Over 25 feet one-way, 4 AWG copper drops 0.5% at 40A on 120V and 0.5% at 80A on 240V: every load here up to 80A stays under 3% at 120V, and every load here up to 80A stays under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 40A | 0.6V | ✅ 0.5% | ✅ 0.3% |
| 50A | 0.8V | ✅ 0.6% | ✅ 0.3% |
| 60A | 0.9V | ✅ 0.8% | ✅ 0.4% |
| 70A | 1.1V | ✅ 0.9% | ✅ 0.4% |
| 80A* | 1.2V | ✅ 1.0% | ✅ 0.5% |
Longest one-way distance that keeps single-phase copper within 3%.
| Load | @ 120V | @ 240V |
|---|---|---|
| 40A | 145 ft | 290 ft |
| 50A | 115 ft | 235 ft |
| 60A | 95 ft | 195 ft |
| 70A | 85 ft | 165 ft |
| 80A | 75 ft | 145 ft |
* Above 70A, 4 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 (0.000308 Ω/ft for 4 AWG copper, from NEC Chapter 9 Table 8 at 75°C), 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. 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 (0.308 Ω per 1000 ft at 75°C), a 25-foot run of 4 AWG copper drops 0.5% at 40A, 0.6% at 50A, 0.8% at 60A, 0.9% at 70A, 1.0% at 80A on 120V, and half those percentages on 240V. All of these meet the 3% branch-circuit guideline at 240V.
To stay within 3% voltage drop, 4 AWG copper can run about 145 ft at 40A on 120V, or 145 ft at 80A on 240V (one-way). Its maximum load is 85A at 75°C; lighter loads can run proportionally farther.
Upsize when your load pushes drop above 3%. At 80A on 240V over 25 feet, 4 AWG drops 0.5%; stepping up to 3 AWG cuts that to 0.4%. For motors, well pumps or sensitive electronics, aim for 2% or less.