Electrical Engineering Glossary
164 key terms used in electrical design, installation, and standards compliance.
A
AC (Alternating Current)
Electric current that periodically reverses direction. Standard frequencies: 60 Hz (North America) or 50 Hz (Europe/Asia).
AFCI (Arc-Fault Circuit Interrupter)
A device that detects dangerous arcing conditions and de-energizes the circuit to prevent fires.
AIC (Ampere Interrupting Capacity)
The maximum fault current that a protective device can safely interrupt without sustaining damage.
Ambient Temperature
The temperature of the surrounding air or environment that affects conductor ampacity and equipment ratings.
Ampacity
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Ampere (A)
The SI unit of electric current, equal to one coulomb per second. Named after André-Marie Ampère.
Arc Flash
A dangerous release of energy caused by an electric arc fault, producing extreme heat, light, pressure, and sound. Incident energy is measured in cal/cm².
Arc Blast
The pressure wave produced by an arc flash event, capable of throwing workers, equipment, and molten metal.
ATS (Automatic Transfer Switch)
A device that automatically transfers power from the utility source to a standby generator or alternate source during a power outage.
AWG (American Wire Gauge)
A standardized wire gauge system used in North America. Smaller numbers indicate larger wire diameters (e.g., 4 AWG is larger than 12 AWG).
B
Back-Fed Breaker
A breaker in which power flows from the load side to the bus, commonly used for solar PV interconnection. Requires hold-down clip.
Ballast
A device used with fluorescent and HID lamps to provide the necessary starting voltage and regulate current during operation.
Bonding
The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and safely conducts fault current.
Bonding Jumper
A reliable conductor to ensure the electrical conductivity between metal parts required to be electrically connected.
Branch Circuit
The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
Breaker (Circuit Breaker)
A device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself.
Bus Duct (Busway)
A prefabricated enclosed power distribution system consisting of busbars in a protective housing, used for high-current applications.
Busbar
A metallic strip or bar, typically copper or aluminum, used for local high-current power distribution within switchgear, panel boards, and bus ducts.
C
Cable Tray
A unit or assembly of units forming a rigid structural system used to securely fasten or support cables and raceways.
Capacitance
The ability of a system to store electric charge. Measured in farads (F). C = Q/V.
Capacitor Bank
A group of capacitors connected to improve power factor by supplying reactive power locally, reducing demand from the utility.
Circular Mil
A unit of area equal to the area of a circle with a diameter of one mil (0.001 inch). Used to measure conductor cross-sections.
Cogeneration
The simultaneous production of electricity and useful thermal energy from a single fuel source, achieving higher overall efficiency.
Conduit
A tube or channel for enclosing and protecting electrical conductors. Types include EMT, IMC, RMC, PVC, LFMC, and FMC.
Conduit Fill
The percentage of a conduit's internal cross-sectional area occupied by conductors. NEC limits: 53% for 1 conductor, 31% for 2, 40% for 3+.
Continuous Load
A load where the maximum current is expected to continue for 3 hours or more. Conductors and OCPDs are typically sized at 125% of continuous load.
Coordination Study
An engineering analysis that determines the proper settings and ratings of protective devices to ensure only the nearest upstream device trips during a fault.
Coulomb
The SI unit of electric charge. One coulomb equals the charge transported by 1 ampere in 1 second.
CT (Current Transformer)
An instrument transformer that produces a reduced current accurately proportional to the primary current, used for metering and protection.
D
DC (Direct Current)
Electric current that flows in one direction only. Common in solar PV, batteries, and electronic circuits.
Dead Front
Equipment with no exposed energized parts on the operating side. Required for safety in most installations.
Delta Connection
A three-phase winding configuration where end-to-end connections form a triangle. Line current = √3 × phase current.
Demand Factor
The ratio of the maximum demand of a system to the total connected load. Applied to reduce feeder and service conductor sizing.
Derating
The reduction of a conductor's ampacity due to conditions such as ambient temperature, conduit fill (more than 3 conductors), or mutual heating.
Disconnect
A device by which conductors can be disconnected from their source of supply. Must be within sight of equipment or lockable.
Diversity Factor
The ratio of the sum of individual maximum demands to the maximum demand of the whole system. Always ≥ 1.
Double-Ended Switchgear
A switchgear arrangement with two utility feeds connected through a tie breaker, providing redundancy for critical facilities.
E
Earthing (Grounding)
Connecting an electrical system or equipment to the earth (ground) to stabilize voltage, provide a fault return path, and protect against electric shock.
EGC (Equipment Grounding Conductor)
The conductive path(s) that provide a ground-fault current path and connect normally non-current-carrying metal parts of equipment to the supply source.
EMT (Electrical Metallic Tubing)
Thin-walled metallic conduit used for indoor wiring. Joined by set-screw or compression fittings, not threaded.
Energized
Electrically connected to, or having, a source of voltage. Opposite of de-energized.
EVSE (Electric Vehicle Supply Equipment)
The complete assembly for delivering energy from the premises wiring to an electric vehicle, including the coupler, cable, and control electronics.
F
Fault Current
An abnormal current flowing through a circuit as a result of an insulation failure, short circuit, or ground fault. May be thousands of amperes.
Feeder
All circuit conductors between the service equipment (or source of a separately derived system) and the final branch-circuit overcurrent device.
FLC (Full-Load Current)
The current required to produce full-load (rated) torque at rated speed from a motor. NEC requires use of table values (not nameplate) for sizing.
FLA (Full-Load Amps)
The nameplate current rating of a motor at rated voltage, frequency, and load. Used for overload protection sizing (distinct from FLC).
Flash Point
The lowest temperature at which a liquid produces enough vapor to form an ignitable mixture with air.
Flexible Metal Conduit (FMC)
A raceway of interlocking metal strip, spiral design, used where flexibility is needed for connections to motors and equipment.
Frequency
The number of complete cycles per second of an AC waveform. Measured in hertz (Hz). Standard: 60 Hz (NA), 50 Hz (EU).
Fuse
An overcurrent protective device with a fusible element that opens the circuit when heated by overcurrent passing through it.
G
GEC (Grounding Electrode Conductor)
A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.
Generator
A machine that converts mechanical energy into electrical energy using electromagnetic induction. Types include diesel, gas, and turbine.
GFCI (Ground-Fault Circuit Interrupter)
A device intended to protect personnel by de-energizing a circuit when the current to ground exceeds 5 mA (±1 mA).
GFPE (Ground-Fault Protection of Equipment)
A system intended to protect equipment from damaging ground-fault currents by operating to open a disconnecting means at currents less than required to trip the OCPD.
Ground Fault
An unintentional, electrically conducting connection between an ungrounded conductor and normally non-current-carrying conductors, metallic enclosures, or earth.
Ground Ring
A grounding electrode consisting of a bare copper conductor ≥2 AWG, buried at least 2.5 feet deep, encircling the building.
Grounding Electrode
A conducting object through which a direct connection to earth is established. Includes rods, plates, rings, and concrete-encased electrodes (Ufer).
H
Harmonic Filter
A device (passive or active) designed to reduce harmonic distortion in power systems, typically tuned to specific harmonic frequencies.
Harmonics
Frequency components of current or voltage that are integer multiples of the fundamental frequency (50 or 60 Hz). Cause overheating, neutral conductor overloading, and equipment malfunction.
Heat Trace
Electric heating cables applied to piping, tanks, or equipment to maintain temperature or prevent freezing.
Hertz (Hz)
The SI unit of frequency, equal to one cycle per second. Named after Heinrich Hertz.
High-Leg Delta
A center-tapped delta transformer configuration where one phase provides 208V to ground (the 'high leg' or 'wild leg'), while the other two provide 120V to ground.
Horsepower (HP)
A unit of power equal to 746 watts. Used to rate motor output. 1 HP = 746W = 0.746 kW.
I
IEC (International Electrotechnical Commission)
An international standards organization for electrical, electronic, and related technologies. Publishes standards like IEC 60364.
Impedance
The total opposition to current flow in an AC circuit, combining resistance (R) and reactance (X). Measured in ohms (Ω). Z = √(R² + X²).
IMC (Intermediate Metal Conduit)
A steel conduit with wall thickness between EMT and RMC. Lighter than RMC but provides similar protection.
Incident Energy
The amount of thermal energy impressed on a surface at a given distance from an arc flash source. Measured in cal/cm² or J/cm². Determines PPE category.
Inrush Current
The initial surge of current when an electrical device is first energized. Transformers may draw 8-12× rated current; motors draw 6-8× FLC.
Insulation Class
The temperature rating of electrical insulation. Common classes: A (105°C), B (130°C), F (155°C), H (180°C).
Interrupting Rating
The highest current at rated voltage that a device is intended to interrupt under standard test conditions.
Inverter
A device that converts DC power to AC power. Used in solar PV systems, UPS systems, and variable frequency drives.
IT System
An earthing system where the neutral is isolated from earth or connected through a high impedance. Provides continuity of supply on first fault.
J
Joule
The SI unit of energy. One joule = one watt × one second. Used in arc flash calculations (J/cm²).
Junction Box
An enclosure for the connection (splicing) of conductors, providing access for maintenance and wire pulling.
K
K-Factor Transformer
A transformer designed to handle non-linear (harmonic-rich) loads. K-ratings: K-4, K-9, K-13, K-20. Higher K handles more harmonics.
kcmil
A unit of measure for large conductor sizes. 1 kcmil = 1000 circular mils. Used for conductors larger than 4/0 AWG (e.g., 250, 350, 500 kcmil).
Kirchhoff's Current Law (KCL)
The algebraic sum of all currents entering and leaving a node equals zero. Current in = current out.
Kirchhoff's Voltage Law (KVL)
The algebraic sum of all voltages around any closed loop in a circuit equals zero.
kVA (kilovolt-ampere)
A unit of apparent power (S) in an AC circuit. S = V × I / 1000. For three-phase: kVA = V × I × √3 / 1000.
kVAR (kilovolt-ampere reactive)
A unit of reactive power (Q) in an AC circuit. Q = V × I × sin(φ) / 1000. Represents non-working power in inductive/capacitive loads.
kW (kilowatt)
A unit of real (active) power. P = V × I × PF / 1000. Represents the useful work-producing component of power.
kWh (kilowatt-hour)
A unit of energy equal to 1,000 watts consumed over one hour. Standard billing unit for electricity.
L
Labeled
Equipment with a label from an organization acceptable to the AHJ (such as UL) indicating conformity with standards.
LFMC (Liquidtight Flexible Metal Conduit)
Flexible metal conduit with a liquid-tight jacket, used in wet locations and where flexibility is needed.
Load Center
A type of panelboard typically used in residential applications, housing circuit breakers for branch circuit distribution.
Load Factor
The ratio of average load to peak load over a specified time period. Higher load factors indicate more uniform power consumption.
Load Shedding
The deliberate disconnection of non-critical loads to maintain power to critical systems during capacity constraints or generator operation.
Locked Rotor Current (LRC)
The steady-state current drawn by a motor with the rotor locked in position. Typically 6-8× FLC for standard induction motors.
Lumen
The SI unit of luminous flux, representing the total quantity of visible light emitted by a source per unit of time.
Lux (lx)
The SI unit of illuminance, equal to one lumen per square meter. 1 foot-candle = 10.764 lux.
M
Main Bonding Jumper
The connection between the grounded conductor (neutral) and the equipment grounding conductor at the service entrance only.
MCC (Motor Control Center)
An assembly of one or more enclosed sections with common bus and motor control units, used to control motors in industrial facilities.
MCM
Thousand circular mils. Older notation for kcmil. 500 MCM = 500 kcmil = 500,000 circular mils.
Metering
The measurement and recording of electrical quantities (kWh, kW, kVA, PF) for billing, monitoring, or control purposes.
Motor Starting Methods
Techniques to reduce starting current and mechanical stress: across-the-line, soft starter, VFD, autotransformer, wye-delta, part-winding.
Motor Starting Current
The initial inrush current drawn by a motor during starting, typically 6-8 times the full-load current for across-the-line starting.
MPPT (Maximum Power Point Tracking)
A technique used in solar inverters and charge controllers to continuously adjust the electrical load to extract maximum power from the PV array.
N
NEC (National Electrical Code)
NFPA 70 — the benchmark for safe electrical design, installation, and inspection in the United States. Updated on a 3-year cycle.
NEMA (National Electrical Manufacturers Association)
A trade association that publishes standards for electrical equipment, including enclosure ratings (NEMA 1, 3R, 4X, etc.).
NEMA Enclosure Rating
Classification for electrical enclosures: Type 1 (indoor general), 3R (outdoor rain), 4 (watertight), 4X (corrosion-resistant), 12 (dust-tight).
Neutral
The conductor connected to the neutral point of a system, intended to carry return current. In wye systems, the center point of the star connection.
Neutral-to-Ground Voltage
Voltage measured between neutral and ground. Should be near zero; elevated values indicate wiring problems, shared neutrals, or harmonic issues.
NFPA (National Fire Protection Association)
The organization that publishes the NEC (NFPA 70), NFPA 70E (Electrical Safety), NFPA 110 (Emergency Power), and other safety standards.
O
OCPD (Overcurrent Protective Device)
A device capable of providing protection for service, feeder, and branch circuits against overcurrent conditions. Includes fuses and circuit breakers.
Ohm (Ω)
The SI unit of electrical resistance. One ohm is the resistance that produces a potential difference of one volt when a current of one ampere flows through it.
Ohm's Law
The fundamental relationship between voltage (V), current (I), and resistance (R): V = I × R. The basis of all electrical calculations.
Overload
Operation of equipment in excess of normal, full-load rating, or conductor current in excess of rated ampacity. Not the same as a short circuit.
P
Panel Board
A single panel or group of panel units including buses and overcurrent devices, with or without switches, for the control of light, heat, or power circuits.
Parallel Conductors
Two or more conductors electrically connected at both ends to form a single conductor, used for large current-carrying requirements.
Peak Demand
The maximum power drawn from the utility over a specified interval (typically 15 minutes). Determines demand charges on commercial bills.
PF (Power Factor)
The ratio of real power (W) to apparent power (VA). PF = cos(φ). Unity (1.0) is ideal. Low PF wastes energy and capacity.
Phase Angle
The angular difference between voltage and current waveforms in an AC circuit. Determines power factor: PF = cos(φ).
Phase Sequence
The order in which the voltage waveforms of a three-phase system reach their maximum values. Designated A-B-C or R-S-T.
Plenum
A space used for air circulation in HVAC systems. Cables in plenums must have low-smoke, fire-resistant jackets (CMP/OFNP).
Point-to-Point Method
A fault current calculation method that accounts for utility contribution, transformer impedance, and conductor impedance at each point in the system.
Power Factor Correction
The practice of adding capacitor banks to improve power factor toward unity, reducing reactive power demand, utility penalties, and conductor losses.
Power Quality
The characteristics of the electrical power supply: voltage stability, frequency, harmonics, sags, swells, and transients.
PUE (Power Usage Effectiveness)
A metric for data center energy efficiency. PUE = Total Facility Power / IT Equipment Power. Ideal is 1.0; typical is 1.5-2.0.
PV (Photovoltaic)
Technology that converts sunlight directly into electricity using semiconductor cells. Solar panels contain multiple PV cells.
R
Raceway
An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars. Includes conduit, tubing, and cable trays.
Rapid Shutdown
NEC 690.12 requirement to reduce PV system voltage to ≤80V within 30 seconds of rapid shutdown initiation for firefighter safety.
Ratchet Clause
A utility billing provision that bases demand charges on the highest peak demand recorded over the previous 11 months, even if current demand is lower.
Reactance
The opposition to AC current flow due to inductance (XL = 2πfL) or capacitance (XC = 1/2πfC). Measured in ohms.
Receptacle
A contact device installed at an outlet for the connection of equipment via an attachment plug (cord-and-plug connection).
Relay
An electrically operated switch that uses a small control signal to operate a larger switching mechanism. Types include protective, control, and time-delay.
Resistance
The opposition to current flow in a conductor. R = ρL/A, where ρ is resistivity, L is length, A is cross-sectional area. Measured in ohms.
RMC (Rigid Metal Conduit)
Heavy-wall threaded steel conduit providing maximum physical protection. Used in industrial, outdoor, and hazardous locations.
S
SCCR (Short-Circuit Current Rating)
The maximum prospective fault current that equipment can safely withstand. Must exceed the available fault current at the installation point.
Selectivity (Coordination)
The ability of protective devices to isolate only the faulted section of a circuit while leaving the remainder of the system energized.
Separately Derived System
A premises wiring system whose power is derived from a generator, transformer, or converter and has no direct electrical connection to the supply.
Service
The conductors and equipment for delivering energy from the serving utility to the wiring system of the premises.
Service Drop
The overhead service conductors from the utility pole to the service point at the building.
Service Entrance
The conductors and equipment for delivering energy from the serving utility to the wiring system of the premises served.
Service Factor (SF)
A multiplier applied to a motor's nameplate HP rating indicating how much above rated load the motor can safely operate. Typical: 1.0 or 1.15.
Short Circuit
An abnormal connection of relatively low impedance between two points of different potential, resulting in extremely high fault current.
Single-Line Diagram (SLD)
A simplified electrical diagram showing the main components and connections of a power system using single lines to represent three-phase circuits.
Skin Effect
The tendency of AC current to flow near the surface of a conductor, effectively reducing the usable cross-section and increasing resistance at higher frequencies.
Soft Starter
An electronic device that reduces motor starting current and torque by gradually increasing voltage during acceleration.
Surge Protector (SPD)
A surge protective device that limits transient overvoltages and diverts surge current. Required at service entrance per NEC 2020+.
Switchboard
A large single panel or assembly of panels with buses, overcurrent devices, and instruments, accessible from the rear or front.
Switchgear
An assembly of switching and interrupting devices with buses, connections, and accessories, enclosed in metal. Rated above 600V or for high fault currents.
T
Tap Rule
NEC provisions allowing conductors to be tapped from feeders without OCPD at the tap point, subject to length and ampacity restrictions.
THD (Total Harmonic Distortion)
A measurement of the harmonic content of a current or voltage waveform as a percentage of the fundamental. IEEE 519 limits: 5% voltage, 5-20% current.
Three-Phase
A power system using three AC waveforms, each offset by 120 electrical degrees. Provides more efficient power transfer for motors and heavy loads.
Time-Current Curve (TCC)
A graph showing the relationship between fault current magnitude and the time a protective device takes to operate. Used for coordination studies.
TN System
An earthing system where the neutral is directly earthed at the source. Variants: TN-S (separate PE), TN-C (combined PEN), TN-C-S (combined then separated).
Torque
The rotational force produced by a motor, measured in lb-ft or N·m. Torque = HP × 5,252 / RPM.
Transformer
A static electrical device that transfers power between circuits through electromagnetic induction, changing voltage levels while maintaining frequency.
Transformer Impedance (%Z)
The percentage of rated voltage required to circulate rated current through the short-circuited winding. Determines fault current magnitude.
TT System
An earthing system where the neutral is earthed at the source and exposed conductive parts are connected to a separate local earth electrode.
U
UL (Underwriters Laboratories)
A global safety certification company that tests and certifies electrical products and components for safety compliance.
Underground Feeder (UF)
A cable type suitable for direct burial without conduit. Has moisture-resistant insulation molded directly onto the conductors.
UPS (Uninterruptible Power Supply)
A device that provides emergency power when the main supply fails, using battery banks and an inverter to maintain power to critical loads.
Utilization Voltage
The voltage at the point of use (equipment terminals), which is typically lower than the nominal system voltage due to voltage drop.
V
Volt (V)
The SI unit of electric potential difference. One volt drives one ampere through one ohm of resistance.
Voltage Drop
The reduction in voltage along a conductor due to its resistance and reactance. NEC recommends ≤3% for branch circuits and ≤5% feeder + branch combined.
Voltage Regulation
The ability of a system to maintain constant voltage at the load. Expressed as a percentage: VR = (Vno-load − Vfull-load) / Vfull-load × 100%.
VFD (Variable Frequency Drive)
A motor controller that varies speed by adjusting the frequency and voltage supplied to the motor. Reduces energy consumption for variable-torque loads.
VA (Volt-Ampere)
The unit of apparent power in an AC circuit. VA = V × I. Used for load calculations per NEC (not watts).
W
Watt (W)
The SI unit of power. One watt equals one joule per second. In DC circuits: P = V × I. In AC circuits: P = V × I × cos(φ).
Withstand Rating (SCCR)
The maximum short-circuit current that equipment can safely withstand without sustaining damage. Must equal or exceed available fault current at point of installation.
Wye (Star) Connection
A three-phase winding configuration where one end of each winding connects to a common neutral point. Line voltage = √3 × phase voltage.
X
X/R Ratio
The ratio of reactance to resistance in a circuit. Affects fault current asymmetry and protective device performance. Higher X/R means more asymmetry.
Z
Zonal Cavity Method
A lighting design method that divides a room into ceiling, room, and floor cavities to accurately calculate the coefficient of utilization.