends on a single compression seal,
                                                                                                                                        diaphragm, or tube to prevent flammable or combustible
                                                                                                                                        fluids from entering the equipment shall be identified for a
                    (A) Approval for Class and Properties                                                                               Class I, Division 2 location even if installed in an unclassified
                    (1) Equipment shall be identified not only for the class of                                                         location. Equipment installed in a Class I, Division 1 location
                    location but also for the explosive, combustible, or ignitible                                                      shall be identified for the Class I, Division 1 location.
                    properties of the specific gas, vapor, dust, fiber, or flyings
                    that will be present. In addition, Class I equipment shall not                                                             FPN: Equipment used for flow measurement is an exam-
                    have any exposed surface that operates at a temperature in                                                                 ple of equipment having a single compression seal, dia-
                                                                                                                                               phragm, or tube.
                    excess of the ignition temperature of the specific gas or
                     --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    vapor. Class II equipment shall not have an external tempera-
                                                                                                                                        (5) Unless otherwise specified, normal operating conditions
                    ture higher than that specified in 500.8(C)(2). Class III equip-
                                                                                                                                        for motors shall be assumed to be rated full-load steady
                    ment shall not exceed the maximum surface temperatures
                                                                                                                                        conditions.
                    specified in 503.5.
                                                                       FPN: Luminaires (lighting fixtures) and other heat-pro-
                                                                                                                                        It is not intended that locked-rotor or other motor overload
                                                                       ducing apparatus, switches, circuit breakers, and plugs
                                                                       and receptacles are potential sources of ignition and are        conditions, such as single phasing, be considered when eval-
                                                                       investigated for suitability in classified locations. Such       uating motor-operating temperatures (internal and external)
                                                                       types of equipment, as well as cable terminations for            in Class I, Division 2 locations. However, such abnormal
                                                                       entry into explosionproof enclosures, are available as           load conditions must be considered when evaluating the
                                                                       listed for Class I, Division 2 locations. Fixed wiring,
                                                                                                                                        external temperatures of explosionproof motors for Class I,
                                                                       however, may utilize wiring methods that are not evalu-
                                                                       ated with respect to classified locations. Wiring products       Division 1 locations and motors such as dust-ignitionproof
                                                                       such as cable, raceways, boxes, and fittings, therefore,         motors for Class II, Division 1 locations. It is important to
                                                                       are not marked as being suitable for Class I, Division 2         be aware of the increase in temperature in some variable-
                                                                       locations. Also see 500.8(B)(6)(a).                              speed motors when they are operated at the lower speed and
                        Suitability of identified equipment shall be determined                                                         are dependent on the fan for cooling.
                    by any of the following:
                    (1) Equipment listing or labeling
                                                                                                                                        (6) Where flammable gases or combustible dusts are or may
                    (2) Evidence of equipment evaluation from a qualified test-
                                                                                                                                        be present at the same time, the simultaneous presence of both
                        ing laboratory or inspection agency concerned with
                                                                                                                                        shall be considered when determining the safe operating
                        product evaluation
                                                                                                                                        temperature of the electrical equipment.
                    (3) Evidence acceptable to the authority having jurisdiction
                        such as a manufacturer’s self-evaluation or an owner’s
                        engineering judgment                                                                                            Examples of where flammable liquid and dust can be present
                                                                                                                                        at the same time are at a coal-handling facility, where there
                    (2) Equipment that has been identified for a Division 1                                                             is methane gas and coal dust, and in an automotive paint
                    location shall be permitted in a Division 2 location of the                                                         spray shop, where flammable paint and powdered metal
                    same class, group, and temperature class and shall comply                                                           flecks are sprayed.
                    with (a) or (b) as applicable.


                 668                                                                                                                                                       2005 National Electrical Code Handbook

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                                                     Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2                                          500.8



                                   FPN: The characteristics of various atmospheric mix-                    Table 500.8(B) Classification of Maximum Surface
                                   tures of gases, vapors, and dusts depend on the specific                Temperature
                                   material involved.
                                                                                                                          Maximum Temperature
                           (B) Marking Equipment shall be marked to show the envi-                                                                                      Temperature Class
                           ronment for which it has been evaluated. Unless otherwise                                            C                                  F        (T Code)
                           specified or allowed in (B)(6), the marking shall include the                                     450                                  842             T1
                           information specified in (B)(1) through (B)(5).                                                   300                                  572             T2
                                                                                                                             280                                  536             T2A
                           The marked operating temperature or temperature range is                                          260                                  500             T2B
                                                                                                                             230                                  446             T2C
                           normally referenced to a 104 F ambient. Unless the equip-
                           ment is provided with thermally actuated sensors that limit                                       215                                  419             T2D
                                                                                                                             200                                  392             T3
                           the temperature to that marked on the equipment, operation                                        180                                  356             T3A
                           in ambient temperatures higher than 104 F increases the                                           165                                  329             T3B
                           operating temperature of the equipment. Many explo-                                               160                                  320             T3C
                           sionproof and dust-ignitionproof motors are equipped with                                         135                                  275             T4
                           thermal protectors. In like manner, operation in ambient                                          120                                  248             T4A
                           temperatures lower than 104 F usually reduces the operating                                       100                                  212             T5
                           temperature.                                                                                       85                                  185             T6



                           (1) Class The marking shall specify the class(es) for which                     more than 100 C shall not be required to have a marked
                           the equipment is suitable.                                                      operating temperature or temperature class.
                           (2) Division The marking shall specify the division if the                             FPN: More than one marked temperature class or op-
                           equipment is suitable for Division 2 only. Equipment suitable                          erating temperature, for gases and vapors, dusts, and
                                                                                                                  different ambient temperatures, may appear.
                           for Division 1 shall be permitted to omit the division mark-
                           ing.                                                                            (5) Ambient Temperature Range For equipment rated for
                                   FPN: Equipment not marked to indicate a division, or                    a temperature range other than –25 C to 40 C, the marking
                                   marked ‘‘Division 1’’ or ‘‘Div. 1,’’ is suitable for both               shall specify the special range of ambient temperatures. The
                                   Division 1 and 2 locations; see 500.8(A)(2). Equipment                  marking shall include either the symbol ‘‘Ta’’ or ‘‘Tamb.’’
                                   marked ‘‘Division 2’’ or ‘‘Div. 2’’ is suitable for Division
                                   2 locations only.                                                              FPN: As an example, such a marking might be ‘‘–30 C
                                                                                                                    Ta     40 C.’’
                           (3) Material Classification Group The marking shall
                           specify the applicable material classification group(s) in ac-                  (6) Special Allowances
                           cordance with 500.6.
                                                                                                           Exception No. 2 and Exception No. 3 to 500.8(B) in the
                           Exception: Fixed luminaires (lighting fixtures) marked for                      2002 Code were changed to mandatory Code language and
                           use only in Class I, Division 2 or Class II, Division 2 loca-                   relocated in a new section, 500.8(B)(6) Special Allowances,
                           tions shall not be required to indicate the group.                              in the 2005 Code. A squirrel-cage induction motor without
                                                                                                           brushes, switching mechanisms, or similar arc-producing
                           (4) Equipment Temperature The marking shall specify
                                                                                                           devices is an example of fixed general-purpose equipment.
                           the temperature class or operating temperature at a 40 C
                                                                                                           See 501.125(B) and its associated commentary for more
                           ambient temperature, or at the higher ambient temperature
                                                                                                           information on motors in Class I, Division 2 locations.
                           if the equipment is rated and marked for an ambient tempera-
                           ture of greater than 40 C. The temperature class, if provided,
                           shall be indicated using the temperature class (T Codes)                             (a) General Purpose Equipment. Fixed general-purpose
                           shown in Table 500.8(B). Equipment for Class I and Class                        equipment in Class I locations, other than fixed luminaires
                           II shall be marked with the maximum safe operating temper-                      (lighting fixtures), that is acceptable for use in Class I,
                           ature, as determined by simultaneous exposure to the combi-                     Division 2 locations shall not be required to be marked
                           nations of Class I and Class II conditions.                                     with the class, division, group, temperature class, or ambient
                           Exception: Equipment of the non–heat-producing type, such                       temperature range.
                           as junction boxes, conduit, and fittings, and equipment of                          (b) Dusttight Equipment. Fixed dusttight equipment,
                           the heat-producing type having a maximum temperature not                        other than fixed luminaires (lighting fixtures), that is accept-

                           National Electrical Code Handbook 2005
                                                                                                                --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                                                            669

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                 500.8                           Article 500 — Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2



                    able for use in Class II, Division 2 and Class III locations                                   Table 500.8(C)(2) Class II Temperatures
                    shall not be required to be marked with the class, division,
                    group, temperature class, or ambient temperature range.                                                                                        Equipment (Such as Motors or
                                                                                                                                                                    Power Transformers) That
                          (c) Associated Apparatus. Associated intrinsically safe                                                      Equipment                       May Be Overloaded
                    apparatus and associated nonincendive field wiring appara-                                                         Not Subject
                    tus that are not protected by an alternative type of protection                                                        to                         Normal                    Abnormal
                    shall not be marked with the class, division, group, or temper-                                                    Overloading                   Operation                  Operation
                                                                                                                   Class II
                    ature class. Associated intrinsically safe apparatus and asso-                                 Group                 C             F             C              F            C      F
                    ciated nonincendive field wiring apparatus shall be marked
                    with the class, division, and group of the apparatus to which                                         E            200           392           200           392            200   392
                    it is to be connected.                                                                                F            200           392           150           302            200   392
                                                                                                                          G            165           329           120           248            165   329
                          (d) Simple Apparatus. ‘‘Simple apparatus’’ as defined
                    in Article 504, shall not be required to be marked with class,
                    division, group, temperature class, or ambient temperature
                    range.                                                                                         (D) Threading All NPT threaded conduit and fittings re-
                                                                                                                   ferred to herein shall be threaded with a National (American)
                    (C) Temperature                                                                                Standard Pipe Taper (NPT) thread that provides a taper of
                    (1) Class I Temperature The temperature marking speci-                                         1 in 16 (3⁄4-in. taper per foot). Conduit and fittings shall be
                    fied in 500.8(B) shall not exceed the ignition temperature                                     made wrenchtight to prevent sparking when fault current
                    of the specific gas or vapor to be encountered.                                                flows through the conduit system, and to ensure the explo-
                                                                                                                   sionproof integrity of the conduit system where applicable.
                    The ignition temperature of a solid, liquid, or gaseous sub-                                   Equipment provided with threaded entries for field wiring
                    stance is the minimum temperature required to initiate or                                      connections shall be installed in accordance with
                    cause self-sustained combustion independent of the heating                                     500.8(D)(1) or (D)(2). Threaded entries into explosionproof
                    or heated element. The flash point is the temperature at                                       equipment shall be made up with at least five threads fully
                    which the material gives off vapors that will ignite when                                      engaged.
                    the temperature reaches the ignition temperature, provided                                     Exception: For listed explosionproof equipment, factory
                    the air-to-fuel ratio is within the proper range. The ignition                                 threaded NPT entries shall be made up with at least 41⁄2
                    temperature and the flash point are unrelated properties,                                      threads fully engaged.
                    except that the flash point is always lower than the ignition
                    temperature.                                                                                   (1) Equipment Provided with Threaded Entries for NPT
                                                                                                                   Threaded Conduit or Fittings For equipment provided
                                                                                                                   with threaded entries for NPT threaded conduit or fittings,
                            FPN: For information regarding ignition temperatures                                   listed conduit, conduit fittings, or cable fittings shall be used.
                            of gases and vapors, see NFPA 497-2004, Recommended
                            Practice for the Classification of Flammable Liquids,                                            FPN: Thread form specifications for NPT threads are
                            Gases, or Vapors, and of Hazardous (Classified) Loca-                                            located in ANSI/ASME B1.20.1-1983, Pipe Threads,
                            tions for Electrical Installations in Chemical Process                                           General Purpose (Inch).
                            Areas.
                                                                                                                   (2) Equipment Provided with Threaded Entries for Met-
                    (2) Class II Temperature The temperature marking speci-
                                                                                                                   ric Threaded Conduit or Fittings For equipment with met-
                    fied in 500.8(B) shall be less than the ignition temperature
                                                                                                                   ric threaded entries, such entries shall be identified as being
                    of the specific dust to be encountered. For organic dusts that
                                                                                                                   metric, or listed adapters to permit connection to conduit or
                    may dehydrate or carbonize, the temperature marking shall
                                                                                                                   NPT-threaded fittings shall be provided with the equipment.
                    not exceed the lower of either the ignition temperature or
                                                                                                                   Adapters shall be used for connection to conduit or NPT-
                    165 C (329 F).
                                                                                                                   threaded fittings. Listed cable fittings that have metric
                            FPN: See NFPA 499-2004, Recommended Practice for                                       threads shall be permitted to be used.
                            the Classification of Combustible Dusts and of Hazard-
                            ous (Classified) Locations for Electrical Installations in                                       FPN: Threading specifications for metric threaded en-
                            Chemical Process Areas, for minimum ignition tempera-                                            tries are located in ISO 965/1-1980, Metric Screw
                            tures of specific dusts.                                                                         Threads, and ISO 965/3-1980, Metric Screw Threads.

                        The ignition temperature for which equipment was ap-                                       All conduit joints must be made up wrenchtight to prevent
                    proved prior to this requirement shall be assumed to be as                                     arcing between the conduit and the coupling, fitting, or en-
                    shown in Table 500.8(C)(2).


                 670                                                              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                     2005 National Electrical Code Handbook

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                                                                           Article 501 — Class I Locations                                                                  500.9



                           closure of the conduit under ground-fault conditions. The                    Contents
                           use of a bonding jumper in lieu of a wrenchtight connection
                                                                                                         I. General
                           is not permitted. The integrity of the ground-fault current
                                                                                                              501.1                    Scope
                           path is critical in hazardous locations in order to prevent
                                                                                                              501.5                    General
                           ignition-capable arcing or sparking.
                                                                                                        II. Wiring
                                The information on metric threads in 500.8(D) has been
                                                                                                             501.10  Wiring Methods
                           included to allow for safe electrical and mechanical connec-
                                                                                                                (A)  Class I, Division 1
                           tions where the enclosure has metric threads and the raceway
                                                                                                                (B)  Class I, Division 2
                           or cable has NPT threads. Equipment with metric threaded
                                                                                                             501.15  Sealing and Drainage
                           entries must be identified or provided with suitable adapters
                                                                                                                (A)  Conduit Seals, Class I, Division 1
                           that permit the connection of conduit and fittings that are
                                                                                                                (B)  Conduit Seals, Class I, Division 2
                           NPT threaded.                                                                        (C)  Class I, Divisions 1 and 2
                                                                                                                (D)  Cable Seals, Class I, Division 1
                                                                                                                (E)  Cable Seals, Class I, Division 2
                           (E) Fiber Optic Cable Assembly Where a fiber optic cable                              (F) Drainage
                           assembly contains conductors that are capable of carrying                         501.20  Conductor Insulation, Class I, Divisions 1
                           current, the fiber optic cable assembly shall be installed in                             and 2
                           accordance with the requirements of Articles 500, 501, 502,                        501.25 Uninsulated Exposed Parts, Class I, Divisions
                           or 503, as applicable.                                                                    1 and 2
                                                                                                              501.30 Grounding and Bonding, Class I, Divisions 1
                                                                                                                     and 2
                           The requirements of Articles 500, 501, 502, or 503 apply,                             (A) Bonding
                           even if the conductor is grounded.                                                    (B) Types of Equipment Grounding Conductors
                                                                                                              501.35 Surge Protection
                                                                                                                 (A) Class I, Division 1
                           500.9 Specific Occupancies                                                            (B) Class I, Division 2
                           Articles 510 through 517 cover garages, aircraft hangars,                          501.40 Multiwire Branch Circuits
                           motor fuel dispensing facilities, bulk storage plants, spray                 III. Equipment
                           application, dipping and coating processes, and health care                       501.100 Transformers and Capacitors
                           facilities.                                                                           (A) Class I, Division 1
                                                                                                                 (B) Class I, Division 2
                                                                                                             501.105 Meters, Instruments, and Relays
                                                                                                                 (A) Class I, Division 1
                                                  ARTICLE 501                                                    (B) Class I, Division 2
                                                 Class I Locations                                           501.115 Switches, Circuit Breakers, Motor Controllers,
                                                                                                                     and Fuses
                                                                                                                 (A) Class I, Division 1
                           Summary of Changes                                                                    (B) Class I, Division 2
                           • General: Restructured and renumbered to provide a scope                         501.120 Control Transformers and Resistors
                             section and parallel numbering systems for Articles 501,                            (A) Class I, Division 1
                             502, and 503.                                                                       (B) Class I, Division 2
                                                                                                             501.125 Motors and Generators
                           • 501.10(B)(6): Revised to require single conductor Type                              (A) Class I, Division 1
                             MV cables to be shielded or metallic-armored.                                       (B) Class I, Division 2
                           • 501.15(B)(2): Revised to permit seals identified for the                        501.130 Luminaires (Lighting Fixtures)
                             purpose of minimizing the passage of gases and vapors                               (A) Class I, Division 1
                             (does not have to be of the explosionproof type) within                             (B) Class I, Division 2
                             the Division 2 portion of the conduit.                                          501.135 Utilization Equipment
                                                                                                                 (A) Class I, Division 1
                           • 501.25: Revised to provide guidance concerning protec-
                                                                                                                 (B) Class I, Division 2
                             tion against explosion and against electric shock in order
                                                                                                             501.140 Flexible Cords, Class I, Divisions 1 and 2
                             to limit the voltage of exposed live parts.
                                                                                                                 (A) Permitted Uses
                           • 501.35: Revised to include TVSS devices.                                            (B) Installation



                           National Electrical Code Handbook 2005                                                                                                            671
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                 501.1                                                       Article 501 — Class I Locations




                            501.145 Receptacles and Attachment Plugs, Class I,                  arcs or sparks should be kept out of Class I locations where
                                    Divisions 1 and 2                                           practicable. If this is not practicable, such apparatus must
                            501.150 Signaling, Alarm, Remote-Control, and                       be approved for the purpose and installed properly. The arc
                                    Communications Systems                                      produced at the contacts of listed or labeled intrinsically
                                (A) Class I, Division 1                                         safe equipment is not ignition-capable because the energy
                                (B) Class I, Division 2                                         available is insufficient to cause ignition.
                                                                                                     Hermetic sealing of all electrical equipment is impracti-
                                                                                                cal, because equipment such as motors, conventional
                    I. General                                                                  switches, and circuit breakers has movable parts that must
                    501.1 Scope                                                                 be operated through the enclosing case; that is, the lever of
                                                                                                a switch or the shaft of a motor must have sufficient clearance
                    Article 501 covers the requirements for electrical and elec-                to operate freely. In addition, in many cases, it is necessary
                    tronic equipment and wiring for all voltages in Class I,                    to have access to the inside of enclosures for installation,
                    Division 1 and 2 locations where fire or explosion hazards                  servicing, or alterations.
                    may exist due to flammable gases or vapors or flammable                          It is practically impossible to make threaded conduit
                    liquids.                                                                    joints gastight. The conduit system and apparatus enclosure
                            FPN: For the requirements for electrical and electronic             ‘‘breathe’’ due to temperature changes, and any flammable
                            equipment and wiring for all voltages in Class I, Zone              gases or vapors in the room may slowly enter the conduit
                            0, Zone 1, or Zone 2 hazardous (classified) locations
                                                                                                or enclosure, creating an explosive mixture. Should an arc
                            where fire or explosion hazards may exist due to flamma-
                            ble gases or vapors or flammable liquids, refer to Article          occur, an explosion could take place.
                            505.                                                                     When an explosion occurs within the enclosure or con-
                                                                                                duit system, the burning mixture or hot gases must be suffi-
                    For the 2005 Code, Article 501 was divided into three parts:                ciently confined within the system to prevent ignition of any
                    I General, II Wiring, and III Equipment. This division pro-                 explosive mixture that might be present in the area outside
                    vides a chronological order to the requirements used in an                  the enclosures or conduit system. An apparatus enclosure
                    electrical installation. Sections 501.2 and 501.3 were moved                must be designed with sufficient strength to withstand the
                    to Part III, Equipment, and renumbered 501.100 and                          maximum pressure generated by an internal explosion in
                    501.105, respectively. Section 501.4, Wiring Methods, was                   order to prevent rupture and the release of burning or hot
                    relocated from 501.4 to Part II, 501.10.                                    gases. Enclosures have been designed to withstand such
                                                                                                internal explosions. The ability to withstand an internal ex-
                                                                                                plosion is one criterion by which explosionproof enclosures
                    501.5 General                                                               are evaluated.
                    The general rules of this Code shall apply to the electric                       During an explosion within an enclosure, gases escape
                    wiring and equipment in locations classified as Class I in                  through any paths or openings that exist, but the gases are
                    500.5.                                                                      sufficiently cooled if they are carried out through an opening
                                                                                                that is long in proportion to its width; that is, the spiral path
                    Exception: As modified by this article.
                                                                                                of at least five fully engaged threads of a screw-on type
                                                                                                junction box cover, as illustrated in Exhibit 501.1. This
                    The most common Class I locations are those areas involved                  principle is also applied in the design of explosionproof
                    in the handling or processing of volatile flammable liquids                 enclosures for apparatus in which a wide machined flange
                    such as gasoline, naphtha, benzene, diethyl ether, and ace-                 on the body of the enclosure and a similar machined flange
                    tone, or flammable gases such as hydrogen, methane, and                     on the cover are provided, as illustrated in Exhibit 501.2.
                    propane.                                                                    These machined flanges are ground so that when the cover
                         Where ignitible concentrations (concentrations within                  is seated in place, the clearance between the two surfaces                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---


                    the flammable or explosive limits) of flammable gases or                    at no point exceeds, for example, 0.0015 in. If an explosion
                    vapors are present, atmospheres exist that are explosive when               occurs within the enclosure, escaping gas travels a consider-
                    ignited by an arc, a spark, or high temperature. NFPA 497,                  able distance through a very small opening. The gas therefore
                    Recommended Practice for the Classification of Flammable                    is cooled sufficiently when it enters and mixes with the
                    Liquids, Gases, or Vapors and of Hazardous (Classified)                     surrounding atmosphere, thus preventing ignition of the ex-
                    Locations for Electrical Installations in Chemical Process                  ternal explosive mixture.
                    Areas, includes information on the explosive limits of flam-                     The clearance between flat surfaces may increase some-
                    mable liquids and gases.                                                    what under explosion conditions because the internal pres-
                         All electrical equipment that may cause ignition-capable               sures created by the explosion tend to force the surfaces


                 672                                                                                                               2005 National Electrical Code Handbook

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                                                                                         Article 501 — Class I Locations                                                              501.10




                                                                        Cool escaping
                                                                        gases

                                 Cover with threaded joint
                                 (5 full threads min.)



                                                                                                                                                   Normal clearance




                                                             Internal
                                                             pressure
                                                                                        Hot flaming
                                                                                        gases
                                                                                                                                             Clearance during explosion

                           Exhibit 501.1 Cooling of hot gases as they pass through the
                           threads of a screw-type cover of an explosionproof junction box.                    Exhibit 501.3 Effect of internal explosion (bottom) on cover-to-
                                                                                                               body joint clearance in an explosionproof enclosure. (Redrawn
                                                                                                               courtesy of Underwriters Laboratories Inc.)


                                                             Hot flaming
                                                             gases
                            Cover with machine-flanged
                                                                                                               II. Wiring




                                                                                                                                                                                               --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                            joint (very small pressure
                            release clearance)
                                                                                                               501.10 Wiring Methods
                                                                                                               Wiring methods shall comply with 501.10(A) or 501.10(B).
                                                                                                               (A) Class I, Division 1
                                                                                                               (1) General In Class I, Division 1 locations, the wiring
                                                                                                               methods in (a) through (d) shall be permitted.
                                                                 Internal
                                                                 pressure          Cool escaping                  (a) Threaded rigid metal conduit or threaded steel inter-
                                                                                   gases                       mediate metal conduit.

                           Exhibit 501.2 Cooling of hot gases as they pass across a ma-                        Rigid metal conduit and intermediate metal conduit must
                           chine-flanged joint. The clearance between the machined sur-
                                                                                                               be threaded with an NPT standard conduit cutting die that
                           faces is kept very small.
                                                                                                               provides a 3⁄4-in. taper per foot. For enclosures that are field-
                                                                                                               threaded, five full threads must be engaged, while for listed
                           apart, as shown in Exhibit 501.3. The amount of increase
                                                                                                               enclosures with factory-threaded NPT entries, 41⁄2 threads
                           in the joint clearance depends on the stiffness of the enclosure
                                                                                                               must be fully engaged. See 500.8(D).
                           parts; the size, strength, and spacing of the bolts; and the
                                                                                                                     The Code recognizes electrical equipment with meti-
                           explosion pressure. Simply measuring the joint width and
                                                                                                               ric threaded entries [see 500.8(D)(2)]. Equipment with metric
                           clearance when there are no internal pressures does not
                                                                                                               threaded entries must be identified to indicate that metric
                           indicate the actual clearances under the dynamic conditions
                                                                                                               threads are provided or be provided with listed adapters to
                           of an explosion. Explosion tests are usually needed to dem-
                                                                                                               allow the connection of NPT-threaded conduit or fittings to
                           onstrate the acceptability of the design.
                                                                                                               the equipment. Each joint must be made up wrenchtight at
                                                                                                               couplings and unions, threaded hubs of junction boxes, de-
                                Equipment listed and marked in accordance with                                 vice boxes, conduit bodies, and so on.
                           505.9(C)(2) for use in Class I, Zone 0, 1, or 2 locations shall
                           be permitted in Class I, Division 2 locations for the same
                           gas and with a suitable temperature class. Equipment listed                         Exception: Rigid nonmetallic conduit complying with Arti-
                           and marked in accordance with 505.9(C)(2) for use in Class                          cle 352 shall be permitted where encased in a concrete
                           I, Zone 0 locations shall be permitted in Class I, Division                         envelope a minimum of 50 mm (2 in.) thick and provided
                           1 or Division 2 locations for the same gas and with a suitable                      with not less than 600 mm (24 in.) of cover measured from
                           temperature class.                                                                  the top of the conduit to grade. The concrete encasement


                           National Electrical Code Handbook 2005                                                                                                                        673

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                 501.10                                               Article 501 — Class I Locations



                    shall be permitted to be omitted where subject to the provis-             The requirement in 501.10(A)(1)(b) specifies that termina-
                    ions of 514.8, Exception No. 2; and 515.8(A). Threaded                    tion fittings used with Type MI cable must be listed for use
                    rigid metal conduit or threaded steel intermediate metal                  in Class I, Division 1 hazardous (classified) locations. In
                    conduit shall be used for the last 600 mm (24 in.) of the                 editions of the Code before 2002, termination fittings used
                    underground run to emergence or to the point of connection                with Type MI cable were required to be approved. (See the
                    to the aboveground raceway. An equipment grounding con-                   definition of approved in Article 100.) This change means
                    ductor shall be included to provide for electrical continuity             that MI cable fittings must be evaluated in accordance with
                    of the raceway system and for grounding of non–current-                   an appropriate product standard or be tested for the specific
                    carrying metal parts.                                                     use. A fitting that is approved must be acceptable to the
                                                                                              authority having jurisdiction, which, in many cases, requires
                    The exception to 501.10(A)(1)(a) permits the use of rigid                 the use of listed equipment as a basis for approval. However,
                    nonmetallic conduit in some underground installations. If                 the term approved does not mandate product evaluation or
                    rigid nonmetallic conduit is used for underground wiring,                 testing. The requirement that fittings used with MI cables
                    threaded rigid metal conduit or threaded steel intermediate               be specifically listed for use in the particular hazardous




                                                                                                                                                                                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    metal conduit must be used for the last 2 ft of the under-                (classified) location class and group involved provides a
                    ground run to the point of emergence or to the point of                   more objective basis for selecting the proper fitting. Type
                    connection to the aboveground raceway. The rigid nonmetal-                MI cable fittings, as shown in Exhibit 501.4, have a clamp-
                    lic conduit, including rigid nonmetallic conduit elbows and               type joint that must be investigated to determine that it is
                    fittings, must be located not less than 2 ft below grade. The             explosionproof. Type MI cable fittings not investigated for
                    conduit must also be encased in not less than 2 in. of concrete.          use in hazardous locations may not be explosionproof. Type
                          The requirements covering the use of rigid nonmetallic              MI cable fittings that are suitable for nonhazardous locations
                    conduit in underground locations per 514.8, Exception No.                 may not be suitable for Class I, Division 1 hazardous (classi-
                    2, and 515.8 do not require concrete encasement. These                    fied) locations.
                    provisions are for specific occupancies where there has been                   Exhibit 501.5 shows an explosionproof junction box
                    considerable experience with underground nonmetallic con-                 with two hubs and a threaded opening for the screw-type
                    duit. The exception to 501.10(A)(1)(a) applies to other occu-             cover. Unused openings must be effectively closed by in-
                    pancies where rigid nonmetallic conduit is installed in an                serting threaded metal plugs that engage at least five full
                    underground location that has been classified as a Class I,               threads [41⁄2 permitted in accordance with 500.8(D), Excep-
                    Division 1 location.                                                      tion] and afford protection equivalent to that of the wall of
                          If rigid nonmetallic conduit is used, an equipment                  the box.
                    grounding conductor must be included and must be bonded
                    to the metal raceways that extend from the underground
                    rigid nonmetallic conduit.                                                      (c) In industrial establishments with restricted public
                                                                                              access, where the conditions of maintenance and supervision
                                                                                              ensure that only qualified persons service the installation,
                         (b) Type MI cable with termination fittings listed for               Type MC-HL cable, listed for use in Class I, Division 1
                    the location. Type MI cable shall be installed and supported              locations, with a gas/vaportight continuous corrugated me-
                    in a manner to avoid tensile stress at the termination fittings.          tallic sheath, an overall jacket of suitable polymeric material,


                    Exhibit 501.4 Type MI cable
                                                                             Threaded gland
                    and fitting listed for use in haz-                                                                                     End seal
                    ardous locations. The screw-
                    on pot contains field-installed
                    sealing compound to seal the
                    end of the cable. The threaded
                    gland has threads for connec-
                    tion to explosionproof enclo-
                    sures. (Courtesy of Pyrotenax                                                                                              Insulating cap
                    Cables, Ltd.)                                         Brass   Brass       NPT        Brass screw-on
                                                                                                         pot (cuts its own                 Anchoring beading
                                                                          gland   gland       taper
                                                                                                         thread as it screws               securing sleeving
                                                                          nut     body        thread
                                                                                                         on to the cable sheath)                             Insulating sleeving
                                                                                                                                                             (may be color coded)




                 674                                                                                                           2005 National Electrical Code Handbook

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                                                                             Article 501 — Class I Locations                                                                         501.10



                                                                                                                  under which Type ITC cable can be used in Class I, Division
                                                                                                                  1 locations parallel those in 501.10(A)(1)(c) for Type MC-
                                                                                                                  HL cable.


                                                                                                                  (2) Flexible Connections Where necessary to employ flex-
                                                                                                                  ible connections, as at motor terminals, flexible fittings listed
                                                                                                                  for Class I, Division 1 locations or flexible cord in accor-
                                                                                                                  dance with the provisions of 501.140 shall be permitted.

                                                                                                                  Flexible connection fittings are available in lengths up to 3 ft
                                                                                                                  for use in Class I, Division 1 locations. A flexible connection
                                                                                                                  fitting consists of a deeply corrugated bronze tube with an
                                                                                                                  internal nonmetallic tubular protective liner and an outer
                                                                                                                  cover of braided fine bronze wires. A threaded fitting is
                                                                                                                  securely attached to each end of the flexible tube. This type
                           Exhibit 501.5 An explosionproof junction box with a screw-type                         of flexible fitting is commonly used at motor connections,
                           cover. (Courtesy of O-Z/Gedney, a division of EGS Electrical                           can withstand continuous vibration for long periods, is explo-
                           Group)                                                                                 sionproof, and affords maximum protection to any enclosed
                                                                                                                  conductors. Limited use of flexible cord in accordance with
                                                                                                                  501.140 is also permitted for specific applications where
                           separate grounding conductors in accordance with 250.122,                              flexibility of the wiring method is made necessary by the
                           and provided with termination fittings listed for the applica-                         type of equipment being supplied.
                           tion.

                                   FPN: See 330.12 for restrictions on use of Type MC                             (3) Boxes and Fittings All boxes and fittings shall be ap-
                                   cable.
                                                                                                                  proved for Class I, Division 1.

                           Due to the potential for physical damage to Type MC cable,                             (B) Class I, Division 2
                           its application is limited to cable that is listed specifically                        (1) General In Class I, Division 2 locations, the following
                           for use in Class I, Division 1 locations and installed at                              wiring methods shall be permitted:
                           facilities that have full-time, qualified maintenance person-
                           nel. This special-use cable is identified as Type MC-HL.                               (1) All wiring methods permitted in Article 501.10(A).
                           Qualified maintenance personnel are those who, in the course                           (2) Threaded rigid metal conduit, threaded steel intermedi-
                           of regular maintenance procedures, would notice whether                                    ate metal conduit.
                           cables were damaged, understand the associated hazards, and                            (3) Enclosed gasketed busways, enclosed gasketed wire-
                           are able to de-energize the circuit to repair the installation.                            ways.
                                                                                                                  (4) Type PLTC cable in accordance with the provisions of
                                                                                                                      Article 725, or in cable tray systems. PLTC shall be
                                 (d) In industrial establishments with restricted public                              installed in a manner to avoid tensile stress at the termi-
                           access, where the conditions of maintenance and supervision                                nation fittings.
                           ensure that only qualified persons service the installation,                           (5) Type ITC cable as permitted in 727.4.
                           Type ITC-HL cable, listed for use in Class I, Division 1                               (6) Type MI, MC, MV, or TC cable with termination fit-
                           locations, with a gas/vaportight continuous corrugated me-                                 tings, or in cable tray systems and installed in a manner
                           tallic sheath, an overall jacket of suitable polymeric material                            to avoid tensile stress at the termination fittings. Single
                           and provided with termination fittings listed for the applica-                             conductor Type MV cables shall be shielded or metallic
                           tion.                                                                                      armored.

                                                                                                                  (2) Flexible Connections Where provision must be made
                           Type ITC-HL cable has a gastight/vaportight continuous                                 for limited flexibility, one or more of the following shall
                           corrugated metallic sheath and a polymeric jacket and is                               also be permitted:
                           listed for use in Class I, Division 1 locations. Type ITC-HL
                           cable is permitted to be used in industrial establishments                             (1) Flexible metal fittings
                           that have specific conditions of operation. The conditions                             (2) Flexible metal conduit with listed fittings
                                                                                                                  (3) Liquidtight flexible metal conduit with listed fittings


                           National Electrical Code Handbook 2005                                                                                                                       675
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                 501.15                                                  Article 501 — Class I Locations



                    (4) Liquidtight flexible nonmetallic conduit with listed fit-           the ignition temperature in degrees Celsius. Where general-
                        tings                                                               purpose enclosures are permitted by 501.10(B)(4), rigid or
                    (5) Flexible cord listed for extra-hard usage and provided              intermediate metal conduit may be used with locknuts and
                        with listed bushed fittings. An additional conductor for            bushings. However, a bonding jumper with proper fittings
                        grounding shall be included in the flexible cord.                   or bonding-type locknuts is required to be used between the
                                                                                            enclosure and the raceway to ensure adequate bonding from
                            FPN: See 501.30(B) for grounding requirements where
                            flexible conduit is used.                                       the hazardous area to the point of grounding at the service
                                                                                            equipment or separately derived system. See 501.30 for
                    (3) Nonincendive Field Wiring Nonincendive field wiring                 grounding and bonding requirements.
                    shall be permitted using any of the wiring methods permitted                 Where limited flexibility is necessary and approved fit-
                    for unclassified locations. Nonincendive field wiring sys-              tings are required for use with flexible metal conduit, liq-
                    tems shall be installed in accordance with the control draw-            uidtight flexible conduit, and extra-hard-usage flexible cord,
                    ing(s). Simple apparatus, not shown on the control drawing,             the fittings are not required to be specifically approved for
                    shall be permitted in a nonincendive field wiring circuit,              Class I locations. Also, where flexible conduit or liquidtight




                                                                                                                                                                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    provided the simple apparatus does not interconnect the                 flexible conduit is used, internal or external bonding jumpers
                    nonincendive field wiring circuit to any other circuit.                 with proper fittings must be provided, in accordance with
                                                                                            501.30(B), unless liquidtight flexible conduit is installed
                    The installation of nonincendive field wiring is covered in             under the conditions described in 501.30(B), Exception.
                    501.10(B)(3). See the commentary following 501.105(B)(1),                    Section 501.10(B)(1) permits a variety of cable types,
                    Exception, and the definitions of nonincendive circuit and              cable tray systems in accordance with 392.3(D), enclosed
                    nonincendive field wiring in 500.2. Many low-voltage, low-              gasketed wireways, and enclosed gasketed busways. The
                    energy circuits are of the nonincendive type. However, a                cable and cable fittings, cable trays, wireways, and busways
                    Class 2 circuit, as defined in Article 725, is not necessarily          are not required to be specifically listed or labeled for Class
                    nonincendive. Testing laboratories, such as Factory Mutual              I, Division 2 locations. For example, if Type ITC or MC
                    Research Corp. and Underwriters Laboratories Inc., list                 cable is used, neither the cable nor the fittings need to be
                    many types of equipment that have nonincendive circuits                 listed for use in hazardous (classified) locations. Type AC
                    intended for connection of nonincendive field wiring. This              cable is not a permitted wiring method in 501.10(B) because
                    equipment is evaluated for use in one or more of the Class              of concern about arcing between convolutions during
                    I gas or vapor groups and is permitted for use only in Divi-            ground-fault conditions.
                    sion 2 locations. Some common telephone circuits and ther-                   Any wiring method suitable for ordinary locations may
                    mocouple circuits are also nonincendive.                                be used for nonincendive field wiring. See 501.10(B)(3).


                            FPN: Simple apparatus is defined in 504.2.                      501.15 Sealing and Drainage
                                                                                            Seals in conduit and cable systems shall comply with
                         Separate nonincendive field wiring circuits shall be in-           501.15(A) through 501.15(F). Sealing compound shall be
                    stalled in accordance with one of the following:                        used in Type MI cable termination fittings to exclude mois-
                    (1) In separate cables                                                  ture and other fluids from the cable insulation.
                    (2) In multiconductor cables where the conductors of each                      FPN No. 1: Seals are provided in conduit and cable
                        circuit are within a grounded metal shield                                 systems to minimize the passage of gases and vapors and
                    (3) In multiconductor cables, where the conductors of each                     prevent the passage of flames from one portion of the
                        circuit have insulation with a minimum thickness of                        electrical installation to another through the conduit. Such
                        0.25 mm (0.01 in.)                                                         communication through Type MI cable is inherently pre-
                                                                                                   vented by construction of the cable. Unless specifically
                    (4) Boxes and Fittings Boxes and fittings shall not be re-                     designed and tested for the purpose, conduit and cable
                                                                                                   seals are not intended to prevent the passage of liquids,
                    quired to be explosionproof except as required by                              gases, or vapors at a continuous pressure differential
                    501.105(B)(1), 501.115(B)(1), and 501.150(B)(1).                               across the seal. Even at differences in pressure across
                                                                                                   the seal equivalent to a few inches of water, there may
                                                                                                   be a slow passage of gas or vapor through a seal and
                    In Class I, Division 2 locations, boxes, fittings, and joints
                                                                                                   through conductors passing through the seal. See
                    are not required to be explosionproof at lighting outlets or                   501.15(E)(2). Temperature extremes and highly corrosive
                    at enclosures containing no arcing devices, such as solenoids                  liquids and vapors can affect the ability of seals to per-
                    and control transformers, if the maximum operating tempera-                    form their intended function. See 501.15(C)(2).
                    ture of any exposed surface does not exceed 80 percent of                      FPN No. 2: Gas or vapor leakage and propagation of
                                                                                                   flames may occur through the interstices between the


                 676                                                                                                           2005 National Electrical Code Handbook

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                                                                                 Article 501 — Class I Locations                                                               501.15



                                   strands of standard stranded conductors larger than 2                    For the purposes of this section, high temperatures shall
                                   AWG. Special conductor constructions, for example,                  be considered to be any temperatures exceeding 80 percent
                                   compacted strands or sealing of the individual strands,
                                   are means of reducing leakage and preventing the propa-             of the autoignition temperature in degrees Celsius of the gas
                                   gation of flames.                                                   or vapor involved.
                                                                                                       Exception to 501.15(A)(1)(1): Seals shall not be required for
                           The sealing compound used in conduit seal fittings is some-                 conduit entering an enclosure where such switches, circuit
                           what porous, so that gases, particularly those under slight                 breakers, fuses, relays, or resistors comply with one of the
                           pressure and those with small molecules such as hydrogen,                   following:
                           can pass slowly through the sealing compound. Also, the
                           seal is around the insulation on the conductor, and gases can               (1) Are enclosed within a chamber hermetically sealed
                           be transmitted slowly through the air spaces (the interstices)                  against the entrance of gases or vapors
                           between strands of stranded conductors. The cable core does                 (2) Are immersed in oil in accordance with
                           not include the interstices of the conductor strands.                           501.115(B)(1)(2)
                                Experience has shown, however, that under normal con-                  (3) Are enclosed within a factory-sealed explosionproof
                           ditions for smaller conductors, and with only normal atmo-                      chamber located within the enclosure, identified for the
                           spheric pressure differentials across the seal, the passage of                  location, and marked ‘‘factory sealed’’ or equivalent,
                           gas through a seal is not sufficient to result in a hazard. For                 unless the enclosure entry is metric designator 53 (trade
                           larger conductors, however, gas or vapor leakage and flame                      size 2) or larger
                           propagation may occur through the interstices between the                   (4) Are in nonincendive circuits
                           strands, of stranded conductors. Special conductor construc-
                           tions, such as compacted strands or sealing individual                            Factory-sealed enclosures shall not be considered to
                           strands, may reduce leakage and prevent flame propagation.                  serve as a seal for another adjacent explosionproof enclosure
                                Sealing fittings should be used only with the sealing                  that is required to have a conduit seal.
                           compound or compounds recommended by the fitting manu-                            Conduit seals shall be installed within 450 mm (18 in.)
                           facturer. Different sealing compounds have different rates of               from the enclosure. Only explosionproof unions, couplings,
                           expansion and contraction that may affect their performance                 reducers, elbows, capped elbows, and conduit bodies similar
                           within a given fitting. Sealing compound must be used as                    to L, T, and Cross types that are not larger than the trade
                           soon as possible on Type MI cable terminations to exclude                   size of the conduit shall be permitted between the sealing
                           moisture from cable insulation.                                             fitting and the explosionproof enclosure.
                                The use of Teflon tapes or joint compounds on conduit
                           threads may weaken the seal fitting and interrupt the equip-                (2) Pressurized Enclosures In each conduit entry into a
                           ment grounding path. Cracks have developed in fittings dur-                 pressurized enclosure where the conduit is not pressurized
                           ing hydrostatic testing in which these materials were used.                 as part of the protection system. Conduit seals shall be
                                                                                                       installed within 450 mm (18 in.) from the pressurized enclo-
                                                                                                       sure.
                           (A) Conduit Seals, Class I, Division 1 In Class I, Division
                           1 locations, conduit seals shall be located in accordance with                     FPN No. 1: Installing the seal as close as possible to the
                           501.15(A)(1) through (A)(4).                                                       enclosure will reduce problems with purging the dead
                                                                                                              airspace in the pressurized conduit.
                           (1) Entering Enclosures In each conduit entry into an ex-                          FPN No. 2: For further information, see NFPA 496-
                           plosionproof enclosure where either of the following apply:                        2003, Standard for Purged and Pressurized Enclosures
                                                                                                              for Electrical Equipment.
                           (1) The enclosure contains apparatus, such as switches, cir-
                               cuit breakers, fuses, relays, or resistors, that may pro-               (3) Two or More Explosionproof Enclosures Where two
                               duce arcs, sparks, or high temperatures that are                        or more explosionproof enclosures for which conduit seals
                               considered to be an ignition source in normal operation.                are required under 501.15(A)(1) are connected by nipples
                           (2) The entry is metric designator 53 (trade size 2) or larger              or by runs of conduit not more than 900 mm (36 in.) long,
                               and the enclosure contains terminals, splices, or taps.                 a single conduit seal in each such nipple connection or run
                                                                                                       of conduit shall be considered sufficient if located not more
                           In each 2-in. or larger conduit, a sealing fitting must be                  than 450 mm (18 in.) from either enclosure.
                           placed within 18 in. of the conduit entrance to any explo-
                           sionproof enclosure, regardless of whether the enclosure                    An example of 501.15(A) requirements for the location of
                           contains arcing or sparking equipment or only splices, taps,                conduit seals in Class I, Division 1 locations is illustrated
                           or terminals.                                                               in Exhibit 501.6. In the example shown in Exhibit 501.6,


                           National Electrical Code Handbook 2005                                                                                                                 677
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                                      501.15                                                                                    Article 501 — Class I Locations




                                                                                                    36 in. max.                     Seal
                                                                                         18 in. max.

                                                                                                       "T"
                                                                       No. 1        18 in.                            18 in.        No. 2




                                                                   Explosionproof                                              Explosionproof
                                                                   enclosure                                                   enclosure
                                                                   requiring                 Seal                              requiring
                                                                                                             18 in.
                                                                   sealing                                   max.              sealing


                                                                                                                                                   Exhibit 501.7 An explosionproof union. (Courtesy of Thomas &
                                                                                                    On                                             Betts Corp.)
                                                                                                        Off            Explosionproof
                                                                                                    No. 3              enclosure
                                                                                                                       requiring
                                                                                                                       sealing                     within 3.05 m (10 ft) of the boundary and shall be designed
                                                                                                                                                   and installed so as to minimize the amount of gas or vapor
                                                                                                                                                   within the Division 1 portion of the conduit from being
                                                                                                                                                   communicated to the conduit beyond the seal. Except for
                --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                   listed explosionproof reducers at the conduit seal, there shall
                                                                                                                                                   be no union, coupling, box, or fitting between the conduit
                                                                                                                                                   seal and the point at which the conduit leaves the Division
                                                                  Exhibit 501.6 Two seals required so that the run of conduit                      1 location.
                                                                  between Enclosure No. 1 and Enclosure No. 2 is sealed. Even                      Exception No. 1: Metal conduit that contains no unions,
                                                                  if Enclosure No. 3 were not required to be sealed, the vertical
                                                                  seal in the vertical run of conduit to Enclosure No. 3 would be
                                                                                                                                                   couplings, boxes, or fittings, and passes completely through
                                                                  required to be sealed within 18 in. of Enclosure No. 1, because                  a Class I, Division 1 location with no fittings less than 300
                                                                  the vertical conduit run to the ‘‘T’’ fitting is a conduit run to                mm (12 in.) beyond each boundary, shall not require a
                                                                  Enclosure No. 1.                                                                 conduit seal if the termination points of the unbroken conduit
                                                                                                                                                   are in unclassified locations.
                                                                  two seals are required so that the run of conduit between                        Exception No. 2: For underground conduit installed in ac-
                                                                  Enclosure No. 1 and Enclosure No. 2 is sealed.                                   cordance with 300.5 where the boundary is beneath the
                                                                       A seal in a conduit prevents an explosion from traveling                    ground, the sealing fitting shall be permitted to be installed
                                                                  through the conduit to another enclosure and minimizes the                       after the conduit leaves the ground, but there shall be no
                                                                  passage of gases or vapors from a hazardous (classified)                         union, coupling, box, or fitting, other than listed explo-
                                                                  location to a nonhazardous location. If the conduit enters an                    sionproof reducers at the sealing fitting, in the conduit be-
                                                                  enclosure that contains arcing or high-temperature equip-                        tween the sealing fitting and the point at which the conduit
                                                                  ment, a sealing fitting must be placed within 18 in. of the                      leaves the ground.
                                                                  enclosure it isolates; conduit bodies (‘‘L,’’ ‘‘T,’’ etc.), cou-
                                                                  plings, unions, and elbows are the only enclosures or fittings                   A sealing fitting is also required at the point where the
                                                                  permitted between the seal and the enclosure. Exhibit 501.6                      conduit leaves a Division 1 location or passes from a Divi-
                                                                  illustrates the placement of conduit seals. See Exhibit 501.7                    sion 2 location to an unclassified location. The sealing fitting
                                                                  for an approved type of union. If two enclosures are spaced                      is permitted on either side of the boundary, and no union,
                                                                  not more than 36 in. apart as is the case with enclosures                        coupling, box, or similar fitting is permitted between the
                                                                  No. 1 and No. 2, a single seal may be placed between two                         seal and the boundary. However, approved explosionproof
                                                                  connecting nipples if the seal is located not more than 18                       reducers are permitted to be installed in conduit seals.
                                                                  in. from either enclosure.                                                            It is preferable to locate the sealing fitting on the nonhaz-
                                                                                                                                                   ardous side of the boundary. A sealing fitting located as
                                                                                                                                                   such serves two purposes: It completes the explosionproof
                                                                  (4) Class I, Division 1 Boundary In each conduit run leav-                       wiring method, and it completes the explosionproof enclo-
                                                                  ing a Class I, Division 1 location. The sealing fitting shall                    sure system. Note that a 1⁄2-in. conduit connected to an explo-
                                                                  be permitted on either side of the boundary of such location


                                     678                                                                                                                                              2005 National Electrical Code Handbook

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                                                                                                                             Article 501 — Class I Locations                                                                             501.15



                                                                          sionproof box that contains only splices, even in a Division                      Additionally Exhibit 501.8 shows a seal provided within
                                                                          1 location, is not required to be sealed within 18 in. of the                18 in. of the switch controlling the lighting. The design of the
                                                                          box. The sealing fitting at the boundary of the Division 1                   luminaire, as required by ANSI/UL 844, Electric Lighting
                                                                          location serves to complete the explosionproof system. The                   Fixtures for Use in Hazardous (Classified) Locations, is
                                                                          sealing fitting at the boundary also prevents the conduit                    such that the explosionproof chamber for the wiring must
                                                                          system from serving as a pipe to transmit flammable mix-                     be separated or sealed from the lamp compartment; hence,
                                                                          tures from a Division 1 or Division 2 location to an unclassi-               a separate seal is not required adjacent to luminaires that
                                                                          fied location.                                                               comply with ANSI/UL 844. The luminaire is suspended on
                                                                               Exhibit 501.8 illustrates a Class I, Division 1 location                a conduit stem threaded into the cover of an explosionproof
                                                                          using threaded rigid metal conduit or threaded intermediate                  ceiling box. See 501.130 for luminaire requirements.
                                                                          metal conduit and explosionproof fittings and equipment,
                                                                          including motors, motor controllers, pushbutton stations,
                                                                          lighting outlets, and junction boxes. The enclosures for the                 (B) Conduit Seals, Class I, Division 2 In Class I, Division
                                                                          disconnecting means and motor controller for the motor                       2 locations, conduit seals shall be located in accordance with
                                                                          (right portion of the drawing) are placed on the other side                  501.15(B)(1) and (B)(2).
                                                                          of the wall in a nonhazardous location and are thus not
                                                                          required to be explosionproof.                                               (1) Entering Enclosures For connections to enclosures
                                                                               In Exhibit 501.8, each of the three conduits is sealed                  that are required to be explosionproof, a conduit seal shall
                                                                          on the nonhazardous side before passing into the hazardous                   be provided in accordance with 501.15(A)(1) and (A)(3).
                                                                          (classified) location. The pigtail leads of both motors are                  All portions of the conduit run or nipple between the seal
                                                                          factory sealed at the motor-terminal housing, and, unless                    and such enclosure shall comply with 501.10(A).
                                                                          the size of the flexible fitting entering the motor-terminal
                                                                          housing is trade size 2 or larger, no other seals are needed                 In Class I, Division 2 locations, a seal is required in each
                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                          at this point. Because the push-button control station and                   conduit entering an enclosure that is required to be explo-
                                                                          the motor controller and disconnect (left portion of the draw-               sionproof, in order to complete the explosionproof enclosure.
                                                                          ing) are considered arc-producing devices, conduits are
                                                                          sealed within 18 in. of the entrance to these enclosures. Seals
                                                                          are required even though the contacts may be immersed in                     (2) Class I, Division 2 Boundary In each conduit run pass-
                                                                          oil.                                                                         ing from a Class I, Division 2 location into an unclassified
                                                                                                                                                       location. The sealing fitting shall be permitted on either side



                                                                          Exhibit 501.8 A Class I, Divi-                                Nonhazardous location area
                                                                          sion 1 location where threaded                                 seal at bondary location
                                                                          metal conduits, sealing fittings,
                                                                          explosionproof fittings, and
                                                                                                                     Class I location
                                                                          equipment for power and lights
                                                                          are used.
                                                                                                                                                               Light fixture
                                                                                                                                                                                                                        Lockable-type
                                                                                                                                                                                                                        disconnect and motor
                                                                                                                                                                                                                        controller located in
                                                                                                                                                                                                                        nonhazardous area

                                                                                                                       Combination
                                                                                                                        switch and                                       Pushbutton
                                                                                                                          starter                                          station
                                                                                                                                                                                             Switch

                                                                                                                                             Motor
                                                                                                                                                                                          Unions


                                                                                                                                                                                                                              Seal at
                                                                                                                     Flexible                                                                                                 boundary
                                                                                                                     conduit                Junction
                                                                                                                                              box




                                                                          National Electrical Code Handbook 2005                                                                                                                            679

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                 501.15                                                                                              Article 501 — Class I Locations



                                                                   of the boundary of such location within 3.05 m (10 ft) of the            or tube to prevent flammable or combustible fluids from
                                                                   boundary. Rigid metal conduit or threaded steel intermediate             entering the conduit system.
                                                                   metal conduit shall be used between the sealing fitting and          (4) The conduit system segment contains only threaded
                                                                   the point at which the conduit leaves the Division 2 location,           metal conduit, unions, couplings, conduit bodies, and
                                                                   and a threaded connection shall be used at the sealing fitting.          fittings in the unclassified location.
                                                                   Except for listed reducers at the conduit seal, there shall be       (5) The conduit system segment is sealed at its entry to
                 --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                   no union, coupling, box, or fitting between the conduit seal             each enclosure or fitting housing terminals, splices, or
                                                                   and the point at which the conduit leaves the Division 2                 taps in Class I, Division 2 locations.
                                                                   location. Conduits shall be sealed to minimize the amount
                                                                                                                                        (C) Class I, Divisions 1 and 2 Seals installed in Class
                                                                   of gas or vapor within the Division 2 portion of the conduit
                                                                                                                                        I, Division 1 and Division 2 locations shall comply with
                                                                   from being communicated to the conduit beyond the seal.
                                                                                                                                        501.15(C)(1) through (C)(6).
                                                                   Such seals shall not be required to be explosionproof but
                                                                   shall be identified for the purpose of minimizing passage            Exception: Seals not required to be explosionproof by
                                                                   of gases under normal operating conditions and shall be              501.15(B)(2) or 504.70.
                                                                   accessible.
                                                                                                                                        (1) Fittings Enclosures for connections or equipment shall
                                                                   Exception No. 1: Metal conduit that contains no unions,              be provided with an integral means for sealing, or sealing
                                                                   couplings, boxes, or fittings, and passes completely through         fittings listed for the location shall be used. Sealing fittings
                                                                   a Class I, Division 2 location with no fittings less than 300        shall be listed for use with one or more specific compounds
                                                                   mm (12 in.) beyond each boundary, shall not be required to           and shall be accessible.
                                                                   be sealed if the termination points of the unbroken conduit
                                                                                                                                        (2) Compound The compound shall provide a seal against
                                                                   are in unclassified locations.
                                                                                                                                        passage of gas or vapors through the seal fitting, shall not
                                                                   Exception No. 2: Conduit systems terminating at an unclas-           be affected by the surrounding atmosphere or liquids, and
                                                                   sified location where a wiring method transition is made to          shall not have a melting point of less than 93 C (200 F).
                                                                   cable tray, cablebus, ventilated busway, Type MI cable, or
                                                                   cable not installed in any cable tray or raceway system,             (3) Thickness of Compounds Except for listed cable seal-
                                                                   shall not be required to be sealed where passing from the            ing fittings, the thickness of the sealing compound in a
                                                                   Class I, Division 2 location into the unclassified location.         completed seal shall not be less than the metric designator
                                                                   The unclassified location shall be outdoors or, if the conduit       (trade size) of the sealing fitting expressed in the units of
                                                                   system is all in one room, it shall be permitted to be indoors.      measurement employed, and in no case less than 16 mm (5⁄8
                                                                   The conduits shall not terminate at an enclosure containing          in.).
                                                                   an ignition source in normal operation.                              (4) Splices and Taps Splices and taps shall not be made
                                                                   Exception No. 3: Conduit systems passing from an enclosure           in fittings intended only for sealing with compound, nor
                                                                   or room that is unclassified as a result of pressurization into      shall other fittings in which splices or taps are made be filled
                                                                   a Class I, Division 2 location shall not require a seal at the       with compound.
                                                                   boundary.
                                                                                                                                        (5) Assemblies In an assembly where equipment that may
                                                                       FPN: For further information, refer to NFPA 496-2003,            produce arcs, sparks, or high temperatures is located in a
                                                                       Standard for Purged and Pressurized Enclosures for               compartment separate from the compartment containing
                                                                       Electrical Equipment.
                                                                                                                                        splices or taps, and an integral seal is provided where conduc-
                                                                   Exception No. 4: Segments of aboveground conduit systems             tors pass from one compartment to the other, the entire
                                                                   shall not be required to be sealed where passing from a              assembly shall be identified for the location. Seals in conduit
                                                                   Class I, Division 2 location into an unclassified location if        connections to the compartment containing splices or taps
                                                                   all of the following conditions are met:                             shall be provided in Class I, Division 1 locations where
                                                                                                                                        required by 501.15(A)(1)(2).
                                                                   (1) No part of the conduit system segment passes through
                                                                       a Class I, Division 1 location where the conduit contains        (6) Conductor Fill The cross-sectional area of the conduc-
                                                                       unions, couplings, boxes, or fittings within 300 mm (12          tors permitted in a seal shall not exceed 25 percent of the
                                                                       in.) of the Class I, Division 1 location.                        cross-sectional area of a rigid metal conduit of the same
                                                                   (2) The conduit system segment is located entirely in out-           trade size unless it is specifically identified for a higher
                                                                       door locations.                                                  percentage of fill.
                                                                   (3) The conduit system segment is not directly connected
                                                                       to canned pumps, process or service connections for              The maximum permitted fill for the conduit is 40 percent;
                                                                       flow, pressure, or analysis measurement, and so forth,           the maximum permitted fill for most conduit seals is 25
                                                                       that depend on a single compression seal, diaphragm,


                 680                                                                                                                                                       2005 National Electrical Code Handbook

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                                                                                                                     Article 501 — Class I Locations                                                                  501.15



                                                                   percent. If the conduit fill exceeds 25 percent of the cross-               Type MC-HL cable has a continuous corrugated metallic
                                                                   sectional area of the sealing fitting, a larger trade size seal             sheath that is gastight/vaportight and an outer nonmetallic
                                                                   may be required. Reducers are allowed for connection of a                   material that enables it to be installed in wet locations. Type
                                                                   larger trade size sealing fitting to the conduit. Exhibit 501.9             MC-HL cables are available with ratings up to 35,000 volts.
                                                                   illustrates the proper sealing of a fitting. A dam must be                  The cable is specifically listed for use in Class I, Division
                                                                   provided to prevent the sealing material, while still in the                1 locations.
                                                                   liquid state, from running out of the fitting. All conductors                    The provisions of 501.15(D) contain the sealing require-
                                                                   must be separated to permit the sealing material to run                     ments for cables installed in Class I, Division 1 locations,
                                                                   between them. The sealing compound must have a minimum                      which differ from the requirements for sealing conduits.
                                                                   thickness of not less than the trade size of the conduit and                Conduits entering explosionproof enclosures must be sealed
                                                                   in no case less than 5⁄8 in. Conduit fittings for sealing are to            if the enclosure contains equipment that produces arcs,
                                                                   be used only with sealing compound that is supplied with                    sparks, or high temperatures or if the conduit entering the
                                                                   the fitting and specified by the manufacturer in instructions               enclosure is trade size 2 or larger. In accordance with
                                                                   furnished with the fitting.                                                 501.15(D)(1), cables must be sealed at all terminations, irre-
                                                                                                                                               spective of the type of equipment contained in the enclosure
                                                                                                                                               or the diameter of the cable. Exhibit 501.10 shows an exam-
                                                                                                                                               ple of a cable sealing fitting for Type MC-HL cable.
                 --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                   Exhibit 501.9 A sealing fitting placed in a run of conduit to
                                                                   minimize the passage of gases from one portion of the electrical            Exhibit 501.10 An explosionproof sealing fitting for Type MC-
                                                                   installation to another. (Courtesy of Appleton Electric Co., EGS            HL cable. (Courtesy of Cooper Crouse-Hinds)
                                                                   Electrical Group)

                                                                                                                                               Exception: Shielded cables and twisted pair cables shall not
                                                                   (D) Cable Seals, Class I, Division 1 In Class I, Division
                                                                                                                                               require the removal of the shielding material or separation of
                                                                   1 locations, cable seals shall be located according to
                                                                                                                                               the twisted pairs, provided the termination is by an approved
                                                                   501.15(D)(1) through (D)(3).
                                                                                                                                               means to minimize the entrance of gases or vapors and
                                                                   (1) At Terminations Cable shall be sealed at all termina-                   prevent propagation of flame into the cable core.
                                                                   tions. The sealing fitting shall comply with 501.15(C). Multi-
                                                                   conductor Type MC-HL cables with a gas/vaportight                           Shielded cables and twisted pair cables, commonly used for
                                                                   continuous corrugated metallic sheath and an overall jacket                 signaling and instrumentation circuits, are permitted by the
                                                                   of suitable polymeric material shall be sealed with a listed                exception to 501.15(D) to be sealed without removal of the
                                                                   fitting after removing the jacket and any other covering so                 outer sheathing or the separation of the twisted conductors.
                                                                   that the sealing compound surrounds each individual insu-                   The need to provide a suitable seal while not adversely
                                                                   lated conductor in such a manner as to minimize the passage                 affecting the operational performance of these cables is ac-
                                                                   of gases and vapors.                                                        complished through this exception.

                                                                   In accordance with 501.10(A)(1)(c), Type MC-HL cable is
                                                                   permitted as a wiring method in Class I, Division 1 areas.                  (2) Cables Capable of Transmitting Gases or Vapors Ca-
                                                                                                                                               bles in conduit with a gas/vaportight continuous sheath capa-

                                                                   National Electrical Code Handbook 2005                                                                                                                681

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                 501.15                                                             Article 501 — Class I Locations



                    ble of transmitting gases or vapors through the cable core                         If cables are run from a Type Z pressurized room or enclosure
                    shall be sealed in the Division 1 location after removing the                      into a Class I, Division 2 location, Exception No. 1 to
                    jacket and any other coverings so that the sealing compound                        501.15(E)(1) allows the cables to be installed without a
                    will surround each individual insulated conductor and the                          sealing fitting at the boundary. This exception correlates
                    outer jacket.                                                                      with a similar allowance for conduit systems found in
                    Exception: Multiconductor cables with a gas/vaportight                             501.15(B)(2), Exception No. 3.
                    continuous sheath capable of transmitting gases or vapors
                    through the cable core shall be permitted to be considered
                    as a single conductor by sealing the cable in the conduit                          Exception No. 2: Shielded cables and twisted pair cables
                    within 450 mm (18 in.) of the enclosure and the cable end                          shall not require the removal of the shielding material or
                    within the enclosure by an approved means to minimize the                          separation of the twisted pairs, provided the termination is
                    entrance of gases or vapors and prevent the propagation of                         by an approved means to minimize the entrance of gases or
                    flame into the cable core, or by other approved methods.                           vapors and prevent propagation of flame into the cable core.
                    For shielded cables and twisted pair cables, it shall not be
                    required to remove the shielding material or separate the                          (2) Cables That Do Not Transmit Gases or Vapors Ca-
                    twisted pair.                                                                      bles that have a gas/vaportight continuous sheath and do not
                                                                                                       transmit gases or vapors through the cable core in excess
                                                                                                       of the quantity permitted for seal fittings shall not be required
                    The intent of the exception to 501.15(D)(2) is to permit flat                      to be sealed except as required in 501.15(E)(1). The mini-
                    computer cables, coaxial cables, and twisted pair cables to                        mum length of such cable run shall not be less than that
                    be treated as single conductors if installed in conduit, because                   length that limits gas or vapor flow through the cable core
                    separating the individual conductors or removing the outer                         to the rate permitted for seal fittings [200 cm3/hr (0.007 ft3/
                    jacket (of a coaxial cable or a twisted pair, for example) is                      hr) of air at a pressure of 1500 pascals (6 in. of water)].
                    impractical and can destroy the electrical properties of the
                    cable.
                         In addition to the cable seal, the end of the cable within                    The ability of a cable to transmit gases or vapors through
                    the enclosure must also be sealed.                                                 the core (primarily between insulated conductors) depends
                                                                                                       not only on how tightly packed the conductors are within
                                                                                                       the outer sheaths and the location and composition of fillers
                    (3) Cables Incapable of Transmitting Gases or Va-                                  but also on how the cable has been handled and the geometry
                    pors Each multiconductor cable in conduit shall be consid-                         of the cable run. If there is any question as to whether or
                    ered as a single conductor if the cable is incapable of                            not the cable run is capable of transmitting gases or vapors
                    transmitting gases or vapors through the cable core. These                         through the core, a sealing fitting should be installed. See
                    cables shall be sealed in accordance with 501.15(A).                               the commentary following 501.15, FPN No. 1.

                    (E) Cable Seals, Class I, Division 2 In Class I, Division
                    2 locations, cable seals shall be located in accordance with                              FPN No. 1: See ANSI/UL 886-1994, Outlet Boxes and
                                                                                                              Fittings for Use in Hazardous (Classified) Locations.
                    501.15(E)(1) through (E)(4).

                    (1) Terminations Cables entering enclosures that are re-                           In a leak rate test, the ends of each individual conductor in
                    quired to be explosionproof shall be sealed at the point of                        the cable are sealed to prevent migration of gases or vapors
                    entrance. The sealing fitting shall comply with 501.15(B)(1).                      between the individual strands of wire. Sealing can be
                    Multiconductor cables with a gas/vaportight continuous                             achieved by dipping the cable end in hot wax. The rate of
                    sheath capable of transmitting gases or vapors through the                         flow through the filler between the insulated conductors can
                    cable core shall be sealed in a listed fitting in the Division                     now be accurately measured, excluding any leakage through
                    2 location after removing the jacket and any other coverings                       the conductor strands. If this is done, the wax should be
                    so that the sealing compound surrounds each individual insu-                       removed before the connections are made and the system
                    lated conductor in such a manner as to minimize the passage                        is placed in service.
                    of gases and vapors. Multiconductor cables in conduit shall
                    be sealed as described in 501.15(D).
                                                                                                              FPN No. 2: The cable core does not include the inter-
                    Exception No. 1: Cables passing from an enclosure or room                                 stices of the conductor strands.
                    that is unclassified as a result of Type Z pressurization into
                    a Class I, Division 2 location shall not require a seal at the                     (3) Cables Capable of Transmitting Gases or Vapors Ca-
                    boundary.                                                                          bles with a gas/vaportight continuous sheath capable of

                 682              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                          2005 National Electrical Code Handbook

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                                                                             Article 501 — Class I Locations                                                                 501.15



                           transmitting gases or vapors through the cable core shall not
                           be required to be sealed except as required in 501.15(E)(1),
                           unless the cable is attached to process equipment or devices
                           that may cause a pressure in excess of 1500 pascals (6 in.
                           of water) to be exerted at a cable end, in which case a seal,
                           barrier, or other means shall be provided to prevent migration
                           of flammables into an unclassified location.
                           Exception: Cables with an unbroken gas/vaportight contin-
                           uous sheath shall be permitted to pass through a Class I,
                           Division 2 location without seals.
                           (4) Cables Without Gas/Vaportight Sheath Cables that
                           do not have gas/vaportight continuous sheath shall be sealed
                           at the boundary of the Division 2 and unclassified location
                           in such a manner as to minimize the passage of gases or
                           vapors into an unclassified location.
                           (F) Drainage
                                                                                                      Exhibit 501.11 A sealing fitting with an automatic drain plug.
                           Unless the additional seal or barrier, described in                        (Courtesy of Appleton Electric Co., EGS Electrical Group)
                           501.15(F)(3), and interconnecting enclosures meet the per-
                           formance requirements of the primary seal, the application
                           of pressure or exposure to extreme temperatures must be
                           prevented at the additional seal or barrier so that the process
                           fluid will not enter the conduit system if the primary seal
                           fails. If the process fluid is a gas or can become a gas under
                           ordinary atmospheric conditions (liquefied natural gas, for
                           example), the drain mentioned in 501.15(F)(3) should be
                           a vent. See the commentary following 501.15(F)(3). The
                           necessary sealing may be accomplished by a sealing fitting
                           and compound.
                                 To eliminate the time-consuming task of field-poured
                           seals, a factory-sealed device with the seal designed into the
                           device is permissible. A wide selection of factory-sealed
                           devices are available for a variety of installations in hazard-            Exhibit 501.12 A combination breather-drainage fitting. (Cour-
                                                                                                      tesy of Appleton Electric Co., EGS Electrical Group)
                           ous (classified) locations. For example, explosionproof mo-
                           tors are normally factory sealed and therefore require no
                           additional seal. Factory-sealed devices are usually marked                 fittings are also available. It is good practice to consider the
                           as such. If a conduit terminates in a motor, however, and if               installation of drain, breather, or combination fittings to
                           the conduit is 2 in. or larger, a seal must be placed within               guard against water accumulation, which can cause future
                           18 in. of the motor terminal housing.                                      insulation failures, even though prevalent conditions may
                                 Exhibit 501.11 shows a sealing fitting designed for use              not indicate a need.
                           in a vertical run of conduit to provide drainage for any
                           condensation of moisture trapped by the seal above the en-
                           closure. Any accumulation of water runs down over the                      (1) Control Equipment Where there is a probability that
                           surface of the sealing compound, flowing through an explo-                 liquid or other condensed vapor may be trapped within enclo-
                           sionproof drain.                                                           sures for control equipment or at any point in the raceway
                                 Exhibit 501.12 shows a combination drain and breather                system, approved means shall be provided to prevent accu-
                                                                                                                                                                                      --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                           fitting. This type of fitting is specifically designed to serve            mulation or to permit periodic draining of such liquid or
                           as a water drain and air vent while providing positive explo-              condensed vapor.
                           sionproof protection. The fitting permits the escape of accu-
                           mulated water through its drain, and the breather allows the               (2) Motors and Generators Where the authority having
                           continuous circulation of air, preventing condensation of any              jurisdiction judges that there is a probability that liquid or
                           moisture that may be present. Individual drain or breather                 condensed vapor may accumulate within motors or genera-
                                                                                                      tors, joints and conduit systems shall be arranged to minimize

                           National Electrical Code Handbook 2005                                                                                                               683

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                 501.15                                                             Article 501 — Class I Locations



                    the entrance of liquid. If means to prevent accumulation or                                               ant for sealing fittings is not designed to withstand high
                    to permit periodic draining are judged necessary, such means                                              pressure or extremely low temperatures, which might be
                    shall be provided at the time of manufacture and shall be                                                 encountered in canned pump installations. Therefore, a sec-
                    considered an integral part of the machine.                                                               ond seal or barrier is required to prevent fluid from entering
                                                                                                                              the electrical conduit or cable system. In addition to this
                    (3) Canned Pumps, Process, or Service Connections, etc.                                                   seal or barrier, a drain, vent, or other similar device that
                    For canned pumps, process, or service connections for flow,                                               indicates failure of the primary seal must be provided.
                    pressure, or analysis measurement, and so forth, that depend                                                   This redundant protection system may be achieved by
                    on a single compression seal, diaphragm, or tube to prevent                                               installing a vented junction (box) enclosure within the classi-
                    flammable or combustible fluids from entering the electrical                                              fied area where the conductors terminate on busbars. Termi-
                    raceway or cable system capable of transmitting fluids, an                                                nating the conductors in this manner allows any fluid that
                    additional approved seal, barrier, or other means shall be                                                escapes through the primary seal and that has traveled
                    provided to prevent the flammable or combustible fluid from                                               through the stranding of the conductors to vent at the termi-
                    entering the raceway or cable system capable of transmitting                                              nations. The circuit continues through the vented enclosure
                    fluids beyond the additional devices or means, if the primary                                             at normal atmospheric pressure to another set of conductors
                    seal fails. The additional approved seal or barrier and the                                               that also must be sealed with a sealing fitting if they travel
                    interconnecting enclosure shall meet the temperature and                                                  into a different classified area. A gas detector can be installed
                    pressure conditions to which they will be subjected upon                                                  in the vicinity of the vented termination box to signal that
                    failure of the primary seal, unless other approved means are                                              the primary seal has failed and allow an orderly shutdown
                    provided to accomplish this purpose. Drains, vents, or other                                              of the process system either automatically or manually.
                    devices shall be provided so that primary seal leakage will                                                    Commentary Table 5.3 summarizes the sealing require-
                    be obvious.                                                                                               ments of 501.15.
                            FPN: See also the fine print notes to 501.15.

                                                                                                                                  Process-connected equipment that is listed and marked
                    Canned pumps and other process equipment that operate                                                     ‘‘Dual Seal’’ shall not require additional process sealing
                    above atmospheric pressure are provided with a primary seal                                               when used within the manufacturer’s ratings.
                    where the electrical conductors enter the pump or equipment
                    containing flammable liquids or gases under pressure. Seal-                                                      FPN: For construction and testing requirements for dual
                                                                                                                                     seal process connected equipment, refer to ISA 12.27.01,



                    Commentary Table 5.3 Conduit and Cable Sealing Requirements

                        Classification                                  Application                                                                                      Location of Seal

                    Conduit Seals                                                                                                          In conduit run within 18 in. of enclosure
                    Class I, Division 1          Switch enclosure
                                                 Circuit breaker enclosure
                                                 Fuse enclosure
                                                 Relay enclosure
                                                 Resistor enclosure
                                                 Arcing or sparking apparatus
                                                 High-temperature apparatus

                                                 Explosionproof enclosure containing arcing or sparking                                    In conduit runs of 11⁄2 in. and smaller, no seal is required.
                                                   contacts that are hermetically sealed against gas or                                       If conduit is larger than 11⁄2 in., in conduit run within
                                                   vapor entry                                                                                18 in. of enclosure
                                                 Explosionproof enclosure containing arcing or sparking
                                                   contacts that are immersed in oil, in accordance with
                                                   501.115(B)(1)(2)

                                                 Enclosure containing terminals, splices, or taps fitting                                  In conduit runs smaller than trade size 2, no seal is
                                                   containing terminals, splices, or taps                                                     required. If conduit is 2 in. or larger, in conduit run
                                                                                                                                              within 18 in. of enclosure




                 684                                                      --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                                 2005 National Electrical Code Handbook

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                                                                               Article 501 — Class I Locations                                                                     501.15



                           Commentary Table 5.3 Continued

                               Classification                          Application                                                               Location of Seal

                                                 Two explosionproof enclosures with a conduit run                  In conduit run within 18 in. of each enclosure. Permitted
                                                   between them of 36 in. or less                                     to use a single seal in each run as long as the seal
                                                                                                                      is within 18 in. of each enclosure

                                                 Two explosionproof enclosures with a conduit run                  In conduit run within 18 in. of each enclosure
                                                   between them greater than 36 in.

                                                 Conduit run leaving Division 1 location                           On either side of boundary. No unions, couplings, boxes,
                                                                                                                     or fittings (other than explosionproof reducers)
                                                                                                                     permitted between the seal fitting and the point where
                                                                                                                     the conduit leaves the Division 1 location.

                                                 Metal conduit containing no unions, couplings, boxes,             Not required to be sealed if the termination points of the
                                                  or fittings that passes completely through a Class I,              unbroken conduit are in unclassified locations.
                                                  Division 1 location, with no fittings less than 12 in.
                                                  beyond each boundary
                           Class I, Division 2   Enclosure required to be explosionproof                           Seal as required for similar equipment in Division 1
                                                                                                                     location

                                                 Conduit run leaving Division 2 location                           On either side of boundary. No unions, couplings, boxes,
                                                                                                                     or fittings (other than explosionproof reducers)
                                                                                                                     permitted between the seal fitting and the point where
                                                                                                                     the conduit leaves the Division 2 location. Not required
                                                                                                                     to be explosionproof seal.

                                                 Metal conduit containing no unions, couplings, boxes,             Not required to be sealed if the termination points of the
                                                  or fittings that passes completely through a Division              unbroken conduit are in unclassified locations.
                                                  2 location with no fittings less than 12 in. beyond each
                                                  boundary




                                                                                                                                                                                              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                 Conduit systems terminating at an unclassified location           Not required to be sealed if passing from the Class I,
                                                   where a wiring method transition is made to cable tray,           Division 2 location into an outdoor unclassified location
                                                   cablebus, ventilated busway, Type MI cable, or open               or an indoor location if the conduit system is all in one
                                                   wiring                                                            room. The conduits do not terminate at an enclosure
                                                                                                                     containing an ignition source in normal operation.
                           Cable Seals           Enclosure with integral seal Multiconductor Type MC-HL            Conduit seal fitting not required Seal at all terminations
                           Class I, Division 1     cables with a gastight/vaportight continuous                      with an approved fitting after removing the jacket and
                                                   corrugated metallic sheath and an overall jacket of               any other covering, so that the sealing compound
                                                   suitable polymeric material                                       surrounds each individual insulated conductor.

                                                 Cables in conduit with a gas/vaportight continuous                Seal in the Division 1 location after removing the jacket
                                                   sheath capable of transmitting gases or vapors                    and any other coverings, so that the sealing compound
                                                   through the cable core                                            surrounds each individual insulated conductor and the
                                                                                                                     outer jacket.

                                                 Multiconductor cables with a gastight/vaportight                  Permitted to be considered a single conductor by sealing
                                                  continuous sheath capable of transmitting gases or                 the cable in the conduit within 18 in. of the enclosure
                                                  vapors through the cable core                                      and the cable end within the enclosure by an approved
                                                                                                                     means, to minimize the entrance of gases or vapors
                                                                                                                     and prevent the propagation of flame into the cable
                                                                                                                     core, or by other approved methods.

                                                                                                                                                                              (continues)




                           National Electrical Code Handbook 2005                                                                                                                       685

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                 501.20                                                                                                   Article 501 — Class I Locations




                    Commentary Table 5.3 Continued

                        Classification                                                                            Application                                                              Location of Seal

                                                                                           For shielded cables and twisted pair cables                       Removal of the shielding material or separation of the
                                                                                                                                                               twisted pair is not required. Sealing the cable in the
                                                                                                                                                               conduit within 18 in. of the enclosure and the cable
                                                                                                                                                               end within the enclosure by an approved means, to
                                                                                                                                                               minimize the entrance of gases or vapors and prevent
                                                                                                                                                               the propagation of flame into the cable core, or by
                                                                                                                                                               other approved methods.

                                                                                           Each multiconductor cable in conduit if the cable is              Considered a single conductor. These cables are sealed
                                                                                             incapable of transmitting gases or vapors through the             in accordance with 501.15(A)
                                                                                             cable core
                    Class I, Division 2                                                    Cables entering enclosures that are required to be                Sealed at the point of entrance
                                                                                             approved for Class I locations

                                                                                           Multiconductor cables in conduit                                  Sealed in accordance with the requirements for Division
                                                                                                                                                               1 locations

                                                                                           Multiconductor cables with a gastight/vaportight                  Sealed in an approved fitting in the Division 2 location
                                                                                            continuous sheath capable of transmitting gases or                 after removing the jacket and any other coverings, so
                                                                                            vapors through the cable core                                      that the sealing compound surrounds each individual
                                                                                                                                                               insulated conductor.

                                                                                           Cables with a gas/vaportight continuous sheath that will          Not required to be sealed unless entering an enclosure
                                                                                             not transmit gases or vapors through the cable core               that is required to be approved for Class I locations.
                                                                                             in excess of the quantity permitted for seal fittings.
                                                                                             The minimum length of such cable run is not less than
                                                                                             that length that limits gas or vapor flow through the
                                                                                             cable core to the rate permitted for seal fittings (0.007
                                                                                             cu ft per hour of air at a pressure of 6 in. of water).

                                                                                           Cables with a gastight/vaportight continuous sheath               Not required to be sealed unless entering an enclosure
                                                                                             capable of transmitting gases or vapors through the               that is required to be approved for Class I locations
                                                                                             cable core                                                        or unless the cable is attached to process equipment
                                                                                                                                                               or devices that may cause a pressure in excess of 6
                                                                                                                                                               in. of water to be exerted at a cable end, in which
                                                                                                                                                               case a seal, barrier, or other means is provided to
                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                               prevent migration of flammables into an unclassified
                                                                                                                                                               area.

                                                                                           Cables with an unbroken gastight/vaportight continuous            No seal required
                                                                                             sheath that pass through a Class I, Division 2 location

                                                                                           Cables that do not have a gastight/vaportight continuous          Sealed at the boundary of the Division 2 and unclassified
                                                                                             sheath                                                            location in such a manner as to minimize the passage
                                                                                                                                                               of gases or vapors into an unclassified location.




                                                                           Requirements for Process Sealing Between Electrical                  shall be of a type identified for use under such conditions;
                                                                           Systems and Potentially Flammable or Combustible Pro-                or the insulation shall be protected by a sheath of lead or
                                                                           cess Fluids.
                                                                                                                                                by other approved means.
                    501.20 Conductor Insulation, Class I,
                    Divisions 1 and 2                                                                                                           Nylon-jacketed conductors, such as Type THWN, that are
                    Where condensed vapors or liquids may collect on, or come                                                                   suitable for use where exposed to gasoline have gained wide-
                    in contact with, the insulation on conductors, such insulation


                 686                                                                                                                                                               2005 National Electrical Code Handbook

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                                                                                                                             Article 501 — Class I Locations                                                                501.30
                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                           spread acceptance because of their ease in handling and                   circuit overcurrent protection is located on the load side of
                                                                           application as well as for economic reasons.                              the disconnecting means.
                                                                                The UL Electrical Construction Materials Directory
                                                                           states in part:                                                           Bonding requirements intended to address the unique haz-
                                                                              Wires, Thermoplastic-Insulated:                                        ards inherent to electrical equipment installed in a Class I,
                                                                                   THWN — wire that is suitable for exposure to                      Division 1 or Division 2 location are covered by 501.30(A)
                                                                              mineral oil, and to liquid gasoline and gasoline vapors                and also by 250.100. These specific bonding methods, in-
                                                                              at ordinary ambient temperature, is marked ‘‘Gasoline                  tended to provide a mechanical/electrical connection that is
                                                                              and Oil Resistant I’’ if suitable for exposure to mineral              low impedance and free from accidental arcing due to loose
                                                                              oil at 60 C or ‘‘Gasoline and Oil Resistant II’’ if the                connections, apply to raceways and raceway-to-enclosure
                                                                              compound is suitable for exposure to mineral oil at                    connections both inside and outside the classified location.
                                                                              75 C. Gasoline-resistant wire has been tested at 23 C                  Section 250.100 specifies this enhanced level of bonding
                                                                              when immersed in gasoline. It is considered inherently                 for all raceways and enclosures, and the requirement is not
                                                                              resistant to gasoline vapors within the limits of the                  contingent on the circuit voltage. Therefore metal raceways
                                                                              temperature rating of the wire type.                                   and enclosures containing signaling, communications, or
                                                                                                                                                     other power-limited circuits are covered.
                                                                                                                                                           A revision to 250.100 for the 2005 Code clarifies that the
                                                                           501.25 Uninsulated Exposed Parts, Class I,                                installation of a wire-type equipment grounding conductor in
                                                                           Divisions 1 and 2                                                         a metal raceway does not negate the special raceway and
                                                                           There shall be no uninsulated exposed parts, such as electric             enclosure bonding requirements. Unless it is an isolated
                                                                           conductors, buses, terminals, or components, that operate at              equipment grounding conductor as permitted by 250.96(B)
                                                                           more than 30 volts (15 volts in wet locations). These parts               or 250.146(D), the wire-type equipment grounding conduc-
                                                                           shall additionally be protected by a protection technique                 tor and the metal raceway will be electrically in parallel and
                                                                           according to 500.7(E), 500.7(F), or 500.7(G) that is suitable             will share ground-fault current based on the impedance of
                                                                           for the location.                                                         the wire and of the metal raceway. For that reason, the
                                                                                                                                                     electrical continuity of raceways and raceway-to-enclosure
                                                                           Exposed live parts are allowed in Class I, Division 1 and 2               connections must always be ensured through compliance
                                                                           locations provided that the voltage does not exceed 30 volts              with 250.100 and 501.30(A), regardless of whether a supple-
                                                                           in dry locations or 15 volts in wet locations. However, cau-              mentary equipment grounding conductor has been installed
                                                                           tion must be observed in working around uninsulated ex-                   in the raceway.
                                                                           posed parts because tool or other conductive items that come                    The exception to 501.30(A) covers the grounding and
                                                                           in contact with the circuit could produce sparks and cause                bonding requirements that are specific to hazardous (classi-
                                                                           an explosion in a hazardous (classified) location.                        fied) locations where the installation occurs at a multibuild-
                                                                                                                                                     ing or multistructure setting. If the service equipment and the
                                                                                                                                                     electrical equipment operating in the hazardous (classified)
                                                                           501.30 Grounding and Bonding, Class I,                                    location are not located in the same building or structure, it is
                                                                           Divisions 1 and 2                                                         not necessary to apply the bonding requirement of 501.30(A)
                                                                           Wiring and equipment in Class I, Division 1 and 2 locations               from the hazardous location back to the service equipment.
                                                                           shall be grounded as specified in Article 250 and with the                It is necessary only to apply the bonding requirement from
                                                                           requirements in 501.30(A) and 501.30(B).                                  the hazardous location back to the grounding electrode on
                                                                                                                                                     the line side of the building or structure disconnecting means.
                                                                           (A) Bonding The locknut-bushing and double-locknut
                                                                                                                                                     This connection must be ahead of the branch circuits that
                                                                           types of contacts shall not be depended on for bonding
                                                                                                                                                     are on the load side of the disconnecting means for the
                                                                           purposes, but bonding jumpers with proper fittings or other
                                                                                                                                                     building or structure.
                                                                           approved means of bonding shall be used. Such means of
                                                                           bonding shall apply to all intervening raceways, fittings,
                                                                           boxes, enclosures, and so forth between Class I locations                        FPN: See 250.100 for additional bonding requirements
                                                                           and the point of grounding for service equipment or point                        in hazardous (classified) locations.
                                                                           of grounding of a separately derived system.                              (B) Types of Equipment Grounding Conductors Where
                                                                           Exception: The specific bonding means shall be required                   flexible metal conduit or liquidtight flexible metal conduit
                                                                           only to the nearest point where the grounded circuit conduc-              is used as permitted in 501.10(B) and is to be relied on to
                                                                           tor and the grounding electrode are connected together on                 complete a sole equipment grounding path, it shall be in-
                                                                           the line side of the building or structure disconnecting means            stalled with internal or external bonding jumpers in parallel
                                                                           as specified in 250.32(A), (B), and (C), provided the branch-             with each conduit and complying with 250.102.

                                                                           National Electrical Code Handbook 2005                                                                                                              687

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                 501.35                                                                      Article 501 — Class I Locations




                         Special consideration is necessary in the grounding and                                and surge-protective capacitors shall be of a type designed
                         bonding of exposed non–current-carrying metal parts of                                 for specific duty. Enclosures shall be permitted to be of the
                         equipment, such as the frames or metal exteriors of motors,                            general-purpose type. Surge protection of types other than
                         fixed or portable lamps, luminaires, enclosures, and race-                             described in this paragraph shall be installed in enclosures
                         ways, to ensure permanent and effective mechanical and                                 identified for Class I, Division 1 locations.
                         electrical connections in order to prevent the possibility of
                         arcs or sparks caused by ineffective or poor grounding meth-                           Article 285, Transient Voltage Surge Suppressors: TVSSs,
                         ods. One example of an external bonding jumper is shown                                was added to the 2002 Code. Nonarcing, sealed type TVSSs
                         in Exhibit 501.13.                                                                     are included as a device permitted in Class I, Division 2
                                                                                                                locations where installed in a general purpose–type enclo-
                                                                                                                sure. Some surge arresters, such as older style lightning
                                                                                                                arresters, are spark-producing devices. Others, such as solid-
                                                                                   Bonding jumper               state types, are not. Nonsparking-type surge arresters need
                                                                                                                no special enclosure in a Class I, Division 2 location. Surge
                                                                       6 ft max.                                arresters should be connected to the service conductors out-
                                                                                                                side the building and should be bonded to the service-en-
                                                                                                                trance raceway system. For services less than 1000 volts,
                                                                                                                the arrester grounding conductor is connected as provided
                                                                                                                in Article 280, Part II. Where the service voltage is less
                                                                      Cable ties or equal                       than 600 volts, the supply system is a secondary system.
                                                                                                                Therefore, the grounded service conductor should always
                                                                                                                be bonded to the equipment grounding conductor, as required
                                                                                                                by Article 250.
                         Exhibit 501.13 A fitting for the connection of an external bonding                          In Class I, Division 1 locations, all surge arresters must
                         jumper used with liquidtight flexible metal conduit.                                   be installed in explosionproof or purged enclosures. In Class
                                                                                                                I, Division 2 locations, only the spark-producing types of
                              To be effective, proper grounding and bonding apply                               surge arresters require such protection. Surge arresters can
                         to all interconnected raceways, fittings, enclosures, and so                           also be installed in oil-filled enclosures or have the arcing or
                         on, between hazardous (classified) locations and the point                             sparking contacts enclosed in hermetically sealed chambers.
                         of grounding for service equipment or the point of grounding
                         of the building disconnecting means that supplies the branch-
                                                                                                                501.40 Multiwire Branch Circuits
                         circuit overcurrent protection. If conduit is used in hazardous
                         (classified) locations, it is preferable that threaded connec-                         In a Class I, Division 1 location, a multiwire branch circuit
                         tions also be employed in the nonhazardous location.                                   shall not be permitted.
                                                                                                                Exception: Where the disconnect device(s) for the circuit
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                opens all ungrounded conductors of the multiwire circuit
                         Exception: In Class I, Division 2 locations, the bonding                               simultaneously.
                         jumper shall be permitted to be deleted where all of the
                         following conditions are met:                                                          The requirement in 501.40 does not permit multiwire branch
                               (1) Listed liquidtight flexible metal conduit 1.8 m (6 ft) or                    circuits in Class I hazardous locations unless the discon-
                                   less in length, with fittings listed for grounding, is used.                 necting device opens all the ungrounded conductors of the
                               (2) Overcurrent protection in the circuit is limited to 10                       multiwire branch circuit simultaneously. See the definition
                                   amperes or less.                                                             of branch circuit, multiwire in Article 100. The requirement
                               (3) The load is not a power utilization load.                                    in 501.40 addresses the inherent hazard of working on a
                                                                                                                multiwire branch circuit in which all the ungrounded conduc-
                         501.35 Surge Protection                                                                tors are not opened by the disconnecting means. Although
                         (A) Class I, Division 1 Surge arresters, transient voltage                             the ungrounded conductor that is supplying equipment being
                         surge suppressors (TVSS), and capacitors shall be installed                            serviced may be disconnected, the common grounded con-
                         in enclosures identified for Class I, Division 1 locations.                            ductor in this circuit configuration is still a current-carrying
                         Surge-protective capacitors shall be of a type designed for                            conductor in a circuit with an ungrounded conductor(s) that
                         specific duty.                                                                         is not disconnected. In a Division 1 or 2 location, the opening
                                                                                                                of the grounded conductor in this scenario could result in
                         (B) Class I, Division 2 Surge arresters and TVSS shall be                              an ignition-capable arc.
                         nonarcing, such as metal-oxide varistor (MOV) sealed type,


                 688                                                                                                                               2005 National Electrical Code Handbook

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                                                                                Article 501 — Class I Locations                                                                  501.105



                              III. Equipment                                                              shall have enclosures identified for Class I, Division 1 loca-
                                                                                                          tions in accordance with 501.105(A).
                              501.100 Transformers and Capacitors
                              (A) Class I, Division 1 In Class I, Division 1 locations,                   Exception: General-purpose enclosures shall be permitted
                              transformers and capacitors shall comply with                               if current-interrupting contacts comply with one of the fol-
                              501.100(A)(1) and (A)(2).                                                   lowing:

                              (1) Containing Liquid That Will Burn Transformers and                       (1) Are immersed in oil
                              capacitors containing a liquid that will burn shall be installed            (2) Are enclosed within a chamber that is hermetically
                              only in vaults that comply with 450.41 through 450.48 and                       sealed against the entrance of gases or vapors
                              with (1) through (4) as follows:
                                                                                                          Generally speaking, there are several types of hermetic seals,
                              (1) There shall be no door or other communicating opening                   including fusion seals such as the glass-to-metal seals in
                                  between the vault and the Division 1 location.                          mercury-tube switches and some reed switches, welded
                              (2) Ample ventilation shall be provided for the continuous                  seals, soldered seals, and seals made with gaskets. Seals of
                                  removal of flammable gases or vapors.                                   the glass-to-metal-fusion type are usually the most reliable.
                              (3) Vent openings or ducts shall lead to a safe location                    Soft soldered seals can be relatively porous, and their effec-
                                  outside of buildings.                                                   tiveness is highly dependent on workmanship. Although
                              (4) Vent ducts and openings shall be of sufficient area to                  gasketed seals can be very effective, depending on the gasket
                                  relieve explosion pressures within the vault, and all                   material used, gasket materials can be damaged and deterio-
                                  portions of vent ducts within the buildings shall be of                 rate rapidly if exposed to atmospheres that contain solvent
                                  reinforced concrete construction.                                       vapors. Gasketed enclosures may be considered hermetically
                              (2) Not Containing Liquid That Will Burn Transformers                       sealed under some conditions; however, in accordance with
                              and capacitors that do not contain a liquid that will burn                  the 500.2 definition of hermetically sealed, such enclosures
                              shall be installed in vaults complying with 501.100(A)(1)                   cannot be used to satisfy those requirements in which her-
                              or be approved for Class I locations.                                       metic sealing is recognized as a protection technique.

                              (B) Class I, Division 2 In Class I, Division 2 locations,
                              transformers and capacitors shall comply with 450.21                        (3) Are in nonincendive circuits
                              through 450.27.                                                             (4) Are listed for Division 2

                              501.105 Meters, Instruments, and Relays                                     See the definitions of the terms nonincendive circuit, nonin-
                              (A) Class I, Division 1 In Class I, Division 1 locations,                   cendive component, and nonincendive equipment in 500.2.
                              meters, instruments, and relays, including kilowatt-hour me-                Nonincendive is similar to intrinsically safe, which is defined
                              ters, instrument transformers, resistors, rectifiers, and therm-            in 504.2 and ANSI/UL 913, Intrinsically Safe Apparatus
                              ionic tubes, shall be provided with enclosures identified for               and Associated Apparatus for Use in Class I, II, and III,
                              Class I, Division 1 locations. Enclosures for Class I, Division             Division 1, Hazardous (Classified) Locations, but nonincen-
                              1 locations include explosionproof enclosures and purged                    dive does not include consideration of faults and all the
                              and pressurized enclosures.                                                 abnormal conditions inherent in the definition of intrinsically
                                                                                                          safe. A circuit may be nonincendive in a Group D atmosphere
                                        FPN: See NFPA 496-2003, Standard for Purged and                   but not in a Group C atmosphere, due to the differences
                                        Pressurized Enclosures for Electrical Equipment.                  in minimum ignition energies for the various flammable
                                                                                                          materials.
                              See the commentary on purged and pressurized enclosures
                              for electrical equipment in hazardous (classified) locations
                              following 500.7(D) and the commentary on explosionproof                     (2) Resistors and Similar Equipment Resistors, resis-
                              enclosures following the exception to 501.5.                                tance devices, thermionic tubes, rectifiers, and similar equip-
                                                                                                          ment that are used in or in connection with meters,
                                                                                                          instruments, and relays shall comply with 501.105(A).
                              (B) Class I, Division 2 In Class I, Division 2 locations,
                              meters, instruments, and relays shall comply with                           Exception: General-purpose-type enclosures shall be per-
                              501.105(B)(1) through (B)(6).                                               mitted if such equipment is without make-and-break or slid-
                                                                                                          ing contacts [other than as provided in 501.105(B)(1)] and
                              (1) Contacts Switches, circuit breakers, and make-and-                      if the maximum operating temperature of any exposed sur-
                              break contacts of pushbuttons, relays, alarm bells, and horns               face will not exceed 80 percent of the ignition temperature

                              National Electrical Code Handbook 2005                                                                                                                 689
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                 501.115                                                         Article 501 — Class I Locations



                    in degrees Celsius of the gas or vapor involved or has been                     provided with enclosures, and the enclosure in each case,
                    tested and found incapable of igniting the gas or vapor. This                   together with the enclosed apparatus, shall be identified as
                    exception shall not apply to thermionic tubes.                                  a complete assembly for use in Class I locations.

                    (3) Without Make-or-Break Contacts Transformer wind-
                                                                                                    Exhibit 501.14 shows an explosionproof panelboard that
                    ings, impedance coils, solenoids, and other windings that
                                                                                                    consists of an assembly of branch-circuit devices enclosed
                    do not incorporate sliding or make-or-break contacts shall be
                                                                                                    in a cast metal explosionproof housing. Explosionproof pan-
                    provided with enclosures. General-purpose-type enclosures
                                                                                                    elboards are provided with bolted access covers and threaded
                    shall be permitted.
                                                                                                    conduit-entry hubs designed to withstand the force of an
                    (4) General-Purpose Assemblies Where an assembly is                             internal explosion.
                    made up of components for which general-purpose enclo-                               Exhibit 501.15 shows a cylindrical-type (spin-top) com-
                    sures are acceptable as provided in 501.105(B)(1), (B)(2),                      bination motor controller, motor control starter, and circuit
                    and (B)(3), a single general-purpose enclosure shall be ac-                     breaker in an explosionproof enclosure. The top and bottom
                    ceptable for the assembly. Where such an assembly includes                      covers are threaded on for quick removal for installation
                    any of the equipment described in 501.105(B)(2), the maxi-                      and servicing. Exhibit 501.16 shows the same type of equip-
                    mum obtainable surface temperature of any component of                          ment in a rectangular enclosure with a hinged, bolted-on
                    the assembly shall be clearly and permanently indicated on                      cover. These types of housings are designed to accommodate
                    the outside of the enclosure. Alternatively, equipment shall                    a wide range of manually or magnetically operated across-
                    be permitted to be marked to indicate the temperature class                     the-line types of motor starters in a variety of ratings.
                    for which it is suitable, using the temperature class (T Code)                       Exhibit 501.17 illustrates a standard toggle switch in
                    of Table 500.8(B).                                                              an explosionproof enclosure.

                    (5) Fuses Where general-purpose enclosures are permitted
                    in 501.105(B)(1) through (B)(4), fuses for overcurrent pro-
                    tection of instrument circuits not subject to overloading in
                    normal use shall be permitted to be mounted in general-
                    purpose enclosures if each such fuse is preceded by a switch
                    complying with 501.105(B)(1).

                    (6) Connections To facilitate replacements, process con-
                    trol instruments shall be permitted to be connected through
                    flexible cord, attachment plug, and receptacle, provided all
                    of the following conditions apply:
                    (1) A switch complying with 501.105(B)(1) is provided so
                        that the attachment plug is not depended on to interrupt
                        current.
                    (2) The current does not exceed 3 amperes at 120 volts,
                        nominal.
                    (3) The power-supply cord does not exceed 900 mm (3 ft),
                        is of a type listed for extra-hard usage or for hard usage
                        if protected by location, and is supplied through an
                        attachment plug and receptacle of the locking and
                        grounding type.
                    (4) Only necessary receptacles are provided.
                    (5) The receptacle carries a label warning against unplug-
                        ging under load.

                    501.115 Switches, Circuit Breakers, Motor
                    Controllers, and Fuses
                    (A) Class I, Division 1 In Class I, Division 1 locations,                       Exhibit 501.14 An explosionproof panelboard. (Courtesy of Ap-
                    switches, circuit breakers, motor controllers, and fuses, in-                   pleton Electric Co., EGS Electrical Group)
                    cluding pushbuttons, relays, and similar devices, shall be



                 690           --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                                                         2005 National Electrical Code Handbook

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                                                                                 Article 501 — Class I Locations                                                                  501.115




                                                                                                           Exhibit 501.16 A magnetic motor starter for use in a Class I,
                                                                                                           Group D location. Note the number of securing bolts and the
                                                                                                           width of the flange. (Courtesy of O-Z/Gedney, a division of EGS
                                                                                                           Electrical Group)



                               Exhibit 501.15 An explosionproof enclosure for a motor control
                               starter and circuit breaker. (Courtesy of Appleton Electric Co.,
                               EGS Electrical Group)

                                   Fuses used for supplementary ballast protection are per-
                               mitted to be installed in high-intensity-discharge and fluores-
                               cent fixtures in accordance with 501.115(B)(4).

                               (B) Class I, Division 2 Switches, circuit breakers, motor
                               controllers, and fuses in Class I, Division 2 locations shall
                               comply with 501.115(B)(1) through (B)(4).
                               (1) Type Required Circuit breakers, motor controllers, and
                               switches intended to interrupt current in the normal perfor-
                               mance of the function for which they are installed shall be
                               provided with enclosures identified for Class I, Division 1
                                                                                                           Exhibit 501.17 A standard toggle switch in an explosionproof
                               locations in accordance with 501.105(A), unless general-
                                                                                                           enclosure. (Courtesy of Appleton Electric Co., EGS Electrical
                               purpose enclosures are provided and any of the following                    Group)
                               apply:
                               (1) The interruption of current occurs within a chamber
                                   hermetically sealed against the entrance of gases and                   (3) The interruption of current occurs within a factory-
                                   vapors.                                                                     sealed explosionproof chamber.
                               (2) The current make-and-break contacts are oil-immersed                    (4) The device is a solid state, switching control without
                                   and of the general-purpose type having a 50-mm (2-                          contacts, where the surface temperature does not exceed
                                   in.) minimum immersion for power contacts and a 25-                         80 percent of the ignition temperature in degrees Celsius
                                   mm (1-in.) minimum immersion for control contacts.                          of the gas or vapor involved.



                               National Electrical Code Handbook 2005                                                                                                                 691
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                 501.120                                                    Article 501 — Class I Locations



                    (2) Isolating Switches Fused or unfused disconnect and                     (1) Identified for Class I, Division 1 locations
                    isolating switches for transformers or capacitor banks that                (2) Of the totally enclosed type supplied with positive-
                    are not intended to interrupt current in the normal perfor-                    pressure ventilation from a source of clean air with
                    mance of the function for which they are installed shall be                    discharge to a safe area, so arranged to prevent energiz-
                    permitted to be installed in general-purpose enclosures.                       ing of the machine until ventilation has been established
                                                                                                   and the enclosure has been purged with at least 10
                    (3) Fuses For the protection of motors, appliances, and
                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                   volumes of air, and also arranged to automatically de-
                    lamps, other than as provided in 501.115(B)(4), standard
                                                                                                   energize the equipment when the air supply fails
                    plug or cartridge fuses shall be permitted, provided they are
                                                                                               (3) Of the totally enclosed inert gas-filled type supplied with
                    placed within enclosures identified for the location; or fuses
                                                                                                   a suitable reliable source of inert gas for pressurizing the
                    shall be permitted if they are within general-purpose enclo-
                                                                                                   enclosure, with devices provided to ensure a positive
                    sures, and if they are of a type in which the operating element
                                                                                                   pressure in the enclosure and arranged to automatically
                    is immersed in oil or other approved liquid, or the operating
                                                                                                   de-energize the equipment when the gas supply fails
                    element is enclosed within a chamber hermetically sealed
                                                                                               (4) Of a type designed to be submerged in a liquid that is
                    against the entrance of gases and vapors, or the fuse is a
                                                                                                   flammable only when vaporized and mixed with air, or
                    nonindicating, filled, current-limiting type.
                                                                                                   in a gas or vapor at a pressure greater than atmospheric
                    (4) Fuses Internal to Luminaires (Lighting Fixtures)                           and that is flammable only when mixed with air; and
                    Listed cartridge fuses shall be permitted as supplementary                     the machine is arranged so to prevent energizing it until
                    protection within luminaires (lighting fixtures).                              it has been purged with the liquid or gas to exclude
                                                                                                   air, and also arranged to automatically de-energize the
                    501.120 Control Transformers and Resistors                                     equipment when the supply of liquid or gas or vapor
                    Transformers, impedance coils, and resistors used as, or in                    fails or the pressure is reduced to atmospheric
                    conjunction with, control equipment for motors, generators,
                    and appliances shall comply with 501.120(A) and                            The intent of 501.125(A)(4) is to permit nonexplosionproof
                    501.120(B).                                                                motors to be submerged in liquefied natural gas (LNG),
                                                                                               liquefied petroleum gas (LP-Gas), gasoline, and other flam-
                    (A) Class I, Division 1 In Class I, Division 1 locations,
                                                                                               mable liquids. The provisions of 501.125(A)(4) do not per-
                    transformers, impedance coils, and resistors, together with
                                                                                               mit nonexplosionproof motors under water, such as in wet
                    any switching mechanism associated with them, shall be
                                                                                               pits, unless the motors are provided with some other system
                    provided with enclosures identified for Class I, Division 1
                                                                                               of explosion protection, for example, if they are purged
                    locations in accordance with 501.105(A).
                                                                                               and pressurized per NFPA 496, Standard for Purged and
                    (B) Class I, Division 2 In Class I, Division 2 locations,                  Pressurized Enclosures for Electrical Equipment.
                    control transformers and resistors shall comply with                            The ASTM test procedure is used to determine the
                    501.120(B)(1) through (B)(3).                                              ignition temperature of some flammable and combustible
                                                                                               liquids.
                    (1) Switching Mechanisms Switching mechanisms used
                    in conjunction with transformers, impedance coils, and resis-
                    tors shall comply with 501.115(B).                                             Totally enclosed motors of the types specified in
                                                                                               501.125(A)(2) or 501.125(A)(3) shall have no external sur-
                    (2) Coils and Windings Enclosures for windings of trans-
                                                                                               face with an operating temperature in degrees Celsius in
                    formers, solenoids, or impedance coils shall be permitted to
                                                                                               excess of 80 percent of the ignition temperature of the gas
                    be of the general-purpose type.
                                                                                               or vapor involved. Appropriate devices shall be provided to
                    (3) Resistors Resistors shall be provided with enclosures;                 detect and automatically de-energize the motor or provide
                    and the assembly shall be identified for Class I locations,                an adequate alarm if there is any increase in temperature of
                    unless resistance is nonvariable and maximum operating                     the motor beyond designed limits. Auxiliary equipment shall
                    temperature, in degrees Celsius, will not exceed 80 percent                be of a type identified for the location in which it is installed.
                    of the ignition temperature of the gas or vapor involved or
                                                                                                      FPN: See D 2155-69, ASTM Test Procedure.
                    has been tested and found incapable of igniting the gas or
                    vapor.                                                                     (B) Class I, Division 2 In Class I, Division 2 locations,
                                                                                               motors, generators, and other rotating electric machinery in
                    501.125 Motors and Generators                                              which are employed sliding contacts, centrifugal or other
                    (A) Class I, Division 1 In Class I, Division 1 locations,                  types of switching mechanism (including motor overcurrent,
                    motors, generators, and other rotating electric machinery                  overloading, and overtemperature devices), or integral resis-
                    shall be one of the following:                                             tance devices, either while starting or while running, shall


                 692                                                                                                              2005 National Electrical Code Handbook

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                                                                                     Article 501 — Class I Locations                                                              501.125



                           be identified for Class I, Division 1 locations, unless such                     the end opposite the shaft end of the motor. A sheet metal
                           sliding contacts, switching mechanisms, and resistance de-                       housing surrounds the fan to reduce the likelihood of an
                           vices are provided with enclosures identified for Class I,                       individual or object coming into contact with the moving
                           Division 2 locations in accordance with 501.105(B). The                          blades and to direct the flow of air. An internal fan on the
                           exposed surface of space heaters used to prevent condensa-                       shaft, as shown in Exhibit 501.19, circulates air around the
                           tion of moisture during shutdown periods shall not exceed                        windings.
                           80 percent of the ignition temperature in degrees Celsius of                          Motors that have arc- or spark-producing devices, such
                           the gas or vapor involved when operated at rated voltage,                        as commutators, internal switches, or other control devices,
                           and the maximum surface temperature [based on a 40 C
                           (104 F) ambient] shall be permanently marked on a visible
                           nameplate mounted on the motor. Otherwise, space heaters
                           shall be identified for Class I, Division 2 locations. In Class
                           I, Division 2 locations, the installation of open or nonexplo-
                           sionproof enclosed motors, such as squirrel-cage induction
                           motors without brushes, switching mechanisms, or similar
                           arc-producing devices that are not identified for use in a
                           Class I, Division 2 location, shall be permitted.

                           It is intended that the phrase ‘‘other rotating electric machin-
                           ery’’ include electric brakes. Listed and labeled electric
                           brakes are available for Class I, Division 1, Group C and
                           D locations.
                                 Many motor heaters are de-energized automatically
                           when the motor is running. However, the heater ratings are
                           usually low when compared with the normal heat generated
                           during motor operation. Unless otherwise indicated on the
                           motor wiring diagram or in instructions provided with the
                           motor, there is no need to de-energize the heater except to
                           save energy. Note the requirement that the heater temperature                    Exhibit 501.18 Terminal housing of a motor listed for use in
                                                                                                            specific hazardous locations. Note integral sealing of the motor.
                           be marked on the motor or that the heaters be identified for
                                                                                                            (Courtesy of General Electric Co.)
                           the location.

                                   FPN No. 1: It is important to consider the temperature
                                   of internal and external surfaces that may be exposed to
                                   the flammable atmosphere.
                                   FPN No. 2: It is important to consider the risk of ignition
                                   due to currents arcing across discontinuities and overheat-
                                   ing of parts in multisection enclosures of large motors
                                   and generators. Such motors and generators may need
                                   equipotential bonding jumpers across joints in the enclo-
                                   sure and from enclosure to ground. Where the presence
                                   of ignitible gases or vapors is suspected, clean-air purging
                                   may be needed immediately prior to and during start-up
                                   periods.


                           Exhibit 501.18 shows a totally enclosed fan-cooled motor
                           listed for use in explosive atmospheres. The main frame and
                           end-bells are designed with sufficient strength to withstand
                           an internal explosion. Flames or hot gases are cooled while
                           escaping because of the wide metal-to-metal joints between
                           the frame and the end-bells and the long, close-tolerance
                           clearance provided for the free turn of the shaft. Air circula-
                           tion outside the motor is maintained by a nonsparking (alu-                      Exhibit 501.19 View showing internal fan of motor in Exhibit
                           minum, bronze, or non-static-generating-type plastic) fan on                     501.18. (Courtesy of General Electric Co.)



                           National Electrical Code Handbook 2005                                   --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                                   693

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                 501.130                                                                                                Article 501 — Class I Locations




                          must be explosionproof. General-purpose squirrel-cage in-                                                        (B) Class I, Division 2 In Class I, Division 2 locations,
                          duction motors without arc- or spark-producing devices may                                                       luminaires (lighting fixtures) shall comply with
                          be used in Division 2 locations.                                                                                 501.130(B)(1) through 501.130(B)(6).
                               Some open-type motors are permitted in Class I, Divi-
                          sion 2 locations. Motor types used where flammable gases                                                         (1) Luminaires (Lighting Fixtures) Where lamps are of
                          or vapors with very low ignition temperatures may be present                                                     a size or type that may, under normal operating conditions,
                          should be selected with great care. Modern motors with                                                           reach surface temperatures exceeding 80 percent of the igni-
                          high-temperature insulation systems, such as Class H [180 C                                                      tion temperature in degrees Celsius of the gas or vapor
                          (356 F)], may operate close to or above the ignition tempera-                                                    involved, fixtures shall comply with 501.130(A)(1) or shall
                          ture of the flammable mixture.                                                                                   be of a type that has been tested in order to determine the
                                                                                                                                           marked operating temperature or temperature class (T Code).
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                           (2) Physical Damage Luminaires (lighting fixtures) shall
                                                                      FPN No. 3: For further information on the application
                                                                      of electric motors in Class I, Division 2 hazardous (classi-         be protected from physical damage by suitable guards or by
                                                                      fied) locations, see IEEE Std. 1349-2001, IEEE Guide                 location. Where there is danger that falling sparks or hot
                                                                      for the Application of Electric Motors in Class I, Division          metal from lamps or fixtures might ignite localized concen-
                                                                      2 Hazardous (Classified) Locations.                                  trations of flammable vapors or gases, suitable enclosures
                                                                                                                                           or other effective protective means shall be provided.
                          501.130 Luminaires (Lighting Fixtures)
                          Luminaires (lighting fixtures) shall comply with 501.130(A)                                                      (3) Pendant Luminaires (Fixtures) Pendant luminaires
                          or (B).                                                                                                          (lighting fixtures) shall be suspended by threaded rigid metal
                                                                                                                                           conduit stems, threaded steel intermediate metal conduit
                          (A) Class I, Division 1 In Class I, Division 1 locations,                                                        stems, or other approved means. For rigid stems longer than
                          luminaires (lighting fixtures) shall comply with                                                                 300 mm (12 in.), permanent and effective bracing against
                          501.130(A)(1) through (A)(4).                                                                                    lateral displacement shall be provided at a level not more
                                                                                                                                           than 300 mm (12 in.) above the lower end of the stem, or
                          (1) Luminaires (Lighting Fixtures) Each luminaire (light-                                                        flexibility in the form of an identified fitting or flexible
                          ing fixture) shall be identified as a complete assembly for                                                      connector shall be provided not more than 300 mm (12 in.)
                          the Class I, Division 1 location and shall be clearly marked                                                     from the point of attachment to the supporting box or fitting.
                          to indicate the maximum wattage of lamps for which it is
                          identified. Luminaires (lighting fixtures) intended for porta-                                                   (4) Portable Lighting Equipment Portable lighting equip-
                          ble use shall be specifically listed as a complete assembly                                                      ment shall comply with 501.130(A)(1).
                          for that use.
                                                                                                                                           Exception: Where portable lighting equipment is mounted
                          (2) Physical Damage Each luminaire (lighting fixture)                                                            on movable stands and is connected by flexible cords, as
                          shall be protected against physical damage by a suitable                                                         covered in 501.140, it shall be permitted, where mounted in
                          guard or by location.                                                                                            any position, if it conforms to 501.130(B)(2).

                          (3) Pendant Luminaires (Lighting Fixtures) Pendant lu-                                                           (5) Switches Switches that are a part of an assembled fix-
                          minaires (lighting fixtures) shall be suspended by and sup-                                                      ture or of an individual lampholder shall comply with
                          plied through threaded rigid metal conduit stems or threaded                                                     501.115(B)(1).
                          steel intermediate conduit stems, and threaded joints shall
                          be provided with set-screws or other effective means to                                                          (6) Starting Equipment Starting and control equipment
                          prevent loosening. For stems longer than 300 mm (12 in.),                                                        for electric-discharge lamps shall comply with 501.120(B).
                          permanent and effective bracing against lateral displacement
                          shall be provided at a level not more than 300 mm (12 in.)                                                       Exhibit 501.20 shows a typical luminaire for Class I, Group
                          above the lower end of the stem, or flexibility in the form                                                      C and D locations. The outlet boxes have an internally
                          of a fitting or flexible connector identified for the Class I,                                                   threaded opening designed to receive the cover. A pendant
                          Division 1 location shall be provided not more than 300                                                          fixture is attached to the cover by threaded rigid metal con-
                          mm (12 in.) from the point of attachment to the supporting                                                       duit or threaded intermediate metal conduit. To prevent loos-
                          box or fitting.                                                                                                  ening from vibration or lamp changing, threaded joints must
                                                                                                                                           be provided with set-screws. The set-screws should not inter-
                          (4) Supports Boxes, box assemblies, or fittings used for                                                         rupt the explosionproof joint. Rigid metal conduit or interme-
                          the support of luminaires (lighting fixtures) shall be identi-                                                   diate metal conduit stems longer than 12 in. require effective
                          fied for Class I locations.


                 694                                                                                                                                                          2005 National Electrical Code Handbook

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                                                                                Article 501 — Class I Locations                                                                  501.135




                           Exhibit 501.20 A typical lighting fixture for use in Class I, Group
                                                                                                          Exhibit 501.21 An explosionproof hand lamp for use in Class I
                           C and D locations. (Courtesy of Appleton Electric Co., EGS Elec-
                                                                                                          locations. (Courtesy of Appleton Electric Co., EGS Electrical
                           trical Group)
                                                                                                          Group)

                           bracing or a flexible fitting approved for the purpose, placed




                                                                                                                                                                                           --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           not more than 12 in. from the point of attachment to the
                           supporting box, cover, or fitting.
                                                                                                          501.135 Utilization Equipment
                                 A globe holder is threaded onto the body of the fixture
                           housing and supports a heavy glass globe, guard, and reflec-                   (A) Class I, Division 1 In Class I, Division 1 locations, all
                           tor. It is available in sizes suitable for lamps from 40 watts                 utilization equipment shall be identified for Class I, Division
                           through 500 watts. In designing any hazardous (classified)                     1 locations.
                           location lighting system, operating temperatures must be
                                                                                                          (B) Class I, Division 2 In Class I, Division 2 locations,
                           considered. Therefore, if the area is Class I, Division 1,
                                                                                                          all utilization equipment shall comply with 501.135(B)(1)
                           fixtures approved for this location that are properly marked
                                                                                                          through (B)(3).
                           must be used. Generally, enclosed and gasketed fixtures
                           (previously called vaportight fixtures) without guards, if                     (1) Heaters Electrically heated utilization equipment shall
                           breakage is unlikely, or fixtures approved for Class I, Divi-                  conform with either item (1) or (2):
                           sion 2 locations are required in Division 2 locations.
                                 Fixtures listed by Underwriters Laboratories Inc. for                    (1) The heater shall not exceed 80 percent of the ignition
                           use in any of the groups under Class I, either Division 1 or                       temperature in degrees Celsius of the gas or vapor in-
                           2 locations, or both, are designed to operate without igniting                     volved on any surface that is exposed to the gas or
                           surrounding flammable gas or vapor atmospheres and are                             vapor when continuously energized at the maximum
                           marked with the operating temperature or temperature class,                        rated ambient temperature. If a temperature controller
                           as shown in Table 500.8(B).                                                        is not provided, these conditions shall apply when the
                                 Exhibit 501.21 shows an explosionproof hand lamp.                            heater is operated at 120 percent of rated voltage.
                           Lamp compartments must be sealed from the terminal com-
                           partment. Provisions must be made for the connection of 3-                     Exception No. 1: For motor-mounted anticondensation
                           conductor (one must be a grounding conductor) flexible,                        space heaters, see 501.125.
                           extra-hard-usage cord. See 501.140(A)(1).                                      Exception No. 2: Where a current-limiting device is applied
                                                                                                          to the circuit serving the heater to limit the current in the
                           Exception: A thermal protector potted into a thermally pro-                    heater to a value less than that required to raise the heater
                           tected fluorescent lamp ballast if the luminaire (lighting                     surface temperature to 80 percent of the ignition tempera-
                           fixture) is identified for the location.                                       ture.


                           National Electrical Code Handbook 2005                                                                                                                    695

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                    501.140                                          Article 501 — Class I Locations



                    (2) The heater shall be identified for Class I, Division 1         (1) Be of a type listed for extra-hard usage
                        locations.                                                     (2) Contain, in addition to the conductors of the circuit, a
                                                                                           grounding conductor complying with 400.23
                    Exception: Electrical resistance heat tracing identified for
                                                                                       (3) Be connected to terminals or to supply conductors in
                    Class I, Division 2 locations.
                                                                                           an approved manner
                    (2) Motors Motors of motor-driven utilization equipment            (4) Be supported by clamps or by other suitable means in
                    shall comply with 501.125(B).                                          such a manner that there is no tension on the terminal
                                                                                           connections
                    (3) Switches, Circuit Breakers, and Fuses Switches, cir-           (5) Be provided with suitable seals where the flexible cord
                    cuit breakers, and fuses shall comply with 501.115(B).                 enters boxes, fittings, or enclosures of the explo-
                                                                                           sionproof type




                                                                                                                                                                     --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    The requirements for utilization equipment in Class I loca-        Exception to (5): Seals shall not be required as provided in
                    tions are virtually identical for Division 1 and 2 locations,      501.10(B) and 501.105(B)(6).
                    except for heaters. Electric pipe heat-tracing systems listed
                    for Class I, Division 2 locations and complying with               (6) Be of continuous length
                    501.135(B)(1)(2), Exception, are available.
                                                                                              FPN: See 501.20 for flexible cords exposed to liquids
                                                                                              having a deleterious effect on the conductor insulation.

                    501.140 Flexible Cords, Class I, Divisions 1 and 2
                                                                                       501.145 Receptacles and Attachment Plugs,
                    (A) Permitted Uses Flexible cord shall be permitted:               Class I, Divisions 1 and 2
                    (1) For connection between portable lighting equipment or          Receptacles and attachment plugs shall be of the type provid-
                        other portable utilization equipment and the fixed por-        ing for connection to the grounding conductor of a flexible
                        tion of their supply circuit.                                  cord and shall be identified for the location.
                    (2) For that portion of the circuit where the fixed wiring
                        methods of 501.10(A) cannot provide the necessary de-          Exception: As provided in 501.105(B)(6).
                        gree of movement for fixed and mobile electrical utiliza-
                        tion equipment, and the flexible cord is protected by          Exhibit 501.22 shows an explosionproof receptacle and at-
                        location or by a suitable guard from damage and only           tachment plug with an interlocking switch. The design of
                        in an industrial establishment where conditions of main-       this device is such that when the switch is in the on position,
                        tenance and engineering supervision ensure that only           the plug cannot be removed. Also, the switch cannot be
                        qualified persons install and service the installation.        placed in the on position when the plug has been removed;
                    (3) For electric submersible pumps with means for removal          that is, the plug cannot be inserted or removed unless the
                        without entering the wet-pit. The extension of the flexi-      switch is in the off position. The receptacle is factory sealed,
                        ble cord within a suitable raceway between the wet-pit         with a provision for threaded-conduit entry to the switch
                        and the power source shall be permitted.

                    Section 501.140(A)(3) recognizes a wet-pit type of installa-
                    tion that is finding increasing acceptance for use in waste-
                    water systems. Section 501.125(A)(4) permits a flexible cord
                    to be run in raceway to a junction box outside the wet
                    pit. See the commentary following 501.125(A)(4) for more
                    information on motors installed in wet pits.
                         Due to the operating conditions associated with electric
                    mixers used in mixing tanks, the mixers are considered
                    portable utilization equipment and may be wired with flexi-
                    ble cord.


                    (4) For electric mixers intended for travel into and out of        Exhibit 501.22 A receptacle and attachment plug of the explo-
                        open-type mixing tanks or vats.                                sionproof type with an interlocking switch. The switch must be
                                                                                       in the off position before the attachment plug can be removed.
                    (B) Installation Where flexible cords are used, the cords          (Courtesy of Appleton Electric Co., EGS Electrical Group)
                    shall comply with all of the following:


                 696                                                                                                      2005 National Electrical Code Handbook

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                                                                                                                              Article 501 — Class I Locations                                                                  501.150



                                                                         compartment. The plug is to be used with Type S or equiva-                     (1) Immersed in oil
                                                                         lent extra-hard-service flexible cord having an equipment                      (2) Enclosed within a chamber hermetically sealed against
                                                                         grounding conductor.                                                               the entrance of gases or vapors
                                                                              Exhibit 501.23 shows a 30-ampere, 4-pole receptacle                       (3) In nonincendive circuits
                                                                         and attachment plug assembly that is suitable for use without
                                                                         a switch. The design is such that the mating parts of the                      See the commentary following 501.105(B)(1), Exception
                                                                         receptacle and plug are enclosed in a chamber that seals the                   (4), for information on nonincendive circuits.
                                                                         arc and, by delayed-action construction, prevents complete
                                                                         removal of the plug until the arc or hot metal has cooled.
                                                                         The receptacle is factory sealed, and the attachment plug is                   (4) Part of a listed nonincendive component
                                                                         designed for use with a 4-conductor cord (3-conductor, 3-                      (2) Resistors and Similar Equipment Resistors, resis-
                                                                         phase circuit with one grounding conductor) or a 3-conductor                   tance devices, thermionic tubes, rectifiers, and similar equip-
                                                                         cord (two circuit conductors and one grounding conductor).                     ment shall comply with 501.105(B)(2).
                                                                                                                                                        (3) Protectors Enclosures shall be provided for lightning
                                                                                                                                                        protective devices and for fuses. Such enclosures shall be
                                                                                                                                                        permitted to be of the general-purpose type.
                                                                                                                                                        (4) Wiring and Sealing All wiring shall comply with
                                                                                                                                                        501.10(B), 501.15(B), and 501.15(C).

                                                                                                                                                        Audible signaling devices, such as bells, sirens, and horns,
                                                                                                                                                        other than electronic types, usually involve make-and-break
                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                        contacts that are capable of producing a spark of sufficient
                                                                                                                                                        energy to cause ignition of a hazardous atmospheric mixture.
                                                                                                                                                        If used in Class I locations, therefore, this type of equipment,
                                                                                                                                                        as shown in Exhibit 501.24, must be contained in explo-




                                                                         Exhibit 501.23 A 4-pole (delayed action) explosionproof recep-
                                                                         tacle and attachment plug suitable for use without a switch. (Cour-
                                                                         tesy of Appleton Electric Co., EGS Electrical Group)



                                                                         501.150 Signaling, Alarm, Remote-Control, and
                                                                         Communications Systems
                                                                         (A) Class I, Division 1 In Class I, Division 1 locations, all
                                                                         apparatus and equipment of signaling, alarm, remote-control,
                                                                         and communications systems, regardless of voltage, shall
                                                                         be identified for Class I, Division 1 locations, and all wiring
                                                                         shall comply with 501.10(A), 501.15(A), and 501.15(C).

                                                                         (B) Class I, Division 2 In Class I, Division 2 locations,
                                                                         signaling, alarm, remote-control, and communications sys-
                                                                         tems shall comply with 501.150(B)(1) through (B)(4).

                                                                         (1) Contacts Switches, circuit breakers, and make-and-
                                                                         break contacts of pushbuttons, relays, alarm bells, and horns
                                                                         shall have enclosures identified for Class I, Division 1 loca-
                                                                         tions in accordance with 501.105(A).
                                                                                                                                                        Exhibit 501.24 An audible signaling device for use in hazardous
                                                                         Exception: General-purpose enclosures shall be permitted                       (classified) locations. (Courtesy of Cooper Crouse-Hinds)
                                                                         if current-interrupting contacts are one of the following:


                                                                         National Electrical Code Handbook 2005                                                                                                                    697

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                 502.1                                                       Article 502 — Class II Locations




                    sionproof or purged and pressurized enclosures, wiring                       III. Equipment
                    methods must comply with 501.10, and sealing fittings must                        502.100 Transformers and Capacitors
                    be provided in accordance with 501.15.                                                (A) Class II, Division 1
                         Explosionproof devices or explosionproof enclosures                              (B) Class II, Division 2
                    may prove more practical than oil-immersed contacts be-                           502.115 Switches, Circuit Breakers, Motor Controllers,
                    cause maintaining the condition and level of the oil can be                               and Fuses
                    a problem. Hermetically sealed enclosures, such as float-                             (A) Class II, Division 1
                    operated mercury-tube switches, are available for some ap-                            (B) Class II, Division 2
                    plications. Electronic signal devices without make-and-                           502.120 Control Transformers and Resistors
                    break contacts usually do not require explosionproof enclo-                           (A) Class II, Division 1
                    sures in Division 2 locations.                                                        (B) Class II, Division 2
                                                                                                      502.125 Motors and Generators
                                                                                                          (A) Class II, Division 1
                                            ARTICLE 502                                                   (B) Class II, Division 2
                                           Class II Locations                                         502.128 Ventilating Piping
                                                                                                          (A) Class II, Division 1
                                                                                                          (B) Class II, Division 2
                    Summary of Changes                                                                502.130 Luminaires (Lighting Fixtures)
                    • General: Restructured and renumbered to provide a scope                             (A) Class II, Division 1
                      section and parallel numbering systems for Articles 501,                            (B) Class II, Division 2
                      502, and 503.                                                                   502.135 Utilization Equipment
                                                                                                          (A) Class II, Division 1
                    • 502.10(A)(2): Revised to add interlocked armor Type MC                              (B) Class II, Division 2
                      cable as an additional method to provide a flexible connec-                     502.140 Flexible Cords — Class II, Divisions 1 and 2
                      tion in Class II, Division 1 locations.                                         502.145 Receptacles and Attachment Plugs
                    • 502.15(4): Added item (4) permitting a raceway to extend                            (A) Class II, Division 1
                      horizontally and downward as an equivalent to a horizon-                            (B) Class II, Division 2
                      tally installed raceway or a vertically installed raceway                       502.150 Signaling, Alarm, Remote-Control, and
                      as a method of sealing dust-ignition proof enclosures.                                  Communications Systems; and Meters,
                    • 502.25: Revised to provide guidance concerning protec-                                  Instruments, and Relays




                                                                                                                                                                               --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                      tion against explosion and against electric shock in order                          (A) Class II, Division 1
                      to limit the voltage of exposed live parts.                                         (B) Class II, Division 2


                                                                                                 I. General
                    Contents                                                                     502.1 Scope
                     I. General                                                                  Article 502 covers the requirements for electrical and elec-
                          502.1              Scope                                               tronic equipment and wiring for all voltages in Class II,
                          502.5              General                                             Division 1 and 2 locations where fire or explosion hazards
                    II. Wiring                                                                   may exist due to combustible dust.
                         502.10             Wiring Methods
                            (A)             Class II, Division 1                                 Two different types of dust environments typically warrant
                            (B)             Class II, Division 2                                 a Class II, Division 1 area classification. The first is where
                         502.15             Sealing, Class II, Divisions 1 and 2                 a cloud of combustible dust is likely to be present continu-
                         502.25             Uninsulated Exposed Parts, Class II, Divsions        ously or intermittently under normal operating conditions
                                            1 and 2                                              as a result of repair or maintenance operations or leakage.
                              502.30        Grounding and Bonding, Class II, Divisions 1         The other environment is one in which a dust layer is likely to
                                            and 2                                                accumulate to a depth greater than 1⁄8 in. on major horizontal
                                 (A)        Bonding                                              surfaces over a defined period of time, generally 24 hours.
                                 (B)        Types of Equipment Grounding Conductors              The size of the dust particle is the primary factor in determin-
                              502.35        Surge Protection — Class II, Divisions 1 and 2       ing whether the area should be classified as Class II or Class
                              502.40        Multiwire Branch Circuits                            III. Combustible dust, as defined in NFPA 499, Recom-


                 698                                                                                                                2005 National Electrical Code Handbook

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                                                                                                                           Article 502 — Class II Locations                                                               502.10



                                                                       mended Practice for the Classification of Combustible Dusts                  3. Accumulation of electrically conductive dust lodged
                                                                       and of Hazardous (Classified) Locations for Electrical In-                      between terminals that have a difference of potential,
                                                                       stallations in Chemical Process Areas, is any finely divided                    thereby causing tracking and glowing hot particles,
                                                                       solid material 420 microns (0.0165 in.) or less in diameter                     short-circuits, or ground faults that may ignite dust accu-
                                                                       (i.e., material passing through a U.S. No. 40 standard sieve)                   mulated in the vicinity
                                                                       that presents a fire or explosion hazard when dispersed.                     4. Deposits of dust that could be ignited by arcs or sparks


                                                                       502.5 General                                                                    In the layout of electrical installations for hazardous
                                                                                                                                                   (classified) locations, it is preferable to locate service equip-
                                                                       The general rules of this Code shall apply to the electric                  ment, switchboards, panelboards, and much of the electrical
                                                                       wiring and equipment in locations classified as Class II                    equipment in less hazardous areas, usually in a separate
                                                                       locations in 500.5(C).                                                      room. The use of pressurized rooms, as described in NFPA
                                                                       Exception: As modified by this article.                                     496, Standard for Purged and Pressurized Enclosures for
                                                                                                                                                   Electrical Equipment, is a common method of protecting
                                                                            Equipment installed in Class II locations shall be able                panelboards and switchboards in grain elevators and similar
                                                                       to function at full rating without developing surface tempera-              locations.
                                                                       tures high enough to cause excessive dehydration or gradual
                                                                       carbonization of any organic dust deposits that may occur.
                                                                           FPN: Dust that is carbonized or excessively dry is highly
                                                                                                                                                   II. Wiring
                                                                           susceptible to spontaneous ignition.                                    502.10 Wiring Methods
                                                                                                                                                   Wiring methods shall comply with 502.10(A) or 502.10(B).
                                                                           Explosionproof equipment and wiring shall not be re-
                                                                       quired and shall not be acceptable in Class II locations unless
                                                                                                                                                   (A) Class II, Division 1
                                                                       identified for such locations.
                                                                                                                                                   (1) General In Class II, Division 1 locations, the wiring
                                                                                                                                                   methods in (1) through (4) shall be permitted.
                                                                       Class II, Division 1 and 2 locations are defined in 500.5(C)
                                                                       as hazardous due to the presence of combustible dust. These                 (1) Threaded rigid metal conduit, or threaded steel interme-
                                                                       locations are separated into three groups: Group E, Group                       diate metal conduit.
                                                                       F, and Group G [see 500.6(B)].                                              (2) Type MI cable with termination fittings listed for the
                                                                            It should be noted that equipment suitable for one class                   location. Type MI cable shall be installed and supported
                     --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                       and group is not necessarily suitable for any other class and                   in a manner to avoid tensile stress at the termination
                                                                       group. Class I equipment is not necessarily better, or even                     fittings.
                                                                       suitable, for a Class II location, because the hazard contem-               (3) In industrial establishments with limited public access,
                                                                       plated in the equipment design is different. Class II equip-                    where the conditions of maintenance and supervision
                                                                       ment is designed to prevent the ignition of layers of dust,                     ensure that only qualified persons service the installa-
                                                                       which may increase the operating temperature of the equip-                      tion, Type MC cable, listed for use in Class II, Division
                                                                       ment. Class I equipment is not designed for dust layering                       1 locations, with a gas/vaportight continuous corrugated
                                                                       unless it is also designed and approved for Class II locations.                 metallic sheath, an overall jacket of suitable polymeric
                                                                       To protect against explosions in hazardous (classified) loca-                   material, separate grounding conductors in accordance
                                                                       tions, all electrical equipment exposed to the hazardous at-                    with 250.122, and provided with termination fittings
                                                                       mosphere must be suitable for such locations. Grain dust,                       listed for the application, shall be permitted.
                                                                       for example, ignites at a temperature lower than that of most               (4) Fittings and boxes shall be provided with threaded
                                                                       flammable vapors. Motors listed for use in Class I locations                    bosses for connection to conduit or cable terminations
                                                                       may not have dust shields on the bearings to prevent entrance                   and shall be dusttight. Fittings and boxes in which taps,
                                                                       of dust into the bearing race, thereby causing overheating                      joints, or terminal connections are made, or that are
                                                                       of the bearing and resulting in ignition of dust on the motor.                  used in Group E locations, shall be identified for Class
                                                                            One or more of the following four hazards may be                           II locations.
                                                                       present in a Class II location:
                                                                        1. An explosive mixture of air and dust in suspension                      In Class II, Division 1 locations, boxes or fittings, such as
                                                                        2. Accumulation of dust that acts as a thermal blanket and                 conduit ‘‘L,’’ ‘‘T,’’ or ‘‘C’’ fittings, that contain splices, taps,
                                                                           interferes with the safe dissipation of heat from electrical            or terminations must be of the threaded type with close-
                                                                           equipment                                                               fitting covers. There can be no holes in the box or fitting,


                                                                       National Electrical Code Handbook 2005                                                                                                                699

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                 502.10                                               Article 502 — Class II Locations



                    such as mounting holes, that would allow dust to enter or             Exception to (6): Type MC cable listed for use in Class II,
                    sparks or burning material to be ejected from the enclosure.          Division 1 locations shall be permitted to be installed without
                    Where used as stated above, the equipment must be approved            the spacings required by (6).
                    for Class II locations, usually of the dust-ignitionproof or
                    pressurized type.                                                     (2) Flexible Connections Where provision must be made
                        Boxes or fittings of the type described in the preceding          for flexibility, 502.10(A)(2) shall apply.
                    paragraph that do not enclose splices, taps, or terminations
                    must be dusttight, with threaded bosses for connection of             Where it is necessary to use flexible connections, liquidtight
                    conduit or cable connectors.                                          flexible conduit or extra-hard-usage flexible cord is permit-
                                                                                          ted. Another method would be to use a flexible fitting, as
                                                                                          described in the commentary following 501.10(A)(2). Where
                    (2) Flexible Connections Where necessary to employ flex-              liquidtight flexible conduit is used, a bonding jumper (inter-
                    ible connections, one or more of the following shall also be          nal or external) must be provided around such conduit. See
                    permitted:                                                            502.30. An additional conductor for grounding must be pro-
                    (1) Dusttight flexible connectors                                     vided where flexible cord is used.
                    (2) Liquidtight flexible metal conduit with listed fittings
                    (3) Liquidtight flexible nonmetallic conduit with listed fit-
                        tings                                                             (3) Nonincendive Field Wiring Nonincendive field wiring
                    (4) Interlocked armor Type MC cable having an overall                 shall be permitted using any of the wiring methods permitted
                        jacket of suitable polymeric material and provided with           for unclassified locations. Nonincendive field wiring sys-
                        termination fittings listed for Class II, Division 1 loca-        tems shall be installed in accordance with the control draw-
                        tions.                                                            ing(s). Simple apparatus, not shown on the control drawing,
                    (5) Flexible cord listed for extra-hard usage and provided            shall be permitted in a nonincendive field wiring circuit,
                        with bushed fittings. Where flexible cords are used, they         provided the simple apparatus does not interconnect the
                        shall comply with 502.140.                                        nonincendive field wiring circuit to any other circuit.
                            FPN: See 502.30(B) for grounding requirements where                  FPN: Simple apparatus is defined in 504.2.
                            flexible conduit is used.
                                                                                               Separate nonincendive field wiring circuits shall be in-
                    Due to the potential for physical damage to Type MC cable,            stalled in accordance with one of the following:
                    its application in Class II, Division 1 locations is limited to
                    cable that is listed specifically for use in Class II, Division       (1) In separate cables
                    1 locations and installed at facilities that have full-time,          (2) In multiconductor cables where the conductors of each
                    qualified maintenance personnel. Qualified maintenance                    circuit are within a grounded metal shield
                    personnel are those who, in the course of regular mainte-             (3) In multiconductor cables where the conductors of each




                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    nance procedures, would notice if cables were damaged,                    circuit have insulation with a minimum thickness of
                    understand the associated hazards, and be able to de-energize             0.25 mm (0.01 in.)
                    the circuit to repair the installation.                               (4) Boxes and Fittings All boxes and fittings shall be dust-
                                                                                          tight.
                    (B) Class II, Division 2
                    (1) General In Class II, Division 2 locations, the following          In Division 1 locations, boxes containing taps, joints, or
                    wiring methods shall be permitted:                                    terminal connections must be dusttight and must be provided
                                                                                          with threaded hubs, as shown in Exhibit 502.1, and must
                    (1) All wiring methods permitted in 502.10(A).                        be identified for use in Class II locations. Threaded hubs
                    (2) Rigid metal conduit, intermediate metal conduit, electri-         also provide adequate bonding in Division 2 locations (see
                        cal metallic tubing, dusttight wireways.                          Exhibit 502.1.)
                    (3) Type MC or MI cable with listed termination fittings.                  Exhibit 502.1 also shows a close-fitting cover, which
                    (4) Type PLTC in cable trays.                                         is required for Class II locations. Standard pressed-steel
                    (5) Type ITC in cable trays.                                          boxes that are not identified for Class II locations are permit-
                    (6) Type MC, MI, or TC cable installed in ladder, ventilated          ted as long as they do not contain taps, joints, or terminal
                        trough, or ventilated channel cable trays in a single             connections, are dusttight, and are provided with a bonding
                        layer, with a space not less than the larger cable diameter       jumper around the box in order to compensate for the absence
                        between the two adjacent cables, shall be the wiring              of threaded hubs.
                        method employed.

                 700                                                                                                         2005 National Electrical Code Handbook

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                                                                                                                          Article 502 — Class II Locations                                                                            502.30



                                                                                                                                                   vertical separation between enclosures of not less than 5 ft
                                                                                                                                                   in a raceway extending downward from a dust-ignitionproof
                                                                                                                                                   enclosure are methods of sealing in Class II locations and
                                                                                                                                                   are shown in Exhibit 502.2.
                                                                                                                                                        The requirement to provide a seal applies if a raceway
                                                                                                                                                   connects an enclosure that is required to be dust-igni-
                                                                                                                                                   tionproof to one that is not required to be dust-ignitionproof.
                                                                                                                                                   If a raceway extends from a dust-ignitionproof enclosure to
                                                                                                                                                   an unclassified location, it is not necessary to provide a seal
                                                                                                                                                   in that raceway. Sealing fittings designed for use in Class I
                                                                                                                                                   locations are acceptable. However, because the Class I loca-
                                                                                                                                                   tion pressure-piling considerations are not inherent to Class
                                                                                                                                                   II locations, conduit seals are not required to be explo-
                                                                                                                                                   sionproof. Conduit seals are expected only to prevent the
                                                                                                                                                   migration of dust into dust-ignitionproof enclosures. No seal-
                                                                                                                                                   ing method is needed in the special, but not unusual, situation
                                                                                                                                                   in which dust cannot enter the raceway in the hazardous
                                                                                                                                                   (classified) location.

                                                                        Exhibit 502.1 Junction box with threaded hubs, suitable for use
                                                                        in Class II, Group E hazardous atmospheres. (Courtesy of Apple-
                                                                        ton Electric Co., EGS Electrical Group)
                                                                                                                                                                                                                          Conduit
                                                                                                                                                                                                 Seal                     termination in
                                                                                                                                                                                                                          an unclassified
                                                                        502.15 Sealing, Class II, Divisions 1 and 2                                                                                                       location
                                                                         Where a raceway provides communication between an en-                          Horizontal
                                                                        closure that is required to be dust-ignitionproof and one                       raceway not less
                                                                                                                                                        than 10 ft
                                                                        that is not, suitable means shall be provided to prevent
                                                                        the entrance of dust into the dust-ignitionproof enclosure
                                                                        through the raceway. One of the following means shall be
                                                                        permitted:
                                                                                                                                                     Dust-ignitionproof
                                                                        (1) A permanent and effective seal                                           enclosure
                                                                        (2) A horizontal raceway not less than 3.05 m (10 ft) long                                                                                        Enclosure not
                                                                                                                                                     Vertical raceway                        Not less
                                                                        (3) A vertical raceway not less than 1.5 m (5 ft) long and                   extending downward                      than 5 ft
                                                                                                                                                                                                                          required to be
                                                                                                                                                                                                                          dust-ignitionproof
                                                                            extending downward from the dust-ignitionproof enclo-
                                                                            sure
                                                                        (4) A raceway installed in a manner equivalent to (2) or                   Exhibit 502.2 Three methods for preventing dust from entering
                                                                            (3) that extends only horizontally and downward from                   a dust-ignitionproof enclosure through the raceway.
                                                                            the dust-ignition proof enclosures.
                                                                                                                                                   502.25 Uninsulated Exposed Parts, Class II,
                                                                            Where a raceway provides communication between an                      Divisions 1 and 2
                                                                        enclosure that is required to be dust-ignitionproof and an
                                                                        enclosure in an unclassified location, seals shall not be re-              There shall be no uninsulated exposed parts, such as electric
                                                                        quired.                                                                    conductors, buses, terminals, or components, that operate at
                                                                            Sealing fittings shall be accessible.                                  more than 30 volts (15 volts in wet locations). These parts
                                                                            Seals shall not be required to be explosionproof.                      shall additionally be protected by a protection technique
                                                                                                                                                   according to 500.7(E), 500.7(F), or 500.7(G) that is suitable
                                                                            FPN: Electrical sealing putty is a method of sealing.                  for the location.
                                                                                                                                                   502.30 Grounding and Bonding, Class II,
                                                                        The requirements of 502.15 provide three suitable ways
                                                                                                                                                   Divisions 1 and 2
                                                                        to prevent dust from entering dust-ignitionproof enclosures
                                                                        through the raceway. A seal in the raceway, a horizontal                   Wiring and equipment in Class II, Division 1 and 2 locations
                                                                        separation between enclosures of not less than 10 ft, or a                 shall be grounded as specified in Article 250 and with the
                                                                                                                                                   requirements in 502.30(A) and 502.30(B).


                                                                        National Electrical Code Handbook 2005                                                                                                                              701

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                 502.35                                                                Article 502 — Class II Locations



                    (A) Bonding The locknut-bushing and double-locknut                                     grounding conductor must not exceed 6 ft and must be routed
                    types of contact shall not be depended on for bonding pur-                             with the raceway or enclosure. See 250.102(E) for equipment
                    poses, but bonding jumpers with proper fittings or other                               bonding jumper installation requirements.
                    approved means of bonding shall be used. Such means of
                    bonding shall apply to all intervening raceways, fittings,
                    boxes, enclosures, and so forth, between Class II locations                            502.35 Surge Protection — Class II,
                    and the point of grounding for service equipment or point                              Divisions 1 and 2
                    of grounding of a separately derived system.                                           Surge arresters and transient voltage surge suppressors
                                                                                                           (TVSS) installed in a Class II, Division 1 location shall be
                    For information regarding bonding in hazardous (classified)                            in suitable enclosures. Surge-protective capacitors shall be
                    locations, see the commentary on 501.30(A). Although that                              of a type designed for specific duty.
                    section addresses bonding in Class I locations, the same
                    requirement for enhanced bonding in Class II locations is                              502.40 Multiwire Branch Circuits
                    found in 502.30(A), and the requirements of 250.100 apply                              In a Class II, Division 1 location, a multiwire branch circuit
                    to Class I, Class II, and Class III hazardous locations.                               shall not be permitted.
                                                                                                           Exception: Where the disconnect device(s) for the circuit
                    Exception: The specific bonding means shall only be re-                                opens all ungrounded conductors of the multiwire circuit
                    quired to the nearest point where the grounded circuit con-                            simultaneously.
                    ductor and the grounding electrode conductor are connected
                    together on the line side of the building or structure discon-                         III. Equipment
                    necting means as specified in 250.32(A), (B), and (C), if the                          502.100 Transformers and Capacitors
                    branch-circuit overcurrent protection is located on the load
                                                                                                           (A) Class II, Division 1 In Class II, Division 1 locations,
                    side of the disconnecting means.
                                                                                                           transformers and capacitors shall comply with
                            FPN: See 250.100 for additional bonding requirements                           502.100(A)(1) through (A)(3).
                            in hazardous (classified) locations.
                                                                                                           (1) Containing Liquid That Will Burn Transformers and
                    (B) Types of Equipment Grounding Conductors Where                                      capacitors containing a liquid that will burn shall be installed
                    flexible conduit is used as permitted in 502.10, it shall be                           only in vaults complying with 450.41 through 450.48, and,
                    installed with internal or external bonding jumpers in parallel                        in addition, (1), (2), and (3) shall apply.
                    with each conduit and complying with 250.102.
                                                                                                           (1) Doors or other openings communicating with the Divi-
                    Exception: In Class II, Division 2 locations, the bonding                                  sion 1 location shall have self-closing fire doors on both
                    jumper shall be permitted to be deleted where all of the                                   sides of the wall, and the doors shall be carefully fitted
                    following conditions are met:                                                              and provided with suitable seals (such as weather strip-
                    (1) Listed liquidtight flexible metal conduit 1.8 m (6 ft) or                              ping) to minimize the entrance of dust into the vault.
                        less in length, with fittings listed for grounding, is used.                       (2) Vent openings and ducts shall communicate only with
                    (2) Overcurrent protection in the circuit is limited to 10                                 the outside air.
                        amperes or less.                                                                   (3) Suitable pressure-relief openings communicating with
                    (3) The load is not a power utilization load.                                              the outside air shall be provided.

                                                                                                           (2) Not Containing Liquid That Will Burn Transformers
                    In Class II locations, a raceway connected to an enclosure                             and capacitors that do not contain a liquid that will burn
                    via a double locknut connection or a single locknut and                                shall be installed in vaults complying with 450.41 through
                    bushing connection is not an acceptable method of bonding.                             450.48 or be identified as a complete assembly, including
                    Bonding jumpers or other approved means with proper fit-                               terminal connections for Class II locations.
                    tings are required for the interconnection of all raceways,
                    junction boxes, fittings, enclosures, and so on, installed be-                         (3) Metal Dusts No transformer or capacitor shall be in-
                    tween the hazardous area and the grounding electrode con-                              stalled in a location where dust from magnesium, aluminum,
                    ductor connection point at the service equipment, the                                  aluminum bronze powders, or other metals of similarly haz-
                    grounding electrode connection at a feeder or branch circuit                           ardous characteristics may be present.
                    disconnecting means at separate buildings, or the grounding
                    electrode connection to the source of a separately derived                             (B) Class II, Division 2 In Class II, Division 2 locations,
                    system. If installed outside the raceway or enclosure, the                             transformers and capacitors shall comply with 502.100(B)(1)
                                                                                                           through (B)(3).

                 702                                                                                                                          2005 National Electrical Code Handbook
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                                                                                                                          Article 502 — Class II Locations                                                                 502.115



                                                                        (1) Containing Liquid That Will Burn Transformers and                       enclosures that shall be designed to minimize the entrance
                                                                        capacitors containing a liquid that will burn shall be installed            of dust and that shall (1) be equipped with telescoping or
                                                                        in vaults that comply with 450.41 through 450.48.                           close-fitting covers or with other effective means to prevent
                                                                                                                                                    the escape of sparks or burning material and (2) have no
                                                                        (2) Containing Askarel Transformers containing askarel                      openings (such as holes for attachment screws) through
                                                                        and rated in excess of 25 kVA shall be as follows:                          which, after installation, sparks or burning material might
                                                                        (1) Provided with pressure-relief vents                                     escape or through which exterior accumulations of dust or
                                                                        (2) Provided with a means for absorbing any gases gener-                    adjacent combustible material might be ignited.
                                                                            ated by arcing inside the case, or the pressure-relief
                                                                                                                                                    (3) Metal Dusts In locations where dust from magnesium,
                                                                            vents shall be connected to a chimney or flue that will
                                                                                                                                                    aluminum, aluminum bronze powders, or other metals of
                                                                            carry such gases outside the building
                                                                                                                                                    similarly hazardous characteristics may be present, fuses,
                                                                        (3) Have an airspace of not less than 150 mm (6 in.) between
                                                                                                                                                    switches, motor controllers, and circuit breakers shall have
                                                                            the transformer cases and any adjacent combustible ma-
                                                                                                                                                    enclosures identified for such locations.
                                                                            terial
                                                                                                                                                    (B) Class II, Division 2 In Class II, Division 2 locations,
                                                                        (3) Dry-Type Transformers Dry-type transformers shall                       enclosures for fuses, switches, circuit breakers, and motor
                                                                        be installed in vaults or shall have their windings and termi-              controllers, including pushbuttons, relays, and similar de-
                                                                        nal connections enclosed in tight metal housings without                    vices, shall be dusttight.
                                                                        ventilating or other openings and shall operate at not over
                                                                        600 volts, nominal.
                                                                                                                                                    The electrical equipment required in Class II locations is
                                                                                                                                                    different from that required for Class I locations. Dust-
                                                                        Where it is necessary to install a transformer, it may be                   ignitionproof enclosures for Class II locations are not re-
                                                                        possible to use a small, low-voltage, dusttight (without venti-             quired to be explosionproof. However, explosionproof
                                                                        lating openings) dry-type transformer, but transformers that                equipment is allowed to be used in Class II locations if the
                                                                        have a primary voltage rating of over 600 volts must be                     equipment is dual rated and identified as suitable for the
                                                                        either less flammable liquid-insulated or installed in a vault.             Class II division and group. Explosionproof enclosures are
                                                                        In almost all cases, transformers can be remotely located                   not necessarily dust-ignitionproof. They may have a different
                                                                        from dust atmospheres.                                                      shape to minimize the accumulation of dust on top of the
                                                                             Capacitors used for power-factor correction of individ-                enclosure or on any protrusions or ledges.
                                                                        ual motors are of sealed construction, but if installed in Class                 Explosionproof enclosures, where used in an environ-
                                                                        II, Division 1 locations, they must also be identified as a                 ment that is only a Class II location, are not required to
                                                                        complete assembly, including dusttight terminal enclosures.                 be sealed within 18 in. of the enclosure to complete the
                                                                        The only special requirement for capacitors in Division 2                   explosionproof assembly, as required in a Class I environ-
                                                                        locations is that they are not permitted to contain oil or any              ment. However, explosionproof enclosures must be provided
                                                                        other liquid that burns; otherwise, they are to be installed                with seals to prevent the entrance of dust into the dust-
                                                                        in vaults.                                                                  ignitionproof enclosure where a raceway provides a means
                                                                                                                                                    for dust to enter the system, such as in a conduit run between
                                                                        502.115 Switches, Circuit Breakers, Motor                                   a dust-ignitionproof enclosure and a general-purpose junc-
                                                                        Controllers, and Fuses                                                      tion box.
                                                                                                                                                         Where equipment is located in a Class II, Division 1
                      --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                        (A) Class II, Division 1 In Class II, Division 1 locations,                 location and is likely to produce arcs or sparks during normal
                                                                        switches, circuit breakers, motor controllers, and fuses shall              operation, it must be installed in a dust-ignitionproof or
                                                                        comply with 502.115(A)(1) through (A)(3).
                                                                                                                                                    pressurized enclosure. In addition, heat-generating equip-
                                                                        (1) Type Required Switches, circuit breakers, motor con-                    ment, such as control transformers, solenoids, impedance
                                                                        trollers, and fuses, including pushbuttons, relays, and similar             coils, resistors, and any associated overcurrent devices or
                                                                        devices that are intended to interrupt current during normal                switching mechanisms, must have dust-ignitionproof enclo-
                                                                        operation or that are installed where combustible dusts of                  sures or pressurized enclosures installed in accordance with
                                                                        an electrically conductive nature may be present, shall be                  NFPA 496, Standard for Purged and Pressurized Enclosures
                                                                        provided with identified dust-ignitionproof enclosures.                     for Electrical Equipment. Caution should be used where
                                                                                                                                                    metal dusts, such as magnesium, aluminum, or other metals
                                                                        (2) Isolating Switches Disconnecting and isolating                          with similar hazardous characteristics, may be present. En-
                                                                        switches containing no fuses and not intended to interrupt                  closures must be approved specifically for that environment
                                                                        current and not installed where dusts may be of an electri-                 (suitable for Class II, Division 1, Group E).
                                                                        cally conductive nature shall be provided with tight metal

                                                                        National Electrical Code Handbook 2005                                                                                                                 703

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                 502.120                                                                           Article 502 — Class II Locations



                          Exhibit 502.3 shows a dust-ignitionproof pushbutton
                    station with pilot light that is suitable for use in Class II,
                    Division 1 hazardous (classified) locations. Dust-
                    ignitionproof equipment enclosures for switching devices
                    can be used in Class II, Division 2 locations, but because
                    of the reduced level of hazard associated with Division 2,
                    dusttight equipment enclosures are also permitted. In addi-
                    tion to being suitable for the specific class and division, this
                    type of equipment must also be suitable for the dust group(s)
                    (i.e., Groups E, F, and G) that will be present in a specific
                    hazardous (classified) location.
                          Exhibit 502.4 shows a panelboard that is suitable for
                    use in Class II locations only. Many, but not all, of the
                    switches or circuit breakers and their associated enclosures
                    that are approved for Class I, Division 1 locations are also
                    approved for Class II locations. It is always important to look
                    for the listing and identification of the hazardous (classified)
                    locations in which the equipment is listed for use.




                                                                                                                       Exhibit 502.4 A dust-ignitionproof panelboard for use in Class
                                                                                                                       II, Group E, F, and G locations. (Courtesy of Cooper Crouse-
                                                                                                                       Hinds)



                                                                                                                       (B) Class II, Division 2 In Class II, Division 2 locations,
                                                                                                                       transformers and resistors shall comply with 502.120(B)(1)
                    Exhibit 502.3 A dust-ignitionproof pushbutton control station
                                                                                                                       through (B)(3).
                    suitable for use in Class II, Group E, F, and G locations. (Courtesy
                                                                                                                       (1) Switching Mechanisms Switching mechanisms (in-
                    of Appleton Electric Co., EGS Electrical Group)
                                                                                                                       cluding overcurrent devices) associated with control trans-
                                                                                                                       formers, solenoids, impedance coils, and resistors shall be
                                                                                                                       provided with dusttight enclosures.
                    502.120 Control Transformers and Resistors
                    (A) Class II, Division 1 In Class II, Division 1 locations,                                        (2) Coils and Windings Where not located in the same
                    control transformers, solenoids, impedance coils, resistors,                                       enclosure with switching mechanisms, control transformers,
                    and any overcurrent devices or switching mechanisms asso-                                          solenoids, and impedance coils shall be provided with tight
                    ciated with them shall have dust-ignitionproof enclosures                                          metal housings without ventilating openings.
                    identified for Class II locations. No control transformer,
                                                                                                                       (3) Resistors Resistors and resistance devices shall have
                    impedance coil, or resistor shall be installed in a location
                                                                                                                       dust-ignitionproof enclosures identified for Class II loca-
                    where dust from magnesium, aluminum, aluminum bronze
                                                                                                                       tions.
                    powders, or other metals of similarly hazardous characteris-
                    tics may be present unless provided with an enclosure identi-                                      Exception: Where the maximum normal operating tempera-
                    fied for the specific location.                                                                    ture of the resistor will not exceed 120 C (248 F), nonadjust-



                 704                                                                                                                                      2005 National Electrical Code Handbook
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                                                                              Article 502 — Class II Locations                                                                 502.128



                             able resistors or resistors that are part of an automatically              The requirements in 502.125(B) permit all types of totally
                             timed starting sequence shall be permitted to have enclosures              enclosed motors in Class II, Division 2 locations if the
                             complying with 502.120(B)(2).                                              external surface temperatures, without a dust blanket, do not
                                                                                                        exceed the temperatures indicated under the maximum full-
                             502.125 Motors and Generators                                              load (normal operation) conditions in 500.8(C)(2). Totally
                             (A) Class II, Division 1 In Class II, Division 1 locations,                enclosed fan-cooled (TEFC) motors are specifically men-
                             motors, generators, and other rotating electrical machinery                tioned. The motor should be examined carefully to be sure
                             shall be in conformance with either of the following:                      there are no external openings, even though the motor may
                                                                                                        be marked TEFC.
                              (1) Identified for Class II, Division 1 locations                              Totally enclosed motors with no special provision for
                              (2) Totally enclosed pipe-ventilated, meeting temperature                 cooling may be used in Class II, Division 2 locations, but
                                  limitations in 502.5                                                  to deliver the same horsepower, they must be considerably
                                                                                                        larger than an open-type, fan-cooled, or pipe-ventilated
                             It is intended that the phrase ‘‘other rotating electrical ma-             motor.
                             chinery’’ include electric brakes. Listed and labeled electric
                             brakes are available for Class II, Group E, F, and G locations.
                                   Although some explosionproof motors for Class I, Divi-               502.128 Ventilating Piping
                             sion 1 locations are dust-ignitionproof and approved for both              Ventilating pipes for motors, generators, or other rotating
                             Class I and II locations, this is not true of all motors. The              electric machinery, or for enclosures for electric equipment,
                             marking should always be checked to be sure the motor is                   shall be of metal not less than 0.53 mm (0.021 in.) in thick-
                             designed and tested for the Class II location involved. If                 ness or of equally substantial noncombustible material and
                             control wiring to the motor is necessary, according to the                 shall comply with all of the following:
                             motor installation instructions, the control circuit must be
                             installed and connected properly. Most motors for Class II                 (1) Lead directly to a source of clean air outside of buildings
                             locations require internal thermal protection to comply with               (2) Be screened at the outer ends to prevent the entrance
                             the temperature limitations in 500.8(C)(2). Integral horse-                    of small animals or birds
                             power Class II motors may require both power and control                   (3) Be protected against physical damage and against rust-
                             circuit wiring from the motor controller to the motor.                         ing or other corrosive influences

                                                                                                            Ventilating pipes shall also comply with 502.128(A)
                             (B) Class II, Division 2 In Class II, Division 2 locations,                and 502.128(B).
                             motors, generators, and other rotating electrical equipment
                             shall be totally enclosed nonventilated, totally enclosed pipe-            (A) Class II, Division 1 In Class II, Division 1 locations,
                             ventilated, totally enclosed water-air-cooled, totally enclosed            ventilating pipes, including their connections to motors or
                             fan-cooled or dust-ignitionproof for which maximum full-                   to the dust-ignitionproof enclosures for other equipment,
                             load external temperature shall be in accordance with                      shall be dusttight throughout their length. For metal pipes,
                             500.8(C)(2) for normal operation when operating in free air                seams and joints shall comply with one of the following:
                             (not dust blanketed) and shall have no external openings.                  (1)    Be riveted and soldered
                             Exception: If the authority having jurisdiction believes ac-               (2)    Be bolted and soldered
                             cumulations of nonconductive, nonabrasive dust will be                     (3)    Be welded
                             moderate and if machines can be easily reached for routine                 (4)    Be rendered dusttight by some other equally effective
                             cleaning and maintenance, the following shall be permitted                        means
                             to be installed:
                                                                                                        (B) Class II, Division 2 In Class II, Division 2 locations,
                              (1) Standard open-type machines without sliding contacts,                 ventilating pipes and their connections shall be sufficiently
                                  centrifugal or other types of switching mechanism (in-                tight to prevent the entrance of appreciable quantities of dust
                                  cluding motor overcurrent, overloading, and overtemp-                 into the ventilated equipment or enclosure and to prevent
                                  erature devices), or integral resistance devices                      the escape of sparks, flame, or burning material that might
                              (2) Standard open-type machines with such contacts,                       ignite dust accumulations or combustible material in the
                                  switching mechanisms, or resistance devices enclosed                  vicinity. For metal pipes, lock seams and riveted or welded
                                  within dusttight housings without ventilating or other                joints shall be permitted; and tight-fitting slip joints shall
                                  openings                                                              be permitted where some flexibility is necessary, as at con-
                              (3) Self-cleaning textile motors of the squirrel-cage type                nections to motors.



                             National Electrical Code Handbook 2005                                                                                                                705
              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---

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                 502.130                                              Article 502 — Class II Locations



                    502.130 Luminaires (Lighting Fixtures)                                be designed to minimize the deposit of dust on lamps and
                    Luminaires (lighting fixtures) shall comply with 502.130(A)           to prevent the escape of sparks, burning material, or hot
                    and 502.130(B).                                                       metal. Each fixture shall be clearly marked to indicate the
                                                                                          maximum wattage of the lamp that shall be permitted without
                    (A) Class II, Division 1 In Class II, Division 1 locations,           exceeding an exposed surface temperature in accordance
                    luminaires (lighting fixtures) for fixed and portable lighting        with 500.8(C)(2) under normal conditions of use.
                    shall comply with 502.130(A)(1) through (A)(4).
                                                                                          (3) Physical Damage Luminaires (lighting fixtures) for
                    (1) Fixtures Each luminaire (fixture) shall be identified for         fixed lighting shall be protected from physical damage by
                    Class II locations and shall be clearly marked to indicate            suitable guards or by location.
                    the maximum wattage of the lamp for which it is designed. In
                                                                                          (4) Pendant Luminaires (Fixtures) Pendant luminaires
                    locations where dust from magnesium, aluminum, aluminum




                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                          (fixtures) shall be suspended by threaded rigid metal conduit
                    bronze powders, or other metals of similarly hazardous char-
                                                                                          stems, threaded steel intermediate metal conduit stems, by
                    acteristics may be present, luminaires (fixtures) for fixed
                                                                                          chains with approved fittings, or by other approved means.
                    or portable lighting and all auxiliary equipment shall be
                                                                                          For rigid stems longer than 300 mm (12 in.), permanent
                    identified for the specific location.
                                                                                          and effective bracing against lateral displacement shall be
                    (2) Physical Damage Each luminaire (fixture) shall be pro-            provided at a level not more than 300 mm (12 in.) above
                    tected against physical damage by a suitable guard or by              the lower end of the stem, or flexibility in the form of an
                    location.                                                             identified fitting or a flexible connector shall be provided
                                                                                          not more than 300 mm (12 in.) from the point of attachment
                    (3) Pendant Luminaires (Fixtures) Pendant luminaires                  to the supporting box or fitting. Where wiring between an
                    (fixtures) shall be suspended by threaded rigid metal conduit         outlet box or fitting and a pendant luminaire (fixture) is not
                    stems, threaded steel intermediate metal conduit stems, by            enclosed in conduit, flexible cord listed for hard usage shall
                    chains with approved fittings, or by other approved means.            be used. Flexible cord shall not serve as the supporting
                    For rigid stems longer than 300 mm (12 in.), permanent                means for a fixture.
                    and effective bracing against lateral displacement shall be
                    provided at a level not more than 300 mm (12 in.) above               (5) Electric-Discharge Lamps Starting and control equip-
                    the lower end of the stem, or flexibility in the form of a            ment for electric-discharge lamps shall comply with the
                    fitting or a flexible connector listed for the location shall be      requirements of 502.120(B).
                    provided not more than 300 mm (12 in.) from the point of
                    attachment to the supporting box or fitting. Threaded joints          Exhibit 502.5 shows a listed fixture suitable for use in Class
                    shall be provided with set-screws or other effective means            II, Group E, F, and G locations. Luminaires, fixed or portable,
                    to prevent loosening. Where wiring between an outlet box              and auxiliary equipment (such as ballasts) must be approved
                    or fitting and a pendant luminaire (fixture) is not enclosed          for use in Group E atmospheres if metal dusts are present.
                    in conduit, flexible cord listed for hard usage shall be used,             Other than the requirement that the fixture be marked
                    and suitable seals shall be provided where the cord enters            to indicate maximum lamp wattage, the only requirement
                    the luminaire (fixture) and the outlet box or fitting. Flexible       for fixtures in Division 2 locations is that lamps be enclosed
                    cord shall not serve as the supporting means for a fixture.           in suitable globes to minimize dust deposits on the lamps
                                                                                          and prevent the escape of sparks or burning material. Metal
                    (4) Supports Boxes, box assemblies, or fittings used for              guards must be provided, unless globe breakage is unlikely.
                    the support of luminaires (lighting fixtures) shall be identi-             Flexible cord of the hard-usage type is permitted with
                    fied for Class II locations.                                          approved sealed connections for the wiring of chain-sus-
                                                                                          pended or hook-and-eye-suspended fixtures. Flexible cords
                    (B) Class II, Division 2 In Class II, Division 2 locations,
                                                                                          are not intended to be used as cord pendants or drop cords.
                    luminaires (lighting fixtures) shall comply with
                                                                                               The portable hand lamp shown in Exhibit 501.21 is
                    502.130(B)(1) through (B)(5).
                                                                                          approved as a complete assembly for use in Class I locations
                    (1) Portable Lighting Equipment Portable lighting equip-              and also in any Class II, Group F or G location.
                    ment shall be identified for Class II locations. They shall
                    be clearly marked to indicate the maximum wattage of lamps
                                                                                          502.135 Utilization Equipment
                    for which they are designed.
                                                                                          (A) Class II, Division 1 In Class II, Division 1 locations,
                    (2) Fixed Lighting Luminaires (lighting fixtures) for fixed           all utilization equipment shall be identified for Class II loca-
                    lighting, where not of a type identified for Class II locations,      tions. Where dust from magnesium, aluminum, aluminum
                    shall provide enclosures for lamps and lampholders that shall         bronze powders, or other metals of similarly hazardous char-

                 706                                                                                                         2005 National Electrical Code Handbook

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                                                                                                                         Article 502 — Class II Locations                                                                 502.150



                                                                                                                                                   (1) Be of a type listed for extra-hard usage

                                                                                                                                                   Exception: Flexible cord listed for hard usage as permitted
                                                                                                                                                   by 502.130(A)(3) and (B)(4).

                                                                                                                                                   (2) Contain, in addition to the conductors of the circuit, a
                                                                                                                                                       grounding conductor complying with 400.23
                                                                                                                                                   (3) Be connected to terminals or to supply conductors in
                                                                                                                                                       an approved manner
                                                                                                                                                   (4) Be supported by clamps or by other suitable means in
                                                                                                                                                       such a manner that there will be no tension on the
                                                                                                                                                       terminal connections
                                                                                                                                                   (5) Be provided with suitable seals to prevent the entrance
                                                                                                                                                       of dust where the flexible cord enters boxes or fittings
                                                                                                                                                       that are required to be dust-ignitionproof

                                                                                                                                                   502.145 Receptacles and Attachment Plugs
                                                                                                                                                   (A) Class II, Division 1 In Class II, Division 1 locations,
                                                                                                                                                   receptacles and attachment plugs shall be of the type provid-
                                                                                                                                                   ing for connection to the grounding conductor of the flexible
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                   cord and shall be identified for Class II locations.
                                                                      Exhibit 502.5 A typical luminaire for use in Class II, Division 1
                                                                      locations. (Courtesy of Cooper Crouse-Hinds)                                 (B) Class II, Division 2 In Class II, Division 2 locations,
                                                                                                                                                   receptacles and attachment plugs shall be of the type that
                                                                                                                                                   provides for connection to the grounding conductor of the
                                                                                                                                                   flexible cord and shall be designed so that connection to the
                                                                                                                                                   supply circuit cannot be made or broken while live parts are
                                                                      acteristics may be present, such equipment shall be identified
                                                                                                                                                   exposed.
                                                                      for the specific location.

                                                                      (B) Class II, Division 2 In Class II, Division 2 locations,                  502.150 Signaling, Alarm, Remote-Control, and
                                                                      all utilization equipment shall comply with 502.135(B)(1)                    Communications Systems; and Meters,
                                                                      through (B)(4).                                                              Instruments, and Relays
                                                                      (1) Heaters Electrically heated utilization equipment shall                         FPN: See Article 800 for rules governing the installation
                                                                      be identified for Class II locations.                                               of communications circuits.
                                                                      Exception: Metal-enclosed radiant heating panel equip-
                                                                                                                                                   (A) Class II, Division 1 In Class II, Division 1 locations,
                                                                      ment shall be dusttight and marked in accordance with
                                                                                                                                                   signaling, alarm, remote-control, and communications sys-
                                                                      500.8(B).
                                                                                                                                                   tems; and meters, instruments, and relays shall comply with
                                                                      (2) Motors Motors of motor-driven utilization equipment                      502.150(A)(1) through (A)(6).
                                                                      shall comply with 502.125(B).
                                                                                                                                                   (1) Wiring Methods The wiring method shall comply with
                                                                      (3) Switches, Circuit Breakers, and Fuses Enclosures for                     502.10(A).
                                                                      switches, circuit breakers, and fuses shall be dusttight.
                                                                                                                                                   (2) Contacts Switches, circuit breakers, relays, contactors,
                                                                      (4) Transformers, Solenoids, Impedance Coils, and Re-                        fuses and current-breaking contacts for bells, horns, howlers,
                                                                      sistors Transformers, solenoids, impedance coils, and resis-                 sirens, and other devices in which sparks or arcs may be
                                                                      tors shall comply with 502.120(B).                                           produced shall be provided with enclosures identified for a
                                                                                                                                                   Class II location.
                                                                      502.140 Flexible Cords — Class II,                                           Exception: Where current-breaking contacts are immersed
                                                                      Divisions 1 and 2                                                            in oil or where the interruption of current occurs within a
                                                                      Flexible cords used in Class II locations shall comply with                  chamber sealed against the entrance of dust, enclosures
                                                                      all of the following:                                                        shall be permitted to be of the general-purpose type.


                                                                      National Electrical Code Handbook 2005                                                                                                                  707

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                 502.150                                             Article 503 — Class III Locations



                    (3) Resistors and Similar Equipment Resistors, trans-                                      ARTICLE 503
                    formers, choke coils, rectifiers, thermionic tubes, and other
                    heat-generating equipment shall be provided with enclosures                              Class III Locations
                    identified for Class II locations.

                    Exception: Where resistors or similar equipment are im-
                                                                                         Summary of Changes
                    mersed in oil or enclosed in a chamber sealed against the            • General: Restructured and renumbered to provide a scope
                    entrance of dust, enclosures shall be permitted to be of the           section and parallel numbering systems for Articles 501,
                    general-purpose type.                                                  502, and 503.
                                                                                         • 503.25: Revised to provide guidance concerning protec-
                    (4) Rotating Machinery Motors, generators, and other ro-               tion against explosion and against electric shock in order
                    tating electric machinery shall comply with 502.125(A).                to limit the voltage of exposed live parts.

                    (5) Combustible, Electrically Conductive Dusts Where
                    dusts are of a combustible, electrically conductive nature,
                    all wiring and equipment shall be identified for Class II            Contents
                    locations.                                                            I. General
                                                                                               503.1            Scope
                    (6) Metal Dusts Where dust from magnesium, aluminum,
                                                                                               503.5            General
                    aluminum bronze powders, or other metals of similarly haz-
                                                                                         II. Wiring
                    ardous characteristics may be present, all apparatus and
                                                                                              503.10  Wiring Methods
                    equipment shall be identified for the specific conditions.
                                                                                                 (A)  Class III, Division 1
                                                                                                 (B)  Class III, Division 2
                    (B) Class II, Division 2 In Class II, Division 2 locations,
                                                                                              503.25  Uninsulated Exposed Parts, Class III,
                    signaling, alarm, remote-control, and communications sys-
                                                                                                      Divisions 1 and 2
                    tems; and meters, instruments, and relays shall comply with
                                                                                               503.30 Grounding and Bonding — Class III,
                    502.150(B)(1) through (B)(5).
                                                                                                      Divisions 1 and 2
                    (1) Contacts Enclosures shall comply with 502.150(A)(2),                      (A) Bonding
                    or contacts shall have tight metal enclosures designed to                     (B) Types of Equipment Grounding Conductors
                    minimize the entrance of dust and shall have telescoping or          III. Equipment
                    tight-fitting covers and no openings through which, after                 503.100 Transformers and Capacitors — Class III,
                    installation, sparks or burning material might escape.                            Divisions 1 and 2
                                                                                              503.115 Switches, Circuit Breakers, Motor Controllers,
                    Exception: In nonincendive circuits, enclosures shall be                          and Fuses — Class III, Divisions 1 and 2
                    permitted to be of the general-purpose type.                              503.120 Control Transformers and Resistors — Class
                                                                                                      III, Divisions 1 and 2
                    (2) Transformers and Similar Equipment The windings                       503.125 Motors and Generators — Class III, Divisions
                    and terminal connections of transformers, choke coils, and                        1 and 2
                    similar equipment shall be provided with tight metal enclo-               503.128 Ventilating Piping — Class III, Divisions 1
                    sures without ventilating openings.                                               and 2
                                                                                              503.130 Luminaires (Lighting Fixtures) — Class III,
                    (3) Resistors and Similar Equipment Resistors, resis-                             Divisions 1 and 2
                    tance devices, thermionic tubes, rectifiers, and similar equip-               (A) Fixed Lighting
                    ment shall comply with 502.130(A)(3).                                         (B) Physical Damage
                                                                                                  (C) Pendant Luminaires (Fixtures)
                    Exception: Enclosures for thermionic tubes, nonadjustable
                                                                                                  (D) Portable Lighting Equipment
                    resistors, or rectifiers for which maximum operating temper-
                                                                                              503.135 Utilization Equipment — Class III, Divisions
                    ature will not exceed 120 C (248 F) shall be permitted to
                                                                                                      1 and 2
                    be of the general-purpose type.
                                                                                                  (A) Heaters
                    (4) Rotating Machinery Motors, generators, and other ro-                      (B) Motors
                    tating electric machinery shall comply with 502.125(B).                       (C) Switches, Circuit Breakers, Motor Controllers,
                                                                                                                                                                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                      and Fuses
                    (5) Wiring Methods The wiring method shall comply with                    503.140 Flexible Cords — Class III, Divisions 1 and 2
                    502.10(B).

                 708                                                                                                        2005 National Electrical Code Handbook

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                                                                                 Article 503 — Class III Locations                                                             503.10



                                                                                                             Fibers or flyings are hazardous not only because they
                                   503.145 Receptacles and Attachment Plugs — Class                     are easily ignited, but also because flames spread through
                                           III, Divisions 1 and 2                                       them quickly. Such fires travel with a rapidity approaching
                                   503.150 Signaling, Alarm, Remote-Control, and Local                  an explosion and are commonly called flash fires.
                                           Loudspeaker Intercommunications Systems                           Class III, Division 1 applies to locations where material
                                           — Class III, Divisions 1 and 2                               is handled, manufactured, or used. Division 2 applies to
                                   503.155 Electric Cranes, Hoists, and Similar                         locations where material is stored or handled but where
                                           Equipment — Class III, Divisions 1 and 2                     no manufacturing processes are performed. Unlike Class I
                                       (A) Power Supply                                                 locations (Groups A, B, C, and D) and Class II locations
                                       (B) Contact Conductors                                           (Groups E, F, and G), there are no material group designa-
                                       (C) Current Collectors
                                                                                                        tions in Class III locations.
                                       (D) Control Equipment
                                   503.160 Storage Battery Charging Equipment — Class
                                           III, Divisions 1 and 2                                       II. Wiring
                                                                                                        503.10 Wiring Methods




                                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           I. General                                                                   Wiring methods shall comply with 503.10(A) or 503.10(B).
                           503.1 Scope                                                                  (A) Class III, Division 1 In Class III, Division 1 locations,
                           Article 503 covers the requirements for electrical and elec-                 the wiring method shall be rigid metal conduit, rigid nonme-
                           tronic equipment and wiring for all voltages in Class III,                   tallic conduit, intermediate metal conduit, electrical metallic
                           Division 1 and 2 locations where fire or explosion hazards                   tubing, dusttight wireways, or Type MC or MI cable with
                           may exist due to ignitible fibers or flyings.                                listed termination fittings.
                                                                                                        (1) Boxes and Fittings All boxes and fittings shall be dust-
                           503.5 General                                                                tight.
                           The general rules of this Code shall apply to electric wiring
                           and equipment in locations classified as Class III locations                 (2) Flexible Connections Where necessary to employ flex-
                           in 500.5(D).                                                                 ible connections, dusttight flexible connectors, liquidtight
                                                                                                        flexible metal conduit with listed fittings, liquidtight flexible
                           Exception: As modified by this article.                                      nonmetallic conduit with listed fittings, or flexible cord in
                                                                                                        conformance with 503.140 shall be used.
                                Equipment installed in Class III locations shall be able
                           to function at full rating without developing surface tempera-                      FPN: See 503.30(B) for grounding requirements where
                           tures high enough to cause excessive dehydration or gradual                         flexible conduit is used.
                           carbonization of accumulated fibers or flyings. Organic ma-                  (3) Nonincendive Field Wiring Nonincendive field wiring
                           terial that is carbonized or excessively dry is highly suscepti-             shall be permitted using any of the wiring methods permitted
                           ble to spontaneous ignition. The maximum surface                             for unclassified locations. Nonincendive field wiring sys-
                           temperatures under operating conditions shall not exceed                     tems shall be installed in accordance with the control draw-
                           165 C (329 F) for equipment that is not subject to overload-                 ing(s). Simple apparatus, not shown on the control drawing,
                           ing, and 120 C (248 F) for equipment (such as motors or                      shall be permitted in a nonincendive field wiring circuit,
                           power transformers) that may be overloaded.                                  provided the simple apparatus does not interconnect the
                                   FPN: For electric trucks, see NFPA 505-2002, Fire                    nonincendive field wiring circuit to any other circuit.
                                   Safety Standard for Powered Industrial Trucks Including
                                   Type Designations, Areas of Use, Conversions, Mainte-                This section was added to the 2005 Code regarding nonin-
                                   nance, and Operation.
                                                                                                        cendive field wiring. The requirements for nonincendive
                                                                                                        field wiring for Class I and Class II were modified and
                           Class III locations usually include textile mills that process               expanded for the 2002 Code, but they were not included
                           cotton, rayon, and so on, where easily ignitible fibers or                   for Class III locations. Section 503.10(A)(3) ensures that
                           combustible flyings are present in the manufacturing pro-                    nonincendive field wiring is permitted in Class III locations.
                           cess. Sawmills and other woodworking plants, where saw-
                           dust, wood shavings, and combustible fibers or flyings are
                           present, may also become hazardous. If wood flour (dust)                            FPN: Simple apparatus is defined in 504.2.
                           is also present, the location is a Class II, Group G location,
                           not a Class III location.                                                         Separate nonincendive field wiring circuits shall be in-
                                                                                                        stalled in accordance with one of the following:

                           National Electrical Code Handbook 2005                                                                                                                 709

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                 503.25                                                     Article 503 — Class III Locations



                    (1) In separate cables                                                      For information regarding bonding in hazardous (classified)
                    (2) In multiconductor cables where the conductors of each                   locations, see the commentary on 501.30(A). Although that
                        circuit are within a grounded metal shield                              section addresses bonding in Class I locations, the same
                    (3) In multiconductor cables where the conductors of each                   requirement for enhanced bonding in Class III locations is
                        circuit have insulation with a minimum thickness of                     found in 502.30(A), and the requirements of 250.100 apply
                        0.25 mm (0.01 in.)                                                      to Class I, Class II, and Class III locations.
                    (B) Class III, Division 2 In Class III, Division 2 locations,
                    the wiring method shall comply with 503.10(A).
                                                                                                Exception: The specific bonding means shall only be re-
                    Exception: In sections, compartments, or areas used solely                  quired to the nearest point where the grounded circuit con-
                    for storage and containing no machinery, open wiring on                     ductor and the grounding electrode conductor are connected
                    insulators shall be permitted where installed in accordance                 together on the line side of the building or structure discon-
                    with Article 398, but only on condition that protection as                  necting means as specified in 250.32(A), (B), and (C), if the
                    required by 398.15(C) be provided where conductors are                      branch-circuit overcurrent protection is located on the load
                    not run in roof spaces and are well out of reach of sources                 side of the disconnecting means.
                    of physical damage.                                                                FPN: See 250.100 for additional bonding requirements
                                                                                                       in hazardous (classified) locations.
                    503.25 Uninsulated Exposed Parts, Class III,
                                                                                                (B) Types of Equipment Grounding Conductors Where
                    Divisions 1 and 2
                                                                                                flexible conduit is used as permitted in 503.10, it shall be
                    There shall be no uninsulated exposed parts, such as electric               installed with internal or external bonding jumpers in parallel
                    conductors, buses, terminals, or components, that operate at                with each conduit and complying with 250.102.
                    more than 30 volts (15 volts in wet locations). These parts
                    shall additionally be protected by a protection technique                   Exception: In Class III, Division 1 and 2 locations, the
                    according to 500.7(E), 500.7(F), or 500.7(G) that is suitable               bonding jumper shall be permitted to be deleted where all
                    for the location.                                                           of the following conditions are met:

                    Exception: As provided in 503.155.                                          (1) Listed liquidtight flexible metal 1.8 m (6 ft) or less in
                                                                                                    length, with fittings listed for grounding, is used.
                                                                                                (2) Overcurrent protection in the circuit is limited to 10
                    Exposed live parts are allowed in Class III, Division 1 and
                                                                                                    amperes or less.
                    2 locations provided that the voltage does not exceed 30
                                                                                                (3) The load is not a power utilization load.
                    volts in dry locations or 15 volts in wet locations. However,
                    caution must be observed in working around uninsulated
                    exposed parts because tool or other conductive items that
                                                                                                III. Equipment
                    come in contact with the circuit could produce sparks and                   503.100 Transformers and Capacitors — Class
                    be a source of ignition in a hazardous (classified) location.               III, Divisions 1 and 2
                                                                                                Transformers and capacitors shall comply with 502.100(B).
                    503.30 Grounding and Bonding — Class III,                                   503.115 Switches, Circuit Breakers, Motor
                    Divisions 1 and 2                                                           Controllers, and Fuses — Class III,
                    Wiring and equipment in Class III, Division 1 and 2 locations               Divisions 1 and 2
                    shall be grounded as specified in Article 250 and with the
                                                                                                Switches, circuit breakers, motor controllers, and fuses, in-
                    following additional requirements in 503.30(A) and
                                                                                                cluding pushbuttons, relays, and similar devices, shall be
                    503.30(B).
                                                                                                provided with dusttight enclosures.
                    (A) Bonding The locknut-bushing and double-locknut
                    types of contacts shall not be depended on for bonding                      See the definition of dusttight in Article 100.
                    purposes, but bonding jumpers with proper fittings or other
                    approved means of bonding shall be used. Such means of
                    bonding shall apply to all intervening raceways, fittings,                  503.120 Control Transformers and Resistors —
                    boxes, enclosures, and so forth, between Class III locations                Class III, Divisions 1 and 2
                    and the point of grounding for service equipment or point                   Transformers, impedance coils, and resistors used as or in
                    of grounding of a separately derived system.                                conjunction with control equipment for motors, generators,



                 710                                                                                                               2005 National Electrical Code Handbook
                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---

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                                                                             Article 503 — Class III Locations                                                                503.135



                           and appliances shall be provided with dusttight enclosures                  quantities of fibers or flyings into the ventilated equipment
                           complying with the temperature limitations in 503.5.                        or enclosure and to prevent the escape of sparks, flame, or
                                                                                                       burning material that might ignite accumulations of fibers
                           See the definition of dusttight in Article 100.                             or flyings or combustible material in the vicinity. For metal
                                                                                                       pipes, lock seams and riveted or welded joints shall be
                                                                                                       permitted; and tight-fitting slip joints shall be permitted
                                                                                                       where some flexibility is necessary, as at connections to
                           503.125 Motors and Generators — Class III,                                  motors.
                           Divisions 1 and 2
                                                                                                       503.130 Luminaires (Lighting Fixtures) — Class
                           In Class III, Divisions 1 and 2 locations, motors, generators,              III, Divisions 1 and 2
                           and other rotating machinery shall be totally enclosed non-
                                                                                                       (A) Fixed Lighting Luminaires (lighting fixtures) for fixed
                           ventilated, totally enclosed pipe ventilated, or totally en-
                                                                                                       lighting shall provide enclosures for lamps and lampholders
                           closed fan cooled.
                                                                                                       that are designed to minimize entrance of fibers and flyings
                           Exception: In locations where, in the judgment of the au-                   and to prevent the escape of sparks, burning material, or hot
                           thority having jurisdiction, only moderate accumulations of                 metal. Each luminaire (fixture) shall be clearly marked to
                           lint or flyings are likely to collect on, in, or in the vicinity            show the maximum wattage of the lamps that shall be permit-
                           of a rotating electric machine and where such machine is                    ted without exceeding an exposed surface temperature of
                           readily accessible for routine cleaning and maintenance,                    165 C (329 F) under normal conditions of use.
                           one of the following shall be permitted:
                                                                                                       (B) Physical Damage A luminaire (fixture) that may be
                           (1) Self-cleaning textile motors of the squirrel-cage type                  exposed to physical damage shall be protected by a suitable
                           (2) Standard open-type machines without sliding contacts,                   guard.
                               centrifugal or other types of switching mechanisms, in-                 (C) Pendant Luminaires (Fixtures) Pendant luminaires
                               cluding motor overload devices                                          (fixtures) shall be suspended by stems of threaded rigid
                           (3) Standard open-type machines having such contacts,                       metal conduit, threaded intermediate metal conduit, threaded
                               switching mechanisms, or resistance devices enclosed                    metal tubing of equivalent thickness, or by chains with ap-
                               within tight housings without ventilating or other open-                proved fittings. For stems longer than 300 mm (12 in.),
                               ings                                                                    permanent and effective bracing against lateral displacement
                                                                                                       shall be provided at a level not more than 300 mm (12 in.)
                           It is intended that the phrase ‘‘other rotating machinery’’ in              above the lower end of the stem, or flexibility in the form
                           503.125 include electric brakes. Listed and labeled electric                of an identified fitting or a flexible connector shall be pro-
                           brakes are available for Class II, Group G locations, and,                  vided not more than 300 mm (12 in.) from the point of
                           according to the UL Hazardous Location Equipment Direc-                     attachment to the supporting box or fitting.
                           tory, these brakes are suitable for Class III locations.
                                                                                                       (D) Portable Lighting Equipment Portable lighting
                                                                                                       equipment shall be equipped with handles and protected
                                                                                                       with substantial guards. Lampholders shall be of the un-
                           503.128 Ventilating Piping — Class III,                                     switched type with no provision for receiving attachment
                                                                                                       plugs. There shall be no exposed current-carrying metal
                           Divisions 1 and 2
                                                                                                       parts, and all exposed non–current-carrying metal parts shall
                           Ventilating pipes for motors, generators, or other rotating                 be grounded. In all other respects, portable lighting equip-
                           electric machinery, or for enclosures for electric equipment,               ment shall comply with 503.130(A).
                           shall be of metal not less than 0.53 mm (0.021 in.) in thick-
                           ness, or of equally substantial noncombustible material, and                503.135 Utilization Equipment — Class III,
                           shall comply with the following:                                            Divisions 1 and 2
                           (1) Lead directly to a source of clean air outside of buildings             (A) Heaters Electrically heated utilization equipment shall
                           (2) Be screened at the outer ends to prevent the entrance                   be identified for Class III locations.
                               of small animals or birds                                               (B) Motors Motors of motor-driven utilization equipment
                           (3) Be protected against physical damage and against rust-                  shall comply with 503.125.
                                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                               ing or other corrosive influences
                                                                                                       (C) Switches, Circuit Breakers, Motor Controllers, and
                                Ventilating pipes shall be sufficiently tight, including               Fuses Switches, circuit breakers, motor controllers, and
                           their connections, to prevent the entrance of appreciable                   fuses shall comply with 503.115.



                           National Electrical Code Handbook 2005                                                                                                                 711

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                 503.140                                             Article 503 — Class III Locations



                    503.140 Flexible Cords — Class III,                                                the contact conductors in case of a fault to ground or gives
                    Divisions 1 and 2                                                                  a visual and audible alarm as long as power is supplied to
                    Flexible cords shall comply with the following:                                    the contact conductors and the ground fault remains.

                    (1) Be of a type listed for extra-hard usage                                       (B) Contact Conductors Contact conductors shall be lo-
                    (2) Contain, in addition to the conductors of the circuit, a                       cated or guarded so as to be inaccessible to other than author-
                        grounding conductor complying with 400.23                                      ized persons and shall be protected against accidental contact
                    (3) Be connected to terminals or to supply conductors in                           with foreign objects.
                        an approved manner                                                             (C) Current Collectors Current collectors shall be ar-
                    (4) Be supported by clamps or other suitable means in such                         ranged or guarded so as to confine normal sparking and
                        a manner that there will be no tension on the terminal                         prevent escape of sparks or hot particles. To reduce sparking,
                        connections                                                                    two or more separate surfaces of contact shall be provided
                    (5) Be provided with suitable means to prevent the entrance                        for each contact conductor. Reliable means shall be provided
                        of fibers or flyings where the cord enters boxes or                            to keep contact conductors and current collectors free of
                        fittings                                                                       accumulations of lint or flyings.

                    503.145 Receptacles and Attachment Plugs —                                         (D) Control Equipment Control equipment shall comply
                                                                                                       with 503.115 and 503.120.
                    Class III, Divisions 1 and 2
                    Receptacles and attachment plugs shall be of the grounding
                                                                                                       In a Class III location, cranes that are installed over accumu-
                    type and shall be designed so as to minimize the accumula-
                                                                                                       lations of fibers or flyings and equipped with rolling or
                    tion or the entry of fibers or flyings, and shall prevent the
                                                                                                       sliding collectors that make contact with bare conductors
                    escape of sparks or molten particles.
                                                                                                       introduce two hazards.
                    Exception: In locations where, in the judgment of the au-                                The first hazard results from any arcing between a con-
                    thority having jurisdiction, only moderate accumulations of                        ductor and a collector rail igniting combustible fibers or lint
                    lint or flyings will be likely to collect in the vicinity of a                     that has accumulated on or near the bare conductor. This
                    receptacle, and where such receptacle is readily accessible                        hazard may be prevented by maintaining the proper align-
                    for routine cleaning, general-purpose grounding-type recep-                        ment of the bare conductor, by using a collector designed
                    tacles mounted so as to minimize the entry of fibers or flyings                    so that proper contact is always maintained, and by using
                    shall be permitted.                                                                guards or shields to confine hot metal particles that result
                                                                                                       from arcing.
                    503.150 Signaling, Alarm, Remote-Control, and                                            The second hazard occurs if enough moisture is present
                    Local Loudspeaker Intercommunications                                              and fibers and flyings accumulating on the insulating sup-
                    Systems — Class III, Divisions 1 and 2                                             ports of the bare conductors form a conductive path between
                                                                                                       the conductors or from one conductor to ground, permitting
                    Signaling, alarm, remote-control, and local loudspeaker in-                        enough current to flow to ignite the fibers. If the system is
                    tercommunications systems shall comply with the require-                           ungrounded, a current flow to ground is unlikely to start a
                    ments of Article 503 regarding wiring methods, switches,                           fire.
                    transformers, resistors, motors, luminaires (lighting fix-                               A suitable recording ground detector sounds an alarm
                    tures), and related components.                                                    and automatically de-energizes contact conductors when the
                                                                                                       insulation resistance is lowered by an accumulation of fibers
                    503.155 Electric Cranes, Hoists, and Similar                                       on the insulators or in case of a fault to ground. A ground-
                    Equipment — Class III, Divisions 1 and 2                                           fault indicator is permitted that maintains an alarm until the
                    Where installed for operation over combustible fibers or                           system is de-energized or the ground fault is cleared.
                    accumulations of flyings, traveling cranes and hoists for
                    material handling, traveling cleaners for textile machinery,
                                                                                                       503.160 Storage Battery Charging Equipment
                    and similar equipment shall comply with 503.155(A)
                    through (D).
                                                                                                       — Class III, Divisions 1 and 2
                                                                                                       Storage battery charging equipment shall be located in sepa-
                    (A) Power Supply Power supply to contact conductors                                rate rooms built or lined with substantial noncombustible
                    shall be electrically isolated from all other systems, un-                         materials. The rooms shall be constructed to prevent the
                    grounded, and shall be equipped with an acceptable ground                          entrance of ignitible amounts of flyings or lint and shall be
                    detector that gives an alarm and automatically de-energizes                        well ventilated.



                 712                                                                                                                     2005 National Electrical Code Handbook
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                                                                          Article 504 — Intrinsically Safe Systems                                                             504.2



                                              ARTICLE 504                                               were in ANSI/ISA RP 12.6-03, Recommended Practice for
                                                                                                        Wiring Methods for Hazardous (Classified) Locations In-
                                        Intrinsically Safe Systems                                      strumentation, Part 1: Intrinsic Safety.

                           Summary of Changes                                                           504.2 Definitions
                           • 504.10(B): Revised to add requirements for the installa-
                             tion of simple apparatus in a hazardous (classified) loca-                 The standard used in the United States for construction and
                             tion.                                                                      performance requirements for intrinsically safe systems is
                           • 504.30(B)(3): Added requirement for spacing between                        ANSI/UL 913, Intrinsically Safe Apparatus and Associated
                             terminals as a means to provide separation between differ-                 Apparatus for Use in Class I, II, and III, Division 1, Hazard-
                             ent intrinsically safe circuits.                                           ous (Classified) Locations. ANSI/UL 913 is similar to stan-
                                                                                                        dards for intrinsically safe equipment in other countries; all
                                                                                                        standards are based on the IEC (International Electrotechni-
                                                                                                        cal Commission) standard. ANSI/UL 60079-11, Electrical
                           Contents                                                                     Apparatus for Explosive Gas Atmospheres — Part II: Intrin-
                            504.1 Scope                                                                 sic Safety ‘‘i’’ is based on the IEC 60079-11 standard and also
                            504.2 Definitions                                                           contains the U.S. deviations that allow it to be compatible for
                            504.3 Application of Other Articles                                         installations in the United States. The NEC offers the choice
                            504.4 Equipment                                                             of designating hazardous (classified) locations as two divi-
                           504.10 Equipment Installation                                                sions (1 and 2) or three zones (0, 1, and 2). ANSI/UL
                              (A) Control Drawing                                                       913 requirements, however, are based on the IEC Zone
                              (B) Location                                                              0 requirements, which are the most stringent. Equipment
                           504.20 Wiring Methods                                                        certified by a testing laboratory for Zone 1 would not neces-
                           504.30 Separation of Intrinsically Safe Conductors                           sarily meet ANSI/UL 913 requirements for Division 1.
                              (A) From Nonintrinsically Safe Circuit Conductors
                              (B) From Different Intrinsically Safe Circuit Conductors                  Associated Apparatus. Apparatus in which the circuits are
                           504.50 Grounding                                                             not necessarily intrinsically safe themselves but that affect
                              (A) Intrinsically Safe Apparatus, Associated Apparatus,                   the energy in the intrinsically safe circuits and are relied on
                                  and Raceways                                                          to maintain intrinsic safety. Associated apparatus may be
                              (B) Connection to Grounding Electrodes                                    either of the following:
                              (C) Shields
                           504.60 Bonding                                                               (1) Electrical apparatus that has an alternative-type protec-
                              (A) Hazardous Locations                                                       tion for use in the appropriate hazardous (classified)
                              (B) Unclassified                                                              location
                           504.70 Sealing                                                               (2) Electrical apparatus not so protected that shall not be
                           504.80 Identification                                                            used within a hazardous (classified) location
                              (A) Terminals                                                                       FPN No. 1: Associated apparatus has identified intrinsi-
                              (B) Wiring                                                                          cally safe connections for intrinsically safe apparatus
                                                                                                                  and also may have connections for nonintrinsically safe
                              (C) Color Coding                                                                    apparatus.
                                                                                                                  FPN No. 2: An example of associated apparatus is an
                           504.1 Scope                                                                            intrinsic safety barrier, which is a network designed to
                                                                                                                  limit the energy (voltage and current) available to the
                           This article covers the installation of intrinsically safe (I.S.)                      protected circuit in the hazardous (classified) location,
                           apparatus, wiring, and systems for Class I, II, and III loca-                          under specified fault conditions.
                           tions.
                                   FPN: For further information, see ANSI/ISA RP                        For an illustration of an intrinsic safety barrier, see Exhibit
                                   12.06.01-2002, Wiring Methods for Hazardous (Classi-                 504.1.
                                   fied) Locations Instrumentation — Part 1: Intrinsic
                                   Safety.
                                                                                                        Control Drawing. See definition in 500.2.
                           Article 504 was first included in the 1990 NEC. Previously,                  Different Intrinsically Safe Circuits. Intrinsically safe cir-
                           the installation requirements for intrinsically safe systems                 cuits in which the possible interconnections have not been
                                                                                                        evaluated and identified as intrinsically safe.



                           National Electrical Code Handbook 2005                                                                                                               713
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                 504.3                                                 Article 504 — Intrinsically Safe Systems



                                                                                               volts and currents in the microampere range, they are notnec-
                                                                                               essarily intrinsically safe. Low-energy devices are normally
                                                                                               connected to amplifiers and power supplies that are con-
                                                                                               nected to 120-volt or higher circuits. Should a fault occur
                                                                                               within the amplifier or power supply, or a voltage surge
                                                                                               occur in the electrical supply system, high-energy arcing,
                                                                                               sparking, or overheating of the low-energy portion of the
                                                                                               circuit could occur.


                                                                                                      FPN: An intrinsically safe system may include more
                                                                                                      than one intrinsically safe circuit.
                                                                                               Simple Apparatus. An electrical component or combina-
                                                                                               tion of components of simple construction with well-defined
                                                                                               electrical parameters that does not generate more than 1.5
                                                                                               volts, 100 milliamps, and 25 milliwatts, or a passive compo-
                                                                                               nent that does not dissipate more than 1.3 watts and is
                                                                                               compatible with the intrinsic safety of the circuit in which
                                                                                               it is used.
                    Exhibit 504.1 A typical intrinsic safety barrier that limits the en-
                    ergy available to the hazardous location. (Courtesy of Cooper                     FPN: The following apparatus are examples of simple
                    Crouse-Hinds)                                                                     apparatus:
                                                                                                           (a) Passive components, for example, switches,
                    Intrinsically Safe Apparatus. Apparatus in which all the                          junction boxes, resistance temperature devices, and sim-
                    circuits are intrinsically safe.                                                  ple semiconductor devices such as LEDs
                                                                                                           (b) Sources of generated energy, for example, ther-
                    Intrinsically Safe Circuit. A circuit in which any spark or                       mocouples and photocells, which do not generate more
                    thermal effect is incapable of causing ignition of a mixture                      than 1.5 V, 100 mA, and 25 mW
                    of flammable or combustible material in air under prescribed
                    test conditions.
                                                                                               The definition of simple apparatus clarifies the use of the
                            FPN: Test conditions are described in ANSI/UL 913-                 term in 504.4, Exception, and 504.10, Exception. The intent
                            1997, Standard for Safety, Intrinsically Safe Apparatus            is to permit the use of apparatus that stores little or no energy
                            and Associated Apparatus for Use in Class I, II, and III,          without requiring the apparatus to be listed or to comply
                            Division 1, Hazardous (Classified) Locations.
                                                                                               with the control drawing. See the fine print note following
                                                                                               the definition of simple apparatus for examples of simple
                    Due to its physical and electrical characteristics, an intrinsi-           apparatus.
                    cally safe circuit does not develop sufficient electrical energy
                    (millijoules) in an arc or spark to cause ignition or sufficient
                    thermal energy resulting from an overload condition to cause               504.3 Application of Other Articles
                    the temperature of the installed circuit to exceed the ignition            Except as modified by this article, all applicable articles of
                    temperature of a specified gas or vapor under normal or                    this Code shall apply.
                    abnormal operating conditions.
                         An abnormal condition may be due to accidental dam-                   Because intrinsically safe wiring must be low-energy wiring
                    age, failure of electrical components, excessive voltage, or               to be intrinsically safe, the wiring itself is most likely to be
                    improper adjustment or maintenance of the equipment.                       Class 2, in accordance with 725.41, or, in a fire-protective
                                                                                               signaling system, power-limited in accordance with 760.41.
                    Intrinsically Safe System. An assembly of interconnected                   See Article 725 or 760, as appropriate, for the requirements
                    intrinsically safe apparatus, associated apparatus, and inter-             for such wiring. The installation may also fall under the
                    connecting cables in that those parts of the system that may               scope of Article 800. The intrinsically safe apparatus and
                                                                                                                                                                             --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    be used in hazardous (classified) locations are intrinsically              associated apparatus, on the other hand, may be supplied by
                    safe circuits.                                                             ordinary power circuits, in which case other Code rules
                                                                                               may apply. It is common for intrinsically safe apparatus or
                    Although low-energy devices, such as thermocouples, crys-                  associated apparatus supplied by a power circuit to be located
                    tal stain transducers, or pressure transducers, generate milli-            in a hazardous (classified) location, with the apparatus pro-


                 714                                                                                                              2005 National Electrical Code Handbook

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                                                                          Article 504 — Intrinsically Safe Systems                                                            504.20



                           tected by one of the protection systems required by Articles                Section 504.10(B) in the 2005 Code has been expanded to
                           500 through 503, 505, or 506, that is, explosionproof, dust-                provide guidance for designers, installers, and inspectors
                           ignitionproof, purged, and pressurized. In this case, Articles              pertaining to simple apparatus installed in hazardous (classi-
                           500 through 503, 505, and 506 also apply. Also, intrinsically               fied) locations. See the definition of simple apparatus and
                           safe systems are not exempt from the grounding and bonding                  the accompanying fine print note in 504.2. Table 504.10(B)
                           requirements of 501.30, 502.30, 503.30, and 505.25.                         provides the surface temperature allowed for T4 classifica-
                                                                                                       tion based on the total surface area of the equipment.
                           504.4 Equipment
                                                                                                       (1) Reference to Table 504.10(B)
                           All intrinsically safe apparatus and associated apparatus shall
                                                                                                       (2) Calculation using the formula:
                           be listed.
                                                                                                                             T = PoRth     Tamb
                           Exception: Simple apparatus, as described on the control                         where:
                           drawing, shall not be required to be listed.                                     T is the surface temperature
                                                                                                            Po is the output power marked on the associated appara-
                           504.10 Equipment Installation                                               tus or intrinsically safe apparatus
                           (A) Control Drawing Intrinsically safe apparatus, associ-                        Rth is the thermal resistance of the simple apparatus
                           ated apparatus, and other equipment shall be installed in                        Tamb is the ambient temperature (normally 40 C) and
                           accordance with the control drawing(s).                                     reference Table 500.8(B)
                           Exception: A simple apparatus that does not interconnect                    Table 504.10(B) Assessment for T4 Classification According
                           intrinsically safe circuits.                                                to Component Size and Temperature
                                   FPN: The control drawing identification is marked on
                                   the apparatus.                                                                                                 Requirement for T4 Classification
                                                                                                        Total Surface Area                            (Based on 40 C Ambient
                                                                                                       Excluding Lead Wires                                Temperature)
                           The control drawing may put limitations on cables and on
                           the separation of circuits in an intrinsically safe system.                         <20 mm2                                 Surface temperature 275 C
                           The control drawing also illustrates what is permitted to be                     20 mm2 10 cm2                              Surface temperature 200 C
                                                                                                                20 mm2                                 Power not exceeding 1.3 W*
                           connected in the system. Compliance with all the provisions
                           in the control drawing is essential if intrinsic safety is to be            *Reduce to 1.2 W with an ambient of 60 C or 1.0 W with 80 C ambient
                           maintained. The investigation of the equipment by third-                    temperature.
                           party testing laboratories is based on installation in accor-
                           dance with the control drawing.                                                  In addition, components with a surface area smaller
                                                                                                       than 10 cm2 (excluding lead wires) may be classified as T5
                                                                                                       if their surface temperature does not exceed 150 C.
                           (B) Location Intrinsically safe apparatus shall be permitted                       FPN: The following apparatus are examples of simple




                                                                                                                                                                                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           to be installed in any hazardous (classified) location for                         apparatus:
                           which it has been identified. General-purpose enclosures                           (1) Passive components, for example, switches, junction
                           shall be permitted for intrinsically safe apparatus.                                   boxes, resistance temperature devices, and simple
                                Associated apparatus shall be permitted to be installed                           semiconductor devices such as LEDs
                                                                                                              (2) Sources of generated energy, for example, thermo-
                           in any hazardous (classified) location for which it has been                           couples and photocells, which do not generate more
                           identified or, if protected by other means, permitted by Arti-                         than 1.5 V, 100 mA, and 25 mW
                           cles 501 through 503 and Article 505.
                                Simple apparatus shall be permitted to be installed in                 504.20 Wiring Methods
                           any hazardous (classified) location in which the maximum                    Intrinsically safe apparatus and wiring shall be permitted to
                           surface temperature of the simple apparatus does not exceed                 be installed using any of the wiring methods suitable for
                           the ignition temperature of the flammable gases or vapors,                  unclassified locations, including Chapter 7 and Chapter 8.
                           flammable liquids, combustible dusts, or ignitible fibers or                Sealing shall be as provided in 504.70, and separation shall
                           flyings present.                                                            be as provided in 504.30.
                                For simple apparatus, the maximum surface temperature
                           can be determined from the values of the output power from                  See the commentary following 504.3 for more information
                           the associated apparatus or apparatus to which it is connected              on the types of circuits typically used as part of an intrinsi-
                           to obtain the temperature class. The temperature class can                  cally safe system.
                           be determined by:

                           National Electrical Code Handbook 2005                                                                                                                   715

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                 504.30                                                                                               Article 504 — Intrinsically Safe Systems



                                                                    504.30 Separation of Intrinsically                                        fault. Because low-voltage, low-energy but nonintrinsically
                                                                    Safe Conductors                                                           safe circuits are permitted by Articles 725 and 800 to be
                                                                    (A) From Nonintrinsically Safe Circuit Conductors                         minimally insulated and may not be protected by being
                                                                                                                                              installed in raceways or cables, particularly in nonhazardous
                                                                    (1) In Raceways, Cable Trays, and Cables Conductors                       locations, it is essential that nonintrinsically safe circuits
                                                                    of intrinsically safe circuits shall not be placed in any race-           and intrinsically safe circuits be physically and electrically
                                                                    way, cable tray, or cable with conductors of any nonintrinsi-             separated.
                                                                    cally safe circuit.
                                                                    Exception No. 1: Where conductors of intrinsically safe cir-              (3) Other (Not in Raceway or Cable Tray Systems) Con-
                                                                    cuits are separated from conductors of nonintrinsically safe              ductors and cables of intrinsically safe circuits run in other
                                                                    circuits by a distance of at least 50 mm (2 in.) and secured,             than raceway or cable tray systems shall be separated by at
                                                                    or by a grounded metal partition or an approved insulating                least 50 mm (2 in.) and secured from conductors and cables
                                                                    partition.                                                                of any nonintrinsically safe circuits.
                                                                        FPN: No. 20 gauge sheet metal partitions 0.91 mm
                                                                        (0.0359 in.) or thicker are generally considered accept-
                                                                        able.                                                                 The provisions of 504.30(A)(3) provide separation require-
                                                                                                                                              ments for intrinsically safe circuit conductors and cables
                                                                    Exception No. 2: Where either (1) all of the intrinsically
                                                                                                                                              that are installed using a wiring method other than a raceway
                                                                    safe circuit conductors or (2) all of the nonintrinsically safe
                                                                                                                                              or a support system other than cable tray.
                                                                    circuit conductors are in grounded metal-sheathed or metal-
                  --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                    clad cables where the sheathing or cladding is capable of
                                                                                                                                              Exception: Where either (1) all of the intrinsically safe cir-
                                                                    carrying fault current to ground.
                                                                                                                                              cuit conductors are in Type MI or MC cables or (2) all of
                                                                        FPN: Cables meeting the requirements of Articles 330                  the nonintrinsically safe circuit conductors are in raceways
                                                                        and 332 are typical of those considered acceptable.
                                                                                                                                              or Type MI or MC cables where the sheathing or cladding
                                                                                                                                              is capable of carrying fault current to ground.
                                                                    Type MI cable with a copper sheath and Type MC cable of
                                                                    smooth metallic sheath or corrugated metallic sheath con-                 (B) From Different Intrinsically Safe Circuit Conduc-
                                                                    struction meet the conditions prescribed in 504.30(A)(1),                 tors Different intrinsically safe circuits shall be in separate
                                                                    Exception No. 2. The metallic sheath of interlocked-tape                  cables or shall be separated from each other by one of the
                                                                    Type MC cable (as opposed to the smooth or corrugated                     following means:
                                                                    continuous outer sheath) is generally not investigated for                (1) The conductors of each circuit are within a grounded
                                                                    suitability as an equipment grounding conductor and, there-                   metal shield.
                                                                    fore, may not be capable of carrying a fault current to ground            (2) The conductors of each circuit have insulation with a
                                                                    as required by Exception No. 2.                                               minimum thickness of 0.25 mm (0.01 in.).
                                                                                                                                              Exception: Unless otherwise identified.
                                                                    (2) Within Enclosures                                                     (3) The clearance between two terminals for connection of
                                                                    (1) Conductors of intrinsically safe circuits shall be sepa-                  field wiring of different intrinsically safe circuits shall
                                                                        rated at least 50 mm (2 in.) from conductors of any                       be at least 6 mm (0.25 in.) unless this clearance is
                                                                        nonintrinsically safe circuits, or as specified in                        permitted to be reduced by the control drawing.
                                                                        504.30(A)(2).
                                                                    (2) All conductors shall be secured so that any conductor
                                                                                                                                              New in the 2005 NEC, terminals for different intrinsically
                                                                        that might come loose from a terminal cannot come in
                                                                                                                                              safe circuits are to have at least a 1⁄4- (0.25-) inch separation.
                                                                        contact with another terminal.
                                                                                                                                              This separation can be reduced if shown in the control draw-
                                                                        FPN No. 1: The use of separate wiring compartments for                ing for the intrinsically safe circuits that are connected to
                                                                        the intrinsically safe and nonintrinsically safe terminals is
                                                                        the preferred method of complying with this requirement.              the adjacent terminals. The minimum clearance requirement
                                                                        FPN No. 2: Physical barriers such as grounded metal                   provides a safeguard against an inadvertent connection be-
                                                                        partitions or approved insulating partitions or approved              tween adjacent terminals (with different IS circuits) that
                                                                        restricted access wiring ducts separated from other such              could occur during maintenance or connection of a new circuit
                                                                        ducts by at least 19 mm (3⁄4 in.) can be used to help                 to an existing terminal block for intrinsically safe circuits.
                                                                        ensure the required separation of the wiring.

                                                                                                                                              504.50 Grounding
                                                                    The intent of 504.30(A) is to prevent the intrusion of unsafe
                                                                    energy into the intrinsically safe system as a result of a wiring         (A) Intrinsically Safe Apparatus, Associated Apparatus,
                                                                                                                                              and Raceways Intrinsically safe apparatus, associated ap-

                 716                                                                                                                                                             2005 National Electrical Code Handbook

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                                                                                  Article 504 — Intrinsically Safe Systems                                                            504.50



                           paratus, cable shields, enclosures, and raceways, if of metal,                           Resistor R2 limits the current in the intrinsically safe
                           shall be grounded.                                                                  circuit to the required value at the output voltage of Z1 and
                                   FPN: Supplementary bonding to the grounding electrode                       Z2. Therefore, even though up to 250 volts ac may be applied
                                   may be needed for some associated apparatus, for exam-                      to the input of the circuit (terminals 1 and 2) as a result of
                                   ple, zener diode barriers, if specified in the control draw-                a fault in the equipment or circuit connected to terminals 1
                                   ing. See ANSI/ISA RP 12.06.01-2002, Wiring Methods
                                                                                                               and 2, the output at terminals 3 and 4 cannot exceed the
                                   for Hazardous (Classified) Locations Instrumentation
                                   Part 1: Intrinsic Safety.                                                   voltage and current permitted by the diodes and resistor.
                                                                                                                    Shunt diode barriers normally have maximum induc-
                                                                                                               tance and capacitance ratings on the intrinsically safe side
                           It is important to maintain a low-impedance path to ground
                                                                                                               because, even though both the voltage and the current are
                           for zener diode barrier systems because such systems shunt
                                                                                                               limited at terminals 3 and 4, too much inductance in the
                           fault currents to ground.
                                                                                                               circuit connected to terminals 3 and 4 could result in the
                                 Exhibit 504.2 illustrates a common type of zener diode
                                                                                                               release of an ignition-capable spark when the circuit is
                           barrier system. F1 limits the duration of the current in Z1 and
                                                                                                               opened. In a like manner, too much capacitance between
                           Z2 to the power rating of the zener diodes under fault condi-
                                                                                                               terminals 3 and 4 could result in the release of an ignition-
                           tions, so that the diodes need not be excessively large. Al-
                                                                                                               capable spark if there were a short-circuit between wiring
                           though the diodes themselves can be considered subject to
                                                                                                               connected to terminals 3 and 4 or between wiring connected
                           open-circuit fault, they are redundant components, and either
                                                                                                               to terminal 3 and ground.
                           one alone provides the necessary protection in the event of a
                                                                                                                    Wiring always has inductance and capacitance associ-
                           first fault, that is, high voltage across input terminals 1 and 2.
                                                                                                               ated with it, depending on the spacing between conductors,
                           Resistor R2 is the current-limiting resistor and has been inves-
                                                                                                               size of conductors, and length of conductors. It is necessary,
                           tigated as a protective component not subject to short-circuit
                                                                                                               therefore, to limit the length of conductors connected to
                           fault. Resistor R1 is used primarily to permit testing to deter-
                                                                                                               terminals 3 and 4, just as it is necessary to limit the induc-
                           mine that the diodes are intact. Terminals 2 and 4 and the ends
                                                                                                               tance and capacitance of connected equipment. The length
                           of the two diodes are connected to a ground bus that, in turn,
                                                                                                               limitation is usually on the order of thousands of feet. The
                           is connected to a ground system to which all grounds in the
                                                                                                               manufacturer’s instructions provide the information on in-
                           intrinsically safe system are connected. A very low imped-
                                                                                                               stallation limitations.
                           ance (1 ohm or less is usually recommended) is necessary
                                                                                                                    By adjusting the values of its components, the barrier
                           so that the voltage level on the ground bus will not be raised
                                                                                                               can be designed for a variety of uses, including different
                           to an unsafe level under high-current fault conditions.
                                                                                                               types of instrument systems and thermocouples. Barriers
                                 A shunt diode barrier system is commonly tested with
                                                                                                               such as described are available commercially from a number
                           250 volts ac across terminals 1 and 2, representing a fault
                                                                                                               of manufacturers.
                           in associated apparatus on the nonhazardous location side
                           of the barrier, which imposes a voltage up to 250 volts ac
                           on these terminals. Diode Z1 conducts at its rated voltage,                         (B) Connection to Grounding Electrodes Where connec-
                           typically 14 or 15 volts for a barrier designed for a 12-volt                       tion to a grounding electrode is required, the grounding
                           (normal operation) system, thus limiting the output voltage                         electrode shall be as specified in 250.52(A)(1), (A)(2),
                           of the circuit to 14 or 15 volts. The voltage at which this                         (A)(3), and (A)(4) and shall comply with 250.30(A)(7). Sec-
                           diode conducts is designed to be higher than the rated input                        tion 250.52(A)(5), (A)(6), and (A)(7) shall not be used if
                           voltage (under no-fault conditions) at terminals 1 and 2. The                       electrodes specified in 250.52(A)(1), (A)(2), (A)(3), or
                           fuse is selected so that it will open before the power rating                       (A)(4) are available.
                           of Z1 is exceeded. Diode Z2 usually conducts within 1 or 2
                           volts of Z1 and serves as a backup to Z1 in the event Z1 fails                      The electrodes specified in 250.52(A)(1) through
                           in an open-circuit condition for any reason.                                        250.52(A)(4) include metal underground water pipes, the
                                                                                                               metal frames of building, concrete-encased electrodes, and
                                                                                                               ground rings. These electrodes usually provide lower resis-
                                                 F1            R1                   R2                         tance grounds than electrodes, such as ground rods and plate
                                  1                                                               3            electrodes, covered in 250.52(A)(5) and 250.52(A)(6).
                                  Nonhazardous           Z1                  Z2      Intrinsically
                                   location side                                      safe side
                                  2                                                               4            (C) Shields Where shielded conductors or cables are used,
                                                                                                               shields shall be grounded.
                           Exhibit 504.2 Fused zener diode barrier.                                            Exception: Where a shield is part of an intrinsically safe
                                                                                                               circuit.


                           National Electrical Code Handbook 2005                                                                                                                        717
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                 504.60                                                                                         Article 505 — Class I, Zone 0, 1, and 2 Locations



                    504.60 Bonding                                                                                                          blue and where no other conductors colored light blue are
                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                    (A) Hazardous Locations In hazardous (classified) loca-                                                                 used. Likewise, color coding shall be permitted to identify
                    tions, intrinsically safe apparatus shall be bonded in the                                                              raceways, cable trays, and junction boxes where they are
                    hazardous (classified) location in accordance with 250.100.                                                             colored light blue and contain only intrinsically safe wiring.

                    (B) Unclassified In unclassified or nonhazardous loca-
                    tions, where metal raceways are used for intrinsically safe                                                                       ARTICLE 505
                    system wiring in hazardous (classified) locations, associated
                    apparatus shall be bonded in accordance with 501.30(A),                                                                 Class I, Zone 0, 1, and 2 Locations
                    502.30(A), 503.30(A), or 505.25, as applicable.

                    504.70 Sealing                                                                                                          Summary of Changes
                    Conduits and cables that are required to be sealed by 501.15,                                                           • 505.8(I): Added requirement that the type of detection
                    502.15, and 505.16 shall be sealed to minimize the passage                                                                equipment, its listing, installation location(s), alarm and
                    of gases, vapors, or dusts. Such seals shall not be required                                                              shutdown criteria, and calibration frequency be docu-
                    to be explosionproof or flameproof.                                                                                       mented when combustible gas detectors are used as a
                                                                                                                                              protection technique.
                    Exception: Seals shall not be required for enclosures that
                    contain only intrinsically safe apparatus, except as required                                                           • 505.9(C)(2): Added two exceptions for associated appara-
                    by 501.15(F)(3).                                                                                                          tus not suitable for hazardous locations and for marking
                                                                                                                                              of simple apparatus.
                    504.80 Identification                                                                                                   • 505.9(E): Added exception for listed explosionproof and
                    Labels required by this section shall be suitable for the                                                                 flameproof equipment regarding the minimum number of
                    environment where they are installed with consideration                                                                   threads required to be engaged.
                    given to exposure to chemicals and sunlight.                                                                            • 505.15(C)(1)(b): Revised to require single conductor
                    (A) Terminals Intrinsically safe circuits shall be identified                                                             Type MV cables to be shielded or metallic-armored.
                    at terminal and junction locations in a manner that will                                                                • 505.16(B)(1): Revised to require conduit seals within 2 in.
                    prevent unintentional interference with the circuits during                                                               of protection ‘‘d’’ or ‘‘e’’ enclosures, with two exceptions
                    testing and servicing.                                                                                                    (Nos. 2 and 3) for protection ‘‘e.’’
                    (B) Wiring Raceways, cable trays, and other wiring meth-                                                                • 505.18: Revised to require every conductor (including
                    ods for intrinsically safe system wiring shall be identified                                                              spares) entering type ‘‘e’’ equipment to be terminated at
                    with permanently affixed labels with the wording ‘‘Intrinsic                                                              a type ‘‘e’’ terminal.
                    Safety Wiring’’ or equivalent. The labels shall be located                                                              • 505.19: Revised to provide guidance concerning protec-
                    so as to be visible after installation and placed so that they                                                            tion against explosion and against electric shock in order
                    may be readily traced through the entire length of the installa-                                                          to limit the voltage of exposed live parts.
                    tion. Intrinsic safety circuit labels shall appear in every sec-
                    tion of the wiring system that is separated by enclosures,
                    walls, partitions, or floors. Spacing between labels shall not                                                          Contents
                    be more than 7.5 m (25 ft).                                                                                              505.1 Scope
                    Exception: Circuits run underground shall be permitted to                                                                505.2 Definitions
                    be identified where they become accessible after emergence                                                               505.3 Other Articles
                    from the ground.                                                                                                         505.4 General
                                                                                                                                               (A) Documentation for Industrial Occupancies
                                                                         FPN No. 1: Wiring methods permitted in unclassified
                                                                                                                                               (B) Reference Standards
                                                                         locations may be used for intrinsically safe systems in
                                                                         hazardous (classified) locations. Without labels to iden-           505.5 Classifications of Locations
                                                                         tify the application of the wiring, enforcement authorities           (A) Classification of Locations
                                                                         cannot determine that an installation is in compliance                (B) Class I, Zone 0, 1, and 2 Locations
                                                                         with this Code.                                                     505.6 Material Groups
                                                                         FPN No. 2: In unclassified locations, identification is               (A) Group IIC
                                                                         necessary to ensure that nonintrinsically safe wire will
                                                                                                                                               (B) Group IIB
                                                                         not be inadvertently added to existing raceways at a later
                                                                         date.                                                                 (C) Group IIA
                                                                                                                                             505.7 Special Precaution
                    (C) Color Coding Color coding shall be permitted to iden-                                                                  (A) Supervision of Work
                    tify intrinsically safe conductors where they are colored light


                 718                                                                                                                                                           2005 National Electrical Code Handbook

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                                                                         Article 505 — Class I, Zone 0, 1, and 2 Locations                                                      505.2



                                                                                                         505.1 Scope
                              (B)        Dual Classification
                              (C)        Reclassification Permitted                                      This article covers the requirements for the zone classifica-
                              (D)        Solid Obstacles                                                 tion system as an alternative to the division classification
                            505.8        Protection Techniques                                           system covered in Article 500 for electrical and electronic
                              (A)        Flameproof ‘‘d’’                                                equipment and wiring for all voltages in Class I, Zone 0,
                              (B)        Purged and Pressurized                                          Zone 1, and Zone 2 hazardous (classified) locations where
                              (C)        Intrinsic Safety                                                fire or explosion hazards may exist due to flammable gases,
                              (D)        Type of Protection ‘‘n’’                                        vapors, or liquids.
                              (E)        Oil Immersion ‘‘o’’                                                    FPN: For the requirements for electrical and electronic
                               (F)       Increased Safety ‘‘e’’                                                 equipment and wiring for all voltages in Class I, Division
                              (G)        Encapsulation ‘‘m’’                                                    1 or Division 2; Class II, Division 1 or Division 2; and
                              (H)        Powder Filling ‘‘q’’                                                   Class III, Division 1 or Division 2 hazardous (classified)
                                                                                                                locations where fire or explosion hazards may exist due
                                (I)      Combustible Gas Detection System
                                                                                                                to flammable gases or vapors, flammable liquids, or com-
                            505.9        Equipment                                                              bustible dusts or fibers, refer to Articles 500 through
                              (A)        Suitability                                                            504.
                              (B)        Listing
                              (C)        Marking                                                         The requirements covering wiring methods, sealing, and
                              (D)        Class I Temperature                                             flexible cord use were added to Article 505 for the 2002
                              (E)        Threading                                                       Code so that the Class I, Zone 0, Zone 1, and Zone 2
                           505.15        Wiring Methods                                                  area classification concept can be used as a separate set of
                              (A)        Class I, Zone 0                                                 requirements that parallel those in Articles 500 and 501
                              (B)        Class I, Zone 1                                                 covering installations in which the Class I, Division 1 and
                              (C)        Class I, Zone 2                                                 Division 2 area classification concept is used. The zone
                           505.16        Sealing and Drainage                                            classification concept offers an alternative method of classi-
                              (A)        Zone 0                                                          fying Class I hazardous locations. The zone classification
                              (B)        Zone 1                                                          method is based on the standards for area classification used
                              (C)        Zone 2                                                          by the International Electrotechnical Commission (IEC).
                              (D)        Class I, Zones 0, 1, and 2                                           The IEC classification scheme includes underground
                              (E)        Drainage                                                        mines. In the United States, mines are under the jurisdiction
                           505.17        Flexible Cords, Class I, Zones 1 and 2                          of the Mine Safety and Health Administration (MSHA) and
                           505.18        Conductors and Conductor Insulation                             are outside of the scope of the NEC.
                              (A)        Conductors
                              (B)        Conductor Insulation
                           505.19        Uninsulated Exposed Parts                                       505.2 Definitions
                           505.20        Equipment Requirements




                                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                         For purposes of this article, the following definitions apply.
                              (A)        Zone 0
                              (B)        Zone 1                                                          Combustible Gas Detection System. A protection tech-
                              (C)        Zone 2                                                          nique utilizing stationary gas detectors in industrial estab-
                              (D)        Manufacturer’s Instructions                                     lishments.
                           505.21        Multiwire Branch Circuits
                                                                                                         Electrical and Electronic Equipment. Materials, fittings,
                           505.22        Increased Safety ‘‘e’’ Motors and Generators
                                                                                                         devices, appliances, and the like that are part of, or in connec-
                           505.25        Grounding and Bonding
                                                                                                         tion with, an electrical installation.
                              (A)        Bonding
                              (B)        Types of Equipment Grounding Conductors                                FPN: Portable or transportable equipment having self-
                                                                                                                contained power supplies, such as battery-operated
                                                                                                                equipment, could potentially become an ignition source
                                                                                                                in hazardous (classified) locations.
                                   FPN: Rules that are followed by a reference in brackets
                                   contain text that has been extracted from NFPA 497-
                                                                                                         Encapsulation ‘‘m.’’ Type of protection where electrical
                                   2004, Recommended Practice for the Classification of                  parts that could ignite an explosive atmosphere by either
                                   Flammable Liquids, Gases, or Vapors and of Hazardous                  sparking or heating are enclosed in a compound in such a
                                   (Classified) Locations for Electrical Installations in                way that this explosive atmosphere cannot be ignited.
                                   Chemical Process Areas. Only editorial changes were
                                   made to the extracted text to make it consistent with this                   FPN: See ANSI/ISA 12.23.01-2002, Electrical Appara-
                                   Code.                                                                        tus for Use in Class I, Zone 1 Hazardous (Classified)


                           National Electrical Code Handbook 2005                                                                                                                719

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                 505.3                                               Article 505 — Class I, Zone 0, 1, and 2 Locations



                            Locations, Type of Protection — Encapsulation ‘‘m’’; and                                           Type of Protection — Oil-Immersion ‘‘o’’; and ANSI/UL
                            ANSI/UL 60079–18, Electrical apparatus for explosive                                               60079-6, Electrical apparatus for explosive gas atmo-
                            gas atmospheres — Part 18: Encapsulation ‘‘m.’’                                                    spheres — Part 6: Oil-immersion ‘‘o.’’

                    Flameproof ‘‘d.’’ Type of protection where the enclosure                                            Powder Filling ‘‘q.’’ Type of protection where electrical
                    will withstand an internal explosion of a flammable mixture                                         parts capable of igniting an explosive atmosphere are fixed
                    that has penetrated into the interior, without suffering dam-                                       in position and completely surrounded by filling material
                    age and without causing ignition, through any joints or struc-                                      (glass or quartz powder) to prevent the ignition of an external
                    tural openings in the enclosure, of an external explosive gas                                       explosive atmosphere.
                    atmosphere consisting of one or more of the gases or vapors                                                FPN: See ANSI/ISA-12.25.01-2002, Electrical Appara-
                    for which it is designed.                                                                                  tus for Use in Class I, Zone 1 Hazardous (Classified)
                                                                                                                               Locations Type of Protection — Powder Filling ‘‘q’’; and
                            FPN: See ANSI/ISA 12.22.01-2002, Electrical Appara-                                                ANSI/UL 60079-5, Electrical apparatus for explosive
                            tus for Use in Class I, Zone 1 and 2 Hazardous (Classi-                                            gas atmospheres — Part 5: Powder filling ‘‘q.’’
                            fied) Locations, Type of Protection — Flameproof ‘‘d’’;                                     Purged and Pressurized. Type of protection for electrical
                            and ANSI/UL 60079–1, Electrical apparatus for explo-
                            sive gas atmospheres — Part 1: Flameproof enclosures
                                                                                                                        equipment that uses the technique of guarding against the
                            ‘‘d.’’                                                                                      ingress of the external atmosphere, which may be explosive,
                                                                                                                        into an enclosure by maintaining a protective gas therein at
                    Increased Safety ‘‘e.’’ Type of protection applied to electri-                                      a pressure above that of the external atmosphere.
                    cal equipment that does not produce arcs or sparks in normal                                               FPN No. 1: See NFPA 496-2003, Standard for Purged
                    service and under specified abnormal conditions, in which                                                  and Pressurized Enclosures for Electrical Equipment.
                    additional measures are applied so as to give increased secu-                                              FPN No. 2: See IEC 60079-2-2000, Electrical Apparatus
                    rity against the possibility of excessive temperatures and of                                              for Explosive Gas Atmospheres — Part 2: Electrical
                    the occurrence of arcs and sparks.                                                                         Apparatus, Type of Protection ‘‘p’’; and IEC 60079-13-
                                                                                                                               1982, Electrical Apparatus for Explosive Gas Atmo-
                            FPN: See ANSI/ISA — 12.16.01-2002, Electrical Appa-                                                spheres — Part 13: Construction and Use of Rooms or
                            ratus for Use in Class I, Zone 1 Hazardous (Classified)                                            Buildings Protected by Pressurization.
                            Locations, Type of Protection — Increased Safety ‘‘e’’;
                            and ANSI/UL 60079-7, Electrical apparatus for explo-
                                                                                                                        Type of Protection ‘‘n.’’ Type of protection where electri-
                            sive gas atmospheres — Part 7: Increased Safety ‘‘e.’’                                      cal equipment, in normal operation, is not capable of igniting
                                                                                                                        a surrounding explosive gas atmosphere and a fault capable
                    Intrinsic Safety ‘‘i.’’ Type of protection where any spark                                          of causing ignition is not likely to occur.
                    or thermal effect is incapable of causing ignition of a mixture                                            FPN: See ANSI/UL 60079-15-2002, Electrical appara-
                    of flammable or combustible material in air under prescribed                                               tus for explosive gas atmospheres — Part 15: Type of
                    test conditions.                                                                                           protection ‘‘n’’; and ANSI/ISA 12.12.02-2003, Electrical
                                                                                                                               apparatus for use in Class I, Zone 2 Hazardous (Classi-
                            FPN No. 1: See ANSI/UL 913-1997, Intrinsically Safe                                                fied) Locations: Type of protection ‘‘n.’’
                            Apparatus and Associated Apparatus for Use in Class I,
                            II, and III, Hazardous Locations; ISA 12.02.01-1999,                                        Unclassified Locations. Locations determined to be neither
                            Electrical Apparatus for Use in Class I, Zones 0, 1 and                                     Class I, Division 1; Class I, Division 2; Class I, Zone 0;
                            2 Hazardous (Classified) Locations — Intrinsic Safety                                       Class I, Zone 1; Class I, Zone 2; Class II, Division 1; Class
                            ‘‘i’’; and ANSI/UL 60079-11, Electrical apparatus for                                       II, Division 2; Class III, Division 1; Class III, Division 2;
                            explosive gas atmospheres — Part II: Intrinsic safety
                                                                                                                        or any combination thereof.
                            ‘‘i.’’
                            FPN No. 2: Intrinsic safety is designated type of protec-                                   505.3 Other Articles
                            tion ‘‘ia’’ for use in Zone 0 locations. Intrinsic safety is
                            designated type of protection ‘‘ib’’ for use in Zone 1                                      All other applicable rules contained in this Code shall apply
                            locations.                                                                                  to electrical equipment and wiring installed in hazardous
                            FPN No. 3: Intrinsically safe associated apparatus, desig-                                  (classified) locations.
                            nated by [ia] or [ib], is connected to intrinsically safe
                            apparatus (‘‘ia’’ or ‘‘ib,’’ respectively) but is located out-
                            side the hazardous (classified) location unless also pro-                                   See the commentary following 500.3 for more information
                            tected by another type of protection (such as flameproof).                                  on other applicable Code requirements that apply to electri-
                                                                                                                        cal equipment and wiring.
                    Oil Immersion ‘‘o.’’ Type of protection where electrical
                    equipment is immersed in a protective liquid in such a way
                    that an explosive atmosphere that may be above the liquid                                           Exception: As modified by Article 504 and this article.
                    or outside the enclosure cannot be ignited.                                                         505.4 General
                            FPN: See ISA 12.26.01-1998, Electrical Apparatus for                                        (A) Documentation for Industrial Occupancies All areas
                            Use in Class I, Zone 1 Hazardous (Classified) Locations,                                    in industrial occupancies designated as hazardous (classi-

                 720                                                --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                           2005 National Electrical Code Handbook

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                                                                           Article 505 — Class I, Zone 0, 1, and 2 Locations                                                       505.5



                           fied) locations shall be properly documented. This documen-                             leum Facilities for Unclassified and Class I, Zone 0,
                           tation shall be available to those authorized to design, install,                       Zone 1, and Zone 2 Locations.
                           inspect, maintain, or operate electrical equipment at the loca-                         FPN No. 6: For further information on the installation
                                                                                                                   of electrical equipment in hazardous (classified) locations
                           tion.                                                                                   in general, see IEC 60079-14-1996, Electrical Apparatus
                                                                                                                   for Explosive Gas Atmospheres — Part 14: Electrical
                                   FPN: For examples of area classification drawings, see
                                                                                                                   Installations in Explosive Gas Atmospheres (Other Than
                                   ANSI/API RP 505-1997, Recommended Practice for
                                                                                                                   Mines), and IEC 60079-16-1990, Electrical Apparatus
                                   Classification of Locations for Electrical Installations at
                                                                                                                   for Explosive Gas Atmospheres — Part 16: Artificial
                                   Petroleum Facilities Classified as Class I, Zone 0, Zone
                                                                                                                   Ventilation for the Protection of Analyzer(s) Houses.
                                   1, or Zone 2; ISA RP12.24.01-1998, Recommended Prac-
                                   tice for Classification of Locations for Electrical Installa-                   FPN No. 7: For further information on application of
                                   tions Classified as Class I, Zone 0, Zone 1, or Zone 2;                         electrical equipment in hazardous (classified) locations
                                   IEC 60079-10-1995, Electrical Apparatus for Explosive                           in general, see ANSI/ISA 12.00.01-2002, Electrical Ap-
                                   Gas Atmospheres, Classification of Hazardous Areas;                             paratus for Use in Class I, Zones 0 and 1, Hazardous
                                   and Model Code of Safe Practice in the Petroleum Indus-                         (Classified) Locations: General Requirements; ISA
                                   try, Part 15: Area Classification Code for Petroleum                            12.01.01-1999, Definitions and Information Pertaining
                                   Installations, IP 15, The Institute of Petroleum, London.                       to Electrical Apparatus in Hazardous (Classified) Loca-
                                                                                                                   tions; and ANSI/UL 60079-0, Electrical apparatus for
                           (B) Reference Standards Important information relating                                  explosive gas atmospheres — Part 0: General require-
                                                                                                                   ments.
                           to topics covered in Chapter 5 may be found in other publica-
                           tions.
                                                                                                            505.5 Classifications of Locations
                                   FPN No. 1: It is important that the authority having                     (A) Classification of Locations Locations shall be classi-
                                   jurisdiction be familiar with recorded industrial experi-
                                   ence as well as with standards of the National Fire Protec-              fied depending on the properties of the flammable vapors,
                                   tion Association (NFPA), the American Petroleum                          liquids, or gases that may be present and the likelihood that
                                   Institute (API), the Instrumentation, Systems, and Auto-                 a flammable or combustible concentration or quantity is
                                   mation Society (ISA), and the International Electrotech-                 present. Where pyrophoric materials are the only materials
                                   nical Commission (IEC) that may be of use in the                         used or handled, these locations shall not be classified. Each
                                   classification of various locations, the determination of
                                   adequate ventilation, and the protection against static                  room, section, or area shall be considered individually in
                                   electricity and lightning hazards.                                       determining its classification.
                                   FPN No. 2: For further information on the classification                        FPN No. 1: See 505.7 for restrictions on area classifica-
                                   of locations, see ANSI/API RP 505-1997, Recommended                             tion.
                                   Practice for Classification of Locations for Electrical
                                   Installations at Petroleum Facilities Classified as Class                       FPN No. 2: Through the exercise of ingenuity in the
                                   I, Zone 0, Zone 1, or Zone 2; ISA RP 12.24.01-1998,                             layout of electrical installations for hazardous (classified)
                                   Recommended Practice for Classification of Locations                            locations, it is frequently possible to locate much of the
                                   for Electrical Installations Classified as Class I, Zone                        equipment in reduced level of classification or in an
                                   0, Zone 1, or Zone 2; IEC 60079-10-1995, Electrical                             unclassified location and, thus, to reduce the amount of
                                   Apparatus for Explosive Gas Atmospheres, Classification                         special equipment required.
                                   of Hazardous Areas; and Model Code of Safe Practice
                                   in the Petroleum Industry, Part 15: Area Classification                      Rooms and areas containing ammonia refrigeration sys-
                                   Code for Petroleum Installations, IP 15, The Institute of                tems that are equipped with adequate mechanical ventilation
                                   Petroleum, London.
                                                                                                            may be classified as ‘‘unclassified’’ locations.
                                   FPN No. 3: For further information on protection against
                                   static electricity and lightning hazards in hazardous (clas-                    FPN: For further information regarding classification
                                   sified) locations, see NFPA 77-2000, Recommended                                and ventilation of areas involving ammonia, see ANSI/
                                   Practice on Static Electricity; NFPA 780-2004, Standard                         ASHRAE 15-1994, Safety Code for Mechanical Re-
                                   for the Installation of Lightning Protection Systems; and                       frigeration; and ANSI/CGA G2.1-1989 (14-39), Safety
                                   API RP 2003-1998, Protection Against Ignitions Arising                          Requirements for the Storage and Handling of Anhydrous
                                   Out of Static Lightning and Stray Currents.                                     Ammonia.
                                   FPN No. 4: For further information on ventilation, see
                                   NFPA 30-2003, Flammable and Combustible Liquids                          (B) Class I, Zone 0, 1, and 2 Locations Class I, Zone 0,
                                   Code, and ANSI/API RP 505-1997, Recommended Prac-                        1, and 2 locations are those in which flammable gases or
                                   tice for Classification of Locations for Electrical Installa-            vapors are or may be present in the air in quantities sufficient
                                   tions at Petroleum Facilities Classified as Class I, Zone
                                   0, Zone 1, or Zone 2.                                                    to produce explosive or ignitible mixtures. Class I, Zone 0,
                                   FPN No. 5: For further information on electrical systems                 1, and 2 locations shall include those specified in 505(B)(1),
                                   for hazardous (classified) locations on offshore oil and                 (B)(2), and (B)(3).
                                   gas producing platforms, see ANSI/API RP 14FZ-2000,
                                   Recommended Practice for Design and Installation of                      (1) Class I, Zone 0 A Class I, Zone 0 location is a location
                                   Electrical Systems for Fixed and Floating Offshore Petro-                in which

                           National Electrical Code Handbook 2005                                                           --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                                                    721

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                 505.5                                                              Article 505 — Class I, Zone 0, 1, and 2 Locations



                    (1) Ignitible concentrations of flammable gases or vapors                                          FPN No. 1: Normal operation is considered the situation
                        are present continuously, or                                                                   when plant equipment is operating within its design pa-
                                                                                                                       rameters. Minor releases of flammable material may be
                    (2) Ignitible concentrations of flammable gases or vapors                                          part of normal operations. Minor releases include the
                        are present for long periods of time.                                                          releases from mechanical packings on pumps. Failures
                                                                                                                       that involve repair or shutdown (such as the breakdown
                            FPN No. 1: As a guide in determining when flammable                                        of pump seals and flange gaskets, and spillage caused
                            gases or vapors are present continuously or for long                                       by accidents) are not considered normal operation.
                            periods of time, refer to ANSI/API RP 505-1997, Recom-
                                                                                                                       FPN No. 2: This classification usually includes locations
                            mended Practice for Classification of Locations for Elec-
                                                                                                                       where volatile flammable liquids or liquefied flammable
                            trical Installations of Petroleum Facilities Classified as
                                                                                                                       gases are transferred from one container to another. In
                            Class I, Zone 0, Zone 1 or Zone 2; ISA 12.24.01-1998,
                                                                                                                       areas in the vicinity of spraying and painting operations
                            Recommended Practice for Classification of Locations
                                                                                                                       where flammable solvents are used; adequately ventilated
                            for Electrical Installations Classified as Class I, Zone
                                                                                                                       drying rooms or compartments for evaporation of flam-
                            0, Zone 1, or Zone 2; IEC 60079-10-1995, Electrical
                                                                                                                       mable solvents; adequately ventilated locations con-
                            Apparatus for Explosive Gas Atmospheres, Classifica-
                                                                                                                       taining fat and oil extraction equipment using volatile
                            tions of Hazardous Areas; and Area Classification Code
                                                                                                                       flammable solvents; portions of cleaning and dyeing
                            for Petroleum Installations, Model Code, Part 15, Insti-
                                                                                                                       plants where volatile flammable liquids are used; ade-
                            tute of Petroleum.
                                                                                                                       quately ventilated gas generator rooms and other portions
                            FPN No. 2: This classification includes locations inside                                   of gas manufacturing plants where flammable gas may
                            vented tanks or vessels that contain volatile flammable                                    escape; inadequately ventilated pump rooms for flamma-
                            liquids; inside inadequately vented spraying or coating                                    ble gas or for volatile flammable liquids; the interiors of
                            enclosures, where volatile flammable solvents are used;                                    refrigerators and freezers in which volatile flammable
                            between the inner and outer roof sections of a floating                                    materials are stored in the open, lightly stoppered, or
                            roof tank containing volatile flammable liquids; inside                                    in easily ruptured containers; and other locations where
                            open vessels, tanks and pits containing volatile flamma-                                   ignitible concentrations of flammable vapors or gases are
                            ble liquids; the interior of an exhaust duct that is used                                  likely to occur in the course of normal operation but not
                            to vent ignitible concentrations of gases or vapors; and                                   classified Zone 0.
                            inside inadequately ventilated enclosures that contain
                            normally venting instruments utilizing or analyzing flam-                           (3) Class I, Zone 2 A Class I, Zone 2 location is a location
                            mable fluids and venting to the inside of the enclosures.
                            FPN No. 3: It is not good practice to install electrical                            (1) In which ignitible concentrations of flammable gases
                            equipment in Zone 0 locations except when the equip-                                    or vapors are not likely to occur in normal operation
                            ment is essential to the process or when other locations
                                                                                                                    and, if they do occur, will exist only for a short period;
                            are not feasible. [See 505.5(A) FPN No. 2.] If it is neces-
                            sary to install electrical systems in a Zone 0 location, it                             or
                            is good practice to install intrinsically safe systems as                           (2) In which volatile flammable liquids, flammable gases,
                            described by Article 504.                                                               or flammable vapors are handled, processed, or used
                                                                                                                    but in which the liquids, gases, or vapors normally are
                    (2) Class I, Zone 1 A Class I, Zone 1 location is a location                                    confined within closed containers of closed systems
                    (1) In which ignitible concentrations of flammable gases                                        from which they can escape, only as a result of acciden-
                        or vapors are likely to exist under normal operating                                        tal rupture or breakdown of the containers or system,
                        conditions; or                                                                              or as a result of the abnormal operation of the equipment
                    (2) In which ignitible concentrations of flammable gases                                        with which the liquids or gases are handled, processed,
                        or vapors may exist frequently because of repair or                                         or used; or
                        maintenance operations or because of leakage; or                                        (3) In which ignitible concentrations of flammable gases
                    (3) In which equipment is operated or processes are carried                                     or vapors normally are prevented by positive mechanical
                        on, of such a nature that equipment breakdown or faulty                                     ventilation but which may become hazardous as a result
                        operations could result in the release of ignitible concen-                                 of failure or abnormal operation of the ventilation equip-
                        trations of flammable gases or vapors and also cause                                        ment; or
                        simultaneous failure of electrical equipment in a mode                                  (4) That is adjacent to a Class I, Zone 1 location, from
                        to cause the electrical equipment to become a source                                        which ignitible concentrations of flammable gases or
                        of ignition; or                                                                             vapors could be communicated, unless such communi-
                    (4) That is adjacent to a Class I, Zone 0 location from which                                   cation is prevented by adequate positive-pressure venti-
                        ignitible concentrations of vapors could be communi-                                        lation from a source of clean air and effective safeguards
                        cated, unless communication is prevented by adequate                                        against ventilation failure are provided.
                        positive pressure ventilation from a source of clean air                                       FPN: The Zone 2 classification usually includes loca-
                        and effective safeguards against ventilation failure are                                       tions where volatile flammable liquids or flammable
                        provided.                                                                                      gases or vapors are used but which would become hazard-



                 722                             --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                   2005 National Electrical Code Handbook

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                                                                           Article 505 — Class I, Zone 0, 1, and 2 Locations                                                      505.7



                                   ous only in case of an accident or of some unusual op-                  Use in Class I, II, III, Division 1, Hazardous (Classified)
                                   erating condition.                                                      Locations, because it is a factor that must be considered in
                                                                                                           the evaluation of intrinsically safe apparatus.
                           505.6 Material Groups
                           For purposes of testing, approval, and area classification,
                           various air mixtures (not oxygen enriched) shall be grouped                            Class I, Zone 0, 1, and 2, groups shall be as follows:
                           as required in 505.6(A), (B), and (C).
                                                                                                           (A) Group IIC Atmospheres containing acetylene, hydro-
                                   FPN: Group I is intended for use in describing atmo-                    gen, or flammable gas, flammable liquid-produced vapor,
                                   spheres that contain firedamp (a mixture of gases, com-                 or combustible liquid-produced vapor mixed with air that
                                   posed mostly of methane, found underground, usually in
                                                                                                           may burn or explode, having either a maximum experimental
                                   mines). This Code does not apply to installations under-
                                   ground in mines. See 90.2(B).                                           safe gap (MESG) value less than or equal to 0.50 mm or
                                                                                                           minimum igniting current ratio (MIC ratio) less than or equal
                                Group II shall be subdivided into IIC, IIB, and IIA, as                    to 0.45. [NFPA 497:3.3]
                           noted in 505.6(A), (B), and (C), according to the nature of                            FPN: Group IIC is equivalent to a combination of Class
                           the gas or vapor, for protection techniques ‘‘d,’’ ‘‘ia,’’ ‘‘ib,’’                     I, Group A, and Class I, Group B, as described in
                           ‘‘[ia],’’ and ‘‘[ib],’’ and, where applicable, ‘‘n’’ and ‘‘o.’’                        500.6(A)(1) and 500.6(A)(2).

                                   FPN No. 1: The gas and vapor subdivision as described                   (B) Group IIB Atmospheres containing acetaldehyde, eth-
                                   above is based on the maximum experimental safe gap                     ylene, or flammable gas, flammable liquid-produced vapor,
                                   (MESG), minimum igniting current (MIC), or both. Test                   or combustible liquid-produced vapor mixed with air that
                                   equipment for determining the MESG is described in IEC
                                   60079-1A-1975, Amendment No. 1 (1993), Construction                     may burn or explode, having either maximum experimental
                                   and Verification Tests of Flameproof Enclosures of Elec-                safe gap (MESG) values greater than 0.50 mm and less than
                                   trical Apparatus; and UL Technical Report No. 58 (1993).                or equal to 0.90 mm or minimum igniting current ratio (MIC
                                   The test equipment for determining MIC is described in                  ratio) greater than 0.45 and less than or equal to 0.80. [NFPA
                                   IEC 60079-11-1999, Electrical Apparatus for Explosive                   497:3.3]
                                   Gas Atmospheres — Part 11: Intrinsic Safety ‘‘i.’’ The
                                   classification of gases or vapors according to their maxi-                     FPN: Group IIB is equivalent to Class I, Group C, as
                                   mum experimental safe gaps and minimum igniting cur-                           described in 500.6(A)(3).




                                                                                                                                                                                          --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                   rents is described in IEC 60079-12-1978, Classification
                                   of Mixtures of Gases or Vapours with Air According to
                                                                                                           (C) Group IIA Atmospheres containing acetone, ammo-
                                   Their Maximum Experimental Safe Gaps and Minimum
                                   Igniting Currents.                                                      nia, ethyl alcohol, gasoline, methane, propane, or flammable
                                   FPN No. 2: Verification of electrical equipment utilizing               gas, flammable liquid-produced vapor, or combustible liq-
                                   protection techniques ‘‘e,’’ ‘‘m,’’ ‘‘p,’’ and ‘‘q,’’ due to            uid-produced vapor mixed with air that may burn or explode,
                                   design technique, does not require tests involving MESG                 having either a maximum experiment safe gap (MESG)
                                   or MIC. Therefore, Group II is not required to be subdi-                value greater than 0.90 mm or minimum igniting current
                                   vided for these protection techniques.
                                                                                                           ratio (MIC ratio) greater than 0.80. [NFPA 497:3.3]
                                   FPN No. 3: It is necessary that the meanings of the
                                   different equipment markings and Group II classifica-                          FPN: Group IIA is equivalent to Class I, Group D as
                                   tions be carefully observed to avoid confusion with Class                      described in 500.6(A)(4).
                                   I, Divisions 1 and 2, Groups A, B, C, and D.
                                                                                                           505.7 Special Precaution
                           In the zone classification system, the gas or vapor group
                                                                                                           Article 505 requires equipment construction and installation
                           order is the approximate inverse of the gas or vapor groups
                                                                                                           that ensures safe performance under conditions of proper
                           specified in Article 500. For example, Group IIC includes
                                                                                                           use and maintenance.
                           Article 500, Groups A and B. Determination of a gas or
                           vapor for the purposes of grouping includes the evaluation                             FPN No. 1: It is important that inspection authorities
                           of the maximum safe experimental gap ratio as well as                                  and users exercise more than ordinary care with regard
                           minimum igniting current ratio. Although the maximum safe                              to the installation and maintenance of electrical equip-
                                                                                                                  ment in hazardous (classified) locations.
                           experimental gap for Group A is less than that for Group
                                                                                                                  FPN No. 2: Low ambient conditions require special con-
                           B in some circumstances, the minimum igniting current ratio
                                                                                                                  sideration. Electrical equipment depending on the protec-
                           is less for hydrogen (Group B) than it is for acetylene (Group                         tion techniques described by 505.8(A) may not be
                           A). This difference has been accounted for in ANSI/UL 913,                             suitable for use at temperatures lower than           20 C
                           Intrinsically Safe Apparatus and Associated Apparatus for                              ( 4 F) unless they are identified for use at lower temper-
                                                                                                                  atures. However, at low ambient temperatures, flammable



                           National Electrical Code Handbook 2005                                                                                                                  723

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                 505.8                                             Article 505 — Class I, Zone 0, 1, and 2 Locations



                            concentrations of vapors may not exist in a location clas-                                     pipes, or other electrical equipment) unless the equipment
                            sified Class I, Zones 0, 1, or 2 at normal ambient tempera-                                    is listed for a smaller distance of separation.
                            ture.

                    (A) Supervision of Work Classification of areas and selec-                                             505.8 Protection Techniques
                    tion of equipment and wiring methods shall be under the                                                Acceptable protection techniques for electrical and elec-
                    supervision of a qualified Registered Professional Engineer.                                           tronic equipment in hazardous (classified) locations shall be
                                                                                                                           as described in 505.8(A) through 505.8(I).
                    Note that 505.7(A) requires area classification, wiring, and                                                    FPN: For additional information, see ANSI/ISA
                    equipment selection to be under the supervision of a qualified                                                  12.00.01-2002, Electrical Apparatus for Use in Class I,
                    Registered Professional Engineer for installations in Class                                                     Zones 0 and 1 Hazardous (Classified) Locations, General
                                                                                                                                    Requirements; ANSI/ISA 12.01.01-2002, Definitions and
                    I, Zone 0, 1, and 2 locations.
                                                                                                                                    Information Pertaining to Electrical Apparatus in Haz-
                                                                                                                                    ardous (Classified) Locations; and ANSI/UL 60079–0,
                                                                                                                                    Electrical apparatus for explosive gas atmospheres —
                    (B) Dual Classification In instances of areas within the                                                        Part 0: General requirements.
                    same facility classified separately, Class I, Zone 2 locations
                    shall be permitted to abut, but not overlap, Class I, Division                                         Where the area is classified in accordance with the zone
                    2 locations. Class I, Zone 0 or Zone 1 locations shall not                                             method, electrical and electronic equipment may be pro-
                    abut Class I, Division 1 or Division 2 locations.                                                      tected by the following methods:
                                                                                                                             1. In Class I, Zone 1, equipment approved as flameproof
                    An installation is permitted to be designed using either the                                                ‘‘d,’’ which is very similar to explosionproof equipment
                    classification scheme of Article 500 or the classification                                                  in the United States
                    scheme of Article 505. Both schemes cannot be used for                                                   2. Purged and pressurized equipment for Class I, Zone 1
                    classifying the same area. In areas within the same facility,                                               or Zone 2 locations for which it is approved
                    Class I, Zone 2 locations are allowed to be adjacent to and                                              3. Intrinsic safety techniques for the Class I, Zone 0, Zone
                    share the same border, but not overlap Class I, Division 2                                                  1, or Zone 2 for which the technique is listed
                    locations. However, Class I, Zone 0 or Zone 1 locations are                                              4. In Class I, Zone 2, equipment approved as type ‘‘n,’’
                    not allowed to be adjacent to and share the same border                                                     which under normal operation is not capable of igniting
                    with Class I, Division 1 or Division 2 locations.                                                           a surrounding explosive gas atmosphere and is not likely
                                                                                                                                to create a fault that is capable of causing ignition
                                                                                                                             5. Oil immersion ‘‘o’’ technique for Class I, Zone 1 or
                    (C) Reclassification Permitted A Class I, Division 1 or                                                     Zone 2, where the equipment or part of the equipment
                    Division 2 location shall be permitted to be reclassified as                                                is immersed in a protective liquid so that explosive
                    a Class I, Zone 0, Zone 1, or Zone 2 location, provided all                                                 gases above or outside the enclosure cannot be ignited
                    of the space that is classified because of a single flammable                                            6. A type of protection technique of increased safety ‘‘e’’
                    gas or vapor source is reclassified under the requirements                                                  approved for Class I, Zone 1 or Zone 2, in which the
                    of this article.                                                                                            electrical equipment involved does not produce arcs or
                                                                                                                                sparks or excessive temperature under normal operation,
                    (D) Solid Obstacles Flameproof equipment with flanged
                                                                                                                                as well as under abnormal conditions, under specified
                    joints shall not be installed such that the flange openings
                                                                                                                                conditions of operation
                    are closer than the distances shown in Table 505.7(D) to
                                                                                                                             7. Encapsulation ‘‘m’’ technique in which the arcing,
                    any solid obstacle that is not a part of the equipment (such
                                                                                                                                sparking, or hot parts are completely surrounded in a
                    as steelworks, walls, weather guards, mounting brackets,
                                                                                                                                compound in such a way that an explosive gas or vapor
                                                                                                                                cannot be ignited in a Class I, Zone 1 or Zone 2 area
                                                                                                                             8. A technique using powder filling ‘‘q’’ in which the
                    Table 505.7(D) Minimum Distance of Obstructions from
                    Flameproof ‘‘d’’ Flange Openings
                                                                                                                                arcing, sparking, or hot parts of the equipment are sur-
                                                                                                                                rounded by a material such as glass or quartz powder
                                                          Minimum Distance                                                      to prevent ignition of an external explosive atmosphere
                                                                                                                                in a Class I, Zone 1 or Zone 2 area
                                  Gas Group               mm                in.
                                                                                                                           The letter in quotation marks following the protection
                                        IIC                40              137⁄64
                                        IIB                30              13⁄16
                                                                                                                           method is the letter used in the marking on the equipment.
                                        IIA                10               25
                                                                               ⁄64                                         See 505.9(C) for marking requirements.


                 724                                                              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                           2005 National Electrical Code Handbook

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                                                                    Article 505 — Class I, Zone 0, 1, and 2 Locations                                                         505.8



                               Of the many protection techniques, intrinsic safety,                    (D) Type of Protection ‘‘n’’ This protection technique
                           flameproof, and increased safety are the most common for                    shall be permitted for equipment in Class I, Zone 2 locations.
                           Zone 1 locations.                                                           Type of protection ‘‘n’’ is further subdivided into nA, nC,
                                                                                                       and nR.

                           (A) Flameproof ‘‘d’’ This protection technique shall be                             FPN: See Table 505.9(C)(2)(4) for the descriptions of
                                                                                                               subdivisions for type of protection ‘‘n’’.
                           permitted for equipment in Class I, Zone 1 or Zone 2 loca-
                           tions.                                                                      (E) Oil Immersion ‘‘o’’ This protection technique shall
                                                                                                       be permitted for equipment in Class I, Zone 1 or Zone 2
                           Flameproof protection is commonly combined with in-                         locations.
                           creased safety protection. For example, motor control and
                           other switching contacts are commonly protected by flame-                   (F) Increased Safety ‘‘e’’ This protection technique shall
                           proof enclosures with the field wiring terminals protected                  be permitted for equipment in Class I, Zone 1 or Zone 2
                           in a separate but attached enclosure by increased safety.                   locations.
                           The conductors between the enclosures are protected by
                           flameproof feed-through insulators. The equipment shown                     The increased safety protection technique is commonly used
                           in Exhibit 505.1 is an example of where this combination                    for motors and generators (see 505.22) and fluorescent lumi-
                           of protection techniques is employed.                                       naires. It is also used for terminal boxes.


                                                                                                       (G) Encapsulation ‘‘m’’ This protection technique shall
                                                                                                       be permitted for equipment in Class I, Zone 1 or Zone 2
                                                                                                       locations.

                                                                                                       (H) Powder Filling ‘‘q’’ This protection technique shall
                                                                                                       be permitted for equipment in Class I, Zone 1 or Zone 2
                                                                                                       locations.

                                                                                                       (I) Combustible Gas Detection System A combustible
                                                                                                       gas detection system shall be permitted as a means of protec-
                                                                                                       tion in industrial establishments with restricted public access
                                                                                                       and where the conditions of maintenance and supervision
                                                                                                       ensure that only qualified persons service the installation.
                                                                                                       Gas detection equipment shall be listed for detection of the
                                                                                                       specific gas or vapor to be encountered. Where such a system
                                                                                                       is installed, equipment specified in 505.8(I)(1), I(2), or I(3)
                           Exhibit 505.1 Typical control stations with the combination of              shall be permitted. The type of detection equipment, its
                           flameproof and increased safety types of protection suitable for
                                                                                                       listing, installation location(s), alarm and shutdown criteria,
                           use in Class I, Zone 1 areas. (Courtesy of Cooper Crouse-Hinds)
                                                                                                       and calibration frequency shall be documented when com-
                                                                                                       bustible gas detectors are used as a protection technique.
                           (B) Purged and Pressurized This protection technique                                FPN No. 1: For further information, see ANSI/ISA-
                           shall be permitted for equipment in those Class I, Zone 1                           12.13.01, Performance Requirements, Combustible Gas
                           or Zone 2 locations for which it is identified.                                     Detectors.
                                                                                                               FPN No. 2: For further information, see ANSI/API RP
                           (C) Intrinsic Safety This protection technique shall be per-                        505, Recommended Practice for Classification of Loca-
                           mitted for apparatus and associated apparatus in Class I,                           tions for Electrical Installations at Petroleum Facilities
                                                                                                               Classified as Class I, Zone 0, Zone 1, and Zone 2.
                           Zone 0, Zone 1, or Zone 2 locations for which it is listed.
                                                                                                               FPN No. 3: For further information, see ISA-
                                                                                                               RP12.13.02, Installation, Operation, and Maintenance
                           The identifying letter for intrinsic safety is ‘‘i’’ followed by                    of Combustible Gas Detection Instruments.
                           either ‘‘a’’ or ‘‘b,’’ identifying whether the equipment is
                           suitable for Zone 0 (ia) or Zone 1 (ib). The associated appara-             (1) Inadequate Ventilation In a Class I, Zone 1 location
                           tus is identified by the same letters in brackets, that is, [ia]            that is so classified due to inadequate ventilation, electrical
                           or [ib].                                                                    equipment suitable for Class I, Zone 2 locations shall be
                                                                                                       permitted.

                           National Electrical Code Handbook 2005                                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---                        725

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                 505.9                                             Article 505 — Class I, Zone 0, 1, and 2 Locations



                    (2) Interior of a Building In a building located in, or with               (3) Temperature                  classification            in      accordance        with
                    an opening into, a Class I, Zone 2 location where the interior                 505.9(D)(1)
                    does not contain a source of flammable gas or vapor, electri-              (2) Zone Equipment Equipment meeting one or more of
                    cal equipment for unclassified locations shall be permitted.               the protection techniques described in 505.8 shall be marked
                    (3) Interior of a Control Panel In the interior of a control               with all of the following in the order shown:
                    panel containing instrumentation utilizing or measuring                    (1) Class
                    flammable liquids, gases, or vapors, electrical equipment                  (2) Zone
                    suitable for Class I, Zone 2 locations shall be permitted.                 (3) Symbol ‘‘AEx’’
                                                                                               (4) Protection technique(s) in accordance with Table
                    505.9 Equipment                                                                505.9(C)(2)(4)
                    (A) Suitability Suitability of identified equipment shall be               (5) Applicable gas classification group(s) in accordance
                    determined by one of the following:                                            with Table 505.9(C)
                                                                                               (6) Temperature classification in accordance with
                    (1) Equipment listing or labeling
                                                                                                   505.9(D)(1)
                    (2) Evidence of equipment evaluation from a qualified test-
                        ing laboratory or inspection agency concerned with
                        product evaluation                                                     The symbol AEx identifies the equipment as meeting Ameri-
                    (3) Evidence acceptable to the authority having jurisdiction               can standards. In European Common Market countries, the
                        such as a manufacturer’s self-evaluation or an owner’s                 symbol is EEx. In the IEC standards, on which American
                        engineering judgment                                                   and European standards are based, the symbol is Ex.

                    (B) Listing
                                                                                               Exception No. 1: Associated apparatus NOT suitable for
                    (1) Equipment that is listed for a Zone 0 location shall be                installation in a hazardous (classified) locations shall be
                        permitted in a Zone 1 or Zone 2 location of the same                   required to be marked only with (3), (4), and (5), but BOTH
                        gas or vapor, provided that it is installed in accordance              the symbol AEx (3) and the symbol for the type of protection
                        with the requirements for the marked type of protection.               (4) shall be enclosed within the same square brackets, for
                        Equipment that is listed for a Zone 1 location shall be                example, [AEx ia] IIC.
                        permitted in a Zone 2 location of the same gas or vapor,               Exception No. 2: Simple apparatus as defined in 504.2 shall
                        provided that it is installed in accordance with the re-               not be required to have a marked operating temperature or
                        quirements for the marked type of protection.                          temperature class.
                    (2) Equipment shall be permitted to be listed for a specific
                        gas or vapor, specific mixtures of gases or vapors, or                       Electrical equipment of types of protection ‘‘e,’’ ‘‘m,’’
                        any specific combination of gases or vapors.                           ‘‘p,’’ or ‘‘q’’ shall be marked Group II. Electrical equipment
                            FPN: One common example is equipment marked for
                            ‘‘IIB. H2.’’                                                       Table 505.9(C)(2)(4) Types of Protection Designation
                    (C) Marking Equipment shall be marked in accordance
                                                                                               Designation                              Technique                              Zone*
                    with 505.9(C)(1) or (C)(2).
                    (1) Division Equipment Equipment identified for Class I,                           d               Flameproof enclosure                                       1
                                                                                                        e              Increased safety                                           1

                                                                                                                                                                                            --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    Division 1 or Class I, Division 2 shall, in addition to being                      ia              Intrinsic safety                                           0
                    marked in accordance with 500.8(B), be permitted to be                             ib              Intrinsic safety                                           1
                    marked with all of the following:                                                 [ia]             Associated apparatus                                  Unclassified
                    (1) Class I, Zone 1 or Class I, Zone 2 (as applicable)                            [ib]             Associated apparatus                                  Unclassified
                                                                                                       m               Encapsulation                                              1
                    (2) Applicable gas classification group(s) in accordance
                                                                                                      nA               Nonsparking equipment                                      2
                        with Table 505.9(C)                                                           nC               Sparking equipment in which the                            2
                                                                                                                       contacts are suitably protected
                    Table 505.9(C) Gas Classification Groups                                                           other than by restricted breathing
                                                                                                                       enclosure
                                                 Gas Group     Comment                                nR               Restricted breathing enclosure                            2
                                                                                                       o               Oil immersion                                             1
                                                    IIC      See 505.6(A)(1)                           p               Purged and pressurized                                  1 or 2
                                                    IIB      See 505.6(A)(2)                           q               Powder filled                                             1
                                                    IIA      See 505.6(A)(3)
                                                                                               *Does not address use where a combination of techniques is used.


                 726                                                                                                              2005 National Electrical Code Handbook

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                                                                                                                           Article 505 — Class I, Zone 0, 1, and 2 Locations                                                        505.9



                                                                           of types of protection ‘‘d,’’ ‘‘ia,’’ ‘‘ib,’’ ‘‘[ia],’’ or ‘‘[ib]’’              Table 505.9(D)(1) Classification of Maximum Surface
                                                                           shall be marked Group IIA, IIB, or IIC, or for a specific                        Temperature for Group II Electrical Equipment
                                                                           gas or vapor. Electrical equipment of types of protection
                                                                                                                                                                           Temperature                           Maximum Surface
                                                                           ‘‘n’’ shall be marked Group II unless it contains enclosed-                                    Class (T Code)                         Temperature ( C)
                                                                           break devices, nonincendive components, or energy-limited
                                                                           equipment or circuits, in which case it shall be marked                                                  T1                                        450
                                                                           Group IIA, IIB, or IIC, or a specific gas or vapor. Electrical                                           T2                                        300
                                                                           equipment of other types of protection shall be marked                                                   T3                                        200
                                                                                                                                                                                    T4                                        135
                                                                           Group II unless the type of protection utilized by the equip-                                            T5                                        100
                                                                           ment requires that it be marked Group IIA, IIB, or IIC, or                                               T6                                        85
                                                                           a specific gas or vapor.
                                                                                FPN No. 1: An example of the required marking for                                Electrical equipment designed for use in the ambient
                                                                                intrinsically safe apparatus for installation in Class I,
                                                                                Zone 0 is ‘‘Class I, Zone 0, AEx ia IIC T6.’’ An explana-
                                                                                                                                                            temperature range between 20 C and 40 C shall require
                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                tion of the marking that is required is shown in FPN                        no additional ambient temperature marking.
                                                                                Figure 505.9(C)(2).                                                              Electrical equipment that is designed for use in a range
                                                                                                                                                            of ambient temperatures other than 20 C and 40 C is
                                                                                                 Example: Class I Zone 0     AEx   ia   IIC   T6            considered to be special; and the ambient temperature range
                                                                           Area classification                                                              shall then be marked on the equipment, including either the
                                                                                                                                                            symbol ‘‘Ta’’ or ‘‘Tamb’’ together with the special range of
                                                                           Symbol for equipment built to American                                           ambient temperatures. As an example, such a marking might
                                                                           standards
                                                                                                                                                            be ‘‘ 30 C Ta            40 C.’’
                                                                           Type(s) of protection designation                                                     Electrical equipment suitable for ambient temperatures
                                                                           Gas classification group (not required for protection
                                                                                                                                                            exceeding 40 C (104 F) shall be marked with both the maxi-
                                                                           techniques indicated in 505.6, FPN No. 2)                                        mum ambient temperature and the operating temperature or
                                                                                                                                                            temperature class at that ambient temperature.
                                                                           Temperature classification
                                                                                                                                                            Exception No. 1: Equipment of the non–heat-producing
                                                                           FPN Figure 505.9(C)(2) Zone Equipment Marking                                    type, such as conduit fittings, and equipment of the heat-
                                                                                                                                                            producing type having a maximum temperature of not more
                                                                                                                                                            than 100 C (212 F) shall not be required to have a marked
                                                                                FPN No. 2: An example of the required marking for
                                                                                intrinsically safe associated apparatus mounted in a
                                                                                                                                                            operating temperature or temperature class.
                                                                                flameproof enclosure for installation in Class I, Zone 1                    Exception No. 2: Equipment identified for Class I, Division
                                                                                is ‘‘Class I, Zone 1 AEx d[ia] IIC T4.’’                                    1 or Division 2 locations as permitted by 505.20(B) and
                                                                                FPN No. 3: An example of the required marking for                           505.20(D) shall be permitted to be marked in accordance
                                                                                intrinsically safe associated apparatus NOT for installa-                   with 500.8(B) and Table 500.8(B).
                                                                                tion in a hazardous (classified) location is ‘‘[AEx ia]
                                                                                IIC.’’                                                                      (E) Threading All NPT threaded conduit and fittings re-
                                                                                                                                                            ferred to herein shall be threaded with a National (American)
                                                                           (D) Class I Temperature The temperature marking speci-                           Standard Pipe Taper (NPT) thread that provides a taper of
                                                                           fied below shall not exceed the ignition temperature of the                      1 in 16 (3⁄4-in. taper per foot). Conduit and fittings shall be
                                                                           specific gas or vapor to be encountered.                                         made wrenchtight to prevent sparking when fault current
                                                                                FPN: For information regarding ignition temperatures                        flows through the conduit system, and to ensure the explo-
                                                                                of gases and vapors, see NFPA 497-2004, Recommended                         sionproof or flameproof integrity of the conduit system
                                                                                Practice for the Classification of Flammable Liquids,                       where applicable. Equipment provided with threaded entries
                                                                                Gases, or Vapors and of Hazardous (Classified) Loca-                        for field wiring connections shall be installed in accordance
                                                                                tions for Electrical Installations in Chemical Process
                                                                                                                                                            with 505.9(E)(1) or 505.9(E)(2). Threaded entries into explo-
                                                                                Areas; and IEC 60079-20-1996, Electrical Apparatus for
                                                                                Explosive Gas Atmospheres, Data for Flammable Gases                         sionproof or flameproof equipment shall be made up with
                                                                                and Vapours, Relating to the Use of Electrical Apparatus.                   at least five threads fully engaged.
                                                                                                                                                            Exception: For listed explosionproof or flameproof equip-
                                                                           (1) Temperature Classifications Equipment shall be
                                                                                                                                                            ment, factory threaded NPT entries shall be made up with
                                                                           marked to show the operating temperature or temperature
                                                                                                                                                            at least 41⁄2 threads fully engaged.
                                                                           class referenced to a 40 C (104 F) ambient. The temperature
                                                                           class, if provided, shall be indicated using the temperature                     (1) Equipment Provided with Threaded Entries for NPT
                                                                           class (T Code) shown in Table 505.9(D)(1).                                       Threaded Conduit or Fittings For equipment provided


                                                                           National Electrical Code Handbook 2005                                                                                                                    727

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                 505.15                                                                                   Article 505 — Class I, Zone 0, 1, and 2 Locations



                    with threaded entries for NPT threaded conduit or fittings,                                                       (A) Class I, Zone 0 In Class I, Zone 0 locations, only
                    listed conduit fittings or cable fittings shall be used.                                                          intrinsically safe wiring methods in accordance with Article
                                                                       FPN: Thread form specifications for NPT threads are            504 shall be permitted.
                                                                       located in ANSI/ASME B1.20.1-1983, Pipe Threads,
                                                                                                                                             FPN: Article 504 only includes protection technique
                                                                       General Purpose (Inch).
                                                                                                                                             ‘‘ia.’’
                    (2) Equipment Provided with Threaded Entries for Met-
                    ric Threaded Conduit or Fittings For equipment with met-                                                          The provision in 505.15(A) is one of the most significant
                    ric threaded entries, such entries shall be identified as being                                                   differences between the zone and division area classification
                    metric, or listed adapters to permit connection to conduit or                                                     requirements. The degree of hazard within a Zone 0 area is
                    NPT-threaded fittings shall be provided with the equipment.                                                       considered so severe that all wiring is this area must be
                    Adapters shall be used for connection to conduit or NPT-                                                          intrinsically safe. In general, only instrumentation and sig-
                    threaded fittings. Listed cable fittings that have metric                                                         naling circuits installed in accordance with Article 504 can
                    threads shall be permitted to be used.                                                                            be used in a Zone 0 area. Explosionproof power utilization
                                                                                                                                      equipment, such as motors and luminaires, is not permitted
                    See the commentary on 500.8(D)(2) FPN for information                                                             in Class I, Zone 0 locations.
                    on metric threads. Exhibit 505.2 is an example of an adapter
                    that provides a means of connecting conduit or fitting with
                    NPT threads to an ‘‘increased safety’’ enclosure that has                                                         (B) Class I, Zone 1
                    metric threads.                                                                                                   (1) General In Class I, Zone 1 locations, the wiring meth-
                                                                                                                                      ods in (B)(1)(a) through (B)(1)(f) shall be permitted.
                                                                                                                                             (a) All wiring methods permitted by 505.15(A).
                                                                                                                                            (b) In industrial establishments with restricted public
                                                                                                                                      access, where the conditions of maintenance and supervision
                                                                                                                                      ensure that only qualified persons service the installation,
                                                                                                                                      and where the cable is not subject to physical damage, Type
                                                                                                                                      MC-HL cable listed for use in Class I, Zone 1 or Division
                                                                                                                                      1 locations, with a gas/vaportight continuous corrugated me-
                                                                                                                                      tallic sheath, an overall jacket of suitable polymeric material,
                                                                                                                                      separate grounding conductors in accordance with 250.122,
                                                                                                                                      and provided with termination fittings listed for the applica-
                                                                                                                                      tion.

                                                                                                                                             FPN: See 330.12 for restrictions on use of Type MC
                     --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                             cable.

                                                                                                                                            (c) In industrial establishments with restricted public
                                                                                                                                      access, where the conditions of maintenance and supervision
                                                                                                                                      ensure that only qualified persons service the installation,
                                                                                                                                      and where the cable is not subject to physical damage, Type
                                                                                                                                      ITC-HL cable, listed for use in Class I, Zone 1 or Division
                                                                                                                                      1 locations, with a gas/vaportight continuous corrugated me-
                                                                                                                                      tallic sheath, an overall jacket of suitable polymeric material
                    Exhibit 505.2 A typical hub providing an NPT threaded entry                                                       and provided with termination fittings listed for the applica-
                    for conduit or cable into an increased safety enclosure. (Courtesy                                                tion.
                    of Cooper Crouse-Hinds)                                                                                                 (d) Type MI cable with termination fittings listed for
                                                                                                                                      Class I, Zone 1 or Division 1 locations. Type MI cable shall
                                                                       FPN: Threading specifications for metric threaded en-          be installed and supported in a manner to avoid tensile stress
                                                                       tries are located in ISO 965/1-1980, Metric Screw              at the termination fittings.
                                                                       Threads; and ISO 965/3-1980, Metric Screw Threads.                   (e) Threaded rigid metal conduit, or threaded steel in-
                                                                                                                                      termediate metal conduit.
                    505.15 Wiring Methods                                                                                                   (f) Rigid nonmetallic conduit complying with Article
                    Wiring methods shall maintain the integrity of protec-                                                            352 shall be permitted where encased in a concrete envelope
                    tion techniques and shall comply with 505.15(A) through                                                           a minimum of 50 mm (2 in.) thick and provided with not
                    505.15 (C).                                                                                                       less than 600 mm (24 in.) of cover measured from the top
                 728                                                                                                                                                     2005 National Electrical Code Handbook

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                                                                      Article 505 — Class I, Zone 0, 1, and 2 Locations                                                       505.16



                           of the conduit to grade. Threaded rigid metal conduit or                    with listed fittings, liquidtight flexible nonmetallic conduit
                           threaded steel intermediate metal conduit shall be used for                 with listed fittings, or flexible cord in accordance with the
                           the last 600 mm (24 in.) of the underground run to emergence                provisions of 505.17 shall be permitted.
                           or to the point of connection to the aboveground raceway.
                                                                                                              FPN: See 505.25(B) for grounding requirements where
                           An equipment grounding conductor shall be included to                              flexible conduit is used.
                           provide for electrical continuity of the raceway system and
                           for grounding of non–current-carrying metal parts.
                                                                                                       505.16 Sealing and Drainage
                           (2) Flexible Connections Where necessary to employ flex-                    Seals in conduit and cable systems shall comply with
                           ible connections, flexible fittings listed for Class I, Zone 1              505.16(A) through 505.16(E). Sealing compound shall be
                           or Division 1 locations or flexible cord in accordance with                 used in Type MI cable termination fittings to exclude mois-
                           the provisions of 505.17 shall be permitted.                                ture and other fluids from the cable insulation.
                           (C) Class I, Zone 2                                                                FPN No. 1: Seals are provided in conduit and cable
                                                                                                              systems to minimize the passage of gases and vapors and
                           (1) General In Class I, Zone 2 locations, the wiring meth-                         prevent the passage of flames from one portion of the
                           ods in (C)(1)(a) through (C)(1)(g) shall be permitted.                             electrical installation to another through the conduit. Such
                                                                                                              communication through Type MI cable is inherently pre-
                                   (a) All wiring methods permitted by 505.15(B).                             vented by construction of the cable. Unless specifically
                                 (b) Types MI, MC, MV, or TC cable with termination                           designed and tested for the purpose, conduit and cable
                           fittings, or in cable tray systems and installed in a manner                       seals are not intended to prevent the passage of liquids,
                                                                                                              gases, or vapors at a continuous pressure differential
                           to avoid tensile stress at the termination fittings. Single                        across the seal. Even at differences in pressure across
                           conductor Type MV cables shall be shielded or metallic-                            the seal equivalent to a few inches of water, there may
                           armored.                                                                           be a slow passage of gas or vapor through a seal and
                                                                                                              through conductors passing through the seal. See
                                   (c) Type ITC cable as permitted in 727.4.                                  505.16(C)(2)(b). Temperature extremes and highly corro-
                                 (d) Type PLTC cable in accordance with the provisions                        sive liquids and vapors can affect the ability of seals to
                           of Article 725, or in cable tray systems. PLTC shall be                            perform their intended function. See 505.16(D)(2).
                           installed in a manner to avoid tensile stress at the termination                   FPN No. 2: Gas or vapor leakage and propagation of
                           fittings.                                                                          flames may occur through the interstices between the
                                                                                                              strands of standard stranded conductors larger than 2
                              (e) Enclosed gasketed busways, enclosed gasketed wir-                           AWG. Special conductor constructions, for example,
                           eways.                                                                             compacted strands or sealing of the individual strands,
                                                                                                              are means of reducing leakage and preventing the propa-
                              (f) Threaded rigid metal conduit, threaded steel inter-                         gation of flames.
                           mediate metal conduit.
                                (g) Nonincendive field wiring shall be permitted using                 (A) Zone 0 In Class I, Zone 0 locations, seals shall be
                           any of the wiring methods permitted for unclassified loca-                  located according to 505.16(A)(1), (A)(2), and (A)(3).
                           tions. Nonincendive field wiring systems shall be installed
                                                                                                       (1) Conduit Seals Seals shall be provided within 3.05 m
                           in accordance with the control drawing(s). Simple apparatus,
                                                                                                       (10 ft) of where a conduit leaves a Zone 0 location. There
                           not shown on the control drawing, shall be permitted in
                                                                                                       shall be no unions, couplings, boxes, or fittings, except listed
                           a nonincendive field wiring circuit, provided the simple
                                                                                                       reducers at the seal, in the conduit run between the seal and
                           apparatus does not interconnect the nonincendive field wir-
                                                                                                       the point at which the conduit leaves the location.
                           ing circuit to any other circuit.
                                                                                                       Exception: A rigid unbroken conduit that passes completely
                                   FPN: Simple apparatus is defined in 504.2.
                                                                                                       through the Zone 0 location with no fittings less than 300
                                Separate nonincendive field wiring circuits shall be in-               mm (12 in.) beyond each boundary shall not be required to
                           stalled in accordance with one of the following:                            be sealed if the termination points of the unbroken conduit
                                                                                                       are in unclassified locations.
                           (1) In separate cables
                           (2) In multiconductor cables where the conductors of each                   (2) Cable Seals Seals shall be provided on cables at the
                               circuit are within a grounded metal shield                              first point of termination after entry into the Zone 0 location.
                           (3) In multiconductor cables where the conductors of each
                               circuit have insulation with a minimum thickness of                     (3) Not Required to Be Explosionproof or Flameproof
                               0.25 mm (0.01 in.)                                                      Seals shall not be required to be explosionproof or flame-
                                                                                                                                                                                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                       proof.
                           (2) Flexible Connections Where provision must be made
                           for limited flexibility, flexible metal fittings, flexible metal            (B) Zone 1 In Class I, Zone 1 locations, seals shall be
                           conduit with listed fittings, liquidtight flexible metal conduit            located in accordance with 505.16(B)(1) through (B)(8).
                           National Electrical Code Handbook 2005                                                                                                                729

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                 505.16
                  --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                             Article 505 — Class I, Zone 0, 1, and 2 Locations



                                                                    (1) Type of Protection ‘‘d’’ or ‘‘e’’ Enclosures Conduit              trade size of the conduit shall be permitted between the
                                                                    seals shall be provided within 50 mm (2 in.) for each conduit         sealing fitting and the explosionproof enclosure.
                                                                    entering enclosures having type of protection ‘‘d’’ or ‘‘e.’’              (c) Where two or more explosionproof enclosures for
                                                                    Exception No. 1: Where the enclosure having type of protec-           which conduit seals are required under 505.16(B)(2) are
                                                                    tion ‘‘d’’ is marked to indicate that a seal is not required.         connected by nipples or by runs of conduit not more than
                                                                                                                                          900 mm (36 in.) long, a single conduit seal in each such
                                                                    Exception No. 2: For type of protection ‘‘e,’’ conduit and            nipple connection or run of conduit shall be considered
                                                                    fittings employing only NPT to NPT raceway joints or fittings         sufficient if located not more than 450 mm (18 in.) from
                                                                    listed for type of protection ‘‘e’’ shall be permitted between        either enclosure.
                                                                    the enclosure and the seal, and the seal shall not be required
                                                                    to be within 50 mm (2 in.) of the entry.                              (3) Pressurized Enclosures Conduit seals shall be pro-
                                                                                                                                          vided in each conduit entry into a pressurized enclosure
                                                                        FPN: Examples of fittings employing other than NPT
                                                                        threads include conduit couplings, capped elbows,                 where the conduit is not pressurized as part of the protection
                                                                        unions, and breather drains.                                      system. Conduit seals shall be installed within 450 mm (18
                                                                                                                                          in.) from the pressurized enclosure.
                                                                    Exception No. 3: For conduit installed between type of pro-
                                                                    tection ‘‘e’’ enclosures employing only NPT to NPT raceway                   FPN No. 1: Installing the seal as close as possible to
                                                                                                                                                 the enclosure reduces problems with purging the dead
                                                                    joints or conduit fittings listed for type of protection ‘‘e,’’ a            airspace in the pressurized conduit.
                                                                    seal shall not be required.                                                  FPN No. 2: For further information, see NFPA 496-
                                                                                                                                                 2003, Standard for Purged and Pressurized Enclosures
                                                                    (2) Explosionproof Equipment Conduit seals shall be pro-                     for Electrical Equipment.
                                                                    vided for each conduit entering explosionproof equipment
                                                                    according to (B)(2)(a), (B)(2)(b), and (B)(2)(c).                     (4) Class I, Zone 1 Boundary Conduit seals shall be pro-
                                                                                                                                          vided in each conduit run leaving a Class I, Zone 1 location.
                                                                         (a) In each conduit entry into an explosionproof enclo-          The sealing fitting shall be permitted on either side of the
                                                                    sure where either (1) the enclosure contains apparatus, such          boundary of such location within 3.05 m (10 ft) of the
                                                                    as switches, circuit breakers, fuses, relays, or resistors, that      boundary and shall be designed and installed so as to mini-
                                                                    may produce arcs, sparks, or high temperatures that are               mize the amount of gas or vapor within the Zone 1 portion
                                                                    considered to be an ignition source in normal operation, or           of the conduit from being communicated to the conduit
                                                                    (2) the entry is metric designator 53 (trade size 2) or larger        beyond the seal. Except for listed explosionproof reducers
                                                                    and the enclosure contains terminals, splices, or taps. For           at the conduit seal, there shall be no union, coupling, box,
                                                                    the purposes of this section, high temperatures shall be con-         or fitting between the conduit seal and the point at which
                                                                    sidered to be any temperatures exceeding 80 percent of the            the conduit leaves the Zone 1 location.
                                                                    autoignition temperature in degrees Celsius of the gas or
                                                                    vapor involved.                                                       Exception: Metal conduit containing no unions, couplings,
                                                                                                                                          boxes, or fittings and passing completely through a Class
                                                                    Exception: Conduit entering an enclosure where such                   I, Zone 1 location with no fittings less than 300 mm (12 in.)
                                                                    switches, circuit breakers, fuses, relays, or resistors comply        beyond each boundary shall not require a conduit seal if
                                                                    with one of the following:                                            the termination points of the unbroken conduit are in unclas-
                                                                                                                                          sified locations.
                                                                    (1) Are enclosed within a chamber hermetically sealed
                                                                        against the entrance of gases or vapors.                          (5) Cables Capable of Transmitting Gases or Vapors
                                                                    (2) Are immersed in oil.                                              Conduits containing cables with a gas/vaportight continuous
                                                                    (3) Are enclosed within a factory-sealed explosionproof               sheath capable of transmitting gases or vapors through the
                                                                        chamber located within the enclosure, identified for the          cable core shall be sealed in the Zone 1 location after remov-
                                                                        location, and marked ‘‘factory sealed’’ or equivalent,            ing the jacket and any other coverings so that the sealing
                                                                        unless the entry is metric designator 53 (trade size              compound surrounds each individual insulated conductor
                                                                        2) or larger. Factory-sealed enclosures shall not be              and the outer jacket.
                                                                        considered to serve as a seal for another adjacent explo-
                                                                                                                                          Exception: Multiconductor cables with a gas/vaportight
                                                                        sionproof enclosure that is required to have a conduit
                                                                                                                                          continuous sheath capable of transmitting gases or vapors
                                                                        seal.
                                                                                                                                          through the cable core shall be permitted to be considered
                                                                         (b) Conduit seals shall be installed within 450 mm (18           as a single conductor by sealing the cable in the conduit
                                                                    in.) from the enclosure. Only explosionproof unions, cou-             within 450 mm (18 in.) of the enclosure and the cable end
                                                                    plings, reducers, elbows, capped elbows, and conduit bodies           within the enclosure by an approved means to minimize the
                                                                    similar to L, T, and cross types that are not larger than the         entrance of gases or vapors and prevent the propagation of

                 730                                                                                                                                                         2005 National Electrical Code Handbook

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                                                                                                                 Article 505 — Class I, Zone 0, 1, and 2 Locations                                                         505.16



                                                                         flame into the cable core, or by other approved methods.                   shall not be required to be sealed where passing from the
                                                                         For shielded cables and twisted pair cables, it shall not be               Class I, Zone 2 location into the unclassified location. The
                                                                         required to remove the shielding material or separate the                  unclassified location shall be outdoors or, if the conduit
                                                                         twisted pair.                                                              system is all in one room, it shall be permitted to be indoors.
                                                                                                                                                    The conduits shall not terminate at an enclosure containing
                                                                         (6) Cables Incapable of Transmitting Gases or Vapors                       an ignition source in normal operation.
                                                                         Each multiconductor cable in conduit shall be considered
                                                                                                                                                    Exception No. 3: Conduit systems passing from an enclosure
                                                                         as a single conductor if the cable is incapable of transmitting
                                                                                                                                                    or room that is unclassified as a result of pressurization into
                                                                         gases or vapors through the cable core. These cables shall
                                                                                                                                                    a Class I, Zone 2 location shall not require a seal at the
                                                                         be sealed in accordance with 505.16(D).
                                                                                                                                                    boundary.
                                                                         (7) Cables Entering Enclosures Cable seals shall be pro-                          FPN: For further information, refer to NFPA 496-2003,
                                                                         vided for each cable entering flameproof or explosionproof                        Standard for Purged and Pressurized Enclosures for
                                                                         enclosures. The seal shall comply with 505.16(D).                                 Electrical Equipment.
                                                                                                                                                    Exception No. 4: Segments of aboveground conduit systems
                                                                         (8) Class I, Zone 1 Boundary Cables shall be sealed at
                                                                                                                                                    shall not be required to be sealed where passing from a
                                                                         the point at which they leave the Zone 1 location.
                                                                                                                                                    Class I, Zone 2 location into an unclassified location if all
                                                                         Exception: Where cable is sealed at the termination point.                 the following conditions are met:

                                                                         (C) Zone 2 In Class I, Zone 2 locations, seals shall be                    (1) No part of the conduit system segment passes through
                                                                         located in accordance with 505.16(C)(1) and (C)(2).                            a Class I, Zone 0 or Class I, Zone 1 location where the
                                                                                                                                                        conduit contains unions, couplings, boxes, or fittings
                                                                         (1) Conduit Seals Conduit seals shall be located in accor-                     within 300 mm (12 in.) of the Class I, Zone 0 or Class
                                                                         dance with (C)(1)(a) and (C)(1)(b).                                            I, Zone 1 location.
                                                                                                                                                    (2) The conduit system segment is located entirely in out-
                                                                             (a) For connections to enclosures that are required to                     door locations.
                                                                         be flameproof or explosionproof, a conduit seal shall be                   (3) The conduit system segment is not directly connected
                                                                         provided in accordance with 505.16(B)(1) and 505.16(B)(2).                     to canned pumps, process or service connections for
                                                                         All portions of the conduit run or nipple between the seal
                       --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                        flow, pressure, or analysis measurement, and so forth,
                                                                         and such enclosure shall comply with 505.16(B).                                that depend on a single compression seal, diaphragm,
                                                                              (b) In each conduit run passing from a Class I, Zone                      or tube to prevent flammable or combustible fluids from
                                                                         2 location into an unclassified location. The sealing fitting                  entering the conduit system.
                                                                         shall be permitted on either side of the boundary of such                  (4) The conduit system segment contains only threaded
                                                                         location within 3.05 m (10 ft) of the boundary and shall be                    metal conduit, unions, couplings, conduit bodies, and
                                                                         designed and installed so as to minimize the amount of gas                     fittings in the unclassified location.
                                                                         or vapor within the Zone 2 portion of the conduit from being               (5) The conduit system segment is sealed at its entry to
                                                                         communicated to the conduit beyond the seal. Rigid metal                       each enclosure or fitting housing terminals, splices, or
                                                                         conduit or threaded steel intermediate metal conduit shall                     taps in Class I, Zone 2 locations.
                                                                         be used between the sealing fitting and the point at which
                                                                         the conduit leaves the Zone 2 location, and a threaded con-                (2) Cable Seals Cable seals shall be located in accordance
                                                                         nection shall be used at the sealing fitting. Except for listed            with (C)(2)(a), (C)(2)(b), and (C)(2)(c).
                                                                         explosionproof reducers at the conduit seal, there shall be                     (a) Explosionproof and Flameproof Enclosures. Cables
                                                                         no union, coupling, box, or fitting between the conduit seal               entering enclosures required to be flameproof or explo-
                                                                         and the point at which the conduit leaves the Zone 2 location.             sionproof shall be sealed at the point of entrance. The seal
                                                                         Exception No. 1: Metal conduit containing no unions, cou-                  shall comply with 505.16(D). Multiconductor cables with a
                                                                         plings, boxes, or fittings and passing completely through a                gas/vaportight continuous sheath capable of transmitting
                                                                         Class I, Zone 2 location with no fittings less than 300 mm                 gases or vapors through the cable core shall be sealed in
                                                                         (12 in.) beyond each boundary shall not be required to be                  the Zone 2 location after removing the jacket and any other
                                                                         sealed if the termination points of the unbroken conduit are               coverings so that the sealing compound surrounds each indi-
                                                                         in unclassified locations.                                                 vidual insulated conductor in such a manner as to minimize
                                                                                                                                                    the passage of gases and vapors. Multiconductor cables in
                                                                         Exception No. 2: Conduit systems terminating at an unclas-                 conduit shall be sealed as described in 505.16(B)(4).
                                                                         sified location where a wiring method transition is made to
                                                                         cable tray, cablebus, ventilated busway, Type MI cable, or                 Exception No. 1: Cables passing from an enclosure or room
                                                                         cable that is not installed in a raceway or cable tray system              that is unclassified as a result of Type Z pressurization into

                                                                         National Electrical Code Handbook 2005                                                                                                               731

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                 505.16                                        Article 505 — Class I, Zone 0, 1, and 2 Locations



                    a Class I, Zone 2 location shall not require a seal at the             be affected by the surrounding atmosphere or liquids, and
                    boundary.                                                              shall not have a melting point less than 93 C (200 F).
                    Exception No. 2: Shielded cables and twisted pair cables
                    shall not require the removal of the shielding material or             (3) Thickness of Compounds In a completed seal, the min-
                    separation of the twisted pairs, provided the termination is           imum thickness of the sealing compound shall not be less
                    by an approved means to minimize the entrance of gases or              than the trade size of the sealing fitting and, in no case, less
                    vapors and prevent propagation of flame into the cable core.           than 16 mm (5⁄8 in.).
                                                                                           Exception: Listed cable sealing fittings shall not be required
                         (b) Cables That Will Not Transmit Gases or Vapors.                to have a minimum thickness equal to the trade size of the
                    Cables with a gas/vaportight continuous sheath and that will           fitting.
                    not transmit gases or vapors through the cable core in excess
                    of the quantity permitted for seal fittings shall not be required      (4) Splices and Taps Splices and taps shall not be made
                    to be sealed except as required in 505.16(C)(2)(a). The mini-          in fittings intended only for sealing with compound, nor
                    mum length of such cable run shall not be less than the                shall other fittings in which splices or taps are made be filled
                    length that limits gas or vapor flow through the cable core            with compound.
                    to the rate permitted for seal fittings [200 cm3/hr (0.007 ft3/
                    hr) of air at a pressure of 1500 pascals (6 in. of water)].            (5) Conductor Fill The cross-sectional area of the conduc-
                                                                                           tors permitted in a seal shall not exceed 25 percent of the
                            FPN No. 1: See ANSI/UL 886-1994, Outlet Boxes and              cross-sectional area of a rigid metal conduit of the same trade
                            Fittings for Use in Hazardous (Classified) Locations.
                                                                                           size unless it is specifically listed for a higher percentage of
                            FPN No. 2: The cable core does not include the inter-
                            stices of the conductor strands.                               fill.

                         (c) Cables Capable of Transmitting Gases or Vapors.               (E) Drainage
                    Cables with a gas/vaportight continuous sheath capable of
                                                                                           (1) Control Equipment Where there is a probability that
                    transmitting gases or vapors through the cable core shall not
                                                                                           liquid or other condensed vapor may be trapped within enclo-
                    be required to be sealed except as required in
                                                                                           sures for control equipment or at any point in the raceway
                    505.16(C)(2)(a), unless the cable is attached to process
                                                                                           system, approved means shall be provided to prevent accu-
                    equipment or devices that may cause a pressure in excess
                                                                                           mulation or to permit periodic draining of such liquid or
                    of 1500 pascals (6 in. of water) to be exerted at a cable end,
                                                                                           condensed vapor.
                    in which case a seal, barrier, or other means shall be provided
                    to prevent migration of flammables into an unclassified area.          (2) Motors and Generators Where the authority having




                                                                                                                                                                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    Exception: Cables with an unbroken gas/vaportight contin-              jurisdiction judges that there is a probability that liquid or
                    uous sheath shall be permitted to pass through a Class I,              condensed vapor may accumulate within motors or genera-
                    Zone 2 location without seals.                                         tors, joints and conduit systems shall be arranged to minimize
                                                                                           entrance of liquid. If means to prevent accumulation or to
                         (d) Cables Without Gas/Vaportight Continuous Sheath.              permit periodic draining are judged necessary, such means
                    Cables that do not have gas/vaportight continuous sheath               shall be provided at the time of manufacture and shall be
                    shall be sealed at the boundary of the Zone 2 and unclassified         considered an integral part of the machine.
                    location in such a manner as to minimize the passage of
                    gases or vapors into an unclassified location.                         (3) Canned Pumps, Process, or Service Connections, and
                            FPN: The cable sheath may be either metal or a nonme-
                                                                                           So Forth For canned pumps, process, or service connec-
                            tallic material.                                               tions for flow, pressure, or analysis measurement, and so
                                                                                           forth, that depend on a single compression seal, diaphragm,
                    (D) Class I, Zones 0, 1, and 2 Where required, seals in                or tube to prevent flammable or combustible fluids from
                    Class I, Zones 0, 1, and 2 locations shall comply with                 entering the electrical conduit system, an additional ap-
                    505.16(D)(1) through (D)(5).                                           proved seal, barrier, or other means shall be provided to
                    (1) Fittings Enclosures for connections or equipment shall             prevent the flammable or combustible fluid from entering
                    be provided with an integral means for sealing, or sealing             the conduit system beyond the additional devices or means
                    fittings listed for the location shall be used. Sealing fittings       if the primary seal fails.
                    shall be listed for use with one or more specific compounds                 The additional approved seal or barrier and the intercon-
                    and shall be accessible.                                               necting enclosure shall meet the temperature and pressure
                                                                                           conditions to which they will be subjected upon failure of
                    (2) Compound The compound shall provide a seal against                 the primary seal, unless other approved means are provided
                    passage of gas or vapors through the seal fitting, shall not           to accomplish the purpose in the preceding paragraph.


                 732                                                                                                          2005 National Electrical Code Handbook

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                                                                         Article 505 — Class I, Zone 0, 1, and 2 Locations                                                          505.20



                               Drains, vents, or other devices shall be provided so that                spares) that enters Type ‘‘e’’ equipment shall be terminated
                           primary seal leakage is obvious.                                             at a Type ‘‘e’’ terminal.
                                   FPN: See also the fine print notes to 505.16.                        (B) Conductor Insulation Where condensed vapors or liq-
                               Process-connected equipment that is listed and marked                    uids may collect on, or come in contact with, the insulation
                           ‘‘Dual Seal’’ shall not require additional process sealing                   on conductors, such insulation shall be of a type identified
                           when used within the manufacturer’s ratings.                                 for use under such conditions, or the insulation shall be
                                                                                                        protected by a sheath of lead or by other approved means.
                                   FPN: For construction and testing requirements for dual
                                   seal process, connected equipment, refer to ISA 12.27.01,
                                   Requirements for Process Sealing Between Electrical
                                                                                                        505.19 Uninsulated Exposed Parts
                                   Systems and Potentially Flammable or Combustible Pro-                There shall be no uninsulated exposed parts, such as electric
                                   cess Fluids.                                                         conductors, buses, terminals, or components that operate at
                                                                                                        more than 30 volts (15 volts in wet locations). These parts
                           505.17 Flexible Cords, Class I, Zones 1 and 2                                shall additionally be protected by type of protection ia, ib,
                           A flexible cord shall be permitted for connection between                    or nA that is suitable for the location.
                           portable lighting equipment or other portable utilization
                           equipment and the fixed portion of their supply circuit. Flexi-              505.20 Equipment Requirements
                           ble cord shall also be permitted for that portion of the circuit             (A) Zone 0 In Class I, Zone 0 locations, only equipment
                           where the fixed wiring methods of 505.15(B) cannot provide                   specifically listed and marked as suitable for the location
                           the necessary degree of movement for fixed and mobile                        shall be permitted.
                           electrical utilization equipment, in an industrial establish-
                           ment where conditions of maintenance and engineering su-                     Exception: Intrinsically safe apparatus listed for use in
                           pervision ensure that only qualified persons install and                     Class I, Division 1 locations for the same gas, or as permitted
                           service the installation, and the flexible cord is protected by              by 505.9(B)(2), and with a suitable temperature class shall
                           location or by a suitable guard from damage. The length of                   be permitted.
                           the flexible cord shall be continuous. Where flexible cords
                           are used, the cords shall comply with all of the following:                  The exception to 505.20(A) results from the fact that ANSI/
                                                                                                        UL 913, the standard used to evaluate intrinsically safe
                           (1) Be of a type listed for extra-hard usage
                                                                                                        systems for Class I, Division 1 locations, is based on the
                           (2) Contain, in addition to the conductors of the circuit, a
                                                                                                        IEC requirements for intrinsically safe equipment for Class
                               grounding conductor complying with 400.23
                                                                                                        I, Zone 0 locations.
                           (3) Be connected to terminals or to supply conductors in
                               an approved manner
                           (4) Be supported by clamps or by other suitable means in                     (B) Zone 1 In Class I, Zone 1 locations, only equipment
                               such a manner that there will be no tension on the                       specifically listed and marked as suitable for the location
                               terminal connections                                                     shall be permitted.
                           (5) Be provided with listed seals where the flexible cord
                                                                                                        Exception No. 1: Equipment identified for use in Class I,
                               enters boxes, fittings, or enclosures that are required to
                                                                                                        Division 1 or listed for use in Class I, Zone 0 locations for
                               be explosionproof or flameproof
                                                                                                        the same gas, or as permitted by 505.9(B)(2), and with a
                           Exception: As provided in 505.16.                                            suitable temperature class shall be permitted.
                                Electric submersible pumps with means for removal                       Exception No. 2: Equipment identified for Class I, Zone 1,
                           without entering the wet-pit shall be considered portable                    or Zone 2 type of protection ‘‘p’’ shall be permitted.
                           utilization equipment. The extension of the flexible cord
                                                                                                        (C) Zone 2 In Class I, Zone 2 locations, only equipment
                           within a suitable raceway between the wet-pit and the power
                                                                                                        specifically listed and marked as suitable for the location
                           source shall be permitted.
                                                                                                        shall be permitted.
                                Electric mixers intended for travel into and out of open-
                           type mixing tanks or vats shall be considered portable utiliza-              Exception No. 1: Equipment listed for use in Class I, Zone
                           tion equipment.                                                              0 or Zone 1 locations for the same gas, or as permitted by
                                                                                                        505.9(B)(2), and with a suitable temperature class, shall be
                                   FPN: See 505.18 for flexible cords exposed to liquids
                                   having a deleterious effect on the conductor insulation.             permitted.
                                                                                                        Exception No. 2: Equipment identified for Class I, Zone 1
                           505.18 Conductors and Conductor Insulation                                   or Zone 2 type of protection ‘‘p’’ shall be permitted.
                           (A) Conductors For type of protection ‘‘e,’’ field wiring                    Exception No. 3: Equipment identified for use in Class I,
                           conductors shall be copper. Every conductor (including                       Division 1 or Division 2 locations for the same gas, or as

                           National Electrical Code Handbook 2005                                                                                                                      733
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                 505.21                                             Article 505 — Class I, Zone 0, 1, and 2 Locations



                    permitted by 505.9(B)(2), and with a suitable temperature                          ternal grounding means between the motor frame and
                    class shall be permitted.                                                          the equipment grounding connection is incorporated
                    Exception No. 4: In Class I, Zone 2 locations, the installa-                       within the housing.
                    tion of open or nonexplosionproof or nonflameproof en-                      (5)    The provisions of Part III of Article 430 shall apply
                    closed motors, such as squirrel-cage induction motors                              regardless of the voltage rating of the motor.
                    without brushes, switching mechanisms, or similar arc-                      (6)    The motors shall be protected against overload by a
                    producing devices that are not identified for use in a Class                       separate overload device that is responsive to motor
                    I, Zone 2 location shall be permitted.                                             current. This device shall be selected to trip or shall be
                            FPN No. 1: It is important to consider the temperature                     rated in accordance with the listing of the motor and
                            of internal and external surfaces that may be exposed to                   its overload protection.
                            the flammable atmosphere.                                           (7)    Sections 430.32(C) and 430.44 shall not apply to such
                            FPN No. 2: It is important to consider the risk of ignition                motors.
                            due to currents arcing across discontinuities and overheat-         (8)    The motor overload protection shall not be shunted or
                            ing of parts in multisection enclosures of large motors                    cut out during the starting period.
                            and generators. Such motors and generators may need
                            equipotential bonding jumpers across joints in the enclo-
                            sure and from enclosure to ground. Where the presence               505.25 Grounding and Bonding
                            of ignitible gases or vapors is suspected, clean air purging
                            may be needed immediately prior to and during start-up
                                                                                                Grounding and bonding shall comply with Article 250 and
                            periods.                                                            the requirements in 505.25(A) and 505.25(B).

                                                                                                (A) Bonding The locknut-bushing and double-locknut
                    See the commentary following 501.125(B), FPN No. 2, for                     types of contacts shall not be depended on for bonding
                    more information on electric motors installed in hazardous                  purposes, but bonding jumpers with proper fittings or other
                    (classified) locations.                                                     approved means of bonding shall be used. Such means of
                                                                                                bonding shall apply to all intervening raceways, fittings,




                                                                                                                                                                              --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                    (D) Manufacturer’s Instructions Electrical equipment in-
                                                                                                boxes, enclosures, and so forth, between Class I locations
                    stalled in hazardous (classified) locations shall be installed
                                                                                                and the point of grounding for service equipment or point
                    in accordance with the instructions (if any) provided by the
                                                                                                of grounding of a separately derived system.
                    manufacturer.
                                                                                                Exception: The specific bonding means shall be required
                    505.21 Multiwire Branch Circuits
                                                                                                only to the nearest point where the grounded circuit conduc-
                    In a Class I, Zone 1 location, a multiwire branch circuit                   tor and the grounding electrode are connected together on
                    shall not be permitted.                                                     the line side of the building or structure disconnecting means
                    Exception: Where the disconnect device(s) for the circuit                   as specified in 250.32(A), (B), and (C), provided the branch-
                    opens all ungrounded conductors of the multiwire circuit                    circuit overcurrent protection is located on the load side of
                    simultaneously.                                                             the disconnecting means.
                    505.22 Increased Safety ‘‘e’’ Motors                                               FPN: See 250.100 for additional bonding requirements
                    and Generators                                                                     in hazardous (classified) locations.
                    In Class I, Zone 1 locations, Increased Safety ‘‘e’’ motors
                                                                                                (B) Types of Equipment Grounding Conductors Where
                    and generators of all voltage ratings shall be listed for Class I,
                                                                                                flexible metal conduit or liquidtight flexible metal conduit
                    Zone 1 locations, and shall comply with all of the following:
                                                                                                is used as permitted in 505.15(C) and is to be relied on to
                    (1) Motors shall be marked with the current ratio, IA/IN,                   complete a sole equipment grounding path, it shall be in-
                        and time, tE.                                                           stalled with internal or external bonding jumpers in parallel
                    (2) Motors shall have controllers marked with the model                     with each conduit and complying with 250.102.
                        or identification number, output rating (horsepower or
                        kilowatt), full-load amperes, starting current ratio (IA/               Exception: In Class I, Zone 2 locations, the bonding jumper
                        IN), and time (tE) of the motors that they are intended                 shall be permitted to be deleted where all of the following
                        to protect; the controller marking shall also include                   conditions are met:
                        the specific overload protection type (and setting, if                       (a) Listed liquidtight flexible metal conduit 1.8 m (6 ft)
                        applicable) that is listed with the motor or generator.                 or less in length, with fittings listed for grounding, is used.
                    (3) Connections shall be made with the specific terminals
                        listed with the motor or generator.                                         (b) Overcurrent protection in the circuit is limited to
                    (4) Terminal housings shall be permitted to be of substan-                  10 amperes or less.
                        tial, nonmetallic, nonburning material, provided an in-                        (c) The load is not a power utilization load.

                 734                                                                                                               2005 National Electrical Code Handbook

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                                                 Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings                                             506.2



                           See the commentary for 501.30 for information on grounding                        506.17       Flexible Cords
                           and bonding requirements in Class I locations.                                    506.20       Equipment Installation
                                                                                                                (A)       Zone 20
                                                                                                                (B)       Zone 21
                                     ARTICLE 506                                                                (C)       Zone 22
                                                                                                                (D)       Manufacturer’s Instructions
                            Zone 20, 21, and 22 Locations for                                                   (E)       Temperature
                             Combustible Dusts, Fibers, and                                                  506.21       Grounding and Bonding
                                                                                                                (A)       Bonding
                                         Flyings                                                                (B)       Types of Equipment Grounding Conductors

                           Summary of Changes
                                                                                                             506.1 Scope
                           • Added article providing an alternative zone (IEC-based)
                             system for locations that include combustible concentra-                        This article covers the requirements for the zone classifica-
                             tions of combustible dust, or of ignitible fibers and flyings.                  tion system as an alternative to the division classification
                                                                                                             system covered in Article 500, Article 502, and Article 503
                                                                                                             for electrical and electronic equipment and wiring for all
                                                                                                             voltages in Zone 20, Zone 21, and Zone 22 hazardous (classi-
                           Contents                                                                          fied) locations where fire and explosion hazards may exist
                            506.1 Scope                                                                      due to combustible dusts, or ignitible fibers or flyings. Com-
                            506.2 Definitions                                                                bustible metallic dusts are not covered by the requirements
                                                                                                             of this article.
                            506.4 General
                              (A) Documentation for Industrial Occupancies                                          FPN No. 1: For the requirements for electrical and elec-
                              (B) Reference Standards                                                               tronic equipment and wiring for all voltages in Class I,
                            506.5 Classification of Locations                                                       Division 1 or Division 2; Class II, Division 1 or Division
                                                                                                                    2; Class III, Division 1 or Division 2; and Class I, Zone
                              (A) Classifications of Locations                                                      0 or Zone 1 or Zone 2 hazardous (classified) locations
                              (B) Zone 20, Zone 21, and Zone 22 Locations                                           where fire or explosion hazards may exist due to flamma-
                            506.6 Special Precaution                                                                ble gases or vapors, flammable liquids, or combustible
                              (A) Implementation of Zone Classification System                                      dusts or fibers, refer to Articles 500 through 505.
                                                                                                                    FPN No. 2: Zone 20, Zone 21, and Zone 22 area classifi-
                              (B) Dual Classification
                                                                                                                    cations are based on the modified IEC area classification
                              (C) Reclassification Permitted                                                        system as defined in ISA 12.10.05, Electrical Apparatus
                              (D) Simultaneous Presence of Flammable Gases and                                      for Use in Zone 20, Zone 21, and Zone 22 Hazardous
                                   Combustible Dusts, Fibers, or Flyings                                            (Classified) Locations — Classification of Zone 20, Zone
                                                                                                                    21, and Zone 22 Hazardous (Classified) Locations
                            506.8 Protection Techniques
                                                                                                                    (IEC61241-10 Mod).
                              (A) Dust Ignitionproof                                                                FPN No. 3: The unique hazards associated with explo-
                              (B) Pressurized                                                                       sives, pyrotechnics, and blasting agents are not addressed
                              (C) Intrinsic Safety                                                                  in this article.
                              (D) Dusttight
                              (E) Nonincendive Circuit                                                       506.2 Definitions
                               (F) Nonincendive Equipment                                                    For purposes of this article, the following definitions apply.
                            506.9 Equipment Requirements
                              (A) Suitability                                                                Associated Nonincendive Field Wiring Apparatus. Ap-
                                                                                                             paratus in which the circuits are not necessarily nonincendive
                              (B) Listing
                                                                                                             themselves but that affect the energy in nonincendive field
                              (C) Marking
                                                                                                             wiring circuits and are relied upon to maintain nonincendive
                              (D) Temperature Classfications
                                                                                                             energy levels. Associated nonincendive field wiring appara-
                              (E) Threading
                                                                                                             tus may be either of the following:
                           506.15 Wiring Methods
                              (A) Zone 20                                                                    (1) Electrical apparatus that has an alternative type of pro-
                              (B) Zone 21                                                                        tection for use in the appropriate hazardous (classified)
                              (C) Zone 22                                                                        location
                           506.16 Sealing                                                                    (2) Electrical apparatus not so protected that shall not be
                                                                                                                 used in a hazardous (classified) location


                           National Electrical Code Handbook 2005                                                                                                                    735
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                 506.4                                                                  Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings



                                                                        FPN: Associated nonincendive field wiring apparatus                   FPN: Conditions are described in ANSI/ISA 12.12.01-
                                                                        has designated associated nonincendive field wiring ap-               2000, Nonincendive Electrical Equipment for Use in
                                                                        paratus connections for nonincendive field wiring appara-             Class I and II, Division 2, and Class III, Divisions 1 and
                                                                        tus and may also have connections for other electrical                2 Hazardous (Classified) Locations.
                                                                        apparatus.
                                                                                                                                       Pressurized. The process of supplying an enclosure with a
                                                                    Dust-Ignitionproof. Equipment enclosed in a manner that            protective gas with or without continuous flow at sufficient
                                                                    excludes dusts and does not permit arcs, sparks, or heat           pressure to prevent the entrance of combustible dust, or an
                                                                    otherwise generated or liberated inside of the enclosure to        ignitible fiber or flying.
                                                                    cause ignition of exterior accumulations or atmospheric sus-
                                                                                                                                              FPN: For further information, see ANSI/ NFPA 496-
                                                                    pensions of a specified dust on or in the vicinity of the                 2003, Purged and Pressurized Enclosures for Electrical
                  --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                    enclosure.                                                                Equipment.
                                                                        FPN: For further information on dust-ignitionproof en-         Zone 20 Hazardous (Classified) Location. An area where
                                                                        closures, see Type 9 enclosure in ANSI/NEMA 250-               combustible dust or ignitible fibers and flyings are present
                                                                        1991, Enclosures for Electrical Equipment, and ANSI/
                                                                                                                                       continuously or for long periods of time in quantities suffi-
                                                                        UL 1203-1994, Explosionproof and Dust-Ignitionproof
                                                                        Electrical Equipment for Hazardous (Classified) Loca-          cient to be hazardous, as classified by 506.5(B)(1).
                                                                        tions.
                                                                                                                                       Zone 21 Hazardous (Classified) Location. An area where
                                                                    Dusttight. Enclosures constructed so that dust will not enter      combustible dust or ignitible fibers and flyings are likely
                                                                    under specified test conditions.                                   to exist occasionally under normal operation in quantities
                                                                                                                                       sufficient to be hazardous, as classified by 506.5(B)(2).
                                                                        FPN No. 1: See ANSI/ISA 12.12.01-2000, Nonincendive
                                                                        Electrical Equipment for Use in Class I and II, Division       Zone 22 Hazardous (Classified) Location. An area where
                                                                        2, and Class III, Divisions 1 and 2 Hazardous (Classified)     combustible dust or ignitible fibers and flyings are not likely
                                                                        Locations, and UL 1604-1994, Electrical Equipment for          to occur under normal operation in quantities sufficient to
                                                                        Use in Class I and II, Division 2 and Class III Hazardous      be hazardous, as classified by 506.5(B)(3).
                                                                        (Classified) Locations.

                                                                    Nonincendive Circuit. A circuit, other than field wiring,          506.4 General
                                                                    in which any arc or thermal effect produced under intended         (A) Documentation for Industrial Occupancies Areas
                                                                    operating conditions of the equipment is not capable, under        designated as hazardous (classified) locations shall be prop-
                                                                    specified test conditions, of igniting the flammable gas–air,      erly documented. This documentation shall be available to
                                                                    vapor–air, or dust–air mixture.                                    those authorized to design, install, inspect, maintain or oper-
                                                                                                                                       ate electrical equipment.
                                                                        FPN: Conditions are described in ANSI/ISA 12.12.01-
                                                                        2000, Nonincendive Electrical Equipment for Use in             (B) Reference Standards Important information relating
                                                                        Class I and II, Division 2, and Class III, Divisions 1 and
                                                                        2 Hazardous (Classified) Locations.
                                                                                                                                       to topics covered in Chapter 5 are found in other publications.
                                                                                                                                              FPN: It is important that the authority having jurisdiction
                                                                    Nonincendive Equipment. Equipment having electrical/                      be familiar with the recorded industrial experience as
                                                                    electronic circuitry that is incapable, under normal operating            well as with standards of the National Fire Protection
                                                                    conditions, of causing ignition of a specified flammable                  Association (NFPA), the ISA, International Society for
                                                                    gas–air, vapor–air, or dust–air mixture due to arcing or ther-            Measurement and Control, and the International Electro-
                                                                    mal means.                                                                technical Commission (IEC) that may be of use in the
                                                                                                                                              classification of various locations, the determination of
                                                                        FPN: Conditions are described in ANSI/ISA 12.12.01-                   adequate ventilation, and the protection against static
                                                                        2000, Nonincendive Electrical Equipment for Use in                    electricity and lightning hazards.
                                                                        Class I and II, Division 2, and Class III, Divisions 1 and
                                                                        2 Hazardous (Classified) Locations.                            506.5 Classification of Locations
                                                                    Nonincendive Field Wiring. Wiring that enters or leaves            (A) Classifications of Locations Locations shall be classi-
                                                                    an equipment enclosure and, under normal operating condi-          fied on the basis of the properties of the combustible dust,
                                                                    tions of the equipment, is not capable, due to arcing or           ignitible fibers or flyings that may be present, and the likeli-
                                                                    thermal effects, of igniting the flammable gas–air, vapor–air,     hood that a combustible or combustible concentration or
                                                                    or dust–air mixture. Normal operation includes opening,            quantity is present. Each room, section, or area shall be
                                                                    shorting, or grounding the field wiring.                           considered individually in determining its classification.
                                                                                                                                       Where pyrophoric materials are the only materials used or
                                                                    Nonincendive Field Wiring Apparatus. Apparatus in-                 handled, these locations are outside of the scope of this
                                                                    tended to be connected to nonincendive field wiring.               article.

                 736                                                                                                                                                      2005 National Electrical Code Handbook

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                                                   Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings                                       506.5



                           (B) Zone 20, Zone 21, and Zone 22 Locations Zone 20,                                 for Use In Zone 20, Zone 21, and Zone 22 Hazardous
                           Zone 21, and Zone 22 locations are those in which combusti-                          (Classified) Locations — Classification of Zone 20, Zone
                                                                                                                21, and Zone 22 Hazardous (Classified) Locations
                           ble dust, ignitible fibers, or flyings are or may be present                         (IEC61241-10 Mod).
                           in the air or in layers, in quantities sufficient to produce                         FPN No. 2: This classification usually includes locations
                           explosive or ignitible mixtures. Zone 20, Zone 21, and Zone                          outside dust containment and in the immediate vicinity
                           22 locations shall include those specified in 506.5(B)(1),                           of access doors subject to frequent removal or opening
                           (B)(2), and (B)(3).                                                                  for operation purposes when internal combustible mix-
                                                                                                                tures are present; locations outside dust containment in
                                   FPN: Through the exercise of ingenuity in the layout of                      the proximity of filling and emptying points, feed belts,
                                   electrical installations for hazardous (classified) loca-                    sampling points, truck dump stations, belt dump over
                                   tions, it is frequently possible to locate much of the                       points, etc. where no measures are employed to prevent
                                   equipment in a reduced level of classification, and, thus,                   the formation of combustible mixtures; locations outside
                                   to reduce the amount of special equipment required.                          dust containment where dust accumulates and where due
                                                                                                                to process operations the dust layer is likely to be dis-
                           (1) Zone 20 A Zone 20 location is a location in which                                turbed and form combustible mixtures; locations inside
                                                                                                                dust containment where explosive dust clouds are likely
                                (a) Ignitible concentrations of combustible dust or ig-                         to occur (but neither continuously, nor for long periods,
                           nitible fibers or flyings are present continuously.                                  nor frequently) as, for example, silos (if filled and/or
                                                                                                                emptied only occasionally) and the dirty side of filters
                                (b) Ignitible concentrations of combustible dust or ig-                         if large self-cleaning intervals are occurring.




                                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           nitible fibers or flyings are present for long periods of time.
                                                                                                         (3) Zone 22 A Zone 22 location is a location
                                   FPN No. 1: As a guide to classification of Zone 20
                                   locations, refer to ISA 12.10.05, Electrical Apparatus                     (a) In which ignitible concentrations of combustible
                                   for Use in Zone 20, Zone 21, and Zone 22 Hazardous                    dust or ignitible fibers or flyings are not likely to occur in
                                   (Classified) Locations — Classification of Zone 20, Zone              normal operation, and if they do occur, will only persist for
                                   21, and Zone 22 Hazardous (Classified) Locations
                                   (IEC61241-10 Mod).                                                    a short period; or
                                   FPN No. 2: Zone 20 classification includes locations                       (b) In which combustible dust, or fibers, or flyings are
                                   inside dust containment systems; hoppers, silos, etc., cy-            handled, processed, or used but in which the dust, fibers, or
                                   clones and filters, dust transport systems, except some               flyings are normally confined within closed containers of
                                   parts of belt and chain conveyors, etc.; blenders, mills,
                                   dryers, bagging equipment, etc.
                                                                                                         closed systems from which they can escape only as a result
                                                                                                         of the abnormal operation of the equipment with which the
                           (2) Zone 21 A Zone 21 location is a location                                  dust, or fibers, or flyings are handled, processed, or used;
                                                                                                         or
                                (a) In which ignitible concentrations of combustible
                                                                                                              (c) That is adjacent to a Zone 21 location, from which
                           dust or ignitible fibers or flyings are likely to exist occasion-
                                                                                                         ignitible concentrations of dust or fibers or flyings could be
                           ally under normal operating conditions; or
                                                                                                         communicated, unless such communication is prevented by
                               (b) In which ignitible concentrations of combustible                      adequate positive pressure ventilation from a source of clean
                           dust or ignitible fibers or flyings may exist frequently be-                  air and effective safeguards against ventilation failure are
                           cause of repair or maintenance operations or because of                       provided.
                           leakage; or
                                                                                                                FPN No. 1: As a guide to classification of Zone 22
                                (c) In which equipment is operated or processes are                             locations, refer to ISA 12.10.05, Electrical Apparatus
                           carried on, of such a nature that equipment breakdown or                             for Use in Zone 20, Zone 21, and Zone 22 Hazardous
                           faulty operations could result in the release of ignitible con-                      (Classified) Locations — Classification of Zone 20, Zone
                           centrations of combustible dust, or ignitible fibers or flyings                      21, and Zone 22 Hazardous (Classified) Locations
                           and also cause simultaneous failure of electrical equipment                          (IEC61241-10 Mod).
                           in a mode to cause the electrical equipment to become a                              FPN No. 2: Zone 22 locations usually include outlets
                                                                                                                from bag filter vents, because in the event of a malfunc-
                           source of ignition; or                                                               tion there can be emission of combustible mixtures; loca-
                                (d) That is adjacent to a Zone 20 location from which                           tions near equipment that has to be opened at infrequent
                           ignitible concentrations of dust or ignitible fibers or flyings                      intervals or equipment that from experience can easily
                                                                                                                form leaks where, due to pressure above atmospheric,
                           could be communicated, unless communication is prevented                             dust will blow out; pneumatic equipment, flexible con-
                           by adequate positive pressure ventilation from a source of                           nections that can become damaged, etc.; storage locations
                           clean air and effective safeguards against ventilation failure                       for bags containing dusty product, since failure of bags
                           are provided.                                                                        can occur during handling, causing dust leakage; and
                                                                                                                locations where controllable dust layers are formed that
                                   FPN No. 1: As a guide to classification of Zone 21                           are likely to be raised into explosive dust/air mixtures.
                                   locations, refer to ISA 12.10.05, Electrical Apparatus                       Only if the layer is removed by cleaning before hazardous


                           National Electrical Code Handbook 2005                                                                                                                737

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                 506.6                           Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings



                            dust–air mixtures can be formed is the area designated                                 (B) Pressurized This protection technique shall be permit-
                            non-hazardous.                                                                         ted for equipment in Zone 21, and Zone 22 locations for
                            FPN No. 3: Locations that normally are classified as                                   which it is identified.
                            Zone 21 can fall into Zone 22 when measures are em-
                            ployed to prevent the formation of explosive dust–air
                                                                                                                   (C) Intrinsic Safety This protection technique shall be per-
                            mixtures. Such measures include exhaust ventilation. The
                            measures should be used in the vicinity of (bag) filling                               mitted for equipment in Zone 20, Zone 21, and Zone 22
                            and emptying points, feed belts, sampling points, truck                                locations for which it is identified. Installation of intrinsically
                            dump stations, belt dump over points, etc.                                             safe apparatus and wiring shall be in accordance with the
                                                                                                                   requirements of Article 504.
                    506.6 Special Precaution
                                                                                                                   (D) Dusttight This protection technique shall be permitted
                    Article 506 requires equipment construction and installation                                   for equipment in Zone 22 locations for which it is identified.
                    that ensures safe performance under conditions of proper
                    use and maintenance.                                                                           (E) Nonincendive Circuit This protection technique shall
                                                                                                                   be permitted for equipment in Zone 22 locations for which
                            FPN: It is important that inspection authorities and users
                                                                                                                   it is identified.
                            exercise more than ordinary care with regard to the instal-
                            lation and maintenance of electrical equipment in hazard-
                            ous (classified) locations.                                                            (F) Nonincendive Equipment This protection technique
                                                                                                                   shall be permitted for equipment in Zone 22 locations for
                    (A) Implementation of Zone Classification System Clas-                                         which it is identified.
                    sification of areas, engineering and design, selection of
                    equipment and wiring methods, installation, and inspection                                     506.9 Equipment Requirements
                    shall be performed by qualified persons.                                                       (A) Suitability Suitability of identified equipment shall be
                                                                                                                   determined by one of the following:
                    (B) Dual Classification In instances of areas within the
                    same facility classified separately, Zone 22 locations shall                                    (1) Equipment listing or labeling
                    be permitted to abut, but not overlap, Class II or Class III,                                   (2) Evidence of equipment evaluation from a qualified test-
                    Division 2 locations. Zone 20 or Zone 21 locations shall                                            ing laboratory or inspection agency concerned with
                    not abut Class II or Class III, Division 1 or Division 2                                            product evaluation
                    locations.                                                                                      (3) Evidence acceptable to the authority having jurisdiction
                                                                                                                        such as a manufacturer’s self-evaluation or an owner’s
                    (C) Reclassification Permitted A Class II or Class III,                                             engineering judgment
                    Division 1 or Division 2 location shall be permitted to be
                    reclassified as a Zone 20, Zone 21, or Zone 22 location,                                       (B) Listing
                    provided that all of the space that is classified because of a                                  (1) Equipment that is listed for Zone 20 shall be permitted
                    single combustible dust or ignitible fiber or flying source is                                      in a Zone 21 or Zone 22 location of the same dust, or
                    reclassified under the requirements of this article.                                                ignitible fiber, or flying. Equipment that is listed for
                                                                                                                        Zone 21 may be used in a Zone 22 location of the same
                    (D) Simultaneous Presence of Flammable Gases and                                                    dust, fiber, or flying.
                    Combustible Dusts, Fibers, or Flyings Where flammable                                           (2) Equipment shall be permitted to be listed for a specific
                    gases or combustible dusts, fibers, or flyings are or may be                                        dust, or ignitible fiber or flying, or any specific combina-
                    present at the same time, the simultaneous presence shall                                           tion of dusts, fibers, or flyings.
                    be considered during the selection and installation of the
                    electrical equipment and the wiring methods, including the                                     (C) Marking Equipment identified for Class II, Division
                    determination of the safe operating temperature of the elec-                                   1 or Class II, Division 2 shall, in addition to being marked
                    trical equipment.                                                                              in accordance with 500.8(B), be permitted to be marked
                                                                                                                   with both of the following:
                    506.8 Protection Techniques                                                                     (1) Zone 20, 21, or 22 (as applicable)
                    Acceptable protection techniques for electrical and elec-                                       (2) Temperature classification in accordance with 506.9(D)
                    tronic equipment in hazardous (classified) locations shall be
                    as described in 506.8(A) through 506.8(F).                                                     (D) Temperature Classifications Equipment shall be
                                                                                                                   marked to show the operating temperature referenced to a
                    (A) Dust Ignitionproof This protection technique shall be                                      40 C (104 F) ambient. Electrical equipment designed for
                    permitted for equipment in Zone 20, Zone 21, and Zone 22                                       use in the ambient temperature range between 20 C and
                    locations for which it is identified.                                                            40 C shall require no additional ambient temperature

                 738                                                               --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                                                                                     2005 National Electrical Code Handbook

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                                                 Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings                                      506.15



                           marking. Electrical equipment that is designed for use in a                       in a manner to avoid tensile stress at the termination
                           range of ambient temperatures other than            20 C and                      fittings.
                              40 C is considered to be special; and the ambient tempera-
                                                                                                      Exception: MI cable and fittings listed for Class II, Division
                           ture range shall then be marked on the equipment, including
                                                                                                      1 locations are permitted to be used.
                           either the symbol ‘‘Ta’’ or ‘‘Tamb’’ together with the special
                           range of ambient temperatures. As an example, such a mark-                 (3) In industrial establishments with limited public access,




                                                                                                                                                                                      --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                           ing might be ‘‘–30 C Ta          40 C.’’ Electrical equipment                  where the conditions of maintenance and supervision
                           suitable for ambient temperatures exceeding 40 C (104 F)                       ensure that only qualified persons service the installa-
                           shall be marked with both the maximum ambient tempera-                         tion, Type MC cable, listed for continuous use in Zone
                           ture and the operating temperature at that ambient tempera-                    20 locations, with a gas/vaportight continuous corru-
                           ture.                                                                          gated metallic sheath, and overall jacket of suitable
                                                                                                          polymeric material, separate grounding conductors in
                           Exception No. 1: Equipment of the non–heat-producing
                                                                                                          accordance with 250.122, and provided with termination
                           type, such as conduit fittings, shall not be required to have
                                                                                                          fittings listed for the application, shall be permitted.
                           a marked operating temperature.
                           Exception No. 2: Equipment identified for Class II, Division               Exception: MC cable and fittings listed for Class II, Division
                           1 or Class II, Division 2 locations as permitted by 506.20(B)              1 locations are permitted to be used.
                           and 506.20(C) shall be permitted to be marked in accordance                (4) Fittings and boxes shall be identified for use in Zone
                           with 500.8(B) and Table 500.8(B).                                              20 locations.
                           (E) Threading All NPT threads referred to herein shall be                  Exception: Boxes and fittings listed for Class II, Division
                           threaded with a National (American) Standard Pipe Taper                    1 locations are permitted to be used.
                           (NPT) thread that provides a taper of 1 in 16 (3⁄4-in. taper
                           per foot). Conduit and fittings shall be made wrenchtight to               (5) Where necessary to employ flexible connections, liq-
                           prevent sparking when the fault current flows through the                      uidtight flexible metal conduit with listed fittings, liq-
                           conduit system, and to ensure the integrity of the conduit                     uidtight flexible nonmetallic conduit with listed fittings,
                           system. Equipment provided with threaded entries for field                     or flexible cord listed for extra-hard usage and provided
                           wiring connections shall be installed in accordance with                       with listed fittings shall be used. Where flexible cords
                           506.9(E)(1) or (E)(2).                                                         are used, they shall also comply with 506.17. Where
                                                                                                          flexible connections are subject to oil or other corrosive
                           (1) Equipment Provided with Threaded Entries for NPT                           conditions, the insulation of the conductors shall be of
                           Threaded Conduit or Fittings For equipment provided                            a type listed for the condition or shall be protected by
                           with threaded entries for NPT threaded conduit or fittings,                    means of a suitable sheath.
                           listed conduit fittings, or cable fittings shall be used.
                                                                                                      Exception: Flexible conduit and flexible conduit and cord
                           (2) Equipment Provided with Threaded Entries for Met-                      fittings listed for Class II, Division 1 locations are permitted
                           ric Threaded Conduit or Fittings For equipment with met-                   to be used.
                           ric threaded entries, such entries shall be identified as being                   FPN: See 506.25 for grounding requirements where flex-
                           metric, or listed adapters to permit connection to conduit or                     ible conduit is used.
                           NPT-threaded fittings shall be provided with the equipment.
                           Adapters shall be used for connection to conduit or NPT-                   (B) Zone 21 In Zone 21 locations, the wiring methods in
                           threaded fittings. Listed cable fittings that have metric                  (B)(1) and (B)(2) shall be permitted.
                           threads shall be permitted to be used.
                                                                                                      (1) All wiring methods permitted in 506.15(A)
                                                                                                      (2) Fittings and boxes that are dusttight, provided with
                           506.15 Wiring Methods                                                          threaded bosses for connection to conduit, in which
                           Wiring methods shall maintain the integrity of the protection                  taps, joints, or terminal connections are not made, and
                           techniques and shall comply with 506.15(A), (B), or (C).                       are not used in locations where metal dust is present,
                                                                                                          may be used
                           (A) Zone 20 In Zone 20 locations, the wiring methods in
                           (1) through (5) shall be permitted.                                        (C) Zone 22 In Zone 22 locations, the wiring methods in
                                                                                                      (1) through (8) shall be permitted.
                           (1) Threaded rigid metal conduit, or threaded steel interme-
                               diate metal conduit.                                                   (1) All wiring methods permitted in 506.15(B).
                           (2) Type MI cable with termination fittings listed for the                 (2) Rigid metal conduit, intermediate metal conduit, electri-
                               location. Type MI cable shall be installed and supported                   cal metallic tubing, dusttight wireways.

                           National Electrical Code Handbook 2005                                                                                                               739

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                 506.16                                                                   Article 506 — Zone 20, 21, and 22 Locations for Combustible Dusts, Fibers, and Flyings



                    (3) Type MC or MI cable with listed termination fittings.                                                            506.20 Equipment Installation
                    (4) Type PLTC in cable trays.                                                                                        (A) Zone 20 In Zone 20 locations, only equipment listed
                    (5) Type ITC in cable trays.                                                                                         and marked as suitable for the location shall be permitted.
                    (6) Type MC, MI, MV, or TC cable installed in ladder,
                        ventilated trough, or ventilated channel cable trays in                                                          Exception: Intrinsically safe apparatus listed for use in
                        a single layer, with a space not less than the larger                                                            Class II, Division 1 locations with a suitable temperature
                        cable diameter between two adjacent cables, shall be                                                             class shall be permitted.
                        the wiring method employed. Single conductor Type
                                                                                                                                         (B) Zone 21 In Zone 21 locations, only equipment listed
                        MV cables shall be shielded or metallic armored.
                                                                                                                                         and marked as suitable for the location shall be permitted.
                    (7) Nonincendive field wiring shall be permitted using any
                        of the wiring methods permitted for unclassified loca-                                                           Exception No. 1: Apparatus listed for use in Class II, Divi-
                        tions. Nonincendive field wiring systems shall be in-                                                            sion 1 locations with a suitable temperature class shall be
                        stalled in accordance with the control drawing(s).                                                               permitted.
                        Simple apparatus, not shown on the control drawing,                                                              Exception No. 2: Pressurized equipment identified for Class
                        shall be permitted in a nonincendive field wiring circuit,                                                       II, Division 1 shall be permitted.
                        provided the simple apparatus does not interconnect the
                        nonincendive field wiring circuit to any other circuit.                                                          (C) Zone 22 In Zone 22 locations, only equipment listed
                                                                                                                                         and marked as suitable for the location shall be permitted.
                                                                           FPN: Simple apparatus is defined in 504.2.
                                                                                                                                         Exception No. 1: Apparatus listed for use in Class II, Divi-
                                                                                                                                         sion 1 or Class II, Division 2 locations with a suitable
                                                                              Separation of nonincendive field wiring circuits shall     temperature class shall be permitted.
                         --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                           be in accordance with one of the following:                   Exception No. 2: Pressurized equipment identified for Class
                                                                           a. Be in separate cables                                      II, Division 1 or Division 2 shall be permitted.
                                                                           b. Be in multiconductor cables where the conductors
                                                                              of each circuit are within a grounded metal shield         (D) Manufacturer’s Instructions Electrical equipment in-
                                                                           c. Be in multiconductor cables where the conductors           stalled in hazardous (classified) locations shall be installed
                                                                              have insulation with a minimum thickness of 0.25           in accordance with the instructions (if any) provided by the
                                                                              mm (0.01 in.)                                              manufacturer.
                    (8) Boxes and fittings shall be dusttight.                                                                           (E) Temperature The temperature marking specified in
                                                                                                                                         506.9(C)(2)(5) shall comply with (E)(1) or (E)(2).
                    506.16 Sealing
                                                                                                                                         (1) For combustible dusts, less than the lower of either
                    Where necessary to protect the ingress of combustible dust,                                                              the layer or cloud ignition temperature of the specific
                    or ignitible fibers, or flyings, or to maintain the type of                                                              combustible dust. For organic dusts that may dehydrate
                    protection, seals shall be provided. The seal shall be identi-                                                           or carbonize, the temperature marking shall not exceed
                    fied as capable of preventing the ingress of combustible dust                                                            the lower of either the ignition temperature or 165 C
                    or ignitible fibers or flyings and maintaining the type of                                                               (329 F).
                    protection but need not be explosionproof or flameproof.                                                             (2) For ignitible fibers or flyings, less than 165 C (329 F)
                                                                                                                                             for equipment that is not subject to overloading, or
                    506.17 Flexible Cords                                                                                                    120 C (248 F) for equipment (such as motors or power
                    Flexible cords used in Zone 20, Zone 21, and Zone 22                                                                     transformers) that may be overloaded.
                    locations shall comply with all of the following:
                                                                                                                                                FPN: See NFPA 499-2004, Recommended Practice for
                    (1) Be of a type listed for extra-hard usage                                                                                the Classification of Combustible Dusts and of Hazard-
                                                                                                                                                ous (Classified) Locations for Electrical Installations in
                    (2) Contain, in addition to the conductors of the circuit, a
                                                                                                                                                Chemical Processing Areas, for minimum ignition tem-
                        grounding conductor in complying with 400.23                                                                            peratures of specific dusts.
                    (3) Be connected to terminals or to supply conductors in
                        an approved manner                                                                                               506.21 Multiwire Branch Circuits
                    (4) Be supported by clamps or by other suitable means in
                                                                                                                                         In Zone 20 and Zone 21 locations, a multiwire branch circuit
                        such a manner to minimize tension on the terminal
                                                                                                                                         shall not be permitted.
                        connections
                    (5) Be provided with suitable seals to prevent the entrance                                                          Exception: Where the disconnect device(s) for the circuit
                        of combustible dust, or ignitible fibers, or flyings where                                                       opens all ungrounded conductors of the multiwire circuit
                        the flexible cord enters boxes or fittings                                                                       simultaneously.


                 740                                                                                                                                                        2005 National Electrical Code Handbook

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                                                                                                           Article 511 — Commercial Garages, Repair and Storage                                                          510.2



                                                                      506.25 Grounding and Bonding                                                because of deposits or accumulations of materials that may
                                                                      Grounding and bonding shall comply with Article 250 and                     be readily ignitible.
                                                                      the requirements in 506.25(A) and 506.25(B).
                                                                                                                                                  510.2 General
                                                                      (A) Bonding The locknut-bushing and double-locknut
                                                                                                                                                  The general rules of this Code and the provisions of Articles
                                                                      types of contacts shall not be depended on for bonding
                                                                                                                                                  500 through 504 shall apply to electric wiring and equipment
                                                                      purposes, but bonding jumpers with proper fittings or other
                                                                                                                                                  in occupancies within the scope of Articles 511 through 517,
                                                                      approved means of bonding shall be used. Such means of
                                                                                                                                                  except as such rules are modified in Articles 511 through
                                                                      bonding shall apply to all intervening raceways, fittings,
                                                                                                                                                  517. Where unusual conditions exist in a specific occupancy,
                                                                      boxes, enclosures, and so forth, between Zone 20, Zone 21,
                                                                                                                                                  the authority having jurisdiction shall judge with respect to
                                                                      and Zone 22 locations and the point of grounding for service
                                                                                                                                                  the application of specific rules.
                                                                      equipment or point of grounding of a separately derived
                                                                      system.
                                                                                                                                                  Some of the requirements contained in Articles 511 through
                                                                      Exception: The specific bonding means shall be required                     517 have been extracted from other NFPA codes and stan-
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                      only to the nearest point where the grounded circuit conduc-                dards. For example, Table 514.3(B)(1), Table 514.3(B)(2),
                                                                      tor and the grounding electrode conductor are connected                     and Table 515.3 are extracted from NFPA 30A, Code for
                                                                      together on the line side of the building or structure discon-              Motor Fuel Dispensing Facilities and Repair Garages, and
                                                                      necting means as specified in 250.32(A), (B), and (C), if the               NFPA 30, Flammable and Combustible Liquids Code. The
                                                                      branch side overcurrent protection is located on the load                   tables were developed by the NFPA Technical Committees
                                                                      side of the disconnecting means.                                            responsible for those documents. The documents were devel-
                                                                          FPN: See 250.100 for additional bonding requirements                    oped through the same process as the NEC; however, the
                                                                          in hazardous (classified) locations.                                    National Electrical Code Committee is not directly responsi-
                                                                                                                                                  ble for the technical content of extracted material.
                                                                      (B) Types of Equipment Grounding Conductors Where                                NFPA publishes a number of standards and recom-
                                                                      flexible conduit is used as permitted in 506.15, it shall be                mended practices that provide requirements or guidance on
                                                                      installed with internal or external bonding jumpers in parallel             the classification of hazardous locations in specific occupan-
                                                                      with each conduit and complying with 250.102.                               cies. Information and copies of standards may be obtained
                                                                                                                                                  from NFPA, 1 Batterymarch Park, Quincy, MA 02169-7471.
                                                                      Exception: In Zone 22 locations, the bonding jumper shall
                                                                      be permitted to be deleted where all of the following condi-
                                                                      tions are met:
                                                                      (1) Listed liquidtight flexible metal conduit 1.8 m (6 ft) or
                                                                                                                                                              ARTICLE 511
                                                                          less in length, with fittings listed for grounding, is used.                   Commercial Garages, Repair
                                                                      (2) Overcurrent protection in the circuit is limited to 10                               and Storage
                                                                          amperes or less.
                                                                      (3) The load is not a power utilization load.
                                                                                                                                                  Summary of Changes
                                                                                                                                                  • 511.3: Revised requirements on which areas are classified
                                                                                   ARTICLE 510                                                      as Class I, Division 2 and the areas that are unclassified.
                                                                                Hazardous (Classified)                                            • 511.4(A): Deleted requirement for raceways embedded in
                                                                                                                                                    a masonry wall or beneath the floor of a classified area
                                                                                Locations — Specific                                                in a commercial garage to be considered as being within
                                                                                                                                                    the Class I location above the floor.
                                                                      Contents
                                                                      510.1 Scope
                                                                      510.2 General
                                                                                                                                                  Contents
                                                                                                                                                   511.1 Scope
                                                                      510.1 Scope
                                                                                                                                                   511.3 Classifications of Locations
                                                                      Articles 511 through 517 cover occupancies or parts of occu-                   (A) Unclassified Locations
                                                                      pancies that are or may be hazardous because of atmospheric                    (B) Classified Locations
                                                                      concentrations of flammable liquids, gases, or vapors, or


                                                                      National Electrical Code Handbook 2005                                                                                                              741

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                 511.1                                       Article 511 — Commercial Garages, Repair and Storage



                                                                                          is covered under the requirements of this article plus the
                      511.4       Wiring and Equipment in Class I Locations               belowgrade work area classification requirements of Article
                        (A)       Wiring Located in Class I Locations                     514. See 511.3(A)(3) and its associated commentary.
                        (B)       Equipment Located in Class I Locations                       Parking, storage, and similar occupancies are not re-
                      511.7       Wiring and Equipment Installed Above Class I            quired to be classified, provided that any repair that occurs
                                  Locations                                               is minor and does not involve the use of electrical equipment.
                          (A)     Wiring in Spaces Above Class I Locations                In accordance with NFPA 88A, Standard for Parking Struc-
                          (B)     Electrical Equipment Installed Above Class I            tures, a mechanical ventilating system that is capable of
                                  Locations                                               continuously providing a ventilation rate of 1 ft3 per minute
                     511.9        Sealing                                                 for each square foot of floor area is required for all enclosed,
                    511.10        Special Equipment
                                                                                          basement, and underground parking garages.
                       (A)        Battery Charging Equipment                                   Operations that involve open flames or electric arcs,
                       (B)        Electric Vehicle Charging Equipment                     including fusion gas welding and electric welding, pre-
                    511.12        Ground-Fault Circuit-Interrupter Protection for         viously were covered by NFPA 88B, Standard for Repair
                                  Personnel
                                                                                          Garages. The NFPA 88B requirements were incorporated
                    511.16        Grounded and Grounding Requirements
                                                                                          into the 2003 edition of NFPA 30A, Code for Motor Fuel
                       (A)        General Grounding Requirements                          Dispensing Facilities and Repair Garages, and NFPA 88B
                       (B)        Supplying Circuits with Grounded and Grounding          was withdrawn as a standard at the 2002 NFPA Fall Meeting.
                                  Conductors in Class I Locations
                                                                                          Repair work that involves an open flame or electric arcs
                                                                                          must be restricted to areas specifically provided for such
                            FPN: Rules that are followed by a reference in brackets
                                                                                          purposes.
                            contain text that has been extracted from NFPA 30A-                Approved suspended unit heaters may be used in com-
                            2003, Code for Motor Fuel Dispensing Facilities and           mercial garages, provided they are located not less than 8
                            Repair Garages. Only editorial changes were made to           ft above the floor and are installed in accordance with the
                            the extracted text to make it consistent with this Code.      conditions of their approval. This requirement also came
                                                                                          from NFPA 88B.
                    511.1 Scope                                                                For the 2005 Code, new Section 555.22 requires that
                    These occupancies shall include locations used for service            the electrical repair facilities for boats and other marine craft
                    and repair operations in connection with self-propelled vehi-         comply with the requirements of Article 511.
                    cles (including, but not limited to, passenger automobiles,
                    buses, trucks, and tractors) in which volatile flammable liq-
                    uids or flammable gases are used for fuel or power.                   511.3 Classifications of Locations




                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                                                                                          (A) Unclassified Locations
                    Article 100 defines garage as ‘‘a building or portion of a
                    building in which one or more self-propelled vehicles can             Section 511.3 was reorganized in the 2005 Code to make
                    be kept for use, sale, storage, rental, repair, exhibition, or        the classification of areas clearer. Unclassified locations are
                    demonstration purposes.’’ Article 511 applies to commercial           described in 511.3(A), and classified locations are found
                    garages in which the primary operation is the service and             in 511.3(B). The Technical Committee on Automotive and
                    repair of self-propelled vehicles that use flammable gases            Marine Service Stations; NFPA 30A, Code for Motor Fuel
                    or liquids for fuel. The commercial garages covered by                Dispensing Facilities and Repair Garages; and NEC Code-
                    Article 511 include automotive service centers; repair gar-           Making Panel 14 have correlated and updated all the electri-
                    ages for commercial vehicles, such as trucks and tractors;            cal requirements between the 2002 Code and the 2005 Code
                    and service garages for fleet vehicles, such as buses, cars,          to eliminate any potential conflicts. Section 511.3 uses the
                    and trucks.                                                           term transferred in place of the old terminology dispensing
                         The requirements of Article 511 are intended to mitigate         in determining the requirements for classifying locations
                    the potential for an ignition-capable arc or spark from electri-      when a significant quantity of flammable or gaseous liquids
                    cal wiring or equipment used in or above hazardous (classi-           will be exposed to the atmosphere by a motor vehicle repair
                    fied) locations. Additionally, there are requirements for             operation (e.g., major engine overhauls or repairs that require
                    personnel protection in occupancies that are frequently wet           draining of the motor vehicle fuel tank). Minor repair gar-
                    or damp in which service personnel are subject to contact             ages, by definition under NFPA 30A, would not be permitted
                    with large grounded surfaces, such as concrete slabs in direct        to conduct such types of repair operations involving the
                    contact with the earth. The increasing number of service              transfer of flammable or gaseous liquids and thus would not
                    operations in which minor repairs, such as oil changes, occur         require a classified location designation.


                 742                                                                                                         2005 National Electrical Code Handbook

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                                                                  Article 511 — Commercial Garages, Repair and Storage                                                                      511.3



                               Section 511.3 correlates the type of repair garage with                                  fied. Repair garages shall be permitted to be unclassified
                           the NFPA 30A definitions for major and minor repair gar-                                     when designed in accordance with 511.3(A)(2) through
                           ages. The definitions from NFPA 30A are as follows:                                          511.3(A)(7).
                                Major Repair Garage. A building or portions of a                                                   FPN: For further information, see NFPA 88A-2002,
                                building where major repairs, such as engine over-                                                 Standard for Parking Structures, and NFPA 30A-2003,
                                hauls, painting, body and fender work, and repairs that                                            Code for Motor Fuel Dispensing Facilities and Repair
                                                                                                                                   Garages.
                                require draining of the motor vehicle fuel tank are
                                performed on motor vehicles, including associated                                       (2) Alcohol-Based Windshield Washer Fluid The stor-
                                floor space used for offices, parking, or showrooms.                                    age, handling, or dispensing into motor vehicles of alcohol-
                                [NFPA 30A-2003, 3.3.12.1]                                                               based windshield washer fluid in areas used for the service
                                Minor Repair Garage. A building or portions of a                                        and repair operations of the vehicles shall not cause such
                                building used for lubrication, inspection, and minor                                    areas to be classified as hazardous (classified) locations.
                                automotive maintenance work, such as engine tune-                                                  FPN: For further information, see 8.3.5, Exception, of
                                ups, replacement of parts, fluid changes (e.g., oil, anti-                                         NFPA 30A-2003, Code for Motor Fuel Dispensing Facil-
                                freeze, transmission fluid, brake fluid, air conditioning                                          ities and Repair Garages.
                                refrigerants, etc.), brake system repairs, tire rotation,
                                                                                                                        (3) Specific Areas Adjacent to Classified Locations Ar-
                                and similar routine maintenance work, including asso-
                                                                                                                        eas adjacent to classified locations in which flammable va-
                                ciated floor space used for offices, parking, or show-
                                                                                                                        pors are not likely to be released, such as stock rooms,
                                rooms. [NFPA 30A-2003, 3.3.12.2]
                                                                                                                        switchboard rooms, and other similar locations, shall not be
                           The term Class I liquid used in 511.3(A) and 511.3(B) refers                                 classified where mechanically ventilated at a rate of four or
                           to flammable liquids as defined in NFPA 30–2003, Flamma-                                     more air changes per hour, or designed with positive air
                           ble and Combustible Liquids Code. Gasoline is a common                                       pressure, or where effectively cut off by walls or partitions.
                           Class I liquid, whereas diesel fuel, because of its flashpoint
                           being above 100 F, is classified as a Class II combustible                                   Section 511.3(A)(3) provides guidance for determining the
                           liquid. The use of Class I, Class II, and Class III in NFPA                                  classification of facilities that primarily offer oil and filter
                           30 for the classification of liquids has no direct correlation                               change and lubrication-type service, not the transfer of fuel.
                           to the use of Class I, Class II, and Class III in the NEC to                                 If the lower-level work area of a lubritorium is provided
                           designate hazardous (classified) locations.                                                  with exhaust ventilation at a rate specified in Table
                                                                                                                        514.3(B)(1) under ‘‘Lubrication or Service Room — Without
                                Flammable Liquid. Any liquid that has a closed-cup
                                                                                                                        Dispensing,’’ the lower level is not a hazardous (classified)
                                flash point below 100 F (37.8 C), as determined by
                                                                                                                        location. Table 514.3(B)(1) is extracted from NFPA 30A,
                                the test procedures and apparatus set forth in 1.7.4.
                                                                                                                        Code for Motor Fuel Dispensing Facilities and Repair Gar-
                                Flammable liquids are classified as Class I as follows:
                                                                                                                        ages.
                                Class I Liquid — any liquid that has a closed-cup
                                flash point below 100 F (37.8 C) and a Reid vapor
                                pressure not exceeding 40 psia (2068.6 mm Hg) at                                        (4) Pits in Lubrication or Service Room Where Class I
                                100 F (37.8 C), as determined by ASTM D 323, Stan-                                      Liquids Are Not Transferred Any pit, belowgrade work
                                dard Method of Test for Vapor Pressure of Petroleum                                     area, or subfloor work area that is provided with exhaust
                                Products (Reid Method). Class I liquids are further                                     ventilation at a rate of not less than 0.3 m3/min/m2 (1 cfm/
                                classified as follows:                                                                  ft2) of floor area at all times that the building is occupied
                                1. Class IA liquids. Those liquids that have flash                                      or when vehicles are parked in or over this area and where
                                   points below 73 F (22.8 C) and boiling points                                        exhaust air is taken from a point within 300 mm (12 in.) of
                                   below 100 F (37.8 C)                                                                 the floor of the pit, belowgrade work area, or subfloor work
                                2. Class IB liquids. Those liquids that have flash                                      area is unclassified. [NFPA 30A:7.4.5.4 and Table 8.3.1]
                                   points below 73 F (22.8 C) and boiling points at
                                                                                                                        (5) Up to a Level of 450 mm (18 in.) Above the Floor in
                                   or above 100 F (37.8 C);
                                                                                                                        Lubrication or Service Rooms Where Class I Liquids
                                3. Class IC liquids. Those liquids that have flash
                                                                                                                        Are Transferred For each floor, the entire area up to a
                                   points at or above 73 F (22.8 C), but below 100 F
                                                                                                                        level of 450 mm (18 in.) above the floor shall be considered
                                   (37.8 C). [NFPA 30-2003, 3.3.25.2]
                                                                                                                        unclassified where there is mechanical ventilation providing
                                                                                                                        a minimum of four air changes per hour or one cubic foot
                           (1) Parking and Repair Garages. Parking garages used                                         per minute of exchanged air for each square foot of floor
                           for parking or storage shall be permitted to be unclassi-                                    area. Ventilation shall provide for air exchange across the


                           National Electrical Code Handbook 2005                                                                                                                            743
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                 511.4                                       Article 511 — Commercial Garages, Repair and Storage



                    entire floor area, and exhaust air shall be taken at a point          (3) Any Ventilated Pit or Depression Below Floor Level.
                    within 0.3 m (12 in.) of the floor.                                       Any ventilated pit or depression in which six air changes
                                                                                              per hour are exhausted from a point within 300 mm (12
                    (6) Flammable Liquids Having Flash Points Below 38 C                      in.) of the floor level of the pit shall be a Class I,
                    (100 F) Where flammable liquids having a flash point                      Division 2 location.
                    below 38 C (100 F) (such as gasoline) or gaseous fuels (such          (4) Space Above an Unventilated Pit or Depression Below
                    as natural gas, hydrogen, or LPG) will not be transferred,                Floor Level. Above a pit, or depression below floor
                    such location shall be considered to be unclassified. unless              level, the space up to 450 mm (18 in.) above the floor
                    the location is required to be classified in accordance with              or grade level and 900 mm (3 ft) horizontally from a
                    511.3(B)(2) or (B)(4).                                                    lubrication pit shall be a Class I, Division 2 location.
                                                                                          (5) Dispenser for Class I Liquids, Other Than Fuels. Within
                    (7) Within 450 mm (18 in.) of the Ceiling In major repair                 900 mm (3 ft) of any fill or dispensing point, extending
                    garages, where lighter-than-air gaseous fuels (such as natural            in all directions shall be a Class I, Division 2 location.
                    gas or hydrogen) vehicles are repaired or stored, the area                See also 511.3(B)(1).
                    within 450 mm (18 in.) of the ceiling shall be considered
                    unclassified where ventilation of at least 1 cfm/sq ft of
                    ceiling area taken from a point within 450 mm (18 in.) of             Section 511.3 was updated in the 2005 Code to address
                    the highest point in the ceiling is provided.                         repair areas where gaseous-fueled vehicles, such as those
                                                                                          fueled by natural gas, hydrogen, or LPG, are serviced. It is
                            FPN: For further information on the definition of major       not the intent of the Code to assume that all repair garages
                            repair garage, see 3.3.12.1 of NFPA 30A-2003, Code for        service gaseous-fueled vehicles. Only such repair garages
                            Motor Fuel Dispensing Facilities and Repair Garages.          that intend to service such types of gaseous-fueled vehicles
                                                                                          need to comply with the requirements for gaseous fuels.
                    (B) Classified Locations
                                                                                          Note that the classified location requirements for gaseous-
                    (1) Flammable Fuel Dispensing Areas Areas in which                    fueled vehicles are based on LPG being a heavier-than-air
                    flammable fuel is dispensed into vehicle fuel tanks shall             fuel, whereas natural gas and hydrogen are lighter-than-air
                    conform to Article 514.                                               fuels.

                    (2) Lubrication or Service Room Where Class I Liquids                 (4) Within 450 mm (18 in.) of the Ceiling In major repair
                    or Gaseous Fuels (Such as Natural Gas, Hydrogen, or                   garages where lighter-than-air gaseous fuel (such as natural
                    LPG) Are Not Transferred The following spaces that are                gas or hydrogen) vehicles are repaired or stored, ceiling
                    not designed in accordance with 511.3(A)(4) shall be classi-          spaces that are not designed in accordance with 511.3(A)(7)
                    fied as Class I, Division 2:                                          shall be classified as Class I, Division 2.
                    (1) Entire area within any unventilated pit, belowgrade                      FPN: For further information on the definition of major




                                                                                                                                                                        --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---
                        work area, or subfloor area.                                             repair garage, see 3.3.12.1 of NFPA 30A, 2003, Code for
                                                                                                 Motor Fuel Dispensing Facilities and Repair Garages.
                    (2) Area up to 450 mm (18 in.) above any such unventilated
                        pit, belowgrade work area, or subfloor work area and              511.4 Wiring and Equipment in
                        extending a distance of 900 mm (3 ft) horizontally from           Class I Locations
                        the edge of any such pit, belowgrade work area, or
                                                                                          (A) Wiring Located in Class I Locations Within Class I
                        subfloor work area.
                                                                                          locations as classified in 511.3, wiring shall conform to
                    (3) Lubrication or Service Room Where Class I Liquids                 applicable provisions of Article 501.
                    or Gaseous Fuels (Such as Natural Gas, Hydrogen, or
                    LPG) Are Transferred The following spaces that are not                In accordance with 511.3(B), specific area classification is
                    designed in accordance with 511.3(A)(5) shall be classified           provided for lubrication and service rooms within a commer-
                    as follows:                                                           cial garage. The classified locations covered in 511.3(B)(2)
                                                                                          typically apply to minor repair garages, while the classified
                    (1) Up to a Level of 450 mm (18 in.) Above the Floor. For             locations covered in 511.3(B)(3) typically apply to major
                        each floor, the entire area up to a level of 450 mm               repair garages. [See the commentary on 511.3(A) for defini-
                        (18 in.) above the floor shall be a Class I, Division 2           tions of major and minor repair garages.]
                        location.                                                              The benchmark for distinguishing which classification
                    (2) Any Unventilated Pit or Depression Below Floor Level.             to use and the extent of the classified area is whether there
                        Any unventilated pit or depression below floor level              will be a transfer of Class I liquids or gaseous fuels. Transfer
                        shall be a Class I, Division 1 location and shall extend          of liquids or gaseous fuels includes the draining of a fuel
                        up to said floor level.


                 744                                                                                                         2005 National Electrical Code Handbook

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                                                                                                           Article 511 — Commercial Garages, Repair and Storage                                                                     511.4



                                                                      tank that is inherent to a tank repair or replacement. In
                                                                      accordance with 511.3(A)(4), (5), and (7), the need to estab-
                                                                      lish classified locations can be mitigated through the use of
                                                                      mechanical ventilation that meets the specified air exchange
                                                                      parameters. Where it is established that there will be classi-
                                                                                                                                                                                          Unclassified area
                                                                      fied locations within a commercial garage, all applicable
                                                                      requirements of Article 501 for installing wiring and equip-                                     ≥ 12 in.                                          ≥ 12 in.
                                                                      ment in Class I, Division 1 and 2 locations of a commercial
                                                                      garage must be followed.                                                                                              Class I area
                                                                            In the 2002 and previous editions of the Code, raceways
                                                                      installed under the Class I, Division 2 areas of a commercial
                                                                      garage were considered to be in that classified location if                                                         Unclassified area
                                                                      the raceway penetrated through the slab or floor and ex-
                                                                      tended into the classified location, even if the extension                                          ≥ 12 in.                                       ≥ 12 in.
                                                                      passed unbroken (without any fittings less than 12 in. from
                                                                      the boundary) through the classified location and into an
                                                                      unclassified location. This below-floor classification is no
                                                                      longer required based on the substantiation that there is
                                                                      typically a very low volume of air in the underground loca-                 Exhibit 511.1 Seals not required for conduits that pass unbroken
                                                                                                                                                  through the Class I location.
                                                                      tions and the air–fuel mixture is below the lower flammable
                                                                      limit (LFL), in other words, too lean to burn or ignite.
                                                                            It should be noted that the general rules of 501.15(A)(4)
                                                                                                                                                      Where mechanical ventilation is provided in the dis-
                                                                      and (B)(2) on providing seals at classified location bound-
                    --``,,``,,`,,`,``,,,,,,`,```,-`-`,,`,,`,`,,`---




                                                                                                                                                  pensing area, the control shall be interlocked so that the
                                                                      aries do apply where raceway installations in a commercial
                                                                                                                                                  dispenser cannot operate without ventilation, as prescribed
                                                                      garage pass from classified to unclassified locations and
                                                                                                                                                  in 500.5(B)(2).
                                                                      conduit fittings, outlet boxes, or both are installed less than
                                                                      12 in. from either side of the boundary. In accordance with
                                                                      501.15(A)(4) Exception No. 4, where a raceway runs from a                   See Figure 514.3 in the Code and Exhibit 514.1 for informa-
                                                                      Class I, Division 1 location into an underground unclassified               tion on classified areas in the vicinity of dispensing units.
                                                                      location and then emerges from below ground into an unclas-
                                                                      sified location, the boundary seal is permitted to be located               (2) Portable Lighting Equipment Portable lighting equip-
                                                                      more than 10 ft from the actual boundary, provided the seal                 ment shall be equipped with handle, lampholder, hook, and
                                                                      is located at the point the conduit emerges from below grade                substantial guard attached to the lampholder or handle. All
                                                                      into the unclassified location. The seal and, if necessary, an              exterior surfaces that might come in contact with battery
                                                                      associated explosionproof union are required to be the first                terminals, wiring terminals, or other objects shall be of non-
                                                                      fitting(s) at the point the conduit emerges from below ground               conducting material or shall be effectively protected with
                                                                      into the unclassified location.                                             insulation. Lampholders shall be of an unswitched type and
                                                                            Exhibit 511.1 depicts two receptacle outlet enclosures                shall not provide means for plug-in of attachment plugs.
                                                                      that are located at least 12 in. above an area that has been                The outer shell shall be of molded composition or other
                                                                      classified as Class I. The rigid metal conduit passes unbroken              suitable material. Unless the lamp and its cord are supported
                                                                      from the outlet boxes through the Class I location into the                 or arranged in such a manner that they cannot be used in
                                                                      unclassified underground location beneath the floor. The con-               the locations classified in 511.3, they shall be of a type
                                                                      duit coupling is located 12 in. or more from the penetration                identified for Class I, Division 1 locations.
                                                                      into the classified location. No seals are required for this in-
                                                                      stallation in accordance with 501.15(A)(4) Exception No. 1.                 Often, a reel-type portable handlamp is used for supplemen-
                                                                                                                                                  tal lighting during vehicle servicing. See Exhibit 511.2 for
                                                                      (B) Equipment Located in Class I Locations Within                           an illustration of a cord reel assembly.
                                                                      Class I locations as defined in 511.3, equipment shall con-                       Where there is no transfer of flammable liquids to vehi-
                                                                      form to applicable provisions of Article 501.                               cle fuel tanks but there is use of electrical diagnostic equip-
                                                                                                                                                  ment, electrically operated tools or machinery, or open
                                                                      (1) Fuel-Dispensing Units Where fuel-dispensing units                       flames such as used for welding or cutting, the area must
                                                                      (other than liquid petroleum gas, which is prohibited) are                  be classified in accordance with 511.3. Fuel dispensing units
                                                                      located within buildings, the requirements of Article 514                   located within the garage are governed by the requirements
                                                                      shall govern.


                                                                      National Electrical Code Handbook 2005                                                                                                                         745

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                 511.7                                                                      Article 511 — Commercial Garages, Repair and Storage




                                                                                                                            Roof                         Commercial garage

                                                                                                                                                                       18 in.
                                                                                                                                                                                                  Stockroom
                                                                                                                                                        Classification of ceiling
                                                                                                                           Unclassified                      area required
                                                                                                                            location                       if lighter-than-air
                                                                                                                                                        gaseous fuel vehicles
                                                                                                                                                               are repaired

                                                                                                                                                                                                       Unclassified
                                                                                                                                                                                                        location

                                                                                                                            No openings                     Transfer of Doorway
                                                                                                                                                            Class I liquids
                                                                                                                                                            or gaseous fuels
                                                                                                                                                            occurs in
                                                                                                                                                            repair area

                                                                                                                            Floor                                      18 in.                    Door sill not less
                                                                                                                                                                                                 than 18 in. above
                                                                                                                                                                                                    garage floor
                                                                                                                                                                       Pit or depression




                                                                                                                                             Roof               Commercial garage (ventilated)
                    Exhibit 511.2 Cord reel. This cord, which is part of a portable
                    lamp assembly, must be arranged so that the lamp cannot be
                    used in a Class I location. Otherwise, the lamp must be an                                                                            No repair of lighter-
                                                                                                                                                         than-air gaseous fuel
                    explosionproof type approved for Class I, Division 1 hazardous                                         At least                         vehicles occurs
                    locations. (Courtesy of Appleton Electric Co., EGS Electrical                                            4 air                           in repair area
                    Group)                                                                                                 changes
                                                                                                                           per hour

                    of Article 514.                                                                                                                                                                       Office
                                                                                                                                                                   Doorway
                         In accordance with 511.3(A) and (B), the Class I, Divi-
                                                                                                                                                        Transfer of Class I
                    sion 2 location above grade within a commercial garage in                                                                            liquids occurs in
                    which Class I liquids or gaseous fuels are transferred extends                                                                          repair area
                                                                                                                                                         Unclassified location
                    18 in. above floor level, unless the authority having jurisdic-                                                                          above floor
                    tion determines otherwise because mechanical ventilation                                                  At least
                    provides at least four air changes per hour.                                                                6 air
                                                                                                                              changes                                                                           Floor
                         The Class I, Division 1 location below grade extends                                                 per hour                                  Pit or depression
                    from the floor of the pit or depression to floor level, unless                                           at pit floor

                    the authority having jurisdiction permits the pit or depression
                    to be classified as Class I, Division 2 because ventilation
                    providing at least six air changes per hour exhausts air at                                                       Division 1                   Division 2
                    the floor level of the pit or depression. Areas suitably cut
                    off and areas adjacent to unclassified, ventilated garages                                           Exhibit 511.3 Classification of locations in commercial garages.
                    are not classified as hazardous. See Exhibit 511.3 for an
                    illustration of classified and unclassified locations in a com-
                                                                                                                         is necessary to ensure that an ignition-capable arc or spark
                    mercial garage.
                                                                                                                         does not migrate into the classified location. Wiring installed
                                                                                                                         above unclassified areas can be selected from the methods
                    511.7 Wiring and Equipment Installed Above                                                           covered in Chapter 3, provided the article covering that
                    Class I Locations                                                                                    wiring method does not contain any restrictions that would
                                                                                                                         limit its use in commercial garages.
                    For the installation of electrical wiring in areas above those
                    designated by 511.3(B) as Class I locations, 511.7 specifies                                         (A) Wiring in Spaces Above Class I Locations
                    the types of raceway and cable systems that are allowed. The
                    integrity of the wiring system above the classified location                                         (1) Fixed Wiring Above Class I Locations All fixed wir-
                                                                                                                         ing above Class I locations shall be in metal raceways, rigid


                 746                                                                                                                                        2005 National Electrical Code Handbook
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                                                                           Article 511 — Commercial Garages, Repair and Storage                                                    511.16



                           nonmetallic conduit, electrical nonmetallic tubing, flexible                     511.10(B)(2) and (B)(3). Flexible cords shall be of a type
                           metal conduit, liquidtight flexible metal conduit, or liq-                       identified for extra-hard usage.
                           uidtight flexible nonmetallic conduit, or shall be Type MC,
                           AC, MI, manufactured wiring systems, or PLTC cable in                            (2) Connector Location No connector shall be located
                           accordance with Article 725, or Type TC cable or Type ITC                        within a Class I location as defined in 511.3.
                           cable in accordance with Article 727. Cellular metal floor                       (3) Plug Connections to Vehicles Where the cord is sus-
                           raceways or cellular concrete floor raceways shall be permit-                    pended from overhead, it shall be arranged so that the lowest
                           ted to be used only for supplying ceiling outlets or extensions                  point of sag is at least 150 mm (6 in.) above the floor. Where
                           to the area below the floor, but such raceways shall have                        an automatic arrangement is provided to pull both cord and
                           no connections leading into or through any Class I location                      plug beyond the range of physical damage, no additional
                           above the floor.                                                                 connector shall be required in the cable or at the outlet.
                           (2) Pendant For pendants, flexible cord suitable for the
                           type of service and listed for hard usage shall be used.                         511.12 Ground-Fault Circuit-Interrupter
                                                                                                            Protection for Personnel
                           (B) Electrical Equipment Installed Above Class I Loca-
                                                                                                            All 125-volt, single-phase, 15- and 20-ampere receptacles
                           tions
                                                                                                            installed in areas where electrical diagnostic equipment,
                           (1) Fixed Electrical Equipment Electrical equipment in a                         electrical hand tools, or portable lighting equipment are to
                           fixed position shall be located above the level of any defined                   be used shall have ground-fault circuit-interrupter protection
                           Class I location or shall be identified for the location.                        for personnel.
                                (a) Arcing Equipment. Equipment that is less than 3.7
                           m (12 ft) above the floor level and that may produce arcs,                       Ground-fault circuit interrupters (GFCIs) intended to protect
                           sparks, or particles of hot metal, such as cutouts, switches,                    personnel from shock hazards are designed to trip when a
                           charging panels, generators, motors, or other equipment (ex-                     ground-fault current of 5 milliamperes (plus or minus 1 mA)
                           cluding receptacles, lamps, and lampholders) having make-                        or greater is detected. The GFCI necessary to comply with
                           and-break or sliding contacts, shall be of the totally enclosed                  the requirement in 511.12 may be either a receptacle type
                           type or constructed so as to prevent the escape of sparks or                     or a circuit-breaker type. This requirement applies to recepta-
                           hot metal particles.                                                             cles supplying specific types of utilization equipment that
                                (b) Fixed Lighting. Lamps and lampholders for fixed                         will be in use by repair personnel in environments where
                           lighting that is located over lanes through which vehicles                       the floor surface (typically, concrete slabs with direct or
                           are commonly driven or that may otherwise be exposed to                          indirect earth contact) and the possibility of dampness or
                           physical damage shall be located not less than 3.7 m (12 ft)                     even standing water increase the potential for electric shock.
                           above floor level, unless of the totally enclosed type or                             See the definition of ground-fault circuit interrupter in
                           constructed so as to prevent escape of sparks or hot metal                       Article 100; also see Exhibit 210.11 and Exhibit 210.12.
                           particles.

                           511.9 Sealing                                                                    511.16 Grounded and Grounding Requirements
                           Seals conforming to the requirements of 501.15 and                               (A) General Grounding Requirements All metal race-
                           501.15(B)(2) shall be provided and shall apply to horizontal                     ways, the metal armor or metallic sheath on cables, and
                           as well as vertical boundaries of the defined Class I locations.                 all non–current-carrying metal parts of fixed or portable
                                                                                                            electrical equipment, regardless of voltage, shall be
                                                                                                            grounded as provided in Article 250.
                           Seals are required if any part of the raceway is in or passes
                           through a Class I, Division 2 location. See the commentary                       (B) Supplying Circuits with Grounded and Grounding
                           on seals following 501.15(F)(3).                                                 Conductors in Class I Locations Grounding in Class I
                                                                                                            locations shall comply with 501.30.
                           511.10 Special Equipment                                                         (1) Circuits Supplying Portable Equipment or Pen-
                           (A) Battery Charging Equipment Battery chargers and                              dants Where a circuit supplies portables or pendants and
                           their control equipment, and batteries being charged, shall                      includes a grounded conductor as provided in Article 200,
                           not be located within locations classified in 511.3.                             receptacles, attachment plugs, connectors, and similar de-
                                                                                                            vices shall be of the grounding type, and the grounded con-
                           (B) Electric Vehicle Charging Equipment                                          ductor of the flexible cord shall be connected to the screw
                           (1) General All electrical equipment and wiring shall be                         shell of any lampholder or to the grounded terminal of any
                           installed in accordance with Article 625, except as noted in                     utilization equipment supplied.


                           National Electrical Code Handbook 2005                                                                                                                     747
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                 513.1                                                                       Article 513 — Aircraft Hangars



                    (2) Approved Means Approved means shall be provided
                                                                                                                   (D) Mobile Servicing Equipment with Electric
                    for maintaining continuity of the grounding conductor be-
                    tween the fixed wiring system and the non–current-carrying