Voltage Drop Calculator

This calculator uses the circular-mils method: voltage drop = (phase factor × K × current × distance) ÷ circular mils. K is the resistivity of the conductor (12.9 for copper, 21.2 for aluminum, in ohm-circular-mils per foot), the phase factor is 2 for single phase and 1.732 for three phase, current is in amps, and distance is the one-way length in feet. Divide the result by your system voltage to get the percent drop.

The NEC does not hard-limit voltage drop in its main rules, but informational notes to 210.19 (branch circuits) and 215.2 (feeders) recommend a maximum of 3% on a branch circuit and 5% total across the feeder and branch combined. These are recommendations for reasonable efficiency, not enforceable code, though your AHJ or project spec may require them.

Aim for 3% or less on the branch circuit that feeds the load, and keep the feeder plus branch combined at 5% or less. Designing to 3% gives you headroom and better efficiency. If a single long run pushes you past 3%, upsizing the conductor one or two sizes usually brings it back into range.

Aluminum has higher electrical resistance than copper, so for the same wire size and run it drops more voltage. That is why aluminum conductors are typically sized one or two AWG larger than copper to carry the same load. This calculator uses K = 12.9 for copper and K = 21.2 for aluminum to reflect that difference.

Enter the one-way distance — the length of the run in a single direction. The calculator already accounts for current traveling out and back through the phase factor (2 for single phase, 1.732 for three phase), so you should not double the distance yourself.

No. This is a DC/AC resistance estimate using the circular-mils method. It does not adjust for power factor, conductor operating temperature, or conduit type, all of which can affect real-world drop. For motor loads, long runs, or critical circuits, verify with the conductor's published AC resistance and reactance values and the NEC.

The most common fix is to upsize the conductor — a larger wire has more circular mils and less resistance, which lowers the drop. You can also shorten the run, reduce the load, or raise the system voltage where the design allows. Re-run the calculation after each change until you are within the 3% / 5% target.

In a balanced three-phase circuit the current returns through the other phase conductors rather than a dedicated neutral, so the effective resistive path is shorter. That is reflected in the phase factor — 1.732 (the square root of 3) for three phase versus 2 for single phase. For the same wire, current, and distance, a three-phase run shows a lower voltage drop.

Yes, the calculator is completely free with no signup required. Your inputs are saved locally in your browser so they persist between visits — nothing is uploaded to a server. Clearing your browser data will erase your saved values.