Journeyman Electrician key terms

The relationship stating that voltage equals current multiplied by resistance (E = I × R) in a purely resistive circuit; knowing any two values lets you calculate the third.

The total opposition to alternating current from resistance plus inductive and capacitive reactance, measured in ohms; it takes the place of resistance in Ohm's Law for AC circuits.

The ratio of real power in watts to apparent power in volt-amperes in an AC circuit; a value below 1.0 means some of the current does no useful work.

The voltage lost along circuit conductors because of their resistance and reactance, roughly current times conductor impedance; too much drop causes dim lights and poor equipment performance.

The principle that all current entering a junction must equal all current leaving it, so the branch currents of a parallel circuit add up to the total circuit current.

A circuit offering only one path for current, so the same current passes through every component, the source voltage divides among them, and resistances simply add.

A circuit with two or more current paths connected across the same voltage, so each branch sees full voltage and the total current is the sum of branch currents.

The maximum continuous current, in amperes, that a conductor can carry under its actual conditions of use without exceeding its rated insulation temperature.

A multiplier applied to table ampacities when the surrounding air is hotter or colder than the 30°C base condition, because hot surroundings reduce a conductor's ability to shed heat.

A percentage reduction of ampacity required when more than three current-carrying conductors share a raceway or cable, since bundled conductors cannot dissipate heat freely.

A load expected to draw its maximum current for three hours or longer; conductors and overcurrent devices serving it are generally sized at 125 percent of that load.

The ratio of the maximum demand on a system to its total connected load, applied because not every load in an installation operates at the same time.

The process of adding general lighting, receptacle, appliance, motor, and special loads and then applying demand factors to find the minimum service or feeder capacity required.

The circuit conductors running from the final overcurrent device that protects the circuit to the outlets or equipment it supplies.

All circuit conductors between the service equipment, a separately derived source, or another supply and the final branch-circuit overcurrent devices that they energize.

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

The highest fault current a circuit breaker or fuse can safely clear at its rated voltage, marked in amperes or thousands of amperes on the device.

The prospective fault current a piece of equipment can withstand without damage; it must equal or exceed the fault current available where the equipment is installed.

A tested combination in which a current-limiting device ahead of downstream breakers lets those breakers be applied where available fault current exceeds their individual interrupting ratings.

The circuit conductor intentionally connected to earth at the source, normally the neutral, which carries return current and is identified by white or gray insulation.

The conductor that ties normally non-current-carrying metal parts of equipment back to the system grounding point, creating a low-impedance path for fault current.

The conductor linking the system grounded conductor or equipment grounding conductor to the grounding electrode, such as a ground rod, water pipe, or building steel.

A reliable conductor that ensures electrical continuity between metal parts required to be connected, equalizing potential and completing the path for fault current.

A device that opens the circuit when it senses a small imbalance, typically 4 to 6 milliamperes, between the hot and neutral conductors, indicating current is leaking to ground.

A system that opens the supply at a higher threshold, commonly 30 milliamperes or more, to limit damage from arcing ground faults on services and large feeders.

The internal volume a box must provide, calculated by counting conductors, devices, clamps, and grounding conductors and assigning volume allowances by conductor size.

The limit on how much of a conduit's internal cross section conductors may occupy, set so they can be pulled without damage and heat can escape.

The clear area that must be kept in front of electrical equipment for safe servicing, with minimum depth, width, and height determined by the voltage and conditions present.

An assembly of busbars and overcurrent devices mounted in a cabinet that receives power from one supply and distributes it to branch circuits or feeders.

A sensing device that opens the motor circuit when current or temperature stays too high for too long, preventing winding damage; it is separate from short-circuit protection.

The fuse or instantaneous breaker selected from code tables for a motor branch circuit so it clears faults yet does not open during normal starting current.

A multiplier on a motor nameplate horsepower showing the continuous overload the motor can carry at rated conditions; it influences how overload devices are sized.

The ratio of primary winding turns to secondary winding turns, which determines the voltage step-up or step-down and inversely sets the primary-to-secondary current relationship.

An area where ignitable gases or vapors are present during normal operations or often during maintenance, requiring the most stringent explosionproof wiring methods and equipment.

A circuit designed so its electrical energy, including any spark or thermal effect, is too low to ignite a hazardous atmosphere, permitting reduced wiring restrictions in classified areas.

Enclosures built to contain an internal explosion and stop flame from escaping through machined joints, so surrounding flammable gases are not ignited.

The distance from energized equipment at which a person could receive a second-degree burn from an arc flash; only qualified workers in proper protective equipment may cross it.

Arc-rated clothing and gear such as face shields, gloves, sleeves, and insulated tools chosen to withstand the thermal and shock hazards of the work being performed.

The procedure of de-energizing equipment, placing a lock and tag on the disconnecting means, and verifying the absence of voltage before any work begins.