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National Electrical Code
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 [hide]
1 General
1.1 Public access to the NEC
2 Structure of the NEC
3 Details of selected NEC requirements
3.1 Conduit and cable protection
3.2 Actual vs maximum current rating
3.3 Temperature rating
4 See also
5 External links
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General
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
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.
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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.
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Structure of the NEC
The NEC is composed of an introduction, nine chapters, annexes A through H, 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, additiional 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.
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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).
Polarized, grounding, 120-volt socket
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).
240 V receptacle faces
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.
A 120-volt GFCI socket
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. This article or
section needs to be updated.
Please update the article to reflect recent events / newly available information,
and remove this template when finished.
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.
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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 manufacturing 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.
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Actual vs maximum current ratingNominal 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.
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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.