Article 90 Introduction --``,`,,,,,,,,,`,``,,``,,,`,`-`-`,,`,,`,`,,`--- 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. references. 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 1 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295. 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- tions 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 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 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. Substation (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 building 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 Gym 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 shop 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 NEC. (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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295. 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 --``,`,,,,,,,,,`,``,,``,,,`,`-`-`,,`,,`,`,,`--- Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 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 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. safe. 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 --``,`,,,,,,,,,`,``,,``,,,`,`-`-`,,`,,`,`,,`--- Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295. 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 NFPA. 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 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 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295. 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 Service panel Communications center 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. direction. Solution 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 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 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. Solution (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 volts. 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295. 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- 12 ⁄ in. 12.7 mm 13 mm 0.51 in. conversion process. U.S. Customary units were used in the 34 ⁄ 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 Copyright National Fire Protection Association Document provided by IHS Licensee=ExxonMobil/1890500101, 05/06/2005 00:58:19 Provided by IHS under license with NFPA MDT Questions or comments about this message: please call the Document Policy Group at 303-397-2295.