The National Electrical Code (NEC) specifies acceptable wiring methods and materials. Local US jurisdictions typically adopt the NEC or other published code and then distribute documents that describe how the local codes vary from the published codes. They cannot distribute the NEC itself for copyright reasons.

The purpose of the NEC is to protect people and property from risks arising from the use of electricity. The NEC is not the electrical code of any jurisdiction per se, but rather it is an influential work of standards that local legislators tend to use as a guide for promulgating local electrical codes. The National Fire Protection Association (NFPA) states that quoted passages from the NEC should carry a disclaimer indicating that such passages do not represent the complete and authoritative position of the NFPA and that the original NEC document should be consulted for 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 large that it must be rebuilt to comply with the current electrical code. In general, facilities are not required to be changed to comply with new codes.

Other code requirements vary by jurisdiction in the United States. In many areas, a homeowner, for example, can carry out the entire home wiring. Some cities have more restrictive rules and require electrical installations to be done by licensed electricians. The work will be inspected at several stages, by a designated authority, before authorization is obtained to energize the wiring. The inspector may be an employee of the state or city, or an employee of an electrical supply facility.

For residential wiring, some of the basic NEC rules are:.

• - The single-phase conductor can have black, red, orange insulator (single-phase 3-wire system), but never green, gray or white (if they are solid colors or in stripes). There are specific exceptions such as a wire that follows a round trip path to a switch, where the white insulated conductor represents the active or live power wire for that switch. Another exception is cables that are used to power a socket for devices or equipment that use a voltage of 250 VAC, consume up to 15 or 20 amps and do not need a neutral socket; In this case, the wire with white insulation represents the active outlet, but must be identified as such, usually with black tape inside junction or distribution boxes.

• - The neutral socket is identified with a cable or conductor with white or gray insulation, either with stripes or marks.

• - In every power cable, the striped conductor is the neutral, and the other is the active one. Standard 400.22(f) of the 2008 revised NEC Code allows the marking of grooved or white-striped grooves on the surface of the power cord. If the cable is covered with transparent insulation, the active conductor is copper-colored, and the neutral is silver-colored.

• - The grounding of a circuit can be a bare wire or conductor or with green insulation or one that has green stripes. Note that all metal pipes in a construction must be joined to the grounding system, such as water, natural gas, air conditioning pipes, among others.

• - Long wires or conductors must have black insulation, which can be properly identified with paint or appropriate adhesive tape.

• - All wiring in a circuit, except for conductors that are part of a device or accessory, must be of the same gauge. It should be noted that cables of different gauges can be used in the same raceway as long as they are all insulated for the maximum voltage of any of these circuits.

• - The NEC Code provides rules for circuit load calculation.

• - Differential circuit breaker protection, also called GFCI, is required in outlets installed in humid locations. These include all small appliance circuits in a kitchen, outlets in a confined space, basements, bathrooms and an outlet for the laundry room, as well as outdoor circuits a short distance from the ground. However, they are not necessary for refrigerators because unattended disconnection of this type of switch can cause food spoilage, nor for garbage disposals. Instead, for refrigerators and other semi-permanent appliances in basements and wet areas, a dedicated unprotected outlet is generally used. Two-wire plugs that do not have a ground conductor may be protected by a residual current circuit breaker located upstream and must be labeled "ungrounded" or similar. Most outlets with GFCI switches allow connection and have GFCI protection for outlets connected downstream. Outlets so protected must be labeled "GFCI protected", "RCD protected" or other similar expression.

• - Most circuits have their metal components interconnected with a ground wire connected to the third round prong of a plug, and to the metal cases and chassis of the device.

• - Stoves "Stove (heating)"), boilers "Boiler (machine)"), heat pumps and central air conditioning units must be connected to dedicated circuits.

• - The code establishes rules for sizing electrical boxes for the number of wires and wiring devices in the box.

• - In a device, the brass screw represents the active, and the silver is the neutral. The ground screw is usually painted green.

The above is only a brief description and should not be used as a substitute for the current NEC Code.

Electrical wiring practices were developed in parallel in many countries in the late 20th century and early 20th century.[2]​ As a result, national and regional variations were developed and are in effect. Some of these are retained for technical reasons, since the security of wiring systems depends not only on the wiring code, but also on technical standards for wiring devices, materials and equipment.

The grounding (grounding) of distribution circuits is a notable difference in practice between wiring systems in the United States and those elsewhere in the world. Since the 1960s, wiring in new construction has required a separate grounding conductor used to electrically connect all those points that do not normally carry electrical current. Portable devices with metal casings also have a bonding conductor in the mains connection cable and plug. The neutral conductor is also grounded at the service entrance panel only, and no other neutral to ground connections are permitted, unlike regulations in other parts of the world.

Lighting and outlet circuits in North American systems typically come from a distribution panel that contains switches to protect each branch circuit.[3] The smallest branch circuit current rating is 15 amps, used for lighting and general purpose outlets, although 20 amp circuits are often used. In residential construction, the highest value circuits are generally dedicated to a single device, for example, fixed kitchen appliances, clothes dryers, and air conditioners. Lighting and general purpose outlets are 120 VAC, with larger devices powered by 240 volt split phase circuits. In commercial construction, three-phase circuits are used. Outlets are generally powered by 120 or 208 VAC and may include special high current outlets for industrial equipment. Lighting is generally powered by 277 VAC, except for lights that use 120 V. Equipment can be connected in the building using the combination of 120/208 VAC or 277/480 VAC.

Certain countries may adopt the NFPA standard as their national electrical code, with local modifications similar to those in United States jurisdictions. Although the Canadian Electrical Code was developed independently of the NFPA code, it is similar in scope and intent to the US NEC code, with minor variations in details on the technical requirements. However, the harmonization of the CSA and NEC codes is intended to facilitate free trade between those countries.

Most residential electrical circuits and modern light construction in North America have installations made with non-metallic sheathed cable, often known by the brand name ROMEX.[4] This type of cable is the least expensive for a given size and is appropriate for applications in dry indoor environments. These cables usually use the designation "NM XX-Y" which indicates, respectively, the type of coating (in this case, non-metallic), the gauge of the main conductors, and the total number of conductors in the circuit (excluding the ground conductor). For example, "NM 14-2" cable contains three conductors (two in addition to the ground conductor) of American 14 gauge, a type typically used for circuits protected at 15 amps. Circuits with larger currents (for example, those for electric furnaces, water heaters, air conditioners, or those supplying electrical power to additional circuit panels) will need to use thicker conductors. Not all jurisdictions in the United States allow the use of non-metallic sheathed cable. The NEC Code does not allow the use of this type of cable in large, fire-resistant, or multi-story structures.[5]​.

In the non-metallic type of cable, the insulation uses a color code for identification, usually one black (hot conductor), one white (neutral conductor), and a bare grounding conductor, as established by the NEC Code.[6]​ The conductors can be re-colored, so these rules are commonly the exception[7]​ In 240 VAC applications that do not require a neutral conductor, the white conductor can be used as a second active conductor, but must be recolored with tape adhesive or by some other method. Four-conductor flexible power cables use red as the fourth color; Unlike European practices, the coding of power cables is the same as that of fixed wiring.

For commercial and industrial uses, non-metallic cable not enclosed in conduits is usually prohibited in certain areas or in an entire region, depending on the use of the building to be wired and local and state building codes. Most wiring is placed in metal conduit, due to its cost and durability. Galvanized metal ducts may be required for certain areas, and additionally, accessories that prevent the entry of vapors, in places where a fire or explosion hazard is present, such as gas stations, chemical factories and silos. Pipes made of PVC are used for underground cables or to be installed where concrete is poured. A bank of PVC conduit is usually made of several of them, embedded in concrete. Flexible metal conduits are used for short runs, such as connections to lighting fixtures. For power circuits, color coding uses the same colors as residential construction, and adds colors for additional wires used for three-phase systems. In 120/208 VAC circuits, the hot wires have black, red, and blue insulators and the neutral wire has white insulator. On the other hand, in 277/480 VAC circuits, the active wires are brown, orange and yellow and gray for the neutral wire. For grounding, regardless of the voltage, cables with green insulation are used.

Other types of systems are used for building wiring in the United States; these include corrugated metal power cables, mineral-insulated cables), and various types of electrical conduit. In industrial applications, cables may be placed in a tray and are Type TC. Special wiring rules apply to humid or corrosive environments,[8]​ and to locations that present explosion hazards.[9]​ Materials used for wiring in the United States must be manufactured and tested to standards set by NEMA and Underwriters Laboratories (UL), carrying in that case, the approval marks established by that organization.

Approved wiring types may vary in various jurisdictions, but not all wiring methods approved by the NEC code are accepted in all areas of the United States.

The types of conductors for electrical wiring in North America are defined by norms or standards published by organizations such as Underwriters Laboratories, Canadian Standards Association, American Society for Testing and Materials, National Electrical Manufacturers Association and Insulated Cable Engineers Association"). Their classification is as follows:

XHHW: It is the acronym for Cross-linked High Heat-resistant, Water-resistant or Cross-linked cable highly resistant to heat and humidity, for its meaning in Spanish[10]​ Conductors and cables of this type are recommended for use in humid locations[11]​ and are commonly used in commercial, institutional and industrial alternating current distribution systems with a range of electrical voltages between 110 and 600 volts. They are manufactured with copper or aluminum conductors[12]​ that can be solid or stranded, depending on their size. According to Standard 44 of Underwriters Laboratories (UL)[13]​ type XHHW cables and conductors are recommended for use in dry locations with temperatures up to 90 °C or humid locations up to 75 °C.

XHHW-2: It is similar to XHHW, and is recommended for use in humid or dry locations with temperatures up to 90 °C.

THWN: It is the acronym for Thermoplastic Heat and Water-resistant Nylon-coated or Thermoplastic Heat and Moisture Resistant Nylon Coated Cable.[14]​ Cables and conductors of this type are made of copper or aluminum (solid or stranded) and are used in alternating current distribution systems in buildings of all types and sizes throughout North America with a range of electrical voltages between 110 and 600 volts.[12]​.

THHN: It stands for Thermoplastic High Heat-resistant Nylon-coated or Thermoplastic High Heat-resistant Nylon-coated Cable[15]​ It is suitable for dry and humid locations[11]​ and has the same use as the THWN type cable.[12]​.

Many conductors are classified as THWN and THHN and are suitable for use in dry locations with temperatures up to 90°C or wet locations with temperatures up to 75°C.

• - American wire gauge.

• - Twist connector.

• - Electrical installation.

• - The NEC at NFPA.org.

• - Summary of NEC Color Code.

The National Electrical Code (NEC) specifies acceptable wiring methods and materials. Local US jurisdictions typically adopt the NEC or other published code and then distribute documents that describe how the local codes vary from the published codes. They cannot distribute the NEC itself for copyright reasons.

The purpose of the NEC is to protect people and property from risks arising from the use of electricity. The NEC is not the electrical code of any jurisdiction per se, but rather it is an influential work of standards that local legislators tend to use as a guide for promulgating local electrical codes. The National Fire Protection Association (NFPA) states that quoted passages from the NEC should carry a disclaimer indicating that such passages do not represent the complete and authoritative position of the NFPA and that the original NEC document should be consulted for 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 large that it must be rebuilt to comply with the current electrical code. In general, facilities are not required to be changed to comply with new codes.

Other code requirements vary by jurisdiction in the United States. In many areas, a homeowner, for example, can carry out the entire home wiring. Some cities have more restrictive rules and require electrical installations to be done by licensed electricians. The work will be inspected at several stages, by a designated authority, before authorization is obtained to energize the wiring. The inspector may be an employee of the state or city, or an employee of an electrical supply facility.

For residential wiring, some of the basic NEC rules are:.

• - The single-phase conductor can have black, red, orange insulator (single-phase 3-wire system), but never green, gray or white (if they are solid colors or in stripes). There are specific exceptions such as a wire that follows a round trip path to a switch, where the white insulated conductor represents the active or live power wire for that switch. Another exception is cables that are used to power a socket for devices or equipment that use a voltage of 250 VAC, consume up to 15 or 20 amps and do not need a neutral socket; In this case, the wire with white insulation represents the active outlet, but must be identified as such, usually with black tape inside junction or distribution boxes.

• - The neutral socket is identified with a cable or conductor with white or gray insulation, either with stripes or marks.

• - In every power cable, the striped conductor is the neutral, and the other is the active one. Standard 400.22(f) of the 2008 revised NEC Code allows the marking of grooved or white-striped grooves on the surface of the power cord. If the cable is covered with transparent insulation, the active conductor is copper-colored, and the neutral is silver-colored.

• - The grounding of a circuit can be a bare wire or conductor or with green insulation or one that has green stripes. Note that all metal pipes in a construction must be joined to the grounding system, such as water, natural gas, air conditioning pipes, among others.

• - Long wires or conductors must have black insulation, which can be properly identified with paint or appropriate adhesive tape.

• - All wiring in a circuit, except for conductors that are part of a device or accessory, must be of the same gauge. It should be noted that cables of different gauges can be used in the same raceway as long as they are all insulated for the maximum voltage of any of these circuits.

• - The NEC Code provides rules for circuit load calculation.

• - Differential circuit breaker protection, also called GFCI, is required in outlets installed in humid locations. These include all small appliance circuits in a kitchen, outlets in a confined space, basements, bathrooms and an outlet for the laundry room, as well as outdoor circuits a short distance from the ground. However, they are not necessary for refrigerators because unattended disconnection of this type of switch can cause food spoilage, nor for garbage disposals. Instead, for refrigerators and other semi-permanent appliances in basements and wet areas, a dedicated unprotected outlet is generally used. Two-wire plugs that do not have a ground conductor may be protected by a residual current circuit breaker located upstream and must be labeled "ungrounded" or similar. Most outlets with GFCI switches allow connection and have GFCI protection for outlets connected downstream. Outlets so protected must be labeled "GFCI protected", "RCD protected" or other similar expression.

• - Most circuits have their metal components interconnected with a ground wire connected to the third round prong of a plug, and to the metal cases and chassis of the device.

• - Stoves "Stove (heating)"), boilers "Boiler (machine)"), heat pumps and central air conditioning units must be connected to dedicated circuits.

• - The code establishes rules for sizing electrical boxes for the number of wires and wiring devices in the box.

• - In a device, the brass screw represents the active, and the silver is the neutral. The ground screw is usually painted green.

The above is only a brief description and should not be used as a substitute for the current NEC Code.

Electrical wiring practices were developed in parallel in many countries in the late 20th century and early 20th century.[2]​ As a result, national and regional variations were developed and are in effect. Some of these are retained for technical reasons, since the security of wiring systems depends not only on the wiring code, but also on technical standards for wiring devices, materials and equipment.

The grounding (grounding) of distribution circuits is a notable difference in practice between wiring systems in the United States and those elsewhere in the world. Since the 1960s, wiring in new construction has required a separate grounding conductor used to electrically connect all those points that do not normally carry electrical current. Portable devices with metal casings also have a bonding conductor in the mains connection cable and plug. The neutral conductor is also grounded at the service entrance panel only, and no other neutral to ground connections are permitted, unlike regulations in other parts of the world.

Lighting and outlet circuits in North American systems typically come from a distribution panel that contains switches to protect each branch circuit.[3] The smallest branch circuit current rating is 15 amps, used for lighting and general purpose outlets, although 20 amp circuits are often used. In residential construction, the highest value circuits are generally dedicated to a single device, for example, fixed kitchen appliances, clothes dryers, and air conditioners. Lighting and general purpose outlets are 120 VAC, with larger devices powered by 240 volt split phase circuits. In commercial construction, three-phase circuits are used. Outlets are generally powered by 120 or 208 VAC and may include special high current outlets for industrial equipment. Lighting is generally powered by 277 VAC, except for lights that use 120 V. Equipment can be connected in the building using the combination of 120/208 VAC or 277/480 VAC.

Certain countries may adopt the NFPA standard as their national electrical code, with local modifications similar to those in United States jurisdictions. Although the Canadian Electrical Code was developed independently of the NFPA code, it is similar in scope and intent to the US NEC code, with minor variations in details on the technical requirements. However, the harmonization of the CSA and NEC codes is intended to facilitate free trade between those countries.

Most residential electrical circuits and modern light construction in North America have installations made with non-metallic sheathed cable, often known by the brand name ROMEX.[4] This type of cable is the least expensive for a given size and is appropriate for applications in dry indoor environments. These cables usually use the designation "NM XX-Y" which indicates, respectively, the type of coating (in this case, non-metallic), the gauge of the main conductors, and the total number of conductors in the circuit (excluding the ground conductor). For example, "NM 14-2" cable contains three conductors (two in addition to the ground conductor) of American 14 gauge, a type typically used for circuits protected at 15 amps. Circuits with larger currents (for example, those for electric furnaces, water heaters, air conditioners, or those supplying electrical power to additional circuit panels) will need to use thicker conductors. Not all jurisdictions in the United States allow the use of non-metallic sheathed cable. The NEC Code does not allow the use of this type of cable in large, fire-resistant, or multi-story structures.[5]​.

In the non-metallic type of cable, the insulation uses a color code for identification, usually one black (hot conductor), one white (neutral conductor), and a bare grounding conductor, as established by the NEC Code.[6]​ The conductors can be re-colored, so these rules are commonly the exception[7]​ In 240 VAC applications that do not require a neutral conductor, the white conductor can be used as a second active conductor, but must be recolored with tape adhesive or by some other method. Four-conductor flexible power cables use red as the fourth color; Unlike European practices, the coding of power cables is the same as that of fixed wiring.

For commercial and industrial uses, non-metallic cable not enclosed in conduits is usually prohibited in certain areas or in an entire region, depending on the use of the building to be wired and local and state building codes. Most wiring is placed in metal conduit, due to its cost and durability. Galvanized metal ducts may be required for certain areas, and additionally, accessories that prevent the entry of vapors, in places where a fire or explosion hazard is present, such as gas stations, chemical factories and silos. Pipes made of PVC are used for underground cables or to be installed where concrete is poured. A bank of PVC conduit is usually made of several of them, embedded in concrete. Flexible metal conduits are used for short runs, such as connections to lighting fixtures. For power circuits, color coding uses the same colors as residential construction, and adds colors for additional wires used for three-phase systems. In 120/208 VAC circuits, the hot wires have black, red, and blue insulators and the neutral wire has white insulator. On the other hand, in 277/480 VAC circuits, the active wires are brown, orange and yellow and gray for the neutral wire. For grounding, regardless of the voltage, cables with green insulation are used.

Other types of systems are used for building wiring in the United States; these include corrugated metal power cables, mineral-insulated cables), and various types of electrical conduit. In industrial applications, cables may be placed in a tray and are Type TC. Special wiring rules apply to humid or corrosive environments,[8]​ and to locations that present explosion hazards.[9]​ Materials used for wiring in the United States must be manufactured and tested to standards set by NEMA and Underwriters Laboratories (UL), carrying in that case, the approval marks established by that organization.

Approved wiring types may vary in various jurisdictions, but not all wiring methods approved by the NEC code are accepted in all areas of the United States.

The types of conductors for electrical wiring in North America are defined by norms or standards published by organizations such as Underwriters Laboratories, Canadian Standards Association, American Society for Testing and Materials, National Electrical Manufacturers Association and Insulated Cable Engineers Association"). Their classification is as follows:

XHHW: It is the acronym for Cross-linked High Heat-resistant, Water-resistant or Cross-linked cable highly resistant to heat and humidity, for its meaning in Spanish[10]​ Conductors and cables of this type are recommended for use in humid locations[11]​ and are commonly used in commercial, institutional and industrial alternating current distribution systems with a range of electrical voltages between 110 and 600 volts. They are manufactured with copper or aluminum conductors[12]​ that can be solid or stranded, depending on their size. According to Standard 44 of Underwriters Laboratories (UL)[13]​ type XHHW cables and conductors are recommended for use in dry locations with temperatures up to 90 °C or humid locations up to 75 °C.

XHHW-2: It is similar to XHHW, and is recommended for use in humid or dry locations with temperatures up to 90 °C.

THWN: It is the acronym for Thermoplastic Heat and Water-resistant Nylon-coated or Thermoplastic Heat and Moisture Resistant Nylon Coated Cable.[14]​ Cables and conductors of this type are made of copper or aluminum (solid or stranded) and are used in alternating current distribution systems in buildings of all types and sizes throughout North America with a range of electrical voltages between 110 and 600 volts.[12]​.

THHN: It stands for Thermoplastic High Heat-resistant Nylon-coated or Thermoplastic High Heat-resistant Nylon-coated Cable[15]​ It is suitable for dry and humid locations[11]​ and has the same use as the THWN type cable.[12]​.

Many conductors are classified as THWN and THHN and are suitable for use in dry locations with temperatures up to 90°C or wet locations with temperatures up to 75°C.

• - American wire gauge.

• - Twist connector.

• - Electrical installation.

• - The NEC at NFPA.org.

• - Summary of NEC Color Code.