Electrical Glossary
NEC 2023 terms and definitions, written for journeyman and master electrician exam candidates. 151+ terms total.
AC (Alternating Current)
Electric current that periodically reverses direction. US standard power cycles at 60 Hz. Used for virtually all power distribution because transformers can efficiently change the voltage level.
Accessible (Equipment)
Equipment that can be reached for operation, renewal, or inspection without needing to climb over obstacles, remove a building finish, or use a portable ladder.
Accessible (Wiring Methods)
Wiring that can be removed or exposed without cutting into or permanently damaging the building's structural elements or finished surfaces.
Adjusted Ampacity
The conductor ampacity after applying both temperature correction and bundling derating factors from the NEC ampacity tables. The adjusted ampacity must equal or exceed the load current.
Ampacity
The maximum current, in amperes, that a conductor can carry continuously under its specific installation conditions without exceeding its temperature rating.
Ampacity Derating — Bundling
A reduction in conductor ampacity when more than three current-carrying conductors share the same raceway or cable, because grouped conductors trap heat. The more conductors, the greater the derating.
Ampacity Temperature Correction
An adjustment to conductor ampacity when the ambient temperature differs from the 30°C assumed in NEC tables. Hot environments reduce ampacity; cool environments can increase it.
Arc Flash
A violent release of energy from an electric arc between phase conductors or between a phase and ground. An arc flash produces intense heat, a pressure blast, and molten metal — one of the most serious electrical hazards.
Arc-Fault Circuit Interrupter (AFCI)
A device that detects the irregular arcing patterns associated with damaged or loose wiring that can cause electrical fires and opens the circuit before ignition. Required in most dwelling unit branch circuits.
Authority Having Jurisdiction (AHJ)
The organization, office, or individual responsible for enforcing a code or standard — typically the local electrical inspector or code-enforcement body.
Bonding
The permanent electrical connection of metallic components to ensure they share the same electrical potential, preventing voltage differences that could cause shock or ignite arcs.
Bonding Jumper — Main (MBJ)
The connection between the grounded conductor (neutral) and the equipment grounding conductor and grounding electrode conductor at the service. The MBJ is the critical link that allows fault current to return to the source and open the OCPD.
Bonding Jumper — System (SBJ)
The connection between the grounded conductor and the EGC at the source of a separately derived system such as a transformer secondary. Performs the same function as the main bonding jumper at the transformer.
Bonding of Metal Piping
The NEC requirement to electrically connect interior metal gas piping, metal water piping, and structural steel to the electrical system to equalize potential and prevent hazardous voltage differences or sparking.
Box Fill Calculation
A volume calculation to verify that a junction, device, or outlet box has enough interior space to safely contain all conductors, devices, fittings, and clamps without crowding.
Branch Circuit
The conductors between the last overcurrent protective device (breaker or fuse) in the panel and the outlets or devices it serves.
Branch Circuit — General Purpose
A branch circuit that supplies two or more receptacles or outlets for lighting and appliances.
Branch Circuit — Individual
A branch circuit that supplies only one piece of utilization equipment, such as a dedicated circuit for a large motor or appliance.
Branch Circuit — Multiwire
A branch circuit using two or more ungrounded conductors sharing a common neutral, with equal voltage between each hot and the neutral. Requires a common or simultaneous-trip breaker.
Bus Bar
A rigid copper or aluminum conductor inside a panelboard or switchboard that serves as a common connection point. Hot bus bars distribute line voltage; the neutral and ground bars are also buses.
Cable — MC (Metal Clad)
A factory assembly of insulated conductors inside flexible metal armor, often with an integral EGC. Used in commercial and industrial applications where flexible installation is needed.
Cable — NM (Nonmetallic Sheathed)
A cable with insulated conductors and a bare or insulated EGC enclosed in a nonmetallic jacket. Commonly called Romex. Permitted in dry locations and specific construction types.
Cable — SE (Service Entrance)
Cable designed for use as an aboveground service entrance conductor, and also permitted for feeder and branch circuit applications in certain conditions. Has a weatherproof braid or jacket.
Cable Tray
A rigid structural system of interconnected rails, channels, or ladder sections used to support cables throughout a facility. Common in industrial and large commercial installations.
Capacitance
The ability of a component to store electrical energy in an electric field. In AC circuits, capacitance creates capacitive reactance that opposes changes in voltage and shifts the current-voltage phase relationship.
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