8 AWG voltage drop at 300 feet
Computed voltage drop for 8 AWG copper conductors on a 300 ft one-way run.
18.34 V
drop on 8 AWG copper, 300 ft one-way, 40 A load · 15.3% of 120 V · 7.6% of 240 V
8 AWG at 300 ft by load
| Load | Drop | % at 120 V | % at 240 V | 3% rule (240 V) |
|---|---|---|---|---|
| 10 A | 4.58 V | 3.8% | 1.9% | OK |
| 20 A | 9.17 V | 7.6% | 3.8% | OK at 240 V up to 5% |
| 30 A | 13.75 V | 11.5% | 5.7% | Upsize |
| 40 A | 18.34 V | 15.3% | 7.6% | Upsize |
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 · 125 ft · 150 ft · 200 ft · 250 ft
FAQ
What is the voltage drop on 8 AWG wire at 300 feet?
At 40 A (80% of its 50 A 75°C rating) 8 AWG copper drops 18.34 V over 300 ft one-way: 15.3% at 120 V or 7.6% 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.