What Size Wire for 15 Amps at 300 Feet?
Voltage-drop-aware wire sizing for a 15-amp circuit running 300 ft (one-way) at 120V — 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 2.3%. For ≤5% drop: 6 (3.7%).
Actual drop 2.4%. For ≤5% drop: 4 (3.8%).
At 300 ft, voltage drop governs: ampacity alone would allow 14 AWG copper, but staying ≤3% requires upsizing to 4.
Voltage Drop at 300 Feet
Voltage drop is calculated for a 120V 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%? |
|---|---|---|---|
| 14 | 23.0% | 37.9% | — |
| 12 | 14.5% | 23.9% | — |
| 10 | 9.1% | 15.0% | — |
| 8 | 5.7% | 9.4% | — |
| 6 | 3.7% | 6.1% | — |
| 4 | 2.3% | 3.8% |
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 15A, 120V, 300 ft to adjust material, temperature and drop target.