NCCER Electrical key terms

A planned procedure in which a circuit or machine is turned off, its energy source isolated, and a lock and tag applied so no one can re-energize it while work is being done.

A sudden release of heat, light, and pressure caused by current jumping through air between conductors or from a conductor to ground; it can burn skin and ignite clothing in an instant.

A protective device that compares the current going out on a circuit with the current coming back and opens the circuit quickly when a small difference shows current is leaking to ground.

The clothing and gear a worker wears, such as hard hat, safety glasses, insulated or arc-rated gloves, and flame-resistant clothing, chosen to match the hazards of the task.

A distance out from exposed energized parts inside which only a qualified person may go; the distance depends on the voltage and on the shock protection required.

The habit of checking a voltage tester on a known live source, then testing the circuit to be worked on, then re-checking the tester, to prove the circuit is truly de-energized.

The relationship that voltage equals current multiplied by resistance (E = I × R), which lets you find any one of the three values when the other two are known.

The rule that all current entering a junction must equal all current leaving it, because charge cannot collect at a point in a circuit.

The total opposition a circuit offers to alternating current, combining plain resistance with the opposing effects of inductance and capacitance, measured in ohms.

The ratio of true power in watts to apparent power in volt-amperes; a value below one means part of the current is not producing useful work.

A circuit in which parts are connected end to end so the same current flows through every part and the total resistance is the sum of the individual resistances.

A circuit in which parts are connected across the same two points so each branch sees the full source voltage, and the total resistance is always less than the smallest branch.

The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without its temperature rising above the rating of its insulation.

A load expected to run at its maximum current for three hours or longer; the code requires it to be counted at 125 percent when sizing conductors and overcurrent protection.

A fuse or circuit breaker that automatically opens the circuit when current exceeds its rating, protecting conductors and equipment from damaging heat.

A multiplier of less than one applied to a total connected load because not all loads operate at the same time, allowing smaller service and feeder equipment.

The clear, unobstructed space required in front of electrical equipment so a person can safely service it; the depth depends on voltage and on whether live parts are exposed.

Lowering the allowable current of a conductor when more than three current-carrying conductors share a raceway, or when the surrounding air is hotter than the table's base temperature.

An enclosed channel such as conduit or tubing that holds and protects wires and cables and lets them be pulled in after the raceway is installed.

The share of a raceway's inside cross-sectional area taken up by conductors, kept below code limits so wires can be pulled without damage and heat can escape.

The tightest curve a raceway or cable may be bent without kinking it, cracking insulation, or shrinking the inside diameter; it depends on the type and size.

A calculation that adds up the volume used by conductors, devices, clamps, and fittings to confirm a box is large enough to hold its contents safely.

A partly punched circle in the side of a box or enclosure that can be removed to create a hole for a raceway, connector, or fitting.

The standard numbering system for conductor sizes in which a smaller number means a larger wire; sizes above No. 1/0 are given in thousands of circular mils (kcmil).

A unit of area equal to a circle one thousandth of an inch across, used to state conductor cross section and to calculate resistance and voltage drop.

A single-conductor wire with thermoplastic insulation and a nylon outer jacket, rated 90 degrees C in dry locations and commonly pulled through conduit.

A factory-made assembly of two or more insulated conductors plus a bare ground, wrapped in an overall jacket and used for indoor wiring in dry locations.

A wiring device with slots that delivers power to a plug-connected load; its grounded slot and blade arrangement must match the circuit and its polarity.

A conducting object, such as a driven rod or a metal water pipe, that touches the earth directly and ties the electrical system to it.

The conductor that ties metal boxes, frames, and enclosures back to the system ground so a fault has a low-resistance path that trips the overcurrent device.

A conductor that permanently joins metal parts so they remain at the same voltage, preventing a difference that could arc, spark, or shock someone touching both.

Unintended current that leaves a live conductor and returns to earth through some other path, such as a person, water, or damaged insulation.

A wiring system fed from a transformer or generator with no direct conductor connection to the supply, so it must be grounded and bonded on its own.

The current a motor draws when running at its nameplate horsepower, voltage, and frequency with full load on the shaft; code tables list it for sizing conductors and protection.

A number on a motor nameplate that shows how much above rated horsepower the motor may be loaded continuously under stated conditions without harm.

A magnetically operated switch that makes and breaks the power circuit of a load such as a motor; by itself it does not sense an overload.

A device that senses excess current in a motor circuit and opens the control circuit to shut the motor down before its windings overheat.

A start-stop circuit in which a momentary start button energizes a coil whose holding contact keeps the circuit closed after release, while the stop button or a power loss drops it out.

An electronic controller that changes the frequency and voltage fed to a motor so the motor's speed can be matched to the load instead of running flat out.