Calculate DC / single-phase AC voltage drop for copper wire by AWG size, length and current. Instant drop in volts and percent.
| Input | Result |
|---|---|
| 120 V, 15 A, 50 ft, 12 AWG | 2.96 V (2.5%) — OK |
| 120 V, 20 A, 100 ft, 12 AWG | 7.90 V (6.6%) — too high |
| 12 V, 10 A, 10 ft, 12 AWG | 0.40 V (3.3%) — marginal |
For DC or single-phase AC: VD = 2 × K × I × L ÷ CM. Example: 15 A over 50 ft of 12 AWG copper: 2 × 12.9 × 15 × 50 ÷ 6,530 = 2.96 V, which is 2.5% of 120 V — within the 3% recommendation.
The same volts-lost is a bigger fraction: a 2.96 V drop is 2.5% at 120 V but 24.7% at 12 V. Low-voltage systems (RV, solar, automotive) need much thicker wire for the same power.
Common practice: ≤3% for a branch circuit, ≤5% total including the feeder. Sensitive electronics and LED lighting dim or misbehave beyond that. Check NEC 210.19(A) Informational Note for the recommendation.