The Voltage Drop Calculator estimates the voltage lost along a wire run from the wire size, material, length, current and voltage.
Voltage drop: 0.49 V (0.41%)
| Voltage drop | 0.490 V |
| Percentage drop | 0.41% |
| Voltage at the end | 119.51 V |
| Wire resistance | 0.245 Ω per 1,000 ft |
| Recommendation | Within the common 3% guideline |
Every wire has resistance, so some voltage is lost as current flows through it. Long runs, thin wires and heavy loads lose more. Too much drop makes motors run hot, lights dim and electronics misbehave.
Single phase: VD = 2 × L × I × R ÷ 1,000. Three phase: VD = √3 × L × I × R ÷ 1,000. L is the one-way length in feet, I the current in amps and R the conductor resistance in ohms per 1,000 ft (copper 3 AWG ≈ 0.245 Ω).
Worked example (defaults): 100 ft of 3 AWG copper at 10 A on 120 V drops 2 × 100 × 10 × 0.245 ÷ 1,000 = 0.49 V, about 0.41%.
The U.S. National Electrical Code recommends (in informational notes) no more than 3% drop on a branch circuit and 5% total from service to outlet. If the drop is too high, use a thicker wire (smaller AWG number), shorten the run, or raise the voltage.
Why is aluminum worse? It has about 64% more resistance than copper.
Does temperature matter? Yes, resistance rises with temperature; values here are typical.
Is this a substitute for code calculations? No, consult an electrician for installations.