Over 100 feet one-way, 10 AWG copper drops 3.1% at 15A on 120V and 3.1% at 30A on 240V: no load in this table stays under 3% at 120V, and loads up to 25A stay under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 15A | 3.7V | ⚠️ 3.1% | ✅ 1.6% |
| 20A | 5.0V | ⚠️ 4.1% | ✅ 2.1% |
| 25A | 6.2V | ❌ 5.2% | ✅ 2.6% |
| 30A | 7.4V | ❌ 6.2% | ⚠️ 3.1% |
Longest one-way distance that keeps single-phase copper within 3%.
| Load | @ 120V | @ 240V |
|---|---|---|
| 15A | 95 ft | 195 ft |
| 20A | 75 ft | 145 ft |
| 25A | 60 ft | 115 ft |
| 30A | 50 ft | 95 ft |
Voltage drop for single-phase circuits: Vdrop = 2 × I × R × L
Where I = current in amps, R = resistance per foot (0.00124 Ω/ft for 10 AWG copper, from NEC Chapter 9 Table 8 at 75°C), and L = one-way distance (100 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 (1.24 Ω per 1000 ft at 75°C), a 100-foot run of 10 AWG copper drops 3.1% at 15A, 4.1% at 20A, 5.2% at 25A, 6.2% at 30A on 120V, and half those percentages on 240V. At 240V, loads up to 25A meet the 3% guideline.
To stay within 3% voltage drop, 10 AWG copper can run about 95 ft at 15A on 120V, or 95 ft at 30A on 240V (one-way). Its maximum load is 30A (the NEC 240.4(D) limit); lighter loads can run proportionally farther.
Upsize when your load pushes drop above 3%. At 30A on 240V over 100 feet, 10 AWG drops 3.1%; stepping up to 8 AWG cuts that to 1.9%. For motors, well pumps or sensitive electronics, aim for 2% or less.