What Size Wire for 15 Amps at 100 Feet?
Voltage-drop-aware wire sizing for a 15-amp circuit running 100 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 1.9%. For ≤5% drop: 12 (4.8%).
Actual drop 2.0%. For ≤5% drop: 10 (5.0%).
At 100 ft, voltage drop governs: ampacity alone would allow 14 AWG copper, but staying ≤3% requires upsizing to 8.
Voltage Drop at 100 Feet
Voltage drop is calculated for a 120V single-phase circuit over a 100 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 | 7.7% | 12.6% | — |
| 12 | 4.8% | 7.9% | — |
| 10 | 3.0% | 5.0% | — |
| 8 | 1.9% | 3.1% | |
| 6 | 1.2% | 2.0% | |
| 4 | 0.8% | 1.3% |
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, 100 ft to adjust material, temperature and drop target.