Voltage Drop Calculator
Voltage drop in volts and percent, the voltage at the load and the smallest wire that stays under 3 %. NEC (AWG) or IEC (mm²), copper or aluminum.
Checks voltage drop only. Size the wire for ampacity and overcurrent protection too.
Electrician Calc: Wire & Panel
Wire size with ampacity and derating, panel planner, motors, solar and 21 calculators with step-by-step working and PDF reports. See what's inside.
Questions
What is the voltage drop formula?
For copper under the NEC, VD = 2 × K × I × L / CM for single phase and 1.732 × K × I × L / CM for three phase, with K ≈ 12.9 Ω·cmil/ft, I the load current, L the one-way length in feet and CM the conductor area in circular mils. This calculator uses the equivalent IEC 60364-5-52 Annex G method with conductor reactance, which matches the K-factor result within about 0.5 % at unity power factor.
What voltage drop is acceptable?
The NEC informational notes recommend no more than 3 % on a branch circuit and 5 % for feeder plus branch combined. IEC 60364-5-52 suggests 3 % for lighting and 5 % for other uses on installations supplied from the public network.
Is the length one way or round trip?
Enter the one-way length from the panel to the load. The formula already counts both conductors (or the three phases).
Why does power factor matter?
On AC circuits the drop depends on resistance and reactance. With motor loads (power factor below 1) the reactive part grows, which matters most on large conductors.