IA Electrician key terms

The relationship in a DC circuit where voltage equals current multiplied by resistance (E = I × R). Knowing any two values lets you calculate the third, making it basic to troubleshooting and sizing.

The total opposition a circuit offers to alternating current, combining resistance with inductive and capacitive reactance. Measured in ohms and written as Z, it determines current flow in AC systems.

The ratio of true power in watts to apparent power in volt-amperes in an AC circuit. A value of one means a purely resistive load; lower values mean more current is needed for the same useful work.

The maximum current a conductor can carry continuously under stated conditions without exceeding its temperature rating. Values come from code tables and are adjusted for ambient temperature and the number of current-carrying conductors.

Reducing the allowable current of conductors or devices below the table value to account for real conditions, such as high ambient temperature, more than three current-carrying conductors in a raceway, or unusual installation circumstances.

A unit of area equal to a circle one-thousandth of an inch in diameter, used to express conductor cross section. One thousand circular mils equals one kcmil, and larger areas mean lower resistance.

The loss of voltage along circuit conductors caused by their resistance and reactance as current flows. Excessive drop leaves equipment with low voltage and can cause poor operation or overheating.

The maximum temperature in degrees Celsius that a conductor's insulation can withstand continuously, such as 60, 75, or 90 degrees. This rating sets the ampacity column used and limits termination temperature.

The circuit conductors between the final overcurrent device protecting the circuit and the outlets or equipment it supplies. Branch circuits feed receptacles, lighting, and individual appliances.

All circuit conductors between the service equipment, a separately derived source, or another supply source and the final branch-circuit overcurrent devices. Feeders supply panelboards and distribution points.

The overhead conductors running from the utility's last pole or transformer to the service point at the building. Service-entrance conductors begin where the drop ends.

A circuit conductor intentionally connected to earth, commonly called the neutral. It carries return current during normal operation and must be identified by white or gray finish.

A load expected to operate at maximum current for three hours or longer. Branch-circuit and overcurrent device sizing must generally include 125 percent of a continuous load.

An enclosed channel of metal or nonmetallic material designed to hold wires, cables, or busbars. Examples include rigid conduit, electrical metallic tubing, and PVC conduit.

The portion of a raceway's cross section occupied by conductors. Codes limit fill so wires can be pulled without damage and heat can dissipate properly.

A factory assembly of insulated conductors covered by an overall nonmetallic jacket, used mainly in dry interior locations such as dwellings. Type NM-B is a common example.

Code requirements for fastening cables and raceways to the building structure and holding them in place along the run at specified intervals, preventing strain and damage.

The calculated volume of an outlet, device, or junction box compared with the space needed by conductors, devices, fittings, and grounding. It prevents overcrowding that could damage insulation.

A removable tab or precut opening in the wall of a box or enclosure that is punched out to accept a conduit or cable connector. Unused openings must be closed.

A receptacle with a third, U-shaped slot connected to the equipment grounding conductor. It provides a fault path for the metal enclosures of cord-and-plug connected equipment.

The conductor running from a switch to the controlled outlet or luminaire. It carries switched power rather than constant unswitched power.

Correct identification of which circuit conductor is grounded versus ungrounded, so that screw shells and identified terminals are wired as intended. Reversed polarity is a shock and safety hazard.

A device that senses the signature of dangerous arcing in branch-circuit wiring and opens the circuit to prevent fire. AFCIs are required for many dwelling branch circuits.

An overcurrent device that opens automatically on excess current and can be reset by hand. Types include thermal-magnetic and electronic, and some include GFCI or AFCI protection.

A fuse or circuit breaker intended to open a circuit when current exceeds a chosen value for a chosen time. It protects conductors and equipment from damaging overheating.

A conductive object in direct contact with the earth, such as a rod, pipe, plate, or concrete-encased electrode. It ties the electrical system's grounding point to earth.

The conductive path connecting normally non-current-carrying metal parts of equipment to the system grounding point. It lets fault current return so the overcurrent device trips.

A conductor, screw, or strap that ensures electrical continuity between metal parts that must stay at the same potential. Bonding prevents dangerous voltage differences during faults.

An unintended path of current from an ungrounded conductor to ground through a person, equipment, or the earth instead of the intended circuit. It can cause shock or fire.

The current a motor draws at its rated load and rated voltage, taken from code tables rather than the nameplate. It is the basis for sizing conductors, overcurrent devices, and disconnects.

A device that protects a motor from sustained excess current and resulting overheating by opening the circuit. It is separate from short-circuit and ground-fault protection.

A multiplier, often 1.15, applied to a motor's nameplate horsepower showing how much above rated load it can carry continuously under rated conditions. It affects overload protection sizing.

A device or group of devices that can safely disconnect all circuit conductors or equipment from the supply for servicing. It must be rated for the load and readily accessible.

An area where fire or explosion risk exists because of flammable gases, vapors, dusts, or fibers. Wiring methods and equipment there are restricted by class and division or zone.

Equipment and wiring designed so available energy and surface temperature are too low to ignite a hazardous atmosphere under normal or fault conditions. It is permitted in classified locations.

A safety procedure in which each energy source is shut off, locked, and tagged by the worker so equipment cannot be re-energized while someone is servicing it.

A sudden release of energy from an electrical arc, producing intense heat, light, pressure, and molten metal. It requires arc-rated protective equipment and safe work practices.

The clear, dedicated area in front of electrical equipment that must stay unobstructed so workers can safely operate and service it. Its depth depends on voltage and conditions.

A chart, usually inside a panelboard door or on plans, listing each branch circuit's number, rating, and the loads it serves. It is used for balancing and identifying circuits.

The ratio of maximum demand over a period to the total connected load. Applying it lets service and feeder conductors be sized to realistic loads rather than every load at once.