AR Electrician key terms

The rule stating that in a resistive direct-current circuit, voltage equals current multiplied by resistance. Knowing any two of the three values lets you calculate the third.

The total opposition an alternating-current circuit offers to current flow, combining resistance with inductive and capacitive reactance. It is measured in ohms and sets the current for a given applied voltage.

The ratio of true power in watts to apparent power in volt-amperes in an alternating-current circuit. A value below one means part of the current performs no useful work.

The reduction in voltage along circuit conductors caused by their resistance and the current flowing through them. Excessive drop leaves equipment running below rated voltage and wastes energy as heat.

A load whose maximum current is expected to continue for three hours or longer. Conductors and overcurrent devices serving it are generally sized at 125 percent of that current.

The ratio of the maximum demand of a system, or part of it, to the total connected load. It lets designers size service and feeder equipment smaller than the sum of every load.

A prescribed method of totaling branch-circuit, feeder, and service loads using code volt-ampere values, demand factors, and allowances to determine the capacity an installation requires.

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

Reducing a conductor's allowable ampacity to account for conditions of use, such as more than three current-carrying conductors in a raceway or a higher ambient temperature.

The maximum continuous insulation temperature a conductor is listed for, such as 60, 75, or 90 degrees Celsius. Termination ratings often limit how much of it may be used for ampacity.

A unit of cross-sectional area equal to the area of a circle one-thousandth of an inch in diameter, used to state conductor sizes and to compute resistance.

The total volume, in cubic inches, that conductors, devices, fittings, and grounding conductors occupy inside an enclosure. It must not exceed the box's marked capacity.

The percentage of a raceway's internal cross-sectional area taken up by conductors. Keeping it within limits controls pulling tension and heat buildup inside the raceway.

The overhead conductors the utility runs from its last pole or transformer to the service point at the building, ending where the customer's wiring begins.

The underground conductors run from the utility supply to the service point of a building, ending where the customer's service-entrance conductors begin.

All circuit conductors between the service equipment, or a separately derived source, and the final branch-circuit overcurrent device that protects the load.

The circuit conductors between the final overcurrent device protecting the circuit and the outlets or utilization equipment that the circuit supplies.

A branch circuit with two or more ungrounded conductors that have a voltage between them and share one neutral. All ungrounded conductors must be opened together.

A circuit conductor intentionally connected to earth at the source. It carries return current during normal operation and is identified by white or gray insulation.

The conductor that connects normally non-current-carrying metal parts of equipment to the grounding system, giving fault current a low-impedance path back to the source.

A rod, pipe, plate, or other approved object in direct contact with the earth, used as the connection point for a building's grounding electrode system.

The conductor or screw that connects the grounded circuit conductor to the equipment grounding conductor at the service, creating the single point where the system is grounded.

A wiring system whose power comes from a transformer, generator, or converter with no direct connection to the supply conductors, so it must be grounded and bonded on its own.

A fuse or circuit breaker that opens a circuit automatically when current exceeds a chosen value for a chosen time, protecting conductors from overheating.

The maximum short-circuit current a supply can deliver at a given point in the system. Equipment installed there must have an interrupting rating at least that high.

The highest fault current a device can safely interrupt at its rated voltage without being damaged or failing dangerously. It is marked on the device and must meet or exceed the available fault current.

A tested combination in which a downstream breaker with a lower interrupting rating is backed up by an upstream current-limiting device, allowing the pair's higher rating to be used.

Choosing overcurrent device ratings and settings so that only the device closest to a fault opens, keeping upstream devices closed and the rest of the system energized.

The current a motor draws at its rated load and voltage, taken from code tables and used to size branch-circuit conductors and protective devices.

A sensing device that interrupts the motor on sustained excess current, protecting the windings from overheating while allowing normal starting current to pass.

A switch or breaker, generally within sight of the motor and controller, that opens all ungrounded conductors so the motor can be worked on safely.

A device that compares current leaving and returning on a circuit and opens it quickly when a small imbalance shows current is leaking to ground.

A device that watches the circuit waveform for the pattern of an arcing fault and opens the circuit, reducing the chance of fire from damaged or deteriorated wiring.

An area where flammable gases, vapors, dusts, or fibers may be present in amounts that could ignite, requiring specially designed equipment and wiring methods.

The clear area required in front of electrical equipment, kept free of obstructions so a person can work safely. Its depth depends on voltage and surrounding conditions.

A rapid release of energy from an arcing fault that produces intense heat, light, and pressure capable of burning or injuring workers near energized equipment.

A procedure in which equipment is de-energized, a personal lock and tag are applied, and the absence of voltage is verified before work begins.

Equipment or material shown in a list published by a nationally recognized testing laboratory, meaning it has been tested and found to meet a recognized standard for its intended use.