National Electrical Code
From Wikipedia, the free encyclopedia
The National Electrical Code (NEC), or NFPA 70, is a United States standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA). "National Electrical Code" and "NEC" are registered trademarks of the NFPA. While the NEC is not itself a U.S. law, NEC use is commonly mandated by state or local law, as well as in many jurisdictions outside of the United States. [1] The NEC codifies the requirements for safe electrical installations into a single, standardized source.
The "Authority Having Jurisdiction" inspects for compliance with these minimum standards.
Contents |
[edit] General
The NEC is developed by NFPA's Committee on the National Electrical Code, which consists of 20 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 2008 Code is the most recent edition, approved on August 15, 2007. 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.
[edit] Public access to the NEC
The NEC is available as a bound book containing approximately 1000 pages. It has been available in electronic form since the 1993 edition. The code is updated every three years. However, some jurisdictions do not immediately adopt the new edition.
The NEC has now become available as a restricted, digitized reference that can be read online free of charge on certain computing platforms. Several external links to this online access are referenced at the end of this article.
However, even though the NEC is accessible via the Internet, the text is still the private property of NFPA and the reader is restricted from downloading or printing the text for offline viewing. For that privilege, the text must still be purchased as either printed media or a CD-ROM.
[edit] Structure of the NEC
The NEC is composed of an introduction, nine chapters, annexes A through G, 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), specific 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 ten tables regarding conductor, cable and conduit properties, among other things. Annexes A-G relate to referenced standards, calculations, examples, additional tables for proper implementation of various code articles (e.g., 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 Fine Print Notes (FPN) -- explanations that are not part of the rules. Articles are coded with numerals and letters, as ###.###(A)(#)(a) e.g., 804.22(C)(3)(b) could 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 1,100-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), 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 (e.g., "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 reliably manufactured according to 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.
[edit] Details of selected NEC requirements
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, eg: outlets in bathrooms, outdoors and kitchens, and, in addition, for dwelling units: crawl-spaces, garages, boathouses, unfinished basements, and within 6 feet (1.8 m) of a wet-bar sink, with limited exceptions. See NEC for details. The NEC also has rules about such things as how many circuits and receptacles/outlets 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 (for example, not more than 12 feet apart, or 4 feet apart on kitchen countertops).
As of 1962 the NEC required that new 120-volt household receptacle outlets, for general purpose use, be both grounded and polarized. NEMA has implemented this in its U.S. standard socket configurations so that:
- There must be a slot for a center-line, rounded pin connected to a common grounding conductor.
- The two blade-shaped slots must be of differing sizes, to prevent ungrounded (2-wire) devices which use "neutral" as their only grounding from being misconnected.
The NEC also has provisions that permit the use of grounding-type receptacles in nongrounded wiring (for example, the retrofit of 2-wire circuits) if a GFCI is used for protection of the new outlet (either itself or "downstream" from a GFCI). Art. 406.3(D)(3).
The 1999 Code required that new 240-volt receptacles be grounded also, which necessitates a fourth slot in their faces. U.S. 240 centertapped single phase has two of these slots being 'hot', with the neutral being the center tap. There is only one standard for these circuits, but 240 V receptacles come in two incompatible varieties. In one the 'neutral' slot accepts a flat blade-prong. In the other the neutral slot accepts a blade with a right angle bend. These are officially NEMA types 14-50R (commonly used with number 8 wire for electric ranges) and 14-30R (commonly used with number 10 wire for electric clothes dryers), respectively, and differ only in current rating (50 A versus 30 A); previous installations would have used the 10-30 or 10-50 configuration.
These changes in standards often cause problems for people living in older buildings.
Unlike traditional circuit breakers and fuses, which only open the circuit when the "hot" 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 is leaked to ground (either directly or through a resistance, such as a person). A GFCI detects an imbalance between the current in the "hot" side and the current in the "neutral" side. Most receptacle outlets with GFCI have the added advantage of protecting other receptacles 'downstream' of them, so that one GFCI receptacle can serve as protection for several conventional receptacles, whether or not they are grounding-type receptacles. GFCI devices come in many configurations including circuit-breakers, portable devices and receptacles.
A GFCI receptacle typically has a pair of small push buttons between its two receptacles: one labeled 'test' and the other 'reset' (or T and R). Pressing 'test' will place a small imbalance in the line sensor, which will trip the device, resulting in an audible "snap". Pressing 'reset' will allow the socket to function normally after a test, or after a faulty appliance has been removed from the circuit or insulated from ground. If a GFCI receptacle fails to trip when the test button is pushed (and the GFCI had been previously armed by first pressing in the reset button), it means the GFCI receptacle must be replaced because it is no longer providing protection against ground faults.
Like fuses and circuit breakers, a GFCI receptacle has a finite number of uses. It must be replaced when a test fails to trip the device.
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 15 and 20 amp circuits to bedrooms, where experience has shown most arc fault fires originate. In the future it is likely that all circuits will require their use.
[edit] Conduit and cable protection
In home construction, wiring is commonly allowed to be installed directly in walls without further protection. However in commercial and industrial buildings, wire needs to be better protected from damage, and so it is more commonly installed inside conduit or ductwork made of metal, plastic, or passageways cast in concrete.
While some types of wiring are available already inside a protective flexible spiraled metal shell, it is more common for conduit and ductwork to be installed empty and the wire added later by threading it through the finished passageways. The NEC spends considerable time documenting safe methods of installing cable in conduit, with the primary concerns being the friction and abrading of insulation due to pulling, damage to the wire or insulation due to sharp bending and kinking, and damage due to too much pulling strain on the cable.
Because most conduit and cabinets are made from metal, it is common for these components to have sharp metal edges due to the manufacuring processes. The NEC specifies a number of protective measures to help protect wire insulation from being cut or damaged by these edges both during installation and later when in actual use. Insulated cables may not be inserted directly through knockouts, for example, due to the sharp edge around nearly all knockout holes. Clamping and other wire protection is often not required for plastic conduit parts, since the plastic is not likely to damage insulation in contact with it.
In potentially hazardous locations, more robust cable protection may be necessary. Common conduit and ductwork protects against direct physical abuse, but is neither airtight nor watertight. In wet locations, conduit may resemble standard threaded pipe in appearance, with sealed 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.
[edit] Actual vs maximum current rating
| Maximum Rated Circuit Capacity |
Actual 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 |
A 15-amp fuse or circuit breaker is not actually intended to ever carry 15 amps of power. In actual fact, a given circuit may only carry up to 80% of its maximum-rated capacity, with the remaining 20% intended as a safety margin.
[edit] Temperature rating
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 either melting, oxidizing, or self-igniting. A full-load wire does heat up slightly due to the metallic resistance to the current, 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. This is necessary because when multiple conductors are carrying full-load power, there is a combined heating of all the cables that may exceed the normal insulation temperature rating. (NEC 310.16)
In construction situations where future expansion is highly likely to occur, it is sometimes economical to install a slightly larger diameter conduit than is necessary for the initial building construction. Larger conduit costs more money, but has a greater fill rating than smaller conduit, so that in the future no additional conduit installations may be required in order to add additional circuits while maintaining the conduit's overall temperature rating.
In certain special 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. The single lone load wires suspended in midair in knob and tube wiring will have a greater heat-dissipation rate than the standard 3-wire NM-2 cable which includes two tightly bundled load and return wires.
[edit] See also
PSE law, Japan Electrical Safety Law.
[edit] External links
- Free, restricted access to the 2005 NEC online
- Free, restricted access to the 2008 NEC online
- List of NEC adopted versions by state
- List of OSHA accredited NRTLs
- Electrical Calculator with 2008 NEC Codes
- everything on NEC's website is free of charge
