Electrical wiring in North America

Electrical wiring in North America follows regulations and standards for installation of building wiring.

Terminology

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

The 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 record what had been common usage.[1]

Electrical codes and standards

The National Electrical Code (NEC) specifies acceptable wiring methods and materials for many states and municipalities in the U.S.[2] It is sponsored by the National Fire Protection Association 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

Electrical wiring practices developed in parallel in many countries in the late 19th and early 20th centuries.[5] 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 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.[6] 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 used. Generally, receptacles are fed by 120 V or 208 V (in place of 240 V in a house), and can include special current rated outlets for industrial equipment. Lighting is usually fed by 277 V (with exception for special-use lights that use 120 V). Equipment can be hard-wired into the building using either 120/208 V or 277/480 V.

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

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.[7] 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.[8]

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;[9] 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.[10] 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. 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.

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 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,[11] and to locations which present an explosion hazard.[12] 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 wire.

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.

XHHW stands for "XLPE (cross-linked polyethylene) High Heat-resistant Water-resistant."[13] XHHW is a designation for a specific insulation material, temperature rating, and condition of use (suitable for wet locations) for electrical wire and cable.[14]

Wires with XHHW insulation are commonly used in the alternating current (AC) electrical distribution systems of commercial, institutional, and industrial buildings and installations, usually at voltage levels (potential difference or electromotive force) ranging from 110-600 volts. This type of insulation is used for both copper and aluminum conductors [15] which are either solid or stranded, depending on size.

According to Underwriters Laboratories (UL) Standard 44,[16] XHHW insulation is suitable for use in dry locations up to 90°C (194°F), or wet locations up to 75°C (167°F).

XHHW-2 insulation, which is similar to XHHW, is suitable for use in dry or wet locations up to 90°C (194°F).

THWN stands for "Thermoplastic Heat and Water-resistant Nylon-coated."[17] THWN is a designation for a specific insulation material, temperature rating, and condition of use (wet locations) for electrical wire and cable.[14]

Wires with THWN insulation are commonly used in the alternating current (AC) electrical distribution systems of buildings of all types and sizes throughout North America, usually at voltage levels (potential difference or electromotive force) ranging from 110-600 volts. This type of insulation is used for both copper and aluminum conductors [15] which are either solid or stranded, depending on size.

THHN stands for "Thermoplastic High Heat-resistant Nylon-coated."[18] THHN is a designation for a specific insulation material, temperature rating, and condition of use (suitable for dry and damp locations) for electrical wire and cable.[14]

Wire with THHN insulation is commonly used in the alternating current (AC) electrical distribution systems of all types and sizes throughout North America, usually at voltage levels (potential difference or electromotive force) ranging from 110-600 volts. This type of insulation is used for both copper and aluminum conductors [15] which are either solid or stranded, depending on size.

Many wires are rated both THWN and THHN, and are suitable for use in dry locations up to 90°C (194°F), or wet locations up to 75°C (167°F).

See also

References

  1. 1 2 3 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 (– Scholar search) 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 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 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. ↑ R. M. Black The History of Electric Wires and Cables, Peter Pergrinus, London 1983 ISBN 0-86341-001-4
  6. ↑ American Electrician's Handbook page 9-87
  7. ↑ 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
  8. ↑ National Electric Code. National Fire Protection Association. 2008. p. 334.10. ISBN 978-0-87765-791-0.
  9. ↑ National Electric Code. National Fire Protection Association. 2008. p. 200.7. ISBN 978-0-87765-791-0.
  10. ↑ National Electric Code. National Fire Protection Association. 2008. p. 200.6(A). ISBN 978-0-87765-791-0.
  11. ↑ National Electric Code. National Fire Protection Association. 2008. p. 110.11. ISBN 978-0-87765-791-0.
  12. ↑ National Electric Code. National Fire Protection Association. 2008. p. 500. ISBN 978-0-87765-791-0.
  13. ↑ What does XHHW stand for? Acronym Finder Definition
  14. 1 2 3 NFPA 70 National Electrical Code (NEC) 2008 edition, Article 310.8 (B)&(C), Table 310.13(A)
  15. 1 2 3 NFPA 70 National Electrical Code (NEC) 2008 edition, Article 310.14
  16. ↑ Scope for UL 44
  17. ↑ What does THWN stand for? Acronym Finder Definition
  18. ↑ What does THHN stand for? Acronym Finder Definition

External links

This article is issued from Wikipedia - version of the 12/2/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.