Estimate voltage drop on a wire run. Enter the load, one-way distance and conductor, and the calculator returns the voltage drop and percentage.
Voltage drop is the voltage lost to conductor resistance over a run of wire. The common estimate uses Vd = (2 × K × I × L) ÷ CM for single phase, where K is the resistivity of the conductor (about 12.9 for copper, 21.2 for aluminum), I is the load in amps, L is the one-way length in feet, and CM is the wire's circular-mil area. Three-phase circuits use a factor of 1.732 instead of 2.
Divide the voltage drop by the source voltage for the percentage. As a rule of thumb, keep voltage drop under 3% on a branch circuit and under 5% total including the feeder. If your run exceeds those limits, increase the wire size—each larger gauge has more circular mils and lowers the drop. This tool is a planning estimate; always design to your local electrical code.