What Size Wire for 40 Amps at 100 Feet?
Voltage-drop-aware wire sizing for a 40-amp circuit running 100 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 2.5%. For ≤5% drop: 8 (2.5%).
Actual drop 2.7%. For ≤5% drop: 8 (4.2%).
At 100 ft, ampacity still governs: 8 AWG copper carries 40A and its drop (2.5%) is already within 3% — no upsizing needed.
Voltage Drop at 100 Feet
Voltage drop is calculated for a 240V 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%? |
|---|---|---|---|
| 8 | 2.5% | 4.2% | |
| 6 | 1.6% | 2.7% | |
| 4 | 1.0% | 1.7% | |
| 3 | 0.8% | 1.3% | |
| 2 | 0.6% | 1.1% | |
| 1 | 0.5% | 0.8% |
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 40A, 240V, 100 ft to adjust material, temperature and drop target.