Master Electrician key terms

The maximum current, in amperes, that a conductor or raceway can carry continuously under the conditions of use without exceeding its rated temperature limit. Adjusted for ambient temperature, number of conductors, and installation method.

The reduction in voltage along a circuit caused by current flowing through conductor impedance. Excessive drop causes poor equipment performance and is limited by code recommendations for feeders and branch circuits.

The ratio of real power in watts to apparent power in volt-amperes in an alternating-current circuit, ranging from zero to one and affecting current draw for a given load.

A point on the wiring system at which current is taken to supply utilization equipment, such as a receptacle, luminaire, or appliance connection.

An enclosed channel of metal or nonmetallic material designed expressly for holding wires, cables, or busbars, with additional functions permitted by code, such as metal conduit, EMT, or PVC.

A compartment or chamber that forms part of the air-handling system, to which one or more ducts are connected and that is used as a supply or return air path. Wiring in it faces stricter rules.

A load in which the maximum current is expected to continue for three hours or more. Feeders, services, and overcurrent devices serving such loads must be sized at 125 percent of the load.

The ratio of the maximum demand on a system to the total connected load of that system. It permits feeders and services to be sized below the sum of all connected loads.

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

The circuit conductors between the final overcurrent device protecting the circuit and the outlet or outlets it supplies. Multiwire and individual branch circuits are common variations.

A fuse or circuit breaker that automatically opens a circuit when current exceeds a predetermined value, protecting conductors and equipment from overheating and damage caused by overloads or short circuits.

An arrangement in which a current-limiting device on the supply side is tested and listed in combination with downstream circuit breakers, allowing the downstream breakers to have interrupting ratings lower than the available fault current.

The design and setting of overcurrent devices so that the device nearest a fault opens first, leaving other circuits energized. It is required for emergency systems and some health-care circuits.

The maximum current that can flow at a given point in a system during a short circuit, determined by the utility source strength, transformer impedance, and conductor impedance. Equipment ratings must equal or exceed it.

A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors protecting the circuit.

A device that senses an imbalance between the current leaving and returning on circuit conductors and trips at a low threshold, typically 4 to 6 milliamperes, to protect people from electric shock.

The conductive path installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or grounding electrode, providing a low-impedance fault path.

A reliable conductor that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed between two or more metal parts that are required to be bonded.

A system conductor, usually the neutral, that is intentionally connected to earth through the grounding electrode system and carries return current under normal operation. It is white or gray in color.

A network of rods, pipes, plates, or concrete-encased electrodes that connects the electrical system to earth, establishing a reference and helping dissipate fault current.

The point of connection between the serving utility's facilities and the premises wiring, often at the service drop, service lateral, or meter, where code rules for services begin.

The conductors between the service point and the service equipment, including any conductors supplying the service disconnecting means and the meter enclosure, sized for the calculated load and available fault current.

The necessary equipment, usually consisting of circuit breakers or switches and fuses with associated accessories, connected to the load end of service conductors to control and cut off the supply.

A single panel or group of panel units designed for assembly in a cabinet, containing buses and automatic overcurrent devices, for controlling and distributing power to branch circuits or feeders.

A device or group of devices, or other means, by which the conductors of a circuit can be disconnected from their source of supply, providing a safe way to isolate equipment.

A device that limits transient voltage surges on circuits by diverting or clamping excess energy, protecting equipment from lightning and switching surges. It may be at service, feeder, or branch level.

A device or sensing means that protects the motor, motor-control apparatus, and branch-circuit conductors against overheating caused by running overloads or failure to start, typically set near the motor's nameplate current.

The current drawn by a motor when operating at rated horsepower, speed, and voltage with rated load. Code tables list values used for sizing branch circuits, feeders, and protective devices.

A fuse, breaker, or motor-circuit protector selected to clear short circuits and ground faults without opening on normal motor starting current; it does not provide overload protection.

The percentage of rated voltage that must be applied to the primary to produce rated current in a short-circuited secondary. It strongly affects available fault current and voltage regulation.

A premises wiring system whose power is derived from a source such as a transformer or generator and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

An area where fire or explosion hazards may exist due to flammable gases, vapors, dusts, or fibers. Such locations are classified by class, division or zone, and group, governing equipment and wiring.

A location where flammable gases or vapors are present in ignitable concentrations under normal operating conditions, or may exist frequently due to repair or maintenance, or where equipment failure could cause ignition.

The clear, unobstructed floor space required in front of electrical equipment, with depth based on voltage and conditions, and width and height dimensions to permit safe examination and servicing.

A dangerous condition associated with the possible release of energy caused by an electric arc, producing intense heat, light, and pressure. It drives labeling, PPE selection, and safe work practices.

A procedure for de-energizing equipment, applying locks and tags, and verifying absence of voltage so that servicing work cannot be re-energized accidentally. Required by safety standards and code.