Voltage drop analysis for 8 AWG copper wire over a 25-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 | 0.47V | ✅ 0.4% | ✅ 0.2% |
| 20A | 0.63V | ✅ 0.5% | ✅ 0.3% |
| 30A | 0.94V | ✅ 0.8% | ✅ 0.4% |
| 40A | 1.26V | ✅ 1.0% | ✅ 0.5% |
| 50A | 1.57V | ✅ 1.3% | ✅ 0.7% |
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 (25 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.