Find the minimum copper or aluminum AWG for your load: ampacity and 3% voltage drop checked together. NEC 75°C ratings.
| Input | Result |
|---|---|
| 20 A, 50 ft, 120 V, copper | 10 AWG (drop 2.1%, 35 A rating) |
| 15 A, 100 ft, 120 V, copper | 8 AWG (drop 2.0%, 50 A rating) |
| 30 A, 25 ft, 240 V, copper | 10 AWG (drop 0.8%, 35 A rating) |
Two checks, both must pass: ① ampacity — the wire must carry the current without overheating (NEC 310.15(B)(16)); ② voltage drop — keep it ≤3% so equipment gets its rated voltage. This calculator picks the first size passing both.
Ampacity alone is fine (25 A ≥ 20 A), but the drop is 2 × 12.9 × 20 × 50 ÷ 6,530 = 3.95 V = 3.3% at 120 V — just over the 3% target. The next size (10 AWG) brings it to 2.1%.
For service entrances and large feeders, yes — but it needs two sizes larger than copper (K = 21.2 vs 12.9) and AL-rated terminals. Never land aluminum on copper-only terminals.