Electrical wiring in North America follows the regulations and standards applicable at the installation location. It is also designed to provide proper function, and is also influenced by history and traditions of the location installation.

The US National Electrical Code is applicable to many areas in the US where state, county or local authorities have adopted it. For electrical wiring in Canada, the Canadian Electrical Code is a very similar standard published in Canada by the Canadian Standards Association.


Although much of the electrician's field terminology matches that of the electrical codes, usages can vary:

United States 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 conform to what had been common usage.[1]

Electrical codes and standards

The National Electrical Code (NEC) specifies minimum acceptable wiring methods and materials for many states and municipalities in the U.S.[2] It is sponsored by the National Fire Protection Association (NFPA) and has been periodically revised since 1897. Local jurisdictions usually adopt the NEC or another published code and then distribute documents describing how local codes vary from the published codes. Governments cannot distribute the NEC itself for copyright reasons, though parts that have been adopted into law are not subject to copyright.

The 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 necessarily 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.

Other code requirements vary by jurisdiction in the United States. In many areas, a homeowner, for example, can perform household wiring for a building which the owner occupies;[3] this may even be complete wiring of a home. A few cities[4] 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 electric utility; the inspector may be an employee of the state or city, or an employee of an electrical supply utility.

For electrical wiring in Canada, the Canadian Electrical Code is a very similar standard published in Canada by the Canadian Standards Association since 1927.

Design and installation conventions

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

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

Main article: Mains electricity by country

Electrical wiring practices developed in parallel in many countries in the late 19th and early 20th centuries.[7] As a result, national and regional variations developed and remain in effect. (see National Electrical Code, electrical wiring, electrical wiring in the United Kingdom). 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 wiring systems of the United States and those of other regions. 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 some other parts of the world.

Lighting and power receptacle circuits in North American systems are typically radial from a distribution panel containing circuit breakers to protect each branch circuit.[8] The smallest branch circuit rating is 15 amperes, used for general purpose receptacles and lighting. Often, 20 ampere circuits are used for general purpose receptacles and lighting. In residential construction, 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.

In commercial construction, three-phase circuits are often used. Common 3 phase configurations within a building are 208v/120 wye, 120/240 center tapped delta and 480v/277v wye. Lighting is usually fed by 277 V or 120v.

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 CEC and NEC codes is intended to facilitate free trade between the two countries.

Wiring methods

12/2 Non-metallic (NM) sheathed cable

Most circuits in the modern North American home and light commercial construction are wired with non-metallic sheathed (NM) cable designated type.[9] 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.[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;[11] and the bare/green conductor, the safety grounding conductor not normally used to carry circuit current. Wires may be re-colored, so these rules are commonly excepted.[12] In 240-volt 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.[citation needed] Four-wire flexible equipment connection cords have red as the fourth color; unlike older European practices, color-coding in flexible cords is the same as for fixed wiring.[citation needed]

In commercial and industrial, unenclosed NM cable is often prohibited in certain areas or altogether (depending on what the building is used for and local/state building codes). Therefore, it is almost never used by commercial electrical contractors. Most wiring is put in non-flexible conduit, usually EMT because of its cost and durability. Rigid may be required for certain areas and additionally, vapor-lock fittings may be required in areas where a fire or explosion hazard is present (such as gas stations, chemical factories, grain silos, etc.) PVC can be used where wire is run underground or where concrete will be poured. A duct bank is usually made of multiple PVC conduits encased in concrete. FMC or Flex is used where EMT or other non-flexible conduit is impractical or for short runs, known as "whips", to lights or other devices. For power circuits, the color-coding uses the same colors as residential construction, and adds the additional wires used for three-phase systems. Black, Red and Blue are used for hot wires and White is used as the neutral wire in a 120/208 V circuit. Brown, Orange and Yellow are used as hot wires and gray is used as the neutral wire in a 277/480 V. For grounding, regardless of the voltage, Green (or a bare wire) is used.

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,[13] and to locations which present an explosion hazard.[14] Wiring materials for use in the United States must generally 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.

Wire types

Heavy duty outdoor electrical cable

When running through conduit, such as in commercial applications, it is typical to pull individual wires rather than a preassembled cable.

Wire is manufactured in a range of conductor sizes, stranding, and materials (copper or aluminum),[15] but the term "wire type" usually refers to the insulation, which determines the environments in which the wire may be used.

Wire types for North American wiring practices are defined by standards issued by Underwriters Laboratories, the Canadian Standards Association, the American Society for Testing and Materials, the National Electrical Manufacturers Association, and the Insulated Cable Engineers Association.

One important property of the insulation which affects the current-carrying capacity of the wire is the maximum conductor temperature. This, in combination with the ambient temperature and ability of the environment to absorb heat, determines the amount of tolerable copper loss in the wire, and therefore its size in relation to the load current.[16]

The most commonly used insulation type in AC electrical distribution systems throughout North America is THHN ("Thermoplastic High Heat-resistant Nylon-coated"[17]). This is a specification for PVC insulation (other thermoplastics are permitted, but rarely used) with a nylon jacket for abrasion resistance. THHN is suitable for dry or damp environments and conductor temperatures up to 90 °C (194 °F).[18]

A second popular type is THWN ("Thermoplastic Heat and Water-resistant Nylon-coated"[19]). This is similar to the preceding, but rated for dry or wet environments and conductor temperatures up to 75 °C (167 °F),[18] and is commonly used in subsurface conduits which may fill with water.

A great deal of THWN wire is actually dual-rated, and meets THHN the specification as well, so may be used in wet environments up to 75°C or dry environments up to 90°C.

An extended specification, THWN-2 permits use in wet locations and conductor temperatures up to 90°C simultaneously.[18]

There are additional restrictions on such wire's use not mentioned here (for example, neither THHN nor THWN may be exposed to sunlight, or directly buried in the ground[20]); see the NEC for details.

XHHW-2 (XLPE High Heat-resistant Water-resistant"[21]) is a less commonly seen insulation type with a thermoset plastic insulation. It has the same moisture and temperature specifications as THWN-2 (the original XHHW was equivalent to THHN/THWN dual-rated insulation),[18] but being a thermoset plastic, it is less affected by temperature, remaining firmer at high temperatures and significantly more flexible at low temperatures. This makes it popular for outdoor wiring applications.[22] The low dielectric constant of the insulation is also beneficial when high-frequency AC is present on the wire, such as the output of variable-frequency drives.

See also


  1. ^ a b c National Electric Code. National Fire Protection Association. 2008. p. 100. ISBN 978-0-87765-791-0.
  2. ^ Edward R. Lipinski (January 19, 1997), "Electrical Codes and Regulations", New York Times, archived from the original on March 21, 2008
  3. ^ http://weststpaul.govoffice.com/vertical/Sites/%7B2CF6FEAE-EDC4-4E50-A078-817B219E41B8%7D/uploads/%7B3186FAFA-5503-46B8-AEFD-2206DE693396%7D.PDF Archived 2011-09-02 at the Wayback Machine City of West St. Paul Electrical Inspections information sheet retrieved 2008 feb 10
  4. ^ http://www.ci.nyc.ny.us/html/dob//html/applications_and_permits/electrical_permit_and_insp.shtml Archived 2007-04-13 at the Wayback Machine New York Electrical permits, retrieved 2008 Feb 10. The city of New York, for example requires all installation work to be done by licensed contractors
  5. ^ NFPA 70: National Electrical Code 2011. National Fire Protection Association. 2010. pp. 70-47 to 70-48. ISBN 978-087765914-3.
  6. ^ "OSHA OFFICE OF TRAINING AND EDUCATION: Electrical Discussion" (PDF). www.osha.gov. p. 14. Retrieved 2018-12-10.
  7. ^ R. M. Black The History of Electric Wires and Cables, Peter Pergrinus, London 1983 ISBN 0-86341-001-4
  8. ^ American Electrician's Handbook page 9-87
  9. ^ Terrell Croft and Wilford Summers (ed), American Electrican's Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-013932-6 page 9-86
  10. ^ National Electric Code. National Fire Protection Association. 2008. p. 334.10. ISBN 978-0-87765-791-0.
  11. ^ National Electric Code. National Fire Protection Association. 2008. p. 200.7. ISBN 978-0-87765-791-0.
  12. ^ National Electric Code. National Fire Protection Association. 2008. p. 200.6(A). ISBN 978-0-87765-791-0.
  13. ^ National Electric Code. National Fire Protection Association. 2008. p. 110.11. ISBN 978-0-87765-791-0.
  14. ^ National Electric Code. National Fire Protection Association. 2008. p. 500. ISBN 978-0-87765-791-0.
  15. ^ NFPA 70 National Electrical Code (NEC) 2008 edition, Article 310.14
  16. ^ NFPA 70 National Electrical Code (NEC) 2020 edition, Table 310.16 Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)
  17. ^ THHN - Thermoplastic High Heat Resistant Nylon Coated (type of wire)
  18. ^ a b c d NFPA 70 National Electrical Code (NEC) 2008 edition, Article 310.8 (B)&(C), Table 310.13(A)
  19. ^ THWN - Thermoplastic Heat and Water Resistant Nylon Coated
  20. ^ NFPA 70 National Electrical Code (NEC) 2018 edition, Article 310.10 (D)&(F)
  21. ^ XHHW - Cross-Linked High Heat Water Resistant Insulated Wire (UL)
  22. ^ "XHHN - XHWN-2". Mike Holt's Forum. 2018-09-18. Our preferred wire for street lighting is XHHW-2. Holds up far better than THHN/W, which has a very thin insulation and easy to damage, ok inside. Underground I have seen THHN/W fail.