Voltage drop analysis for 8 AWG copper wire over a 75-foot one-way run. See whether this gauge is suitable for your circuit at various loads and voltages.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 15A | 1.41V | ✅ 1.2% | ✅ 0.6% |
| 20A | 1.88V | ✅ 1.6% | ✅ 0.8% |
| 30A | 2.83V | ✅ 2.4% | ✅ 1.2% |
| 40A | 3.77V | ⚠️ 3.1% | ✅ 1.6% |
| 50A | 4.71V | ⚠️ 3.9% | ✅ 2.0% |
Voltage drop for single-phase circuits: Vdrop = 2 × I × R × L
Where I = current in amps, R = resistance per foot (0.000628 Ω/ft for 8 AWG copper), and L = one-way distance (75 ft).
The factor of 2 accounts for both the supply and return conductors. NEC recommends a maximum 3% voltage drop for branch circuits and 5% for the total circuit (feeder + branch).
If voltage drop is too high: Use the next larger wire size. Going from 8 to 7 AWG roughly halves the voltage drop. Alternatively, for 120V loads, consider rewiring for 240V — same absolute drop but half the percentage.