NEC definitions

The NEC glossary for journeyman electricians.

The journeyman exam lives and dies on definitions — grounded vs grounding, EGC vs GEC, feeder vs branch circuit. Here's every term that trips people up, defined in plain language and tied to its NEC reference.

NEC 2017 2020 2023

Defined terms

Each term links to its NEC reference and the questions that test it.

Article 100
Ampacity
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Open →Article 100
Bonding
The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to safely conduct any current likely to be imposed.
Open →Article 100
Bonding Jumper (Main / System / Equipment)
A conductor installed to ensure required electrical conductivity between metal parts that must be electrically connected; the NEC identifies three distinct types — main, system, and equipment bonding jumpers — each serving a specific grounding and bonding function.
Open →Article 100
Branch Circuit
The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) it serves.
Open →Article 100
Multiwire Branch Circuit (MWBC)
A branch circuit consisting of two or more ungrounded conductors with voltage between them and a shared grounded conductor that has an equal voltage between it and each ungrounded conductor; MWBCs must be protected by a 2-pole or handle-tied breaker at the origin.
Open →Article 100
Continuous Load
A load where the maximum current is expected to continue for 3 hours or more; conductors and overcurrent devices serving a continuous load must be sized at no less than 125% of the continuous load current.
Open →310.15
Current-Carrying Conductor (CCC)
A conductor that carries load current during normal operation; the count of CCCs in a raceway or cable determines whether ampacity adjustment factors for bundling apply under NEC 310.15.
Open →Article 100
Demand Factor
The ratio of the maximum demand of a system or part of a system to the total connected load; demand factors from Article 220 allow the calculated service or feeder load to be reduced to a value closer to actual expected demand.
Open →Article 100
Device
A unit of an electrical system, other than a conductor, whose principal function is to carry or control electric energy — for example, a switch, receptacle, or circuit breaker.
Open →Article 100
Disconnecting Means
A device, group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply; the NEC specifies location and accessibility requirements for each type of equipment.
Open →Article 100
Dwelling Unit
A single unit providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, eating, cooking, and sanitation.
Open →Article 100
Equipment
A general term including material, fittings, devices, appliances, luminaires, apparatus, machinery, and the like used as a part of, or in connection with, an electrical installation.
Open →250.122
Equipment Grounding Conductor (EGC)
The conductive path installed to connect normally non-current-carrying metal parts of equipment back to the system grounded conductor and grounding electrode conductor; its minimum size is selected from Table 250.122 based on the rating of the overcurrent device protecting the circuit.
Open →Article 100
Feeder
All circuit conductors between the service equipment (or separately derived system source) and the final branch-circuit overcurrent device; feeders are sized using Article 215 and Article 220 load calculations.
Open →430.248
Full Load Current (FLC / FLA)
The current a motor draws when operating at its nameplate horsepower and voltage; the NEC requires using Table 430.248 (single-phase) or Table 430.250 (three-phase) values — not the nameplate amps — when sizing motor branch-circuit conductors and overcurrent devices.
Open →Article 100
GFCI (Ground-Fault Circuit Interrupter)
A device that de-energizes a circuit within a specific time when current to ground exceeds the Class A threshold (approximately 4–6 mA), providing shock protection in areas where contact with water or earth is possible.
Open →Article 100
AFCI (Arc-Fault Circuit Interrupter)
A device that detects the electrical signature of dangerous arcing and de-energizes the circuit before the arc can ignite a fire; required in most dwelling unit areas under Articles 210 and 440.
Open →Article 100
Ground
The earth itself, or a conductive body that serves in place of the earth; grounding establishes a reference point and a fault-current return path to enable overcurrent devices to operate.
Open →Article 100
Ground Fault
An unintentional conducting connection between an ungrounded conductor and any grounded conductor, metal enclosure, raceway, equipment, or the earth; ground faults are a type of overcurrent and trigger GFCI protection.
Open →Article 100
Grounded Conductor
A system or circuit conductor that is intentionally grounded — typically the neutral wire — identified by white or gray insulation or three white stripes, and required to be kept insulated throughout the wiring system.
Open →Article 100
Grounding Conductor
A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode; unlike the grounded conductor, a grounding conductor does not carry current during normal operation.
Open →250.52
Grounding Electrode
A conducting object through which a direct connection to earth is established; NEC 250.52 lists acceptable electrodes including metal underground water pipe, concrete-encased electrodes (Ufer grounds), ground rings, and ground rods.
Open →250.66
Grounding Electrode Conductor (GEC)
The conductor that connects the system grounded conductor (neutral) at the service or separately derived system to the grounding electrode system; its minimum size is determined by Table 250.66 based on the size of the service entrance conductors.
Open →250.50
Grounding Electrode System
The network formed by bonding together all grounding electrodes present at a premises — including the water pipe, concrete-encased electrode, ground ring, and rod or plate electrodes — as required by NEC 250.50.
Open →Article 100
Hermetic Motor-Compressor
A combination of a compressor and motor enclosed in the same sealed housing with no external shaft or shaft seals, where the motor windings operate in the refrigerant; this construction is the basis for the special sizing rules in Article 440.
Open →Article 100
Identified Conductor
A conductor distinguished by white or gray insulation (or three white stripes on other-than-green insulation) to indicate it is the grounded conductor — typically the neutral — of the circuit.
Open →Article 100
In Sight / Within Sight
Visible and not more than 50 ft (15 m) distant from the equipment in question; the NEC uses this term to specify where disconnecting means must be located so an equipment operator can see the disconnect from the equipment.
Open →110.9
Interrupting Rating (AIC)
The highest current at rated voltage that an overcurrent protective device is tested and listed to interrupt; NEC 110.9 requires every overcurrent device to have an interrupting rating not less than the available fault current at its point of installation.
Open →Article 100
Listed
Equipment, materials, or services that appear in a list published by an organization acceptable to the authority having jurisdiction (AHJ), such as UL or ETL, indicating the equipment has been evaluated and found to comply with the applicable standards.
Open →Article 100
Luminaire
A complete lighting unit consisting of a light source (lamp or lamps) together with the components designed to distribute the light, protect the lamp, and connect the unit to the power supply.
Open →Article 100
Motor Control Center (MCC)
An assembly of one or more enclosed sections with a common power bus, principally containing motor control units (starters, disconnects, and overcurrent devices), used to control and protect multiple motors from a central location.
Open →Article 100
Neutral Conductor
The conductor connected to the neutral point of a system and intended to carry current under normal conditions; in a balanced load the neutral carries only the difference current, but it must be sized to handle the maximum unbalanced load.
Open →Article 100
Overcurrent
Any current in excess of the rated current of equipment or the ampacity of a conductor; overcurrent can result from overload, short circuit, or ground fault, and must be interrupted by overcurrent protective devices.
Open →240.6
Overcurrent Protective Device (OCPD)
A fuse or circuit breaker rated to interrupt a circuit when current exceeds a predetermined value, protecting conductors and equipment; standard OCPD ampere ratings are listed in NEC 240.6(A).
Open →Article 100
Overload
Operation of equipment in excess of its normal full-load rating, or of a conductor beyond its rated ampacity, that when sustained for sufficient time would cause damage or dangerous overheating — distinguished from short circuits and ground faults by being a moderate, sustained excess rather than a sudden fault.
Open →Article 100
Panelboard
A single panel or assembly of panel units containing buses and overcurrent devices (and sometimes switches), mounted in a cabinet, used to control and protect branch circuits and feeders supplying a building or portion thereof.
Open →Article 100
Raceway
An enclosed channel of metallic or nonmetallic materials designed to hold and protect wires, cables, or busbars; types include EMT, RMC, PVC, ENT, wireways, and auxiliary gutters, each with its own NEC article.
Open →Article 100
Separately Derived System (SDS)
An electrical source — such as a transformer secondary, generator, or UPS — having no direct electrical connection to the supply-side circuit conductors of another system except through grounding and bonding connections; an SDS requires its own grounding electrode connection.
Open →Article 100
Service
The conductors and equipment that deliver electrical energy from the serving utility to the wiring system of the premises; the service is the boundary between utility and owner responsibility.
Open →Article 100
Service Conductors (Lateral / Drop / Entrance)
The conductors from the utility service point to the service disconnecting means, classified as service-drop conductors (overhead), service-lateral conductors (underground), or service-entrance conductors (from the weatherhead or meter base to the main disconnect).
Open →Article 100
Service Equipment
The necessary equipment — typically a main circuit breaker or fused switch and its accessories — located near the entry of service conductors into a building to provide the main control and disconnect for the electrical supply.
Open →Article 100
Short Circuit
An overcurrent fault caused by a low-impedance connection between ungrounded conductors (phase-to-phase) or between an ungrounded and a grounded conductor (phase-to-neutral), producing fault currents far exceeding normal overcurrent levels and requiring overcurrent devices rated for the available fault current.
Open →230.67
Surge Protective Device (SPD)
A device that limits transient overvoltages and diverts surge currents by clamping voltage spikes; required at the service of all new dwelling units per NEC 230.67 (2020 NEC) and at new feeders to dwellings per NEC 225.35 (2023 NEC).
Open →Article 100
Switchboard
A large single panel or assembly of panels bearing switches, overcurrent devices, buses, and instruments, accessible from the front and generally from the rear as well; switchboards are used for service equipment and large distribution applications.
Open →Article 100
Nominal Voltage
A designated voltage value used to identify a circuit or system class — for example, 120 V, 240 V, or 480 V — that may differ slightly from actual measured voltage; the NEC uses nominal voltage to determine applicable wiring rules.
Open →210.19
Voltage Drop
The reduction in voltage between the source and the load caused by conductor resistance; the NEC informally recommends no more than 3% voltage drop on a branch circuit and no more than 5% combined on the feeder and branch circuit, as noted in the fine print notes to NEC 210.19 and 215.2.
Open →Article 100
Weatherproof vs. Weather-Resistant
Weatherproof means constructed or protected so that exposure to weather will not interfere with successful operation; weather-resistant is a separate listing designation indicating equipment tested to resist rain and wet conditions per specific product standards — the two terms have distinct meanings in the NEC and are not interchangeable.
Open →

A huge share of journeyman exam mistakes come down to definitions: confusing the grounded conductor with the grounding conductor, the equipment grounding conductor (250.122) with the grounding electrode conductor (250.66), or a feeder with a branch circuit. Article 100 and the defined terms scattered through the NEC are the foundation everything else is built on.

Each entry here is written to be understood and remembered, then drilled — Loomi turns these definitions into spaced-repetition flashcards and NEC-referenced questions so they stick until exam day.

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