Craftsman Electricity key terms

The unobstructed floor space and headroom that must be kept in front of electrical equipment so a person can work on it safely without contacting live parts or blocking an escape path.

The distance from exposed live parts within which a person could receive a second-degree burn from an arc flash; it depends on available fault current and clearing time.

A procedure for de-energizing equipment, applying a personal lock and tag, and verifying the absence of voltage so the equipment cannot be re-energized while someone works on it.

A product or method that is acceptable to the authority having jurisdiction, typically because it is listed by a recognized testing laboratory or meets an adopted standard.

The organization or individual responsible for enforcing the electrical code and approving equipment, installations, and materials within a jurisdiction, such as a local electrical inspector.

Equipment that appears in a published list maintained by a nationally recognized testing laboratory and is marked to show it has been evaluated to a specific safety standard.

The relationship among voltage, current, and resistance in a circuit: voltage equals current multiplied by resistance, and current equals voltage divided by resistance.

The ratio of real power in watts to apparent power in volt-amperes in an alternating-current circuit; a value below one means some current does no useful work.

The product of root-mean-square voltage and root-mean-square current in a circuit, expressed in volt-amperes, and the value used to size conductors and equipment.

A load in which current lags voltage because of a magnetic field, such as a motor winding or transformer coil, causing a lagging power factor and additional reactive current.

A circuit with only one path for current, so the same current flows through every component and the total resistance is the sum of the individual resistances.

A circuit in which components are connected across the same two points, so each branch sees the same voltage and the total current is the sum of branch currents.

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

A multiplier applied to a conductor's ampacity when the surrounding air or earth is hotter than the temperature on which the tabulated value is based, because heat shortens insulation life.

A reduction factor applied when several current-carrying conductors share a raceway or cable, because their combined heat cannot dissipate as readily as a single conductor's.

The required minimum volume of an outlet, junction, or device box based on the number and size of conductors, devices, clamps, and equipment grounds it contains.

The limit on how much of a conduit's cross-sectional area may be occupied by conductors, set so wires can be pulled without damage and heat can dissipate.

A load expected to operate at maximum current for three hours or more, which must be sized at no less than 125 percent of its current when calculating branch circuits and feeders.

The ratio of the maximum demand on a system to the total connected load, used to size service and feeder capacity because not all loads operate at once.

The sum of the nameplate ratings of all equipment connected to a system, before any demand factors or diversity allowances are applied.

All circuit conductors between the service equipment, source of a separately derived system, or output of a power supply, and the final branch-circuit overcurrent device.

The circuit conductors between the final overcurrent device protecting the circuit and the outlet or outlets it serves, such as receptacles or luminaires.

The conductors that carry power from the utility's service point to the service equipment disconnect, sized for the calculated load and often subject to utility or code rules.

The conductive path that connects normally non-current-carrying metal parts of equipment to the system grounding point, providing a low-impedance fault path that trips the overcurrent device.

A conductor that ensures electrical continuity between metal parts that must be connected, such as service equipment enclosures, metal raceways, or water piping.

The conductor that connects the grounded conductor or equipment grounding conductor to the grounding electrode system, such as a ground rod or metal water pipe.

A device that de-energizes a circuit within a set time when it senses a small current imbalance between the ungrounded and grounded conductors, indicating current is leaking to ground.

A fuse or circuit breaker that opens a circuit automatically when current exceeds its rating for a set time, protecting conductors and equipment from overheating.

The maximum current that would flow at a given point in a system during a short circuit or ground fault, used to select equipment with adequate interrupting rating.

The current a motor draws at rated load and rated voltage, taken from code tables for conductor and overload sizing rather than from the motor nameplate.

A device, such as a thermal overload relay, that protects the motor from sustained excess current due to mechanical overload or phase loss, separate from short-circuit protection.

The brief, high current a motor or transformer draws when first energized, before its magnetic field is established, which short-circuit protection must tolerate without nuisance tripping.

A motor control assembly that connects the motor directly to the supply through contactors and overload relays, providing full voltage at starting and simple forward operation.

The ratio of primary to secondary winding turns, which determines the ratio of primary to secondary voltage and inversely relates the currents in the two windings.

The apparent power a transformer can deliver continuously without exceeding its temperature rise, used with voltage to calculate rated primary and secondary current.

A transformer connection with a delta primary and a wye secondary, which provides a neutral for line-to-neutral loads and shifts secondary voltage by 30 degrees from the primary.

An instrument used to confirm the presence or absence of voltage on a conductor or terminal before work begins, and to verify the tester on a known live source first.

An instrument that applies a high direct voltage to insulation and measures resistance, used to detect moisture, contamination, or deterioration in windings and cables.

A meter whose jaws encircle a single conductor and measure current by sensing its magnetic field, allowing load readings without breaking the circuit.