Over 100 feet one-way, 12 AWG copper drops 5.0% at 15A on 120V and 3.3% at 20A on 240V: no load in this table stays under 3% at 120V, and loads up to 15A stay under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 15A | 5.9V | ⚠️ 5.0% | ✅ 2.5% |
| 20A | 7.9V | ❌ 6.6% | ⚠️ 3.3% |
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 (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 (1.98 Ω per 1000 ft at 75°C), a 100-foot run of 12 AWG copper drops 5.0% at 15A, 6.6% at 20A on 120V, and half those percentages on 240V. At 240V, loads up to 15A meet the 3% guideline.
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 100 feet, 12 AWG drops 3.3%; stepping up to 10 AWG cuts that to 2.1%. For motors, well pumps or sensitive electronics, aim for 2% or less.