Everything about a 300 ft one-way run on one page: at 240 V and a 3% budget, a 20 A load needs 6 AWG copper and a 50 A load needs 2 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 | 4 AWG | 8 AWG | 4 AWG |
| 20 A | 3 AWG | 6 AWG | 4 AWG |
| 30 A | 2 AWG | 4 AWG | 2 AWG |
| 40 A | 1/0 AWG | 3 AWG | 1 AWG |
| 50 A | 2/0 AWG | 2 AWG | 1/0 AWG |
| 60 A | 2/0 AWG | 2 AWG | 2/0 AWG |
| 80 A | 4/0 AWG | 1/0 AWG | 3/0 AWG |
| 100 A | 250 kcmil | 2/0 AWG | 4/0 AWG |
| Copper \ Amps | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 23.0 | 30.7 | 46.0† | 61.4† | 76.8† | 92.1† | 122.8† | 153.5† |
| 12 AWG | 14.5 | 19.3 | 29.0† | 38.6† | 48.2† | 57.9† | 77.2† | 96.5† |
| 10 AWG | 9.1 | 12.1 | 18.2 | 24.2† | 30.2† | 36.3† | 48.4† | 60.5† |
| 8 AWG | 5.7 | 7.6 | 11.5 | 15.3 | 19.1 | 22.9† | 30.6† | 38.2† |
| 6 AWG | 3.7 | 4.9 | 7.4 | 9.8 | 12.3 | 14.7 | 19.6† | 24.6† |
| 4 AWG | 2.3 | 3.1 | 4.6 | 6.2 | 7.7 | 9.2 | 12.3 | 15.4† |
| 3 AWG | 1.8 | 2.5 | 3.7 | 4.9 | 6.1 | 7.3 | 9.8 | 12.2 |
| 2 AWG | 1.5 | 1.9 | 2.9 | 3.9 | 4.9 | 5.8 | 7.8 | 9.7 |
| 1 AWG | 1.2 | 1.5 | 2.3 | 3.1 | 3.9 | 4.6 | 6.2 | 7.7 |
| 1/0 AWG | 0.9 | 1.2 | 1.8 | 2.4 | 3.1 | 3.7 | 4.9 | 6.1 |
| 2/0 AWG | 0.7 | 1.0 | 1.5 | 1.9 | 2.4 | 2.9 | 3.9 | 4.8 |
| 3/0 AWG | 0.6 | 0.8 | 1.1 | 1.5 | 1.9 | 2.3 | 3.1 | 3.8 |
| 4/0 AWG | 0.5 | 0.6 | 0.9 | 1.2 | 1.5 | 1.8 | 2.4 | 3.0 |
| Copper \ Amps | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 11.5 | 15.4 | 23.0† | 30.7† | 38.4† | 46.0† | 61.4† | 76.8† |
| 12 AWG | 7.2 | 9.7 | 14.5† | 19.3† | 24.1† | 29.0† | 38.6† | 48.2† |
| 10 AWG | 4.5 | 6.0 | 9.1 | 12.1† | 15.1† | 18.2† | 24.2† | 30.2† |
| 8 AWG | 2.9 | 3.8 | 5.7 | 7.6 | 9.6 | 11.5† | 15.3† | 19.1† |
| 6 AWG | 1.8 | 2.5 | 3.7 | 4.9 | 6.1 | 7.4 | 9.8† | 12.3† |
| 4 AWG | 1.2 | 1.5 | 2.3 | 3.1 | 3.9 | 4.6 | 6.2 | 7.7† |
| 3 AWG | 0.9 | 1.2 | 1.8 | 2.5 | 3.1 | 3.7 | 4.9 | 6.1 |
| 2 AWG | 0.7 | 1.0 | 1.5 | 1.9 | 2.4 | 2.9 | 3.9 | 4.9 |
| 1 AWG | 0.6 | 0.8 | 1.2 | 1.5 | 1.9 | 2.3 | 3.1 | 3.9 |
| 1/0 AWG | 0.5 | 0.6 | 0.9 | 1.2 | 1.5 | 1.8 | 2.4 | 3.1 |
| 2/0 AWG | 0.4 | 0.5 | 0.7 | 1.0 | 1.2 | 1.5 | 1.9 | 2.4 |
| 3/0 AWG | 0.3 | 0.4 | 0.6 | 0.8 | 1.0 | 1.1 | 1.5 | 1.9 |
| 4/0 AWG | 0.2 | 0.3 | 0.5 | 0.6 | 0.8 | 0.9 | 1.2 | 1.5 |
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%, 300 ft needs 8 AWG at 15 A, 6 AWG at 20 A, and 2 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.