Formula
| V_drop (single-phase) | = 2 · I · R_per_length · length |
|---|---|
| V_drop (three-phase) | = √3 · I · R_per_length · length |
| NEC recommendation | ≤ 3% on feeders, ≤ 3% on branch (5% combined) |
Copper resistance
| AWG | Ω / 1 000 ft | Ω / km |
|---|---|---|
| 4/0 | 0.049 | 0.161 |
| 2/0 | 0.078 | 0.256 |
| 1/0 | 0.098 | 0.323 |
| 2 | 0.156 | 0.513 |
| 4 | 0.248 | 0.815 |
| 6 | 0.395 | 1.296 |
| 8 | 0.628 | 2.061 |
| 10 | 0.999 | 3.277 |
| 12 | 1.588 | 5.211 |
| 14 | 2.525 | 8.286 |
| 16 | 4.016 | 13.17 |
| 18 | 6.385 | 20.95 |
| 20 | 10.15 | 33.31 |
| 22 | 16.14 | 52.96 |
Quick-pick examples
| Load | Length (one-way) | Min AWG | V_drop @ 120 V |
|---|---|---|---|
| 15 A @ 120 V | 50 ft | 14 AWG | < 3% |
| 15 A @ 120 V | 100 ft | 12 AWG | < 3% |
| 20 A @ 120 V | 100 ft | 10 AWG | < 3% |
| 30 A @ 240 V | 100 ft | 10 AWG | < 3% |
| 50 A @ 240 V | 150 ft | 6 AWG | < 3% |
Notes
- NEC voltage-drop figures are recommendations (not code requirements in most jurisdictions) but widely followed.
- Aluminum wire has ~60% the conductivity of copper — size up 2 AWG larger for the same drop.
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