Electrical wiring (United States)

Electrical wiring in general refers to conductors used to carry electricity and their accessories. General aspects of electrical wiring as used to provide power in or to buildings and structures, commonly referred to as building wiring, are described in Electrical wiring. Electrical wiring in the United States is generally in compliance with the National Electrical Code, a standard developed by the National Fire Protection Association which has been periodically revised since 1882. Local amendments or supplements to this model code are common in American cities or states.

Terminology (field)

Although much of the electrician's field terminology matches that of the National Electrical Code (US), usages can and do vary.

* Neutral refers to a conductor with continuity to the electrical system's center tap of the power company transformer of a single-phase system, or the center of the wye connection of a polyphase system. American electrical codes require that the neutral be connected to earth at the "service panel" only and at no other point within the building wiring system. Formally the neutral is called the "grounded conductor"; as of the 2008 NEC, the terms "neutral conductor" and "neutral point" have been defined in the Code to record what had been common usage.
* Hot is any conductor (wire or otherwise) connected with an electrical system that has electric potential to electrical ground or neutral (ground and neutral should be approximately equipotential in a correctly configured system). Because a person is more likely to provide a path from a conductor in the electrical system to an inadvertent ground/neutral (the floor, a pipe, equipment housings, etc.), rather than the opposite, such a person is likely to experience 'hot' wires as hot, and neutral or ground wires as not-so-hot.
* Ground is a conductor with continuity to earth, whether intentional or not.
* Leg as in 'hot leg' refers to one of multiple hot conductors in an electrical system. One such leg will have a higher potential to another hot leg than to ground or neutral, typically 208 V or 240 V, depending on the electrical service for the system. The most common service in the U.S., single-phase, 240 V, features a neutral and two hot legs, 240 V to each other, and 120 V each to the neutral.
*Switch-leg is a wiring configuration in which the full-potential circuit is available at the fixture location, while one half of the circuit (hot in new installations, but often neutral in older knob-and-tube systems) is routed from the fixture location, through a switch, and back and into the fixture itself.
*An outlet is called a receptacle in the NEC. In the NEC an outlet is a device for easily connecting a utilization device by inserting a mating plug.

National Electrical Code and other codes and standards

The National Electrical Code (NEC) specifies acceptable wiring methods and materials in the United States. [citation|url=http://query.nytimes.com/gst/fullpage.html?res=9803E5DB173BF93AA25752C0A961958260|author=Edward R. Lipinski|date=January 19, 1997|title=Electrical Codes and Regulations|publisher=New York Times] Local jurisdictions usually adopt the NEC or another published code and then distribute documents describing how local codes vary from the published codes. They cannot distribute the NEC itself for copyright reasons. The stated purpose of the NEC is to protect persons and property from hazards arising from the use of electricity. The NEC is not any jurisdiction's electrical code per se; rather, it is an influential work of standards that local legislators (e.g., city council members, state legislators, etc. as appropriate) tend to use as a guide when enacting local electrical codes. The NFPA states that excerpts quoted from the National Electrical Code must have a disclaimer indicating that the excerpt is not the complete and authoritative position of the NFPA and that the original NEC document must be consulted as the definitive reference.

New construction, additions or major modifications must follow the relevant code for that jurisdiction, which is not necessarly the latest version of the NEC. Regulations in each jurisdiction will indicate when a change to an existing installation is so great that it must then be rebuilt to comply with the current electrical code. Generally existing installations are not required to be changed to meet new codes.

Enforcement of code requirements varies by jurisdiction in the United States. In many areas, a homeowner, for example, can perform household wiring for a building which the owner occupies; [ http://weststpaul.govoffice.com/vertical/Sites/%7B2CF6FEAE-EDC4-4E50-A078-817B219E41B8%7D/uploads/%7B3186FAFA-5503-46B8-AEFD-2206DE693396%7D.PDF City of West St. Paul Electrical Inspections information sheet retrieved 2008 feb 10 ] this may even be complete wiring of a home. A few cities [ http://www.ci.nyc.ny.us/html/dob//html/applications_and_permits/electrical_permit_and_insp.shtml New York Electrical permits, retrieved 2008 Feb 10. The city of New York, for example requires all installation work to be done by licenced contractors ] have more restrictive rules and require electrical installations to be done by licensed electricians. The work will be inspected by a designated authority at several stages before permission is obtained to energize the wiring from the local utility; the inspector may be an employee of the state or city, or an employee of an electrical supply utility.

Design and installation conventions

For residential wiring, some basic rules given in the NEC are:

*"phase" wire in a circuit may be black, red, orange (high leg delta) insulated wire, sometimes other colors, but never green, gray, or white (whether these are solid colors or stripes).
*"neutral" wire is connected to the center tap of the final step-down transformer and is identified by gray or white insulated wire, perhaps with stripes; most commonly bonded to earth for a fixed known path to stabilize the voltages only at the main service panel; many times called the grounded wire. Note that all metallic systems in a building are to be bonded to the panel; e.g., water, natural gas, HVAC piping, etc.
*"Grounding" wire of circuit may be bare or identified insulated wire of green or having green stripes.
*Larger wires are furnished only in black; these may be properly identified with suitable paint or tape. The phase wire for a switch "leg" is the white wire of a two insulated wire cable; the black wire is connected to the light.
*All wiring in a circuit except for the leads that are part of a device or fixture must be the same gauge. Note that different size wires may be used in the same raceway so long as they are all insulated for the maximum voltage of any of these circuits.
*There is no maximum number of receptacles on a circuit, but the Code gives rules for calculating circuit loading.
*Ground-fault circuit interrupter (GFCI) protection is required on receptacles in wet locations. This includes all small appliance circuits in a kitchen, receptacles in a crawl space, basements, bathrooms and a receptacle for the laundry room, as well as outdoor circuits within easy reach of the ground. However, they are not required for refrigerators because unattended disconnection could cause spoilage of food, nor for garbage disposals. Instead, for refrigerators and other semi-permanent appliances in basements and wet areas, use a one-outlet non-GFCI dedicated receptacle. Two-wire outlets having no grounding conductor may be protected by an upstream gfci and must be labelled "no grounding". Most GFCI receptacles allow the connection and have GFCI protection for down-stream connected receptacles. Receptacles protected in this manner should be labeled "GFCI protected".
*All branch circuits with receptacles must have arc-fault circuit interrupter protection, unless wired with specific types of armored cable. [cite book | authorlink = David Mumford| title = National Electric Code| publisher = National Fire Protection Association year = 2008| isbn = 978-087765791-0|page=210.12(B) ]
*Most circuits have the metallic components interconnected with a grounding wire connected to the third, round prong of a plug, and to metal boxes and appliance chassis.
*furnaces, water heaters, heat pumps, central air conditioning units, stoves on dedicated circuits
*Use exterior components for exterior lighting and outlets
*Electrical boxes must be properly sized to prevent heat build-up, especially from joints, and just having so many items in too small a space.

The foregoing is just a brief overview and must not be used as a substitute for the actual National Electrical Code.

Comparison of US practices with other countries

Electrical wiring practices developed in parallel in many countries in the late 19th and early 20th centuries [ R. M. Black "The History of Electric Wires and Cables", Peter Pergrinus, London 1983 ISBN 0 86341 001 4] . As a result, national and regional variations developed and remain in effect. (see National Electrical Code (US), Electrical wiring, Electrical wiring (UK)). Some of these are retained for technical reasons, since the safety of wiring systems depends not only on the wiring code but also on the technical standards for wiring devices, materials, and equipment.

Grounding (earthing) of distribution circuits is a notable difference in practice between United States wiring systems and those elsewhere in the world. Since the early 1960s, wiring in new construction has required a separate grounding conductor used to bond (electrically connect) all normally non-current carrying parts of an electrical installation. Portable appliances with metal cases also have a bonding conductor in the flexible cable and plug connecting them to the distribution system. The circuit return conductor ("neutral") is also connected to ground at the service entrance panel only; no other connections from neutral to ground are allowed, unlike regulations in other parts of the world.

Lighting and power receptacle circuits in US systems are typically radial from a distribution panel containing circuit breakers to protect each branch circuit. [ American Electrician's Handbook page 9-87 ] The smallest branch circuit rating is 15 amperes, used for general purpose receptacles and lighting. Branch circuits for higher ratings are usually dedicated to one appliance, for example, fixed cooking appliances, electric clothes dryers, and air conditioners. Lighting and general purpose receptacles are at 120 volts AC, with larger devices fed by three wire single-phase circuits at 240 volts.

Countries such as Mexico may adopt the NFPA standard as their national electrical code, with local amendments similar to those in United States jurisdictions. The Canadian electrical code, while developed independently from the NFPA code, is similar in scope and intent to the US NEC, with only minor variations in technical requirement details; harmonization of the CSA and NEC codes is intended to facilitate free trade between the two countries.

Wiring methods

Most circuits in the modern North American home and light commercial construction are wired with non-metallic sheathed cable designated type (often referred to by the brand name Romex). [ Terrell Croft and Wilford Summers (ed), "American Electricans' Handbook, Eleventh Edition", McGraw Hill, New York (1987) ISBN 0-07013932-6 page 9-86 ] This type of cable is the least expensive for a given size and is appropriate for dry indoor applications. The designation NM XX-Y indicates, respectively, the type of sheathing (in this case, non-metallic), the size of the main conductors, and the total number of circuit conductors (exclusive of the grounding conductor). For example, NM 14-2 cable contains three conductors (two plus one ground) at 14 gauge, a size typically used for circuits protected at 15 amperes. Circuits with larger currents (such as for electric furnaces, water heaters, air conditioners, or sub-mains to additional circuit panels) will have larger conductors. Not all US jurisdictions permit use of non-metallic sheathed cable. The NEC does not permit use of NM cable in large, fire-resistant, or high-rise structures. [ NEC 2005 Article 334.10 ]

In type NM cable, conductor insulation is color-coded for identification, typically one black, one white, and a bare grounding conductor. The National Electrical Code (NEC) specifies that the black conductor represent the "hot" conductor, with significant voltage to earth ground; the white conductor represent the identified or neutral conductor, near ground potential [ NEC 2005 article 200.7 ] ; and the bare/green conductor, the safety grounding conductor not normally used to carry circuit current. In 240 V applications not requiring a neutral conductor, the white wire may be used as the second hot conductor, but must be recolored with tape or by some other method. Four-wire flexible equipment connection cords have red as the fourth color; unlike European practices, color-coding in flexible cords is the same as for fixed wiring.

Several other types of wiring systems are used for building wiring in the United States; these include corrugated metal armored cable, mineral-insulated cable, other types of power cable, and various types of electrical conduit. In industrial applications cables may be laid in cable trays. Cable type TC is especially intended for use in tray systems. Special wiring rules apply to wet or corrosive locations [ NEC 2005 article 110.11] , and to locations which present an explosion hazard [ NEC 2005 Article 500 ] .. Wiring materials for use in the United States must be made and tested to product standards set by NEMA and Underwriters Laboratories (UL) and must bear approval marks such as those set by UL.

Approved wiring types can vary by jurisdiction.Not all wiring methods approved in the NEC are accepted in all areas of the United States.

See also

*American wire gauge
*Wire nut

References

External links

* [http://www.nfpa.org/nec/TheNEC/index.asp The NEC at NFPA.org]
* [http://www.repeater-builder.com/tech-info/ac-power-info.html Summary of NEC Color Code]