By ampacity alone, a 100 A circuit needs 3 AWG copper (or 1 AWG aluminum) at the 75 °C column. But past a certain distance, voltage drop — not ampacity — picks the size. The chart below shows the real answer.
| One-way distance | 120 V copper | 240 V copper | 240 V aluminum |
|---|---|---|---|
| 25 ft | 3 AWG | 3 AWG | 1 AWG |
| 50 ft | 3 AWG | 3 AWG | 1 AWG |
| 75 ft | 2 AWG | 3 AWG | 1 AWG |
| 100 ft | 1 AWG | 3 AWG | 1 AWG |
| 150 ft | 2/0 AWG | 2 AWG | 1/0 AWG |
| 200 ft | 3/0 AWG | 1 AWG | 2/0 AWG |
| 300 ft | 250 kcmil | 2/0 AWG | 4/0 AWG |
| 500 ft | 400 kcmil | 4/0 AWG | 300 kcmil |
Short runs are governed by ampacity; long runs by drop. For this load the crossover happens where the table first steps above 3 AWG. For a 5% total budget the distances stretch 1.67×. Continuous loads (3+ hours) must also be breakered at 125% — size the overcurrent device accordingly.
Only where the drop stays acceptable. 3 AWG copper handles 100 A thermally, but on a long run the voltage drop will exceed 3% — check the distance chart or the calculator above.
The conductor must be protected at or below its ampacity (per NEC 240.4); continuous loads are calculated at 125%. Breaker sizing is a separate check from voltage drop.