Estimate voltage drop across a copper wire run based on current, length, and gauge.
Voltage drop occurs because every real wire has resistance, and that resistance eats a small amount of voltage proportional to current and distance. Formula: Vdrop = 2 × K × I × L ÷ CM, where K is copper's resistivity constant (12.9), L is one-way length, and CM is the wire's circular-mil area.
The '2×' accounts for the full circuit — current travels out and back, so both directions of wire contribute resistance.
This matters most on long runs and high-current circuits — a garage sub-panel, a well pump, or an EV charger circuit are common places where an undersized gauge causes a real, measurable drop.
20A over a 100ft one-way run on 12 AWG copper: voltage drop ≈ 7.90V — potentially significant on a 120V circuit.
The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total from source to farthest outlet — beyond that, equipment can underperform or overheat.
Yes, slightly — resistivity increases with temperature. This calculator uses a standard mid-range value; very hot or cold installations will vary a little from the estimate.
Use a thicker gauge (lower AWG number) or shorten the run. Doubling wire cross-sectional area roughly halves voltage drop for the same current and length.