8 AWG voltage drop at 125 feet
Computed voltage drop for 8 AWG copper conductors on a 125 ft one-way run.
7.64 V
drop on 8 AWG copper, 125 ft one-way, 40 A load · 6.4% of 120 V · 3.2% of 240 V
8 AWG at 125 ft by load
| Load | Drop | % at 120 V | % at 240 V | 3% rule (240 V) |
|---|---|---|---|---|
| 10 A | 1.91 V | 1.6% | 0.8% | OK |
| 20 A | 3.82 V | 3.2% | 1.6% | OK |
| 30 A | 5.73 V | 4.8% | 2.4% | OK |
| 40 A | 7.64 V | 6.4% | 3.2% | OK at 240 V up to 5% |
Vd = 2 × length × R × I ÷ 1000, R = 0.764 Ω/1000 ft (copper, NEC Ch. 9 Table 8). Aluminum is about 1.6× higher.
Same wire, other distances
25 ft · 50 ft · 75 ft · 100 ft · 150 ft · 200 ft · 250 ft · 300 ft
FAQ
What is the voltage drop on 8 AWG wire at 125 feet?
At 40 A (80% of its 50 A 75°C rating) 8 AWG copper drops 7.64 V over 125 ft one-way: 6.4% at 120 V or 3.2% at 240 V.
How far can I run 8 AWG at 40 amps?
To stay within 3% at 120 V, about 59 ft one-way; at 240 V about 118 ft.