Voltage drop for 12 V / 24 V / 48 V DC systems (RV, solar, automotive). Same 2×K×I×L÷CM physics, much less margin than mains voltage.
| Input | Result |
|---|---|
| 12 V, 10 A, 10 ft, 12 AWG | 0.40 V (3.3%) — marginal |
| 12 V, 10 A, 10 ft, 8 AWG | 0.16 V (1.3%) — OK |
| 48 V, 10 A, 10 ft, 12 AWG | 0.40 V (0.8%) — OK |
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.
Required CM for ≤3% at 12 V: 2 × 12.9 × 15 × 15 ÷ 0.36 = 16,125 → 8 AWG copper. At 12 V almost everything needs thicker wire than the ampacity table suggests.
For the wire resistance itself, no — the same 2×K×I×L÷CM applies (the factor 2 covers the out-and-back loop). AC additionally has reactance, which is why long AC runs use impedance tables.