The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. The NEC, while having no legally binding regulation as written, can be and often is adopted by states, municipalities and cities in an effort to standardize their enforcement of safe electrical practices within their respective jurisdiction. In some cases, the NEC is amended, altered and may even be rejected in lieu of regional regulations as voted on by the governing bodies of any given locale.
The NEC codifies the requirements for safe electrical installations into a single, standardized source. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA), and while not itself a U.S. law, NEC use is commonly mandated by state or local law.[1]
The "authority having jurisdiction" inspects for compliance with these minimum standards.
("National Electrical Code" and "NEC" are registered trademarks of the NFPA).
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The NEC is developed by NFPA's Committee on the National Electrical Code, which consists of 19 code-making panels and a technical correlating committee. Work on the NEC is sponsored by the National Fire Protection Association. The NEC is approved as an American national standard by the American National Standards Institute (ANSI). It is formally identified as ANSI/NFPA 70.
First published in 1897, the NEC is updated and published every three years. The 2011 NEC is the current edition (effective date August 25, 2010). Most states adopt the most recent edition within a couple of years of its publication. As with any "uniform" code, a few jurisdictions regularly omit or modify some sections, or add their own requirements (sometimes based upon earlier versions of the NEC, or locally accepted practices). However, the NEC is the least amended model code, even with it setting minimum standards. No court has faulted anyone for using the latest version of the NEC, even when the local code was not updated.
In the U.S., anyone, including the city issuing building permits, may face a civil liability lawsuit (be sued) for negligently creating a situation that results in loss of life or property. Those who fail to adhere to well known best practices for safety have been held negligent. This means that the city should adopt and enforce building codes that specify standards and practices for electrical systems (as well as other departments such as water and fuel-gas systems). This creates a system whereby a city can best avoid lawsuits by adopting a single, standard set of building code laws. This has led to the NEC becoming the de facto standard set of electrical requirements. A licensed electrician will have spent years of apprenticeship studying and practicing the NEC requirements prior to obtaining his or her license.
The NEC is available as a bound book containing approximately 1000 pages. It has been available in electronic form since the 1993 edition. Although the code is updated every three years, some jurisdictions do not immediately adopt the new edition.
The NEC is also available as a restricted, digitized coding model that can be read online free of charge on certain computing platforms. An external link to this online access is referenced at the end of this article.
In the United States, statutory law cannot be copyrighted and is freely accessible and copyable by anyone.[2] When a standards organization develops a new coding model and it is not yet accepted by any jurisdiction as law, it is still the private property of the standards organization and the reader may be restricted from downloading or printing the text for offline viewing. For that privilege, the coding model must still be purchased as either printed media or a CD-ROM. Once the coding model has been accepted as law, it loses copyright protection and may be freely obtained at no cost.
Archive.org and many state or local government sites allow download of the NEC without the registration that the NFPA requires.
The NEC is composed of an introduction, nine chapters, annexes A through I, and the index. The introduction sets forth the purpose, scope, enforcement and rules or information that are general in nature. The first four chapters cover definitions and rules for installations (voltages, connections, markings, etc.), circuits and circuit protection, methods and materials for wiring (wiring devices, conductors, cables, etc.), and general-purpose equipment (cords, receptacles, switches, heaters, etc.). The next three chapters deal with special occupancies (high risk to multiple persons), special equipment (signs, machinery, etc.) and special conditions (emergency systems, alarms, etc.). Chapter 8 is specific to additional requirements for communications systems (telephone, radio/TV, etc.) and chapter 9 is composed of tables regarding conductor, cable and conduit properties, among other things. Annexes A-I relate to referenced standards, calculations, examples, additional tables for proper implementation of various code articles (for example, how many wires fit in a conduit) and a model adoption ordinance.
The introduction and the first 8 chapters contain numbered articles, parts, sections (or lists or tables) italicized exceptions, and Informational notes – explanations that are not part of the rules. Articles are coded with numerals and letters, as ###.###(A)(#)(a). For example 804.22(C)(3)(b) would be read as "section 804 point 22(C)(3)(b)." and would be found in chapter 8. For internal references, some lengthy articles are further broken into "parts" with Roman-numerals (parts I, II, III, etc.).
Each code article is numbered based on the chapter it is in. Those wiring methods acceptable by the NEC are found in chapter 3, thus all approved wiring method code articles are in the 300s. Efforts have been underway for some time to make the code easier to use. Some of those efforts include using the same extension in those code articles for the support of wiring methods.
The NFPA also publishes a 1,497-page NEC Handbook (for each new NEC edition) that contains the entire code, plus additional illustrations and explanations, and helpful cross-references within the code and to earlier versions of the code. The explanations are only for reference and are not enforceable.
Many NEC requirements refer to "listed" or "labeled" devices and appliances, and this means that the item has been designed, manufactured, tested or inspected, and marked in accordance with requirements of the listing agency. To be listed, the device has to meet the testing and other requirements set by a listing agency such as Underwriters Laboratories (UL), MET Laboratories, Inc. (MET), Intertek Group (ETL), Canadian Standards Association (CSA), and FM Approvals (FM). These are examples of "national recognized testing laboratories" (NRTLs) approved by the United States Department of Labor, Occupational Safety and Health Administration (OSHA) under the requirements of 29CFR1910.7. Only a listed device can carry the listing brand (or "mark") of the listing agency. Upon payment of an investigation fee to determine suitability, an investigation is started. To be labeled as fit for a particular purpose (for example "wet locations", "domestic range") a device must be tested for that specific use by the listing agency and then the appropriate label applied to the device. A fee is paid to the listing agency for each item so labeled, that is, for each label. Most NRTLs will also require that the manufacturer's facilities and processes be inspected as evidence that a product will be manufactured reliably and with the same qualities as the sample or samples submitted for evaluation. An NRTL may also conduct periodic sample testing of off-the-shelf products to confirm that safety design criteria are being upheld during production. Because of the reputation of these listing agencies, the "authority having jurisdiction" ( or "AHJ" – as they are commonly known) usually will quickly accept any device, appliance, or piece of equipment having such a label, provided that an end user or installer uses the product in accordance with manufacturer instructions and the limitations of the listing standard. However, an AHJ, under the National Electrical Code provisions, has the authority to deny approval for even listed and labeled products. Likewise, an AHJ may make a written approval of an installation or product that does not meet either NEC or listing requirements, although this is normally done only after an appropriate review of the specific conditions of a particular case or location.
The 2008 Code has user-friendly features to aid the reader in seeing changes. Revisions or additions to the articles from the 2005 version are highlighted in gray shading. Where sections have been deleted, a bullet (•) is shown between the paragraphs that remain.
Articles 210 addresses "branch circuits" (as opposed to service or feeder circuits) and receptacles and fixtures on branch circuits. There are requirements for the minimum number of branches, and placement of receptacles, according to the location and purpose of the receptacle outlet. A ground fault circuit interrupter (GFCI) is required for all receptacles in wet locations defined in the Code. The NEC also has rules about how many circuits and receptacles should be placed in a given residential dwelling, and how far apart they can be in a given type of room, based upon the typical cord-length of small appliances.
As of 1962 the NEC required that new 120-volt household receptacle outlets, for general purpose use, be both grounded and polarized. NEMA connectors implement these requirements.
The NEC also permits grounding-type receptacles in nongrounded wiring protected by a GFCI. The 1999 Code required that new 240-volt receptacles be grounded also, which necessitates a fourth slot in their faces. These changes in standards often cause problems for people living in older buildings.
Unlike circuit breakers and fuses, which only open the circuit when the current exceeds a fixed value for a fixed time, a GFCI device will interrupt electrical service when more than 4 to 6 milliamperes of current in either conductor leaks to ground. A GFCI detects an imbalance between the current in the "hot" side and the current in the "neutral" side. One GFCI receptacle can serve as protection for several downstream conventional receptacles. GFCI devices come in many configurations including circuit-breakers, portable devices and receptacles.
Another safety device introduced with the 1999 code is the arc-fault circuit interrupter (AFCI). This device detects arcs from hot to neutral that can develop when insulation between wires becomes frayed or damaged. While arcs from hot to neutral would not trip a GFCI device since current is still balanced, circuitry in an AFCI device detects those arcs and will shut down a circuit. AFCI devices generally replace the circuit breaker in the circuit. They are required in new construction on all 15A and 20A 125V circuits to bedrooms, where most arc fault fires originate.
In home construction, wiring is commonly allowed to be installed directly in walls without additional protection. However, in commercial and industrial buildings, wiring needs to be protected from damage, so it is more commonly installed inside conduit or ductwork made of metal, plastic, or passageways cast in concrete.
While some types of cable are protected by flexible spiraled metal armor, it is more common for conduit and ductwork to be installed empty and the wire pulled in later. The NEC spends considerable time documenting safe methods of installing cable in conduit, the primary concerns being the abrading of insulation, damage to the wire or insulation caused by sharp bends, kinking, and damage due to excess pulling strain.
A wire pulled with excessive force may break inside the conduit, requiring costly removal and replacement. However, a wire pulled with enough force to stretch the wire, but not break it, creates a hazard of future failure or fire. The stretched wire section will have a thinner cross section and higher resistance than other parts of the cable, and may have damaged insulation. Breaks may form in the stretched insulation, which may not be discovered until the circuit is powered and damage from arcing or shorting has occurred.
Wires must be protected from sharp metal edges of cut conduits or cabinet holes. The NEC specifies measures to protect wire insulation from damage by these edges during installation and use. For example, insulated cables may not be inserted directly through knockouts, due to the sharp edge around nearly all knockout holes. Clamping and other wire protection is often not required for plastic conduit parts, as plastic is not likely to damage insulation in contact with it.
In hazardous locations, more robust cable protection may be necessary. Common conduit and ductwork protects against direct physical abuse, but is neither air nor water tight. In wet locations, conduit may resemble standard threaded pipe in appearance, with gasketed box openings to keep moisture out. Areas with potentially explosive gases need further protection to prevent electrical sparks from igniting the gases, and internal conduit gas-tight barriers to prevent potentially ignited gases from traveling inside the conduit to other parts of the building.
| Nominal Rated Circuit Capacity |
Continuous Rated Circuit Capacity |
|---|---|
| 5 amps | 4 amps |
| 10 amps | 8 amps |
| 15 amps | 12 amps |
| 20 amps | 16 amps |
| 30 amps | 24 amps |
| 50 amps | 40 amps |
| 100 amps | 80 amps |
| 200 amps | 160 amps |
Most commonly available circuit breakers are rated to carry no more than 80% of their nominal rating continuously (3 hours or more) (NEC Art. 100). 100%-rated circuit breakers are manufactured for and may carry 100% of their nominal rating continuously.
The temperature rating of a wire or cable is generally the maximum safe ambient temperature that the wire can carry full-load power without the cable insulation melting, oxidizing, or self-igniting. A full-load wire does heat up slightly due to the metallic resistance of the wire, but this wire heating is factored into the cable's temperature rating. (NEC 310.10)
The NEC specifies acceptable numbers of conductors in crowded areas such as inside conduit, referred to as the fill rating. If the accepted fill rating is exceeded, then all the cables in the conduit are derated, lowering their acceptable maximum ambient operating temperature. Derating is necessary because multiple conductors carrying full-load power generate heat that may exceed the normal insulation temperature rating. (NEC 310.16)
In construction situations where future expansion is highly likely, it is sometimes more economical to install a slightly larger diameter conduit than is necessary for the initial building construction. Larger conduit costs more, but has a greater fill rating. This may eliminate the need for additional conduit installation while maintaining the conduit's overall temperature rating.
In certain situations, the temperature rating can be higher than normal, such as for knob and tube wiring where two or more load-carrying wires are never likely to be in close proximity. A knob and tube installation uses wires that are suspended in air. This gives them a greater heat dissipation rating than standard 3-wire NM-2 cable which includes two tightly bundled load and return wires.