Electrical wiring (United States)
Encyclopedia
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...
which has been periodically revised since 1897. Local amendments or supplements to this model code are common in American cities or states. For electrical wiring in Canada, the Canadian Electrical Code
Canadian Electrical Code
The Canadian Electrical Code, CE code, or CSA C22.1 is a standard published by the Canadian Standards Association pertaining to the installation and maintenance of electrical equipment in Canada....
is a very similar standard published in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
by the Canadian Standards Association
Canadian Standards Association
The Canadian Standards Association, also known as the CSA, is a not-for-profit Standards organization with the stated aim of developing standards for use in 57 different areas of specialisation...
since 1927.
Terminology
Although much of the electricianElectrician
An 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. Electricians may also...
's field terminology
Terminology
Terminology is the study of terms and their use. Terms are words and compound words that in specific contexts are given specific meanings, meanings that may deviate from the meaning the same words have in other contexts and in everyday language. The discipline Terminology studies among other...
matches that of the electrical code
Electrical code
An electrical code is a set of regulations for electrical wiring. The intention of an electrical code is to provide standards to ensure electrical wiring systems that are safe and unlikely to produce either electric shock or fires. Electrical codes are usually devised by national or international...
s, usages can vary.
- A neutralGround and 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...
wire is the return leg of a circuit; in building wiring systems the neutral wire is connected to earth ground at least at one point. North American standards state that the neutral is neither switched nor fused. The neutral is connected to the center tap of the power company transformerTransformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
of a split-phase system, or the center of the wye connection of a polyphase power 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 potentialElectric potentialIn classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
relative to electrical ground or neutral. - Grounded is a conductor with continuity to earth.
- Leg as in 'hot leg' refers to one of multiple hot conductors in an electrical system. The most common residential and small commercial service in the U.S., single split-phase, 240 V, features a neutral and two hot legs, 240 V to each other, and 120 V each to the neutral. The most common three-phase system will have three "hot" legs, 208 V to each other and 120 V each to the neutral. An older but still widely used three phase system has two "hot" legs 240 V to each other, and 120 V each to the neutral and a third ""High Leg Hot" 240 V to each of the other "hots" and 208 V to the Neutral. http://en.wikipedia.org/wiki/High-leg_delta For larger commercial installations 277/480 V three phase is common
- An outlet is called a receptacle in the NEC. In the NEC an outlet is "a point in the wiring system at which current is taken to utilization equipment."
Electrical codes and standards
The National Electrical Code (NEC) specifies acceptable wiring methods and materials. 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 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; 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 electric utility
Electric utility
An electric utility is a company that engages in the generation, transmission, and distribution of electricity for sale generally in a regulated market. The electrical utility industry is a major provider of energy in most countries. It is indispensable to factories, commercial establishments,...
; 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). Specific exceptions apply, such as a cable running to a switch and back (known as a traveler) where the white wire will be the hot wire feeding that switch. Another is for a cable used to feed an outlet for 250VAC 15 or 20 amp appliances that do not need a neutral, there the white is hot (but should be identified as being hot, usually with black tape inside junction boxes).
- The neutral wire is identified by gray or white insulated wire, perhaps using stripes or markings.
- With lamp cord wire the ribbed wire is the neutral, and the smooth wire is the hot.NEC2008 400.22(f) allows surface marking with ridged, grooves or white stripes on the surface of lamp cord. With transparent cord the hot wire is copper colored, and the neutral is silver colored.
- Grounding wire of circuit may be bare or identified insulated wire of green or having green stripes. Note that all metallic systems in a building are to be bonded to the building grounding system, such as water, natural gas, HVAC piping, and others.
- Larger wires are furnished only in black; these may be properly identified with suitable paint or tape.
- 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.
- 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, a one-outlet non-GFCI dedicated receptacle is generally used. 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.
- FurnaceFurnaceA furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
s, water heaters, heat pumpHeat pumpA heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
s, central air conditioningAir conditioningAn air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
units and stoveStoveA 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 must be on dedicated circuits - The code provides rules for sizing electrical boxes for the number of wires and wiring devices in the box.
- In a fixture, the brass screw is hot, and the silver screw is neutral. The grounding screw is usually painted green.
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, electrical wiringElectrical wiring
Electrical 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. This article is intended to...
, 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)
Ground and 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...
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 North American 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. 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
Split phase
A 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
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
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
Canadian Electrical Code
The Canadian Electrical Code, CE code, or CSA C22.1 is a standard published by the Canadian Standards Association pertaining to the installation and maintenance of electrical equipment in Canada....
, 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
North American Free Trade Agreement
The North American Free Trade Agreement or NAFTA is an agreement signed by the governments of Canada, Mexico, and the United States, creating a trilateral trade bloc in North America. The agreement came into force on January 1, 1994. It superseded the Canada – United States Free Trade Agreement...
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
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada 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 Code
National Electrical Code (US)
The National Electrical Code , or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment...
(NEC) specifies that the black conductor represent the hot conductor, with significant voltage to earth ground; the white conductor represent the identified or neutral
Ground and 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...
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, 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 120/208V circuit. Brown, Orange and Yellow are used as hot wires and gray 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 cable
Mineral-insulated copper-clad cable
Mineral-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 cable
Power cable
A 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...
, and various types of electrical conduit
Electrical conduit
An electrical conduit is an electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fiber, 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 generally be made and tested to product standards set by NEMA
Néma
Néma 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 Laboratories
Underwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...
(UL) and must bear approval marks such as those set by UL.
Approved wiring types can vary by jurisdiction
Jurisdiction
Jurisdiction is the practical authority granted to a formally constituted legal body or to a political leader to deal with and make pronouncements on legal matters and, by implication, to administer justice within a defined area of responsibility...
. Not all wiring methods approved in the NEC are accepted in all areas of the United States.
Wire types
Wire types for North American wiring practices are defined by standards issued by Underwriters LaboratoriesUnderwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...
, the Canadian Standards Association
Canadian Standards Association
The Canadian Standards Association, also known as the CSA, is a not-for-profit Standards organization with the stated aim of developing standards for use in 57 different areas of specialisation...
, the American Society for Testing and Materials, the National Electrical Manufacturers Association
National Electrical Manufacturers Association
is the association of electrical and medical imaging equipment manufacturers. Founded in 1926 and headquartered near Washington, D.C., its approximately manufacture products used in the generation, transmission, distribution, control, and end use of electricity. These products are used in utility,...
and the Insulated Cable Engineers Association
Insulated Cable Engineers Association
Formed in the United States in 1925, the Insulated Cables Engineers Association, Inc. , sometimes in conjunction with other organizations like NEMA, produces technical standards for the manufacture and use of power cable, data, and control cable....
.
XHHW stands for "XLPE (cross-linked polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
) High Heat-resistant Water-resistant." XHHW is a designation for a specific insulation material, temperature rating, and condition of use (suitable for wet locations) for electrical wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
and cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
.
Wires with XHHW insulation are commonly used in the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) electrical distribution systems of commercial, institutional, and industrial buildings and installations, usually at voltage levels (potential difference or electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
) ranging from 110-600 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s. This type of insulation is used for both copper and aluminum conductors which are either solid or stranded, depending on size.
According to Underwriters Laboratories (UL) Standard 44, 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 High Water-resistant Nylon-coated." THWN is a designation for a specific insulation material, temperature rating, and condition of use (wet locations) for electrical wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
and cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
.
Wires with THWN insulation are commonly used in the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) electrical distribution systems of buildings of all types and sizes throughout North America, usually at voltage levels (potential difference or electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
) ranging from 110-600 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s. This type of insulation is used for both copper and aluminum conductors which are either solid or stranded, depending on size.
THHN stands for "Thermoplastic High Heat-resistant Nylon-coated." THHN is a designation for a specific insulation material, temperature rating, and condition of use (suitable for dry and damp locations) for electrical wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
and cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
.
Wire with THHN insulation is commonly used in the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(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 which are either solid or stranded, depending on size.