How it works
Every wire has resistance, so some voltage is lost along the run. Long runs at high current can leave motors, lights and chargers underpowered. This calculator uses DC resistance at 75 °C from NEC Chapter 9, Table 8.
For single-phase and DC circuits the current flows out and back, so the one-way length is doubled. For balanced three-phase circuits the multiplier is √3. Results ignore conductor reactance, which matters mainly for large conductors in steel conduit at AC.
Voltage drop is separate from ampacity. A wire can pass the voltage-drop check and still be too small for the breaker, so size for ampacity first (NEC 310.16), then upsize if the drop is too high.
Formula
Single-phase / DC: VD = 2 × L × I × R ÷ 1000 Three-phase: VD = √3 × L × I × R ÷ 1000 (L in feet, R in Ω per 1,000 ft)
Questions people ask
What voltage drop is acceptable?
NEC informational notes in 210.19 and 215.2 recommend no more than 3% on a branch circuit or feeder and 5% total from service to the farthest outlet. It is a recommendation, not a requirement, except in some special cases such as fire pumps.
Should I enter the round-trip length?
No. Enter the one-way distance from the panel to the load. The calculator doubles it for single-phase and DC circuits and applies √3 for three-phase.
Why is aluminum drop higher?
Aluminum has about 61% of the conductivity of copper, so the same gauge has roughly 1.6 times the resistance. Aluminum is usually upsized by about two AWG sizes to match copper.
Does temperature matter?
Yes. Copper resistance rises about 0.39% per °C. These values assume 75 °C conductors, which is conservative for lightly loaded circuits in cool locations.