Over 150 feet one-way, 14 AWG copper drops 7.8% at 10A on 120V and 5.9% at 15A on 240V: no load in this table stays under 3% at 120V, and no load in this table stays under 3% at 240V.
Single phase, copper conductor, one-way distance. ✅ = under 3%, ⚠️ = 3-5%, ❌ = over 5%
| Load | V-Drop | @ 120V | @ 240V |
|---|---|---|---|
| 10A | 9.4V | ❌ 7.8% | ⚠️ 3.9% |
| 15A | 14.1V | ❌ 11.8% | ❌ 5.9% |
Longest one-way distance that keeps single-phase copper within 3%.
| Load | @ 120V | @ 240V |
|---|---|---|
| 10A | 55 ft | 115 ft |
| 15A | 40 ft | 75 ft |
Voltage drop for single-phase circuits: Vdrop = 2 × I × R × L
Where I = current in amps, R = resistance per foot (0.00314 Ω/ft for 14 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 (3.14 Ω per 1000 ft at 75°C), a 150-foot run of 14 AWG copper drops 7.8% at 10A, 11.8% at 15A on 120V, and half those percentages on 240V. None meet the 3% guideline, so a larger wire is needed.
To stay within 3% voltage drop, 14 AWG copper can run about 55 ft at 10A on 120V, or 75 ft at 15A on 240V (one-way). Its maximum load is 15A (the NEC 240.4(D) limit); lighter loads can run proportionally farther.
Upsize when your load pushes drop above 3%. At 15A on 240V over 150 feet, 14 AWG drops 5.9%; stepping up to 12 AWG cuts that to 3.7%. For motors, well pumps or sensitive electronics, aim for 2% or less.