- Ampacity
- The maximum current a conductor can carry continuously under specified conditions without exceeding its temperature rating.
- Apparent power (kVA)
- The product of voltage and current, combining real and reactive power. kVA = √3 × V × I ÷ 1,000 for three-phase.
- Arc flash
- A dangerous release of energy from an electric arc through air. See the Safety Handbook.
- AIC / interrupting rating
- The maximum fault current a protective device can safely interrupt.
- Balanced load
- A load drawing equal current on all three phases at the same power factor.
- Busbar
- A rigid copper or aluminum conductor that distributes power within panels and switchgear.
- Delta (Δ) connection
- Windings connected end-to-end in a closed triangle with no neutral. See Wye vs. Delta.
- Full-load amps (FLA)
- The current a motor draws at rated load, voltage and frequency, from its nameplate.
- Ground fault
- An unintended connection between a phase conductor and ground or grounded metal.
- Harmonics
- Currents or voltages at multiples of the fundamental 60 Hz frequency, caused by non-linear loads.
- High-leg delta
- A 240 V delta system center-tapped on one winding to provide 120 V; the high leg measures about 208 V to neutral and must be marked orange.
- Induction motor
- A motor whose rotor current is induced by the stator's rotating field. See Types of Motors.
- Line voltage
- Voltage measured between any two phase conductors.
- Lockout/tagout (LOTO)
- Procedures that isolate energy sources and prevent re-energization during work.
- Motor control center (MCC)
- An assembly of motor starters, drives and controls sharing a common bus.
- Neutral
- The conductor connected to the star point of a wye system, carrying unbalanced and harmonic current.
- Phase
- One of the three alternating voltages in a three-phase system, offset by 120°.
- Phase rotation
- The order (A-B-C or A-C-B) in which phases reach their peaks; determines motor direction.
- Phase voltage
- Voltage between a phase conductor and neutral in a wye system.
- Reactive power (kVAR)
- Power that oscillates between source and load to sustain magnetic and electric fields; does no net work.
- Real power (kW)
- Power that does useful work. kW = √3 × V × I × PF ÷ 1,000.
- Separately derived system
- A system, such as a transformer secondary, with no direct conductor connection to the supply's circuit conductors.
- Service factor
- A multiplier on motor nameplate horsepower showing permissible short-term overload.
- Single-phasing
- Loss of one phase in a three-phase supply, which can overheat motors.
- Slip
- The difference between synchronous speed and actual rotor speed in an induction motor.
- Switchgear
- Assemblies of switching and protective devices for controlling and protecting power distribution.
- Synchronous speed
- Speed of the rotating magnetic field: 120 × frequency ÷ poles.
- Unbalance
- Unequal voltages or currents between phases; small voltage unbalance causes large motor heating.
- Variable frequency drive (VFD)
- An electronic controller that varies motor speed by changing frequency and voltage. See Choosing a VFD.
- Wye (Y) connection
- Windings joined at a common neutral point. Also called star.