Electrical wiring in general refers to conductors used to carry
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
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 wiringElectrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring...
. Electrical wiring in the United States is generally in compliance with the National Electrical Code, a standard sponsored by the
National Fire Protection AssociationThe National Fire Protection Association is a U.S. organization charged with creating and maintaining minimum standards and requirements for fire prevention and suppression activities, training, and equipment, as well as other life-safety codes and standards...
which has been periodically revised since 1897. Local amendments or supplements to this model code are common in American cities or states.
Terminology (field)
Although much of the
electricianAn electrician is a tradesman specializing in electrical wiring of buildings, stationary machines and related equipment. Electricians may be employed in the installation of new electrical components or the maintenance and repair of existing electrical infrastructure...
's field terminology matches that of the
National Electrical Code (US)The National Electrical Code , 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 . "National Electrical Code" and "NEC" are registered trademarks...
, usages can and do vary.
- Neutral
Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...
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
At a point in space, the electric potential is potential energy divided by charge that is associated with a static electric field. It is a scalar quantity, typically measured in volts....
to electrical ground or neutral (ground and neutral should be approximately equipotentialEquipotential or isopotential in mathematics and physics refers to a region in space where every point in it is at the same potential. This usually refers to a scalar potential, although it can also be applied to vector potentials...
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.
- Grounded 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
Electrical service, in building wiring, refers to the wiring that connects the electric utility's cables in the street to the building. Specifically, electrical service is the wiring from the street, through the meter and up to the panelboard, but no farther....
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. 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 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.
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; this may even be complete wiring of a home. A few cities 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
Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
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".
- 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.
- furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven. The earliest furnace was excavated at Balakot, a site of the Indus Valley Civilization, dating back to its mature phase...
s, water heaters, heat pumpA heat pump is a machine or device that moves heat from one location to another location using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink...
s, central air conditioningAn air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area, or provide heat to an area. The cooling is done using a simple refrigeration cycle...
units, stoveA stove is an enclosed heated space. The term is commonly taken to mean an enclosed space in which fuel is burned to provide heating, either to heat the space in which the stove is situated or to heat the stove itself, and items placed on it...
s 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 . As a result, national and regional variations developed and remain in effect. (see
National Electrical Code (US)The National Electrical Code , 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 . "National Electrical Code" and "NEC" are registered trademarks...
,
Electrical wiringElectrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring...
,
Electrical wiring (UK)The modern UK standards and regulations for electrical wiring no longer differ substantially from those in other European countries. However, there are a number of noteworthy national peculiarities, habits and traditions associated with domestic electrical wiring in the UK that differ...
). 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)Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...
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. The smallest branch circuit rating is 15 amperes, 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-phaseA split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial applications. It is the AC equivalent of the original Edison 3-wire direct current system...
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 amperage 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
CanadianEstablished in 1919, the Canadian Standards Association is a not-for-profit association composed of representatives from government, industry, and consumer groups...
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). 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
sizeAmerican wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States for the diameters of round, solid, nonferrous, electrically conducting wire...
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.
In type NM cable, conductor insulation is color-coded for identification, typically one black, one white, and a bare grounding conductor. The
National Electrical CodeThe National Electrical Code , 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 . "National Electrical Code" and "NEC" are registered trademarks...
(NEC) specifies that the black conductor represent the
hot conductor, with significant voltage to earth ground; the white conductor represent the identified or
neutralSince the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...
conductor, near ground potential ; and the bare/green conductor, the safety grounding conductor not normally used to carry circuit current. Wires may be re-coloured, so these rules are commonly excepted. 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 European practices, color-coding in flexible cords is the same as for fixed wiring.
In commercial and industrial, 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. MC 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 120/208V circuit. Brown, Orange and Yellow are used as hot wires and Grey is used as the neutral wire in a 277/480V. 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 cableMineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro...
, other types of
power cableA power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed...
, and various types of
electrical conduitAn electrical conduit is a purpose-designed electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fibre, or fired clay. Flexible conduit is available for special purposes....
. 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 , and to locations which present an explosion hazard. Wiring materials for use in the United States must be made and tested to product standards set by
NEMANéma is a town in southeastern Mauritania, close to the border with Mali. It is located at around . It is the capital of Hodh Ech Chargui Region and of the Néma Department....
and
Underwriters LaboratoriesUnderwriters Laboratories® Inc. is an independent product safety certification organization. Based in Northbrook, Illinois, UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing with product safety...
(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.
External links