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.
Use the voltage drop calculator to estimate voltage loss and percentage drop for a wire run. It is useful for comparing conductor material, wire size, distance, load and source voltage.
Start with the calculator that matches how your material is sold: area, length, volume, count, weight or electrical load. Enter one work area at a time, use the unit selector beside each measurement, then review the order quantity and cost fields before comparing supplier quotes.
Most calculators can save the current estimate in your browser, reload named estimates, print a copy, switch to dark mode or create a shareable link. Those tools are intended for planning and communication; always confirm final quantities, coverage rates, density, code requirements and pricing with the supplier or contractor responsible for the work.