What Size Wire for 125 Amps at 300 Feet?
Voltage-drop-aware wire sizing for a 125-amp circuit running 300 ft (one-way) at 240V — because over distance, voltage drop, not ampacity, often sets the wire size.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
Actual drop 2.4%. For ≤5% drop: 1 (4.8%).
Actual drop 2.6%. For ≤5% drop: 2/0 (5.0%).
At 300 ft, voltage drop governs: ampacity alone would allow 1 AWG copper, but staying ≤3% requires upsizing to 3/0.
Voltage Drop at 300 Feet
Voltage drop is calculated for a 240V single-phase circuit over a 300 ft one-way run (conductor length doubled for the return). Sizes meeting the ≤3% target are highlighted.
| Wire size | Copper drop | Aluminum drop | Within 3%? |
|---|---|---|---|
| 1 | 4.8% | 7.9% | — |
| 1/0 | 3.8% | 6.3% | — |
| 2/0 | 3.0% | 5.0% | — |
| 3/0 | 2.4% | 3.9% | |
| 4/0 | 1.9% | 3.1% | |
| 250 kcmil | 1.6% | 2.6% |
Drop % uses NEC Chapter 9, Table 8 DC resistance — an accepted approximation for typical power-factor AC branch circuits. The wire must also meet ampacity; both checks are applied above.
Different distance or voltage?
Open the calculator pre-filled with 125A, 240V, 300 ft to adjust material, temperature and drop target.