Everything about a 400 ft one-way run on one page: at 240 V and a 3% budget, a 20 A load needs 4 AWG copper and a 50 A load needs 1 AWG copper. The quick-answer table and the full percent-drop matrices are below.
| Load | 120 V copper | 240 V copper | 240 V aluminum |
|---|---|---|---|
| 15 A | 3 AWG | 6 AWG | 4 AWG |
| 20 A | 2 AWG | 4 AWG | 3 AWG |
| 30 A | 1/0 AWG | 3 AWG | 1 AWG |
| 40 A | 2/0 AWG | 2 AWG | 1/0 AWG |
| 50 A | 3/0 AWG | 1 AWG | 2/0 AWG |
| 60 A | 4/0 AWG | 1/0 AWG | 3/0 AWG |
| 80 A | 250 kcmil | 2/0 AWG | 4/0 AWG |
| 100 A | 300 kcmil | 3/0 AWG | 250 kcmil |
| Copper \ Amps | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 30.7 | 40.9 | 61.4† | 81.9† | 102.3† | 122.8† | 163.7† | 204.7† |
| 12 AWG | 19.3 | 25.7 | 38.6† | 51.5† | 64.3† | 77.2† | 102.9† | 128.7† |
| 10 AWG | 12.1 | 16.1 | 24.2 | 32.3† | 40.3† | 48.4† | 64.5† | 80.7† |
| 8 AWG | 7.6 | 10.2 | 15.3 | 20.4 | 25.5 | 30.6† | 40.7† | 50.9† |
| 6 AWG | 4.9 | 6.5 | 9.8 | 13.1 | 16.4 | 19.6 | 26.2† | 32.7† |
| 4 AWG | 3.1 | 4.1 | 6.2 | 8.2 | 10.3 | 12.3 | 16.4 | 20.5† |
| 3 AWG | 2.5 | 3.3 | 4.9 | 6.5 | 8.2 | 9.8 | 13.1 | 16.3 |
| 2 AWG | 1.9 | 2.6 | 3.9 | 5.2 | 6.5 | 7.8 | 10.3 | 12.9 |
| 1 AWG | 1.5 | 2.1 | 3.1 | 4.1 | 5.1 | 6.2 | 8.2 | 10.3 |
| 1/0 AWG | 1.2 | 1.6 | 2.4 | 3.3 | 4.1 | 4.9 | 6.5 | 8.1 |
| 2/0 AWG | 1.0 | 1.3 | 1.9 | 2.6 | 3.2 | 3.9 | 5.2 | 6.4 |
| 3/0 AWG | 0.8 | 1.0 | 1.5 | 2.0 | 2.6 | 3.1 | 4.1 | 5.1 |
| 4/0 AWG | 0.6 | 0.8 | 1.2 | 1.6 | 2.0 | 2.4 | 3.2 | 4.1 |
| Copper \ Amps | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 15.4 | 20.5 | 30.7† | 40.9† | 51.2† | 61.4† | 81.9† | 102.3† |
| 12 AWG | 9.7 | 12.9 | 19.3† | 25.7† | 32.2† | 38.6† | 51.5† | 64.3† |
| 10 AWG | 6.0 | 8.1 | 12.1 | 16.1† | 20.2† | 24.2† | 32.3† | 40.3† |
| 8 AWG | 3.8 | 5.1 | 7.6 | 10.2 | 12.7 | 15.3† | 20.4† | 25.5† |
| 6 AWG | 2.5 | 3.3 | 4.9 | 6.5 | 8.2 | 9.8 | 13.1† | 16.4† |
| 4 AWG | 1.5 | 2.1 | 3.1 | 4.1 | 5.1 | 6.2 | 8.2 | 10.3† |
| 3 AWG | 1.2 | 1.6 | 2.5 | 3.3 | 4.1 | 4.9 | 6.5 | 8.2 |
| 2 AWG | 1.0 | 1.3 | 1.9 | 2.6 | 3.2 | 3.9 | 5.2 | 6.5 |
| 1 AWG | 0.8 | 1.0 | 1.5 | 2.1 | 2.6 | 3.1 | 4.1 | 5.1 |
| 1/0 AWG | 0.6 | 0.8 | 1.2 | 1.6 | 2.0 | 2.4 | 3.3 | 4.1 |
| 2/0 AWG | 0.5 | 0.6 | 1.0 | 1.3 | 1.6 | 1.9 | 2.6 | 3.2 |
| 3/0 AWG | 0.4 | 0.5 | 0.8 | 1.0 | 1.3 | 1.5 | 2.0 | 2.6 |
| 4/0 AWG | 0.3 | 0.4 | 0.6 | 0.8 | 1.0 | 1.2 | 1.6 | 2.0 |
Read across your load, down your gauge: ≤3.0 passes the NEC branch recommendation, ≤5.0 the total budget. Cells marked † exceed the 75 °C copper ampacity for that load — undersized regardless of drop. For aluminum multiply any value by ~1.61; for three-phase multiply by 0.866.
It depends on the load: at 240 V and 3%, 400 ft needs 6 AWG at 15 A, 4 AWG at 20 A, and 1 AWG at 50 A. At 120 V, sizes step up roughly two gauges — see the table above.
One-way — source to load. The return path is already in the formula (×2 single-phase, ×√3 three-phase), so don't double your measurement.