0.2 NEC 2005 Hanbook Introduction

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					                                                             Article 90


                                                                                              90.1 Purpose
                           Summary of Changes                                                 (A) Practical Safeguarding The purpose of this Code is
                           • 90.2(B): Added FPN providing information on utilities            the practical safeguarding of persons and property from haz-
                           • 90.2(C): Added paragraph describing bracketed                    ards arising from the use of electricity.
                                                                                              The National Electrical Code (NEC) is prepared by the
                                                                                              National Electrical Code Committee, which consists of a
                                                                                              Technical Correlating Committee and 19 code-making pan-
                           Contents                                                           els. The code-making panels have specific subject responsi-
                           90.1 Purpose                                                       bility within the Code. The scope of the National Electrical
                                (A) Practical Safeguarding                                    Code Committee is as follows:
                                (B) Adequacy                                                      This committee shall have primary responsibility for
                                (C) Intention
                                                                                                  documents on minimizing the risk of electricity as a
                                (D) Relation to Other International Standards
                                                                                                  source of electric shock and as a potential ignition
                           90.2 Scope
                                                                                                  source of fires and explosions. It shall also be responsi-
                                (A) Covered
                                                                                                  ble for text to minimize the propagation of fire and
                                (B) Not Covered                                                   explosions due to electrical installations.
                                (C) Special Permission
                           90.3 Code Arrangement                                                  In addition to its overall responsibility for the National
                           90.4 Enforcement
                                                                                              Electrical Code, the Technical Correlating Committee is
                           90.5 Mandatory Rules, Permissive Rules, and Explanatory            responsible for NFPA 70A, National Electrical Code Re-
                                Material                                                      quirements for One- and Two-Family Dwellings, and for
                                (A) Mandatory Rules                                           correlation of the following:
                                (B) Permissive Rules
                                (C) Explanatory Material                                       1. NFPA 70B, Recommended Practice for Electrical
                           90.6 Formal Interpretations                                            Equipment Maintenance
                           90.7 Examination of Equipment for Safety                            2. NFPA 70E, Standard for Electrical Safety in the Work-
                           90.8 Wiring Planning                                                   place
                                (A) Future Expansion and Convenience                           3. NFPA 73, Electrical Inspection Code for Existing
                                (B) Number of Circuits in Enclosures                              Dwellings
                           90.9 Units of Measurement                                           4. NFPA 79, Electrical Standard for Industrial Machinery
                                (A) Measurement System of Preference                           5. NFPA 110, Standard for Emergency and Standby Power
                                (B) Dual System of Units                                          Systems
                                (C) Permitted Uses of Soft Conversion                          6. NFPA 111, Standard on Stored Electrical Energy Emer-
                                (D) Compliance                                                    gency and Standby Power Systems


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                 90.2                                                             Article 90 — Introduction

                     (B) Adequacy This Code contains provisions that are con-                     calls attention to the fact that installations meeting the re-
                     sidered necessary for safety. Compliance therewith and                       quirements of the NEC are also in compliance with the
                     proper maintenance results in an installation that is essen-                 fundamental principles outlined in IEC 60364-1, Electrical
                     tially free from hazard but not necessarily efficient, conve-                Installations of Buildings, Section 131. That TIA allowed
                     nient, or adequate for good service or future expansion of                   countries that do not have formalized rules for electrical
                     electrical use.                                                              installations to adopt the NEC and by so doing to be fully
                                                                                                  compatible with the safety principles of IEC 60364-1, Sec-
                            FPN: Hazards often occur because of overloading of
                            wiring systems by methods or usage not in conformity                  tion 131. The addition of 90.1(D) will promote acceptance
                            with this Code. This occurs because initial wiring did                and adoption of the NEC internationally.
                            not provide for increases in the use of electricity. An                    The NEC is an essential part of the safety system of
                            initial adequate installation and reasonable provisions for           the Americas, and its future will be enhanced by increased
                            system changes provide for future increases in the use                international acceptance.
                            of electricity.

                     Consideration should always be given to future expansion                     90.2 Scope
                     of the electrical system. Future expansion might be unlikely                 (A) Covered This Code covers the installation of electrical
                     in some occupancies, but for others it is wise to plan an                    conductors, equipment, and raceways; signaling and com-
                     initial installation comprised of service-entrance conductors                munications conductors, equipment, and raceways; and opti-
                     and equipment, feeder conductors, and panelboards that                       cal fiber cables and raceways for the following:
                     allows for future additions, alterations, designs, and so on.
                                                                                                  (1) Public and private premises, including buildings, struc-
                                                                                                      tures, mobile homes, recreational vehicles, and floating
                     (C) Intention This Code is not intended as a design specifi-                     buildings
                     cation or an instruction manual for untrained persons.                       (2) Yards, lots, parking lots, carnivals, and industrial substa-
                     The NEC is intended for use by capable engineers and electri-                       FPN to (2): For additional information concerning such
                     cal contractors in the design and/or installation of electrical                     installations in an industrial or multibuilding complex,
                                                                                                         see ANSI C2-2002, National Electrical Safety Code.
                     equipment; by inspection authorities exercising legal juris-
                     diction over electrical installations; by property insurance
                     inspectors; by qualified industrial, commercial, and residen-                Requirements for locations such as these are found through-
                     tial electricians; and by instructors of electrical apprentices              out the Code. Specific items such as outside feeders and
                     or students.                                                                 branch circuits can be found in Article 225, grounding in
                                                                                                  Article 250, surge arresters in Article 280, switches in Article
                                                                                                  404, outside lighting in Article 410, transformers in Article
                     (D) Relation to Other International Standards The re-                        450, and carnivals in Article 525.
                     quirements in this Code address the fundamental principles
                     of protection for safety contained in Section 131 of Interna-
                                                                                                  (3) Installations of conductors and equipment that connect
                     tional Electrotechnical Commission Standard 60364-1, Elec-
                                                                                                      to the supply of electricity
                     trical Installations of Buildings.
                            FPN: IEC 60364-1, Section 131, contains fundamental                   Often, but not always, the source of supply of electricity is
                            principles of protection for safety that encompass protec-
                            tion against electric shock, protection against thermal
                                                                                                  the serving electric utility. The point of connection from a
                            effects, protection against overcurrent, protection against           premises wiring system to a serving electric utility system
                            fault currents, and protection against overvoltage. All of            is, by definition, referred to as the service point. The conduc-
                            these potential hazards are addressed by the requirements             tors on the premises side of the service point are, by defini-
                            in this Code.                                                         tion, referred to as service conductors. (These definitions
                                                                                                  are found in Article 100.) The requirements for service con-
                     In addition to being the most widely adopted code for the                    ductors as well as for service-related equipment are found
                     built environment in the United States, the NEC is also                      in Article 230. Article 230 applies only where the source of
                     adopted and used extensively in many foreign countries.                      supply of electricity is from a utility.
                     Section 90.1(D) makes it clear that the NEC is compatible                         Where the source of supply of electricity is not the

                     with international safety principles. Added as a Tentative                   serving electric utility, the source may be a generator, a
                     Interim Amendment (TIA) to the 1999 Code, this section                       battery system, a solar photovoltaic system, a fuel cell, or

                 2                                                                                                                   2005 National Electrical Code Handbook

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                                                                                                                                 Article 90 — Introduction                                                                                    90.2

                                                                           a combination of those sources. Requirements for such
                                                                           sources of supply are found in Article 445 and Articles 700
                                                                           through 702 for generators, Article 480 for storage batteries,
                                                                           Article 690 for solar photovoltaic systems, and Article 692
                                                                           for fuel cells. The associated delivery wiring requirements
                                                                           are found in Chapters 2 and 3 (except Article 230) and in
                                                                           Articles 700 through 702 for emergency, legally required,
                                                                           and optional standby power system circuits.

                                                                           (4) Installations used by the electric utility, such as office
                                                                               buildings, warehouses, garages, machine shops, and rec-                          Generation control
                                                                                                                                                                and transmission                                               Distribution
                                                                               reational buildings, that are not an integral part of a                                                                                        and metering
                                                                               generating plant, substation, or control center.
                                                                                                                                                                                        NEC does not apply.
                                                                           Section 90.2(A), which was rewritten for the 2002 Code,
                                                                           provides order and clarity concerning the portions of electric                                 Garage
                                                                           utility facilities covered by the NEC. [See 90.2(B) and the
                                                                           related commentary for information on facilities and specific                                                                                   Office
                                                                           lighting not covered by the NEC.] Exhibit 90.1 illustrates

                                                                           the distinction between electric utility facilities to which the
                                                                           NEC does and does not apply.                                                      Warehouse
                                                                                 Industrial and multibuilding complexes and campus-
                                                                           style wiring often include substations and other installations
                                                                           that employ construction and wiring similar to those of elec-
                                                                           tric utility installations. Although such nonutility installa-
                                                                           tions are within the scope of the NEC, the NEC requirements
                                                                           may not always be all-inclusive, for example, in clearances
                                                                           of conductors or in clearances from buildings or structures                                                                         Machine
                                                                           for nominal voltages over 600 volts. In such cases, the user
                                                                           can find additional information in the National Electrical                                             NEC applies to these buildings.
                                                                           Safety Code (NESC), published by the Institute of Electrical
                                                                           and Electronics Engineers, Inc., P.O. Box 1331, 445 Hoes                    Exhibit 90.1 Typical electric utility complexes showing examples
                                                                           Lane, Piscataway, NJ 08855-1331.                                            of facilities covered and not covered by the provisions of the

                                                                           (B) Not Covered This Code does not cover the following:                     ments for floating buildings are covered in the NEC and are
                                                                           (1) Installations in ships, watercraft other than floating                  found in Article 553.
                                                                               buildings, railway rolling stock, aircraft, or automotive
                                                                               vehicles other than mobile homes and recreational                       (2) Installations underground in mines and self-propelled
                                                                               vehicles                                                                    mobile surface mining machinery and its attendant elec-
                                                                                                                                                           trical trailing cable
                                                                               FPN: Although the scope of this Code indicates that the
                                                                               Code does not cover installations in ships, portions of                 (3) Installations of railways for generation, transformation,
                                                                               this Code are incorporated by reference into Title 46,                      transmission, or distribution of power used exclusively
                                                                               Code of Federal Regulations, Parts 110–113.                                 for operation of rolling stock or installations used exclu-
                                                                                                                                                           sively for signaling and communications purposes
                                                                           The NEC does not specifically cover shipboard wiring. Title                 (4) Installations of communications equipment under the
                                                                           46 of the Code of Federal Regulations, Parts 110–113, how-                      exclusive control of communications utilities located
                                                                           ever, does contain many specific NEC-referenced require-                        outdoors or in building spaces used exclusively for such
                                                                           ments. These requirements, which originated in the NEC,                         installations
                                                                           are enforced by the U.S. Coast Guard. Installation require-                 (5) Installations under the exclusive control of an electric
                                                                                                                                                           utility where such installations

                                                                           National Electrical Code Handbook 2005                                                                                                                               3

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                 90.3                                                                              Article 90 — Introduction

                           a. Consist of service drops or service laterals, and asso-                              generally; Chapters 5, 6, and 7 apply to special occupancies,
                              ciated metering, or                                                                  special equipment, or other special conditions. These latter
                           b. Are located in legally established easements, rights-                                chapters supplement or modify the general rules. Chapters
                              of-way, or by other agreements either designated by                                  1 through 4 apply except as amended by Chapters 5, 6, and
                              or recognized by public service commissions, utility                                 7 for the particular conditions.
                              commissions, or other regulatory agencies having                                          Chapter 8 covers communications systems and is not
                              jurisdiction for such installations, or                                              subject to the requirements of Chapters 1 through 7 except
                           c. Are on property owned or leased by the electric                                      where the requirements are specifically referenced in Chap-
                              utility for the purpose of communications, metering,                                 ter 8.
                              generation, control, transformation, transmission, or                                     Chapter 9 consists of tables.
                              distribution of electric energy.                                                          Annexes are not part of the requirements of this Code
                            FPN to (4) and (5): Examples of utilities may include                                  but are included for informational purposes only.
                            those entities that are typically designated or recognized
                            by governmental law or regulation by public service/                                   The reference to ‘‘the introduction’’ is intended to include
                            utility commissions and that install, operate, and maintain
                                                                                                                   Article 90 in the application of the Code. Chapters 1 through
                            electric supply (such as generation, transmission, or dis-
                            tribution systems) or communication systems (such as                                   4 apply generally, except as amended or specifically refer-
                            telephone, CATV, Internet, satellite, or data services).                               enced in Chapters 5, 6, and 7 (Articles 500 through 780).
                            Utilities may be subject to compliance with codes and                                  For example, 300.22 (Chapter 3) is modified by 725.3(C)
                            standards covering their regulated activities as adopted                               and 760.3(B) and is specifically referenced in 800.133(D)
                            under governmental law or regulation. Additional infor-
                                                                                                                   and 830.3(B). A graphic explanation of the NEC arrange-
                            mation can be found through consultation with the appro-
                            priate governmental bodies, such as state regulatory                                   ment, Figure 90.3, was added to the 2002 Code.
                            commissions, Federal Energy Regulatory Commission,
                            and Federal Communications Commission.

                                                                                                                     Chapter 1 — General
                     An FPN was added to the 2005 Code to clarify the use of
                     the word utility as used in 90.2(B)(4) and 90.2(B)(5). This                                     Chapter 2 — Wiring and Protection                                Applies generally
                     explanatory information now provides the authority having                                                                                                         to all electrical
                                                                                                                     Chapter 3 — Wiring Methods and Materials                            installations
                     jurisdiction a basis for judgment concerning this issue.
                          It is not the intent of 90.2(B)(5) to exclude the NEC as                                   Chapter 4 — Equipment for General Use
                     an installation regulatory document. After all, the NEC is
                     fully capable of being utilized for electrical installations in
                     most cases, and 90.2(B)(5) does not pertain to areas where                                                                                     Chapter 5 — Special Occupancies
                     portions of the NEC could not be used. Rather, 90.2(B)(5)                                            Supplements or modifies                   Chapter 6 — Special Equipment
                                                                                                                           Chapters 1 through 4
                     lists specific areas where the nature of the installation re-
                     quires specialized rules or where other installation rules,                                                                                    Chapter 7 — Special Conditions

                     standards, and guidelines have been developed for specific
                     uses and industries. For example, the electrical utility indus-                                                                                         Chapter 8 is not subject
                                                                                                                                                                             to the requirements of
                     try uses the NESC as its primary requirement in the genera-                                                                                             Chapters 1 through 7 except
                     tion, transmission, distribution, and metering of electrical                                   Chapter 8 — Communications Systems                       where the requirements are
                                                                                                                                                                             specifically referenced in
                     energy. See Exhibit 90.1 for examples of electric utility                                                                                               Chapter 8.
                     facilities that may or may not be covered by the Code.                                          Chapter 9 — Tables                                       Applicable as referenced

                                                                                                                     Annex A through Annex G                                  Informational only;
                                                                                                                                                                              not mandatory
                     (C) Special Permission The authority having jurisdiction
                     for enforcing this Code may grant exception for the installa-
                     tion of conductors and equipment that are not under the                                       Figure 90.3 Code Arrangement.
                     exclusive control of the electric utilities and are used to
                     connect the electric utility supply system to the service-                                    90.4 Enforcement
                     entrance conductors of the premises served, provided such                                     This Code is intended to be suitable for mandatory applica-
                     installations are outside a building or terminate immediately                                 tion by governmental bodies that exercise legal jurisdiction
                     inside a building wall.                                                                       over electrical installations, including signaling and commu-
                                                                                                                   nications systems, and for use by insurance inspectors. The
                     90.3 Code Arrangement                                                                         authority having jurisdiction for enforcement of the Code
                     This Code is divided into the introduction and nine chapters,                                 has the responsibility for making interpretations of the rules,
                     as shown in Figure 90.3. Chapters 1, 2, 3, and 4 apply                                        for deciding on the approval of equipment and materials,
                 4                                                                                                                                    2005 National Electrical Code Handbook

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                                                                                 Article 90 — Introduction                                                                    90.5

                           and for granting the special permission contemplated in a                    90.5 Mandatory Rules, Permissive Rules, and
                           number of the rules.                                                         Explanatory Material
                                                                                                        (A) Mandatory Rules Mandatory rules of this Code are
                           Some localities do not adopt the NEC, but even in those                      those that identify actions that are specifically required or
                           localities, installations that comply with the current Code                  prohibited and are characterized by the use of the terms
                           are prima facie evidence that the electrical installation is                 shall or shall not.
                                Section 90.4 advises that all materials and equipment                   Section 90.5, which was revised and reorganized for the
                           used under the requirements of the Code are subject to the                   1999 Code, clarifies that two distinctive types of rules are
                           approval of the authority having jurisdiction. The text of                   stated in the Code. Mandatory rules, characterized by the
                           90.7, 110.2, and 110.3, along with the definitions of the terms              terms shall and shall not, are covered in 90.5(A).
                           approved, identified (as applied to equipment), labeled, and
                           listed, is intended to provide a basis for the authority having
                           jurisdiction to make the judgments that fall within that partic-             (B) Permissive Rules Permissive rules of this Code are
                           ular area of responsibility.                                                 those that identify actions that are allowed but not required,
                                The phrase ‘‘including signaling and communication
                                                                                                        are normally used to describe options or alternative methods,
                           systems’’ was added to the 2002 Code to emphasize that,                      and are characterized by the use of the terms shall be permit-
                           indeed, these systems are also subject to enforcement.
                                                                                                        ted or shall not be required.

                                By special permission, the authority having jurisdiction                Permissive rules are simply options or alternative methods
                           may waive specific requirements in this Code or permit                       of achieving equivalent safety — they are not requirements.
                           alternative methods where it is assured that equivalent objec-               A close reading of permissive terms is important, because
                           tives can be achieved by establishing and maintaining effec-                 permissive rules are often misinterpreted. For example, the
                           tive safety.                                                                 frequently used permissive term shall be permitted can be
                                                                                                        mistaken for a requirement. Substituting ‘‘the inspector must
                           It is the responsibility of the authority having jurisdiction                allow [item A or method A]’’ for ‘‘[item A or method A]
                           to interpret the specific rules of the Code. This paragraph                  shall be permitted’’ generally clarifies the interpretation.
                           empowers the authority having jurisdiction, using special
                           permission (written consent), to permit alternative methods
                           where specific rules are not established in the Code. For                    (C) Explanatory Material Explanatory material, such as
                           example, the authority having jurisdiction may waive spe-                    references to other standards, references to related sections of
                           cific requirements in industrial occupancies, research and                   this Code, or information related to a Code rule, is included in
                           testing laboratories, and other occupancies where the specific               this Code in the form of fine print notes (FPNs). Fine print
                           type of installation is not covered in the Code.                             notes are informational only and are not enforceable as re-
                                                                                                        quirements of this Code.
                                                                                                              Brackets containing section references to another NFPA
                                This Code may require new products, constructions, or                   document are for informational purposes only and are pro-
                           materials that may not yet be available at the time the Code                 vided as a guide to indicate the source of the extracted text.
                           is adopted. In such event, the authority having jurisdiction                 These bracketed references immediately follow the extracted
                           may permit the use of the products, constructions, or materi-                text.
                           als that comply with the most recent previous edition of this
                           Code adopted by the jurisdiction.                                            A number of requirements in the NEC have been extracted
                                                                                                        from other NFPA codes and standards. Therefore, a second
                           This paragraph of 90.4 permits the authority having jurisdic-                paragraph was added for the 2005 Code to prevent any
                           tion to waive a new Code requirement during the interim                      misunderstanding about the purpose of bracketed references
                           period between acceptance of a new edition of the NEC and                    to other NFPA codes and standards — they are provided
                           the availability of a new product, construction, or material                 only to indicate the section of the NFPA document from
                           redesigned to comply with the increased safety required by                   which the material in the NEC was extracted. Although
                           the latest edition. It is difficult to establish a viable future             NEC requirements based on extracted material are under the
                           effective date in each section of the NEC because the time                   jurisdiction of the technical committee responsible for the
                           needed to change existing products and standards, as well                    particular document in which the extracted material resides,
                           as to develop new materials and test methods, usually is not                 this revision to 90.5(C) makes it clear that the NEC require-
                           known at the time the latest edition of the Code is adopted.                 ments stand on their own and that extracted material

                           National Electrical Code Handbook 2005                                                                                                               5

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                 90.6                                                         Article 90 — Introduction

                     with bracketed references does not indicate that other NFPA              tions Governing Committee Projects. These regulations are
                     documents are adopted through reference.                                 included in the NFPA Directory, which is published annually
                           Fine print notes (FPNs) do not contain statements of               and can be obtained from the Secretary of the NFPA Stan-
                     intent or recommendations. They present additional supple-               dards Council. The Formal Interpretations procedure can be
                     mentary material that aids in the application of the require-            found in Section 6 of the Regulations.
                     ment. In addition to explanatory material being in fine print                 The National Electrical Code Committee cannot be re-
                     (small type), the material is further identified in the Code             sponsible for subsequent actions of authorities enforcing the
                     by the abbreviation FPN preceding the paragraph. Fine print              NEC that accept or reject its findings. The authority having
                     notes are not requirements of the NEC and are not enforce-               jurisdiction is responsible for interpreting Code rules and
                     able.                                                                    should attempt to resolve all disagreements at the local level.
                           Footnotes to tables, although also in fine print, are not          Two general forms of Formal Interpretations are recognized:
                     explanatory material unless they are identified by the abbre-            (1) those that are interpretations of the literal text and (2)
                     viation FPN. Table footnotes are part of the tables and are              those that are interpretations of the intent of the Committee
                     necessary for proper use of the tables. For example, the                 at the time the particular text was issued.
                     footnotes at the end of Table 310.13 are necessary for the                    Interpretations of the NEC not subject to processing are
                     use of the table and therefore are mandatory and enforceable             those that involve (1) a determination of compliance of a
                     Code text.                                                               design, installation, product, or equivalency of protection;
                           Additional explanatory material is also found in the               (2) a review of plans or specifications or judgment or knowl-
                     annexes at the back of this handbook. Annex A is a reference             edge that can be acquired only as a result of on-site inspec-

                     list of product safety standards used for product listing where          tion; (3) text that clearly and decisively provides the
                     that listing is required by the Code. Annex B provides guid-             requested information; or (4) subjects not previously consid-
                     ance on the use of the general formula for ampacity found                ered by the Technical Committee or not addressed in the
                     in 310.15(C). Annex C consists of wire fill tables for conduit           document. Formal Interpretations of Code rules are pub-
                     and tubing. Annex D contains example calculations. Annex                 lished in several venues, including necdigest , the NFPA
                     E presents various tables showing fire resistance ratings for            Electrical Section News segment found in the NFPA Journal,
                     Types I-V construction to correlate with the uses of Type NM             in NFPA News, and in the National Fire Codes subscription
                     cable, Annex F contains cross-reference tables for Chapter 3             service and are sent to interested trade publications.
                     realignment with the 2002 edition, and Annex G contains                       Most interpretations of the NEC are rendered as the
                     model administration and enforcement legislation.                        personal opinions of NFPA electrical engineering staff or of
                                                                                              an involved member of the National Electrical Code Com-
                            FPN: The format and language used in this Code follows            mittee because the request for interpretation does not qualify
                            guidelines established by NFPA and published in the               for processing as a Formal Interpretation in accordance with
                            NEC Style Manual. Copies of this manual can be obtained           NFPA Regulations Governing Committee Projects. Such
                            from NFPA.                                                        opinions are rendered in writing only in response to written
                                                                                              requests. The correspondence contains a disclaimer indicat-
                     This fine print note informs the user that a style manual is             ing that it is not a Formal Interpretation issued pursuant to
                     available for the NEC. A style manual is basically a ‘‘how-              NFPA Regulations and that any opinion expressed is the
                     to’’ pamphlet for editors. The NEC Style Manual contains                 personal opinion of the author and does not necessarily
                     a list of rules and regulations used by the panels and editors           represent the official position of NFPA or the National Elec-
                     who prepare the NEC. The NEC Style Manual, which was                     trical Code Committee.
                     revised for the 2002 edition of the Code, is available from
                                                                                              90.7 Examination of Equipment for Safety
                     90.6 Formal Interpretations                                              For specific items of equipment and materials referred to in
                     To promote uniformity of interpretation and application of               this Code, examinations for safety made under standard
                     the provisions of this Code, formal interpretation procedures            conditions provide a basis for approval where the record is
                     have been established and are found in the NFPA Regulations              made generally available through promulgation by organiza-
                     Governing Committee Projects.                                            tions properly equipped and qualified for experimental test-
                                                                                              ing, inspections of the run of goods at factories, and service-
                     The procedures for implementing Formal Interpretations of                value determination through field inspections. This avoids
                     the provisions of the NEC are outlined in the NFPA Regula-               the necessity for repetition of examinations by different ex-
                                                                                              aminers, frequently with inadequate facilities for such work,

                 6                                                                                                               2005 National Electrical Code Handbook

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                                                                                   Article 90 — Introduction                                                                    90.9

                           and the confusion that would result from conflicting reports                  ways, and additional spaces allow for future increases in
                           on the suitability of devices and materials examined for a                    electric power and communication circuits. Distribution cen-
                           given purpose.                                                                ters located in readily accessible locations provide conve-
                                It is the intent of this Code that factory-installed internal            nience and safety of operation.
                           wiring or the construction of equipment need not be in-
                           spected at the time of installation of the equipment, except
                                                                                                         The requirement for providing the exclusively dedicated
                           to detect alterations or damage, if the equipment has been
                                                                                                         equipment space mandated by 110.26(F) supports the intent

                           listed by a qualified electrical testing laboratory that is recog-
                                                                                                         of 90.8(A) regarding future increases in the use of electricity.
                           nized as having the facilities described in the preceding
                                                                                                         The phrase ‘‘and communications circuits’’ was added for
                           paragraph and that requires suitability for installation in
                                                                                                         the 2005 Code to point out the importance of considering
                           accordance with this Code.
                                                                                                         communications circuits when planning future needs. Elec-
                                                                                                         trical and communications distribution centers should con-
                                   FPN No. 1: See requirements in 110.3.                                 tain additional space and capacity for future additions and
                                   FPN No. 2: Listed is defined in Article 100.                          should be conveniently located for easy accessibility.
                                   FPN No. 3: Annex A contains an informative list of                         Where electrical and communications distribution
                                   product safety standards for electrical equipment.                    equipment is installed so that easy access cannot be achieved,
                                                                                                         a spare raceway(s) or pull line(s) should be run at the initial
                           Testing laboratories, inspection agencies, and other organiza-                installation, as illustrated in Exhibit 90.2.
                           tions concerned with product evaluation publish lists of
                           equipment and materials that have been tested and meet
                                                                                                         (B) Number of Circuits in Enclosures It is elsewhere
                           nationally recognized standards or that have been found
                                                                                                         provided in this Code that the number of wires and circuits
                           suitable for use in a specified manner. The Code does not
                                                                                                         confined in a single enclosure be varyingly restricted.
                           contain detailed information on equipment or materials but
                                                                                                         Limiting the number of circuits in a single enclosure
                           refers to products as ‘‘listed,’’ ‘‘labeled,’’ or ‘‘identified.’’
                                                                                                         minimizes the effects from a short circuit or ground fault
                           See Article 100 for definitions of these terms.
                                                                                                         in one circuit.
                                NFPA does not approve, inspect, or certify any installa-
                           tions, procedures, equipment, or materials, nor does it ap-
                           prove or evaluate testing laboratories. In determining the                    These limitations minimize the heating effects inherently
                           acceptability of installations or procedures, equipment, or                   present wherever current-carrying conductors are grouped
                           materials, the authority having jurisdiction may base accep-                  together. See 408.35 for restrictions on the number of over-
                           tance on compliance with NFPA or other appropriate stan-                      current devices on one panelboard.
                           dards. In the absence of such standards, the authority may
                           require evidence of proper installation, procedures, or use.
                           The authority having jurisdiction may also refer to the listing               90.9 Units of Measurement
                           or labeling practices of an organization concerned with prod-                 (A) Measurement System of Preference For the purpose
                           uct evaluations that is able to determine compliance with                     of this Code, metric units of measurement are in accordance
                           appropriate standards for the current production of listed                    with the modernized metric system known as the Interna-
                           items.                                                                        tional System of Units (SI).
                                Annex A contains a list of product safety standards used
                           for product listing. The list includes only product safety
                           standards for which a listing is required by the Code. For                    According to a recent report titled ‘‘A Metric for Success’’
                           example, 344.6 requires that rigid metal conduit, Type RMC,                   by the National Institute of Standards and Technology
                           be listed. By using Annex A, the user finds that the listing                  (NIST), most U.S. industries that do business abroad are
                           standard for rigid metal conduit is UL 6, Rigid Metal Con-                    predominantly metric already because of global sourcing of
                           duit. Because associated conduit fittings are required to be                  parts, service, components, and production. However, quite
                           listed, UL 514B, Fittings for Cable and Conduit, is found                     a few domestic industries still use U.S. Customary units.
                           in Annex A also.                                                              The NIST report warns that domestic industries that ignore
                                                                                                         global realities and continue to design and manufacture with
                                                                                                         nonmetric measures will find that they risk increasing their
                           90.8 Wiring Planning                                                          costs. Nonmetric modular products (the building construc-
                                                                                                         tion industry uses great quantities of modular parts) and those
                           (A) Future Expansion and Convenience Plans and speci-
                                                                                                         that interface with outside industry products are especially
                           fications that provide ample space in raceways, spare race-

                           National Electrical Code Handbook 2005                                                                                                                 7

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                                                                  90.9                                                    Article 90 — Introduction

                                                                      Exhibit 90.2 A residential dis-
                                                                      tribution system showing spare
                                                                      raceways or pull lines that                                                                                                 Spare raceways or pull lines
                                                                      allow for future circuits and                                                                                               from electrical service panel
                                                                                                                                                                                                  and communications center
                                                                      loads for both electrical and                                                                                               to accessible attic space
                                                                      communications systems.

                                                                                                                                       Junction                         Raceway
                                                                                                                                       boxes                            or pull
                                                                                                                                       Unfinished             Finished lines
                                                                                                                                       utility area        basement area


                                                                      vulnerable to the added costs of adapting to a metric environ-        NFPA documents), and safety calculations (e.g., minimum
                                                                      ment.                                                                 distances taken from Table 110.31).
                                                                           Metric standards are beginning to appear in the domestic                                                  Example
                                                                      building construction industry because our national stan-
                                                                                                                                            Using the hard-conversion method, determine the equivalent
                                                                      dards are being harmonized with international standards.
                                                                                                                                            metric conversion for 24 in., generally the minimum cover
                                                                      The National Electrical Code is an important building con-
                                                                                                                                            requirements for direct burial cables and conductors in non-
                                                                      struction standard and moves another step in the metric
                                                                                                                                            specific locations taken from row 1 of Table 300.5.
                                                                                                                                            Step 1.
                                                                      (B) Dual System of Units SI units shall appear first, and
                                                                      inch-pound units shall immediately follow in parentheses.                                    25.4 mm
                                                                                                                                                                 24 in.          609.6 mm
                                                                      Conversion from inch-pound units to SI units shall be based                                    1 in.
                                                                      on hard conversion except as provided in 90.9(C).                     Step 2. Because the calculation is being performed as a hard
                                                                                                                                            conversion, the 609.6 mm dimension may be changed, and
                                                                      Hard conversion is explained in FPN No. 1 following                   the selected equivalent cover requirement is 600 mm.
                                                                      90.9(D). Calculations to convert measurements from inch-                   For the 2005 Code as well as the 2002 Code, the mea-
                                                                      pound units to metric units must be made using hard conver-           surements of 600 mm and 24 in. appear in Table 300.5 for
                                                                      sion. The hard-conversion method is mandatory except for              the minimum cover requirements for direct burial cables and
                                                                      trade sizes [e.g., raceway sizes in Table 300.1(C)], extracted        conductors in nonspecific locations. For the 1999 NEC, the
                                                                      material (e.g., class and zone measurements from other                selected SI unit of measure was required to be 609.6 mm.

                                                                  8                                                                                                            2005 National Electrical Code Handbook

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                                                                                 Article 90 — Introduction                                                                    90.9

                           The 2005 Code (as well as the 2002 Code) permits much                                                              Example 1
                           more latitude for the final selected dimension, and so the                  Determine the equivalent metric conversion for 10 ft where
                           equivalent minimum cover requirement of 600 mm is a more                    the calculation could have a negative impact on safety, such
                           practical solution. Basically, a hard conversion permits a                  as the minimum distance of 10 ft given in Table 110.31,
                           change in a dimension or allows rounding up or down to                      and where the measurement is from a fence to a live part
                           better fit the physical constraints of the installation.                    from 601 volts to 13,799 volts.
                           (C) Permitted Uses of Soft Conversion The cases given                       Step 1.                              0.3048 m
                                                                                                                              10 ft                                3.048 m
                           in 90.9(C)(1) through (C)(4) shall not be required to use hard                                                      1 ft
                           conversion and shall be permitted to use soft conversion.                   Step 2. Round up the calculation to 3.05 m, because a dis-
                                                                                                       tance less than 3.048 could have a negative impact on safety.
                           (1) Trade Sizes Where the actual measured size of a prod-
                                                                                                       The answer, 3.05 m, matches the minimum distance in Table
                           uct is not the same as the nominal size, trade size designators
                                                                                                       110.31 from a fence to a live part from 601 volts to 13,799
                           shall be used rather than dimensions. Trade practices shall
                           be followed in all cases.
                                                                                                            Because safety is a concern for this conversion calcula-
                                                                                                       tion, the original Code distance (the U.S. Customary units
                           Metric trade sizes (metric designators) of conduits were                    for this example) remains the shortest permitted distance.
                           added in the 1996 Code as fine print notes in each raceway                  The final metric equivalent ends up slightly larger. The exact
                           article. Since the 2002 Code, these metric designators appear               difference is of no practical concern, however, because 0.2
                           in the Code text, preceding the trade size equivalents, in the              mm is less than 1/32 in. From a practical point of view, a
                           raceway articles.                                                           variance of 1/32 in. in a length of 10 ft is insignificant.
                                For example, in 350.20(A) of this Code, the size require-
                           ment is stated as follows: ‘‘LFMC smaller than metric desig-                                                       Example 2
                           nator 16 (trade size 1/2) shall not be used.’’ In 351-5(a) of               Using the soft-conversion method, determine the equivalent

                           the 1999 NEC, the size requirement was stated as follows:                   metric conversion for 30 in. where the calculation could
                           ‘‘Liquidtight flexible metallic conduit smaller than 1/2-in.                have a negative impact on safety, such as a 30 in. minimum
                           electrical trade size shall not be used.’’                                  horizontal working space requirement in the rear of equip-
                                This change does not reflect a technical change but                    ment that requires access to nonelectrical parts according to
                           rather provides acceptable language to both domestic and                    110.26(A)(1)(a).
                           international users of the NEC. For ease of use, in Table 4
                           of Chapter 9, metric designators are separate columns.                      Solution
                                                                                                       Step 1.                                25.4 mm
                                                                                                                             30 in.                                762 mm
                                                                                                                                                1 in.
                           (2) Extracted Material Where material is extracted from                     Step 2. Do not round off the calculation, because even a
                           another standard, the context of the original material shall                slight reduction in the original distance could have a negative
                           not be compromised or violated. Any editing of the extracted                impact on safety. The answer is 762 mm, which matches
                           text shall be confined to making the style consistent with                  the minimum distance of 110.26(A)(1)(a) for a minimum
                           that of the NEC.                                                            horizontal working space.
                           (3) Industry Practice Where industry practice is to express
                           units in inch-pound units, the inclusion of SI units shall not              (4) Safety Where a negative impact on safety would result,
                           be required.                                                                soft conversion shall be used.

                           The following examples illustrate conversions from U.S.                     (D) Compliance Conversion from inch-pound units to SI
                           Customary units to SI units. Example 1 shows the process                    units shall be permitted to be an approximate conversion.
                           of converting a dimension from feet to meters, where safety                 Compliance with the numbers shown in either the SI system
                           is a concern. Table 110.31 contains minimum permitted dis-                  or the inch-pound system shall constitute compliance with
                           tances from a fence to a live part for voltages 601 and greater.            this Code.
                           Example 1 calculates the equivalent metric conversion for                          FPN No. 1: Hard conversion is considered a change in
                           10 ft using the minimum distance of 10 ft in Table 110.31                          dimensions or properties of an item into new sizes that
                           where the measurement is from a fence to a live part from                          might or might not be interchangeable with the sizes
                           601 volts to 13,799 volts.                                                         used in the original measurement. Soft conversion is
                                                                                                              considered a direct mathematical conversion and involves

                           National Electrical Code Handbook 2005                                                                                                               9

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                 90.9                                                                                                     Article 90 — Introduction

                                                                         a change in the description of an existing measurement           Commentary Table 90.1 Conversions Using the Hard-
                                                                         but not in the actual dimension.                                 Conversion Method
                                                                         FPN No. 2: SI conversions are based on IEEE/ASTM
                                                                         SI 10-1997, Standard for the Use of the International
                                                                                                                                            U.S.                    Soft                    Hard
                                                                         System of Units (SI): The Modern Metric System.
                                                                                                                                          Customary             Conversions,             Conversions,            Equivalent U. S.
                                                                                                                                            Units                 SI Units                 SI Units              Customary Units
                    Commentary Table 90.1 offers some examples of the hard-
                                                                                                                                               ⁄ in.               12.7 mm                   13 mm                       0.51 in.
                    conversion process. U.S. Customary units were used in the
                                                                                                                                               ⁄ in.                19 mm                    19 mm                       0.75 in.
                    1993, 1996, and 1999 Code and were still valid for the 2002
                                                                                                                                                1 in.              25.4 mm                   25 mm                       0.98 in.
                    Code. Soft-conversion SI units were used in the 1996 and
                                                                                                                                                4 in.              102 mm                   100 mm                       3.94 in.
                    1999 Code. The hard-conversion SI units, which were added                                                                 12 in.               305 mm                   300 mm                      11.81 ft
                    to the 2002 Code, were listed with their equivalent U.S.                                                                    2 ft               610 mm                   600 mm                       1.97 ft
                    Customary units. The equivalent U.S. units are given only                                                                   3 ft               914 mm                   900 mm                       2.95 ft
                    to show the small variance between customary units and the                                                                  6 ft                1.83 m                   1.8 m                       5.91 ft
                    hard-conversion units.                                                                                                    15 ft                 4.57 m                   4.5 m                      14.76 ft
                         Warning signs that state specific clearances, such as
                    required in 513.10(B), permit distance measurements in ei-
                    ther inch-pound units or metric units.

                 10                                                                                                                                                          2005 National Electrical Code Handbook

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