Over 25 feet one-way, 12 AWG copper drops 1.2% at 15A on 120V and 0.8% at 20A on 240V: every load here up to 20A stays under 3% at 120V, and every load here up to 20A stays under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 15A | 1.5V | ✅ 1.2% | ✅ 0.6% |
| 20A | 2.0V | ✅ 1.7% | ✅ 0.8% |
Longest one-way distance that keeps single-phase copper within 3%.
| Load | @ 120V | @ 240V |
|---|---|---|
| 15A | 60 ft | 120 ft |
| 20A | 45 ft | 90 ft |
Voltage drop for single-phase circuits: Vdrop = 2 × I × R × L
Where I = current in amps, R = resistance per foot (0.00198 Ω/ft for 12 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 (1.98 Ω per 1000 ft at 75°C), a 25-foot run of 12 AWG copper drops 1.2% at 15A, 1.7% at 20A 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, 12 AWG copper can run about 60 ft at 15A on 120V, or 90 ft at 20A on 240V (one-way). Its maximum load is 20A (the NEC 240.4(D) limit); lighter loads can run proportionally farther.
Upsize when your load pushes drop above 3%. At 20A on 240V over 25 feet, 12 AWG drops 0.8%; stepping up to 10 AWG cuts that to 0.5%. For motors, well pumps or sensitive electronics, aim for 2% or less.