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Electrical Glossary

47 terms from the vocabulary of voltage drop and conductor sizing, defined in plain English.

AHJ: Authority Having Jurisdiction — the inspector or office that interprets and enforces the electrical code locally. The final word on any sizing question.
ABYC: American Boat and Yacht Council — publishes marine wiring standards, including the 3% (critical) / 10% (non-critical) voltage drop columns used for boats.
Ampacity: The maximum current a conductor can carry continuously without exceeding its insulation temperature rating. Defined per installation conditions in NEC Table 310.16.
AWG: American Wire Gauge — the North American conductor sizing ladder. Smaller numbers are larger wires; three steps changes resistance by 2×.
Branch circuit: The wiring between the final overcurrent device and the outlets it serves. NEC recommends ≤3% voltage drop here.
Circular mil: The area of a circle 0.001 inch in diameter; the NEC's conductor area unit. 1 cmil = 5.067×10⁻⁴ mm².
Code letter: The NEMA nameplate letter giving a motor's locked-rotor kVA per horsepower — the key to its inrush multiple (Code G ≈ 6× FLC).
Conductor: The current-carrying wire — copper or aluminum in building work.
Continuous load: A load expected to run 3+ hours, which the NEC sizes at 125% for ampacity (voltage drop uses actual current).
Copper-clad aluminum (CCA): Aluminum conductor with a thin copper skin; ~40% more resistance than solid copper. Common in cheap data cable, failing PoE budgets.
Current-carrying conductor: A conductor counted for bundling derates under 310.15(C) — hots always, neutrals sometimes (see multiwire and harmonics).
Derating: Reducing usable ampacity for hostile conditions — high ambient temperature or many conductors bundled together.
EGC: Equipment Grounding Conductor — the safety ground. Sized from NEC 250.122; full-size in each raceway of a parallel run.
EMT: Electrical Metallic Tubing — thin-wall steel raceway; the default commercial conduit and the reference type in fill tables.
Feeder: Conductors between service equipment and a downstream panelboard. Budget ≤2% drop here to leave 3% for branches.
FLC: Full-Load Current — a motor's rated running amps. Motor conductors are sized at 125% of FLC per NEC 430.22.
GFCI: Ground-Fault Circuit Interrupter — shock protection that trips on milliamp-level leakage. Required outdoors and near water; doesn't change conductor sizing.
Harmonics: Distorted, non-sinusoidal currents from electronic loads; triplen harmonics add on the neutral and can make it a current-carrying conductor.
Impedance: The AC opposition to current — resistance plus reactance. Matters most for large conductors in steel raceway.
Inrush current: The brief surge a load draws at energization — up to 6–8× running current for motors, the cause of starting voltage sag.
Insulation rating: The temperature class of a conductor's jacket (60/75/90 °C) — sets which ampacity column applies, capped by termination ratings.
K-factor: The constant in Vd = 2KIL/CM: 12.9 for copper, 21.2 for aluminum at 75 °C.
kcmil: Thousand circular mils — the size unit above 4/0 AWG.
Line-to-line / line-to-neutral: Two ways to measure polyphase voltage: between phases (480 V) or phase to neutral (277 V). They differ by √3 in wye systems.
Locked-rotor current: The current a motor draws with its shaft stalled — effectively its starting current. See code letter.
MCA: Minimum Circuit Ampacity — the nameplate value HVAC equipment gives you; size conductors from it directly.
Multiwire branch circuit: Two hots sharing one neutral on opposite phases — the neutral carries only the imbalance, halving copper for paired 120 V circuits.
mV/A/m: Millivolts per amp per meter — how UK and AU/NZ tables express cable voltage drop. 2.5 mm² copper ≈ 18 mV/A/m.
NEMA MG-1: The motor standard defining voltage unbalance and its derating curve — design assumption ≤1%, do-not-operate above 5%.
Nominal voltage: The system's named voltage (120, 240, 480 V) — the denominator in percent-drop calculations even though actual supply varies.
OCPD: Overcurrent Protective Device — the breaker or fuse. Protects the conductor's ampacity; sized separately from voltage drop.
Ohm's law: V = I × R — the entire physics of voltage drop in five characters.
One-way length: Distance from source to load, not the total wire. Formulas double it (or apply √3) internally.
Parallel conductors: Multiple conductors per phase sharing current. NEC 310.10(G) requires 1/0 AWG minimum and matched lengths.
Power factor: The ratio of real to apparent power in AC circuits; reactive loads (motors) have PF below 1, which alters effective voltage drop.
Raceway: Any channel for conductors — conduit, tubing, wireway. Fill limits and bundling derates apply inside it.
Reactance: The frequency-dependent part of impedance, from magnetic fields around conductors; grows with conductor size and steel raceway.
Resistivity: A material's intrinsic resistance — ~10.37 Ω·cmil/ft for copper at 20 °C, ~17.0 for aluminum; the root of every table on this site.
Skin effect: AC current crowding toward a conductor's surface, raising effective resistance — negligible below 4/0 at 60 Hz, real above it.
Soft starter / VFD: Electronics that ramp a motor up gradually, cutting inrush from ~6× to 2–4× and taming starting voltage sag.
THHN / THWN-2: Common 90 °C nylon-jacketed building-wire insulations; the basis of standard conduit-fill tables.
UF-B: Underground Feeder cable — direct-burial rated; the conduit-free way to feed outbuildings.
Voltage drop: The voltage lost between source and load due to conductor impedance — the subject of this entire site.
Voltage at load: Source voltage minus the drop; what the equipment actually receives.
Voltage unbalance: Phases of a three-phase system disagreeing with each other (NEMA: max deviation ÷ average) — a motor killer distinct from drop.
XHHW-2: Cross-linked polyethylene 90 °C insulation — slightly larger OD than THHN, common in feeders and wet locations.
Wye / Delta: The two three-phase connection geometries — wye provides a neutral and two voltage levels (480Y/277); delta does not.