Definitions for common terms found on electrician licensing exams
A device that provides protection from the effects of arc faults by recognizing the characteristics unique to arcing and de-energizing the circuit. Required by NEC 210.12 in most residential dwelling areas.
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Determined per NEC Tables 310.16–310.21.
The base SI unit of electric current. One ampere equals one coulomb of charge passing a point per second.
A sudden release of electrical energy through the air when a high-voltage gap exists and there is a flashover of electric current. NFPA 70E addresses arc flash hazard analysis.
The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and capacity to safely conduct any fault current imposed. Distinct from grounding.
The circuit conductors between the final overcurrent protective device protecting the circuit and the outlet(s). Defined in NEC Article 100.
A branch circuit consisting of two or more ungrounded conductors with a voltage between them, and a grounded conductor with equal voltage between it and each ungrounded conductor (NEC 100).
The percentage of a conduit's cross-sectional area that may be occupied by conductors. NEC Chapter 9 Table 1: 53% for 1 conductor, 31% for 2 conductors, 40% for 3+ conductors.
A load where the maximum current is expected to continue for 3 hours or more. Overcurrent devices and conductors must be sized at 125% of the continuous load (NEC 210.19, 215.2).
An instrument transformer used to measure or monitor alternating current. The CT steps down high currents to a safe, measurable level (typically 5A secondary).
The ratio of the maximum demand of a system to the total connected load of the system. Used in load calculations to account for the fact that not all loads operate simultaneously.
A device used to disconnect the electrical supply from the equipment it serves. Service disconnects are covered under NEC Article 230.
The conductive path installed to connect normally non–current-carrying metal parts of equipment to the system grounded conductor, the grounding electrode conductor, or both. Provides a low-impedance fault current path (NEC 250.118).
A thin-walled steel or aluminum conduit used as a raceway for electrical conductors. Listed under NEC Article 358. Not suitable for direct burial or concrete encasement without special listings.
All circuit conductors between the service equipment or the source of a separately derived system and the final branch-circuit overcurrent device (NEC 100).
A device intended for the protection of personnel that de-energizes a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value (typically 5mA ± 1mA). Required in bathrooms, kitchens, garages, outdoors, and other damp/wet locations per NEC 210.8.
The conductor used to connect the system grounded conductor or the equipment to a grounding electrode or a point on the grounding electrode system (NEC 100).
A steel raceway with a wall thickness between EMT and rigid metal conduit. Listed under NEC Article 342. Can be threaded.
Equipment that converts DC power to AC power. Commonly used in PV (solar) systems. NEC Article 690 covers PV systems; Article 705 covers interconnected electric power production sources.
A constant used in voltage drop calculations. k = 12.9 for copper, k = 21.2 for aluminum. Formula: VD = (k × I × L) / CM, where L is one-way distance in feet and CM is circular mils.
The process of determining the total electrical load on a service or feeder. Governed by NEC Article 220. Two methods: Standard (220.82 for dwellings) and Optional (220.83–220.87).
A conduit body with one opening on each end and one opening on the back (90°). Used to make 90° turns in conduit runs. The "L" denotes the back opening, "B" denotes a removable cover.
The current drawn by a motor running at rated load and speed. Taken from NEC Tables 430.247–430.252 for sizing conductors, overload protection, and short-circuit protection — NOT from the motor nameplate FLA.
The actual current drawn by a specific motor at rated conditions, shown on the nameplate. Used for setting overload protection (NEC 430.32). Different from Table FLC.
NFPA 70 — the standard for safe electrical design, installation, and inspection in the United States. Published by the NFPA and updated every 3 years. The 2023 NEC is the most current edition.
The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. In a 120/240V single-phase system, the neutral carries the unbalanced load.
Standard for Electrical Safety in the Workplace. Establishes requirements for safe work practices, arc flash hazard analysis, and personal protective equipment (PPE) for work on or near energised electrical equipment.
Any fuse, circuit breaker, or other device used to open a circuit automatically at a predetermined overcurrent level to protect conductors and equipment (NEC 240).
The fundamental relationship between voltage (V), current (I), and resistance (R): V = I × R. Extended with P = I × V for power calculations.
The ratio of real power (kW) to apparent power (kVA). A PF of 1.0 is unity. Low PF causes higher current draw for the same real power. Relevant for motor and transformer sizing.
Rigid or flexible non-metallic conduit made of polyvinyl chloride. Listed under NEC Article 352 (rigid). Suitable for direct burial and concrete encasement. Expansion fittings required for long runs due to thermal expansion.
The heaviest-walled steel or aluminum conduit. Listed under NEC Article 344. Threaded; provides strong physical protection. Approved for nearly all locations including direct burial and concrete.
An enclosed channel of metallic or nonmetallic materials designed expressly for holding wires, cables, or busbars (NEC 100). Includes conduit, wireways, cable trays, and surface raceways.
A premises wiring system whose power is derived from a source of electric energy or equipment having no direct electrical connection to the supply conductors originating in another system (NEC 100). Examples: transformers, generators.
The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served. Governed by NEC Article 230.
The maximum short-circuit current that equipment can withstand without damage. Must be verified against the available fault current at the installation point.
A measure of the distortion of a sinusoidal waveform caused by harmonics. High THD from non-linear loads (VFDs, computers) can cause overheating of neutral conductors and transformers.
A static electrical device that transfers energy between circuits via electromagnetic induction. Used to step voltage up or down. Key formulas: V₁/V₂ = N₁/N₂ = I₂/I₁.
An electronic controller that adjusts the speed of an AC motor by varying the frequency and voltage supplied to the motor. Used for energy efficiency in HVAC, pump, and conveyor applications.
The reduction in voltage in an electrical circuit between the source and the load. NEC recommends max 3% on branch circuits and 5% total from service to load (NEC 210.19 FPN, 215.2 FPN). Formula: VD = (2 × k × I × L) / CM.
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