LEW key terms

The fixed relationship in a DC circuit where voltage equals current multiplied by resistance. Knowing any two values lets you calculate the third, making it the basis for predicting circuit behavior and verifying measurements.

A circuit with a single current path, so the same current flows through every component while voltage divides in proportion to resistance. Total resistance is the sum of individual resistances, and one open stops all current.

A circuit where components connect across the same two nodes, so voltage is common and current divides among branches. Total resistance is always lower than the smallest branch resistance, and one branch can fail without stopping others.

The loss of potential along conductors carrying current, caused by conductor resistance. Excessive drop at the load causes dim lights, overheated motors, and poor equipment performance, so code limits it on feeders and branch circuits.

The ratio of real power in watts to apparent power in volt-amperes in an AC circuit. A low value means more current for the same useful work, increasing conductor and transformer loading and often triggering utility penalties.

The total power supplied to an AC load, measured in volt-amperes as the product of RMS voltage and RMS current without regard to phase angle. Sizing of conductors, transformers, and generators depends on it.

The maximum continuous current a conductor can carry under stated conditions without exceeding its temperature rating. It depends on material, size, insulation, ambient temperature, and the number of current-carrying conductors bundled together.

Selecting conductor cross-sectional area so ampacity after adjustment and correction is at least the required load current, while also satisfying voltage drop, short-circuit withstand, and terminal temperature limits.

An adjustment applied to a conductor's tabulated ampacity when the surrounding air or soil is hotter than the rating basis, because higher ambient temperature reduces the allowable current before insulation damage occurs.

The required internal volume of an outlet, junction, or device box based on counting each conductor, device, fitting, and grounding conductor at specified volumes. Undersized boxes overheat conductors and damage insulation during folding.

A support system of rails and rungs that carries and routes cables, often permitting easier additions than conduit. Its use requires attention to fill, securement, and derating for bundled conductors.

An enclosed channel such as conduit, tubing, or duct designed expressly for holding wires, cables, or busbars. Fill limits and bend radius rules keep conductors from jamming and insulation from damage during pulling.

Any current exceeding the rated current of equipment or the ampacity of a conductor, whether from overload, short circuit, or ground fault. Protective devices must clear it before thermal or mechanical damage occurs.

A current above normal full-load value that flows in an otherwise healthy circuit, typically gradual and small in magnitude. Motors and conductors heat over time, so overload protection responds with a time delay.

An unintended low-impedance connection between energized conductors or between an energized conductor and ground, producing very high current. Protective devices must interrupt it quickly, and equipment must withstand the magnetic and thermal stress.

Unintended current flowing from an energized conductor to normally non-current-carrying metal parts or earth. It may be low enough to escape a standard breaker, so sensitive detection or equipment grounding is essential to clear it safely.

A fuse or circuit breaker intended to open a circuit automatically when current exceeds a set value for a set time. Its rating, trip curve, and interrupting capacity must match the circuit and available fault current.

The maximum short-circuit current a protective device can safely interrupt at its rated voltage. If available fault current exceeds it, the device may fail violently instead of clearing the fault.

Selecting protective device ratings and trip characteristics so that the device nearest a fault opens first, limiting outage scope. It requires comparing time-current curves and may conflict with the need for fast overall clearing.

The conductor that connects exposed metal enclosures and equipment frames to the system grounding point, providing a low-impedance path for fault current so protective devices operate promptly.

The permanent joining of metal parts to form an electrically conductive path that ensures electrical continuity and capacity to conduct any likely fault current safely.

The set of electrodes, such as rods, plates, pipes, or concrete-encased electrodes, that connects the electrical system to earth. It stabilizes voltage to ground and helps dissipate lightning and surge energy.

A device or relay that senses motor current and opens the control circuit when sustained current exceeds a set level, protecting windings from overheating caused by mechanical overload, low voltage, or single phasing.

An assembly combining a contactor for switching motor current with overload protection and control logic. It starts and stops the motor remotely and prevents restart after an overload trip until reset.

A multiplier indicating how much a motor may exceed its nameplate horsepower continuously under rated conditions without damage. It is used to set overload protection and to judge acceptable loading.

The current a motor draws at rated voltage with the rotor stationary, typically several times full-load current. It determines branch-circuit and controller sizing and influences voltage sag during starting.

The apparent power a transformer can deliver continuously without exceeding its temperature rise limit. It is set by winding and cooling design and must be matched to the connected load's total volt-amperes.

Reducing a device's allowable load or a conductor's ampacity because of adverse conditions such as high ambient temperature, altitude, harmonics, or enclosure confinement. It preserves insulation life and prevents nuisance tripping.

Voltages or currents at integer multiples of the fundamental frequency, produced mainly by electronic power supplies and variable-speed drives. They heat neutral conductors and transformer cores and distort waveform quality.

A dangerous release of energy caused by an electric arc between conductors or from conductor to ground. It produces intense heat, blast pressure, and molten metal, so energized work requires assessment and appropriate protective equipment.

A procedure that isolates energy sources, applies locks and tags, and verifies de-energization before work begins, preventing unexpected energization or release of stored energy such as capacitor charge.

A distance from exposed energized parts that unqualified persons must not cross unless escorted and protected. It helps plan work so that shock risk is controlled by distance, barriers, and personal protective equipment.

Confirming a circuit is de-energized with a tester proven functional on a known live source before and after the test. This step guards against meter failure and misidentified circuits.

A de-energized check using a meter or signal source to confirm a low-resistance path exists through conductors, switches, or connections, verifying integrity where full current cannot be applied.

An insulation resistance measurement made with a high DC test voltage to detect moisture, contamination, or degradation in insulation. Readings are compared over time and corrected for temperature and humidity.

A device that compares current leaving and returning through a circuit and trips when the imbalance indicates leakage to ground, protecting people from shock in wet or otherwise hazardous locations.