NEC Architecture key terms

The maximum continuous current, in amperes, that a conductor can carry under the conditions of use without exceeding its temperature rating. It depends on conductor material, size, insulation type, and ambient conditions.

A percentage reduction applied to conductor ampacity when more than three current-carrying conductors share a raceway or cable without maintained spacing, because mutual heating reduces the ability to dissipate heat.

A multiplier applied to a conductor's base ampacity to account for surrounding air temperatures above the value assumed by the insulation rating, since hotter surroundings reduce heat dissipation.

A load expected to operate at maximum current for three hours or more. Conductors and overcurrent devices serving continuous loads must be sized at no less than 125 percent of the load.

A load that operates at maximum current for less than three hours in any given period. It is not subject to the 125 percent sizing adder that applies to continuous loads.

A device such as a circuit breaker or fuse that automatically opens a circuit when current exceeds a predetermined value, protecting conductors and equipment from overheating and fault damage.

A device that de-energizes a circuit within a set time when current to ground exceeds a threshold well below the level needed to trip a standard breaker, protecting people from shock.

The conductor that connects the non-current-carrying metal parts of equipment to the grounding electrode system, providing a low-impedance fault path that lets overcurrent devices clear faults.

The set of grounding electrodes and their connecting conductors that establishes a direct, permanent connection to earth, anchoring the electrical system's reference to ground potential.

A conductor, screw, or strap that ensures electrical continuity between metal parts that must be electrically connected, equalizing potential and providing a path for fault current.

An enclosed channel of metal or nonmetallic material designed expressly for holding wires, cables, or busbars, with additional functions permitted by the code depending on its type.

The percentage of a conduit's internal cross-sectional area occupied by conductors. Limits vary by conductor count and conduit type to allow pulling and prevent insulation damage from heat.

A sheet-metal or nonmetallic trough with a removable cover, used to house and support conductors and cables, often where multiple circuits run in accessible locations.

The required minimum volume of an outlet, junction, or device box based on the number and size of conductors, devices, and fittings it contains, preventing overcrowding and insulation damage.

The point where the serving utility's conductors connect to the premises wiring, marking the boundary between utility-owned and customer-owned equipment for code purposes.

The conductors that carry power from the service point to the service equipment, including both overhead and underground service conductors on the customer side of the demarcation.

The necessary equipment, usually a switchboard or panelboard with breakers, that controls and cuts off power to a building's wiring and contains the service disconnecting means.

All circuit conductors between the service equipment, source of a separately derived system, or other supply source and the final branch-circuit overcurrent device.

The circuit conductors between the final overcurrent device protecting the circuit and the outlets or loads it serves, forming the last segment of the distribution system.

The ratio of the maximum demand on a system to the total connected load, used to size service and feeder equipment realistically because not all loads operate at once.

A structured method of summing lighting, receptacle, appliance, motor, and other loads, applying demand factors, to determine the minimum capacity of service and feeder conductors and equipment.

The required clear floor space and headroom in front of electrical equipment, kept free of obstructions, allowing safe inspection, servicing, and maintenance of energized parts.

The area above and in front of switchboards, panelboards, and similar equipment reserved exclusively for electrical wiring and equipment, excluding piping and ducts unrelated to the electrical installation.

A dangerous condition in which an electric arc releases intense heat, light, and pressure, requiring equipment labeling and safe work practices for personnel who service energized electrical systems.

A device or group of devices that can disconnect the conductors of a circuit from their supply source, allowing equipment to be isolated for servicing and maintenance.

A combination of overcurrent devices where a downstream breaker relies on an upstream device to interrupt high available fault current, typically marked and tested as a listed combination.

The maximum prospective fault current a piece of equipment can safely withstand or interrupt without damage, used to verify equipment is not applied where available fault current is higher.

An area where fire or explosion hazards may exist due to flammable gases, vapors, dusts, or fibers, requiring special wiring methods and equipment approved for the specific hazard class and division or zone.

A circuit designed so that any spark or thermal effect produced under normal or fault conditions cannot ignite a flammable atmosphere, typically by limiting voltage and current to low levels.

Enclosures designed to contain an internal explosion and prevent ignition of surrounding flammable atmosphere, used in certain classified locations such as Class I hazardous areas.

A system of circuits and equipment arranged to supply power to critical patient care areas, life safety, and equipment branches so essential functions continue during a utility outage.

An alternate source of electric power, such as a generator, that supplies designated loads when the normal supply fails, typically required for life safety and critical operations.

A device that switches load connections from the normal power source to an alternate source and back, preventing inadvertent interconnection of the two supplies.

A dedicated circuit supplying fire pump equipment, required to remain energized during a fire and be protected and routed so that failure of other circuits does not disable the pump.

A circuit with limited power and voltage, as defined by its supply source, that poses low fire and shock risk and may use lighter wiring methods than power circuits.

A circuit with power and voltage limits above Class 2 but still restricted, used for signaling and control, allowing different wiring methods than line-voltage power circuits.

A circuit that carries signals for telephone, data, cable television, or similar communications, subject to separate article requirements and permitted to share certain raceways under conditions.

A space used for air handling, such as above a suspended ceiling, where cable routing and materials are restricted because smoke from burning materials would spread through the building's air system.

The loss of voltage along conductors due to their resistance and reactance, which increases with current and distance and can impair equipment performance if not limited by proper conductor sizing.