On long wire runs, resistance causes the voltage to drop. This single-phase formula estimates the drop for a copper conductor.
VD = (2 × K × I × L) ÷ CM
What the terms mean
| Symbol | Meaning |
|---|---|
| VD | Voltage drop, in volts |
| K | Resistivity (about 12.9 for copper) |
| I | Current, in amperes |
| L | One-way length, in feet |
| CM | Conductor area, in circular mils |
Worked example
20 A over 100 ft of 12 AWG (6,530 CM) copper: (2 × 12.9 × 20 × 100) ÷ 6,530 = about 7.9 V, or 6.6% on 120 V.
Keep it under 3%. Branch circuits should hold voltage drop to about 3%. Past that, step up a wire gauge to lower resistance.
See the Power Formula and Voltage Drop Calculator.