NEC Electrical Engineering key terms

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

Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault, and must be interrupted by a protective device.

Operation of equipment in excess of its normal full-load rating, or of a conductor in excess of its ampacity, that persists long enough to cause damage. Overloads are typically moderate currents, unlike short circuits.

An abnormal connection of relatively low impedance between two points of different voltage, allowing excessive current to flow. Short-circuit current is limited mainly by the impedance of the circuit conductors and source.

An unintentional electrical connection between a normally current-carrying conductor and ground, or between equipment and ground. It is cleared by overcurrent devices or by ground-fault protective equipment.

A device that senses an imbalance in current between the ungrounded and grounded conductors of a circuit and opens the circuit at a low trip threshold to protect people from electric shock.

A device intended to detect dangerous arcing in branch-circuit wiring and de-energize the circuit. It mitigates fire ignition from damaged conductors, loose connections, or degraded insulation.

The conductive path installed to connect normally non-current-carrying metal parts of equipment to the system grounded conductor or grounding electrode system. It provides a low-impedance fault-return path.

The collection of grounding electrodes and their bonding jumpers that establishes a direct connection to earth. It includes rods, pipes, plates, and concrete-encased electrodes as permitted by code.

The conductor that connects the grounding electrode system to the grounded conductor or equipment grounding terminal at the service or source. It carries fault and lightning currents to earth.

The connection between the grounded circuit conductor and the equipment grounding conductor at the service disconnecting means. It establishes the single point where the two systems are joined.

A reliable conductor that ensures electrical continuity between metal parts required to be electrically connected. It equalizes potential and provides a fault-current path across joints and enclosures.

The switch or breaker that disconnects all service-entrance conductors from the building wiring. It must be readily accessible, properly rated, and marked to indicate its function.

The conductors that carry power from the utility service point to the service equipment disconnecting means. Their ampacity and sizing are governed by calculated load and code tables.

All circuit conductors between the service equipment or source of a separately derived system and the final branch-circuit overcurrent device. It supplies one or more downstream panelboards or loads.

The circuit conductors between the final overcurrent device protecting the circuit and the outlet or load. Branch circuits are classified as general-purpose, appliance, or individual circuits.

A load where the maximum current is expected to continue for three hours or longer. Continuous loads require conductors and overcurrent devices sized at 125 percent of the load.

The ratio of the maximum demand of a system, or part of it, to the total connected load. It allows service and feeder sizing based on realistic simultaneous use rather than total connected load.

The sum of the continuous and noncontinuous ratings of all equipment supplied by a system or part of it. It is the starting point for applying demand factors in load calculations.

A fuse or circuit breaker that opens a circuit when current exceeds a predetermined value. It protects conductors and equipment from overloads and short circuits and must be properly coordinated.

The maximum current that can flow at a given point in a system during a short circuit or ground fault. Equipment interrupting ratings must equal or exceed this value at its installation point.

The highest current a device is rated to safely interrupt at its rated voltage without damage. It must be at least equal to the available fault current where the device is installed.

A rating applied to a combination of a current-limiting device and downstream overcurrent devices, permitting the downstream device to have a lower individual interrupting rating. The combination must be tested and listed.

Localization of an overcurrent condition to restrict outages to the circuit or equipment affected. It is achieved by time-current coordination so the device nearest the fault opens first.

Adjustment of a conductor's tabulated ampacity for actual ambient temperature and for the number of current-carrying conductors bundled together. These factors reduce allowable current to safe levels.

The temperature surrounding a conductor or equipment under normal operating conditions. It is used to select insulation and to correct ampacity values taken from standard tables.

Reducing the allowable ampacity of conductors or equipment because of adverse conditions such as high ambient temperature, conduit fill, or continuous loading. Derating keeps operating temperatures within ratings.

The percentage of a raceway's interior cross-sectional area occupied by conductors. Fill limits prevent insulation damage during pulling and allow heat dissipation from current-carrying conductors.

An enclosed channel designed expressly for holding wires, cables, or busbars. Examples include conduit, tubing, and wireways, and each has specific installation and support requirements.

A unit or assembly of units forming a rigid structural system used to support and secure cables and raceways. It provides mechanical support but is not itself an enclosure.

An outlet where one or more receptacles are installed. Its placement, spacing, and circuit requirements vary by occupancy and are specified for dwelling units and other locations.

The current a motor draws at rated voltage, frequency, and load as listed in code tables. It is used to size branch-circuit conductors, overload protection, and disconnecting means.

A device or protective scheme that senses excessive motor current or temperature and opens the circuit to prevent winding damage. It is sized to the motor's service factor and temperature rise.

A switch or device that isolates a motor and controller from the power source for maintenance. It must be rated for the motor's voltage and horsepower and be within sight of the controller.

An area where fire or explosion hazards may exist due to flammable gases, vapors, dusts, or fibers. Equipment and wiring must meet class, division or zone, and group requirements.

A protection technique that limits the electrical and thermal energy in a circuit to a level too low to ignite a hazardous atmosphere. It is commonly used for instrumentation in classified areas.

Enclosures designed to contain an internal explosion and prevent ignition of the surrounding atmosphere. They also keep external surface temperatures below the ignition temperature of the hazard.

A safety procedure that de-energizes equipment and physically locks or tags controls so it cannot be re-energized during service. It protects workers from unexpected energization or release of stored energy.

A sudden, high-energy electrical discharge through air caused by a fault or switching event. It produces intense heat, light, and pressure, and drives requirements for arc-rated PPE and labeling.

Acceptable to the authority having jurisdiction. Approval may be based on listing, labeling, or field evaluation, and it confirms suitability for a specific purpose under the code.

Equipment, materials, or services included in a published list by an organization acceptable to the authority having jurisdiction. Listing indicates the product has been evaluated to a recognized standard.

The organization, office, or individual responsible for enforcing the code and approving equipment, installations, and procedures. The AHJ interprets requirements and may grant special permission.