Over 150 feet one-way, 8 AWG copper drops 3.9% at 20A on 120V and 4.9% at 50A on 240V: no load in this table stays under 3% at 120V, and loads up to 30A stay under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 20A | 4.7V | ⚠️ 3.9% | ✅ 1.9% |
| 30A | 7.0V | ❌ 5.8% | ✅ 2.9% |
| 40A | 9.3V | ❌ 7.8% | ⚠️ 3.9% |
| 50A* | 11.7V | ❌ 9.7% | ⚠️ 4.9% |
Longest one-way distance that keeps single-phase copper within 3%.
| Load | @ 120V | @ 240V |
|---|---|---|
| 20A | 115 ft | 230 ft |
| 30A | 75 ft | 155 ft |
| 40A | 60 ft | 115 ft |
| 50A | 45 ft | 95 ft |
* Above 40A, 8 AWG needs 75°C-rated terminations at both ends (NEC 110.14(C)); most breakers and panels rated 100A or less assume 60°C unless marked 75°C.
Voltage drop for single-phase circuits: Vdrop = 2 × I × R × L
Where I = current in amps, R = resistance per foot (0.000778 Ω/ft for 8 AWG copper, from NEC Chapter 9 Table 8 at 75°C), and L = one-way distance (150 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. Each single AWG step up in size cuts voltage drop by about 21% — it takes three steps to roughly halve it, since each step increases the conductor area by a factor of 1.26. Alternatively, for 120V loads, consider rewiring for 240V — same absolute drop but half the percentage.
It depends on the load and voltage. With NEC Table 8 resistance (0.778 Ω per 1000 ft at 75°C), a 150-foot run of 8 AWG copper drops 3.9% at 20A, 5.8% at 30A, 7.8% at 40A, 9.7% at 50A on 120V, and half those percentages on 240V. At 240V, loads up to 30A meet the 3% guideline.
To stay within 3% voltage drop, 8 AWG copper can run about 115 ft at 20A on 120V, or 95 ft at 50A on 240V (one-way). Its maximum load is 50A at 75°C; lighter loads can run proportionally farther.
Upsize when your load pushes drop above 3%. At 50A on 240V over 150 feet, 8 AWG drops 4.9%; stepping up to 6 AWG cuts that to 3.1%. For motors, well pumps or sensitive electronics, aim for 2% or less.