Wire Voltage Drop Calculator

This calculator finds the voltage lost along a wire run, that loss as a percentage of the supply, and the voltage left at the load. It flags whether the drop is within the widely used 3% guideline.

It is used when running power to a detached garage, shed, well pump or EV charger; laying low-voltage landscape lighting; and wiring 12 V solar, RV and marine systems, where a volt or two matters far more. Too much drop makes motors run hot, lights dim and electronics reset.

You need the wire's resistance per 1,000 ft, which depends on gauge and material; the table on this page lists common copper sizes. The method treats the run as a single-phase or DC circuit with a supply and return conductor. Once you know the drop is acceptable, confirm the wire can carry the current safely with the wire gauge ampacity calculator.

Resistance values come from NEC Chapter 9, Table 8 (DC resistance at 75 °C). For balanced three-phase circuits, replace the 2 with √3 (1.732). For large AC conductors (roughly 1/0 and above), reactance adds to the drop and NEC Table 9 values give a more accurate answer. The NEC's 3% branch / 5% total figure is an informational recommendation for efficiency, not a mandatory rule, though some local codes and equipment makers require it.

DC resistance of uncoated stranded copper at 75 °C, from NEC Chapter 9, Table 8. Aluminum conductors have roughly 1.6 times the resistance of copper at the same gauge.

• Mains branch circuits: 3% on the branch and 5% in total from service to outlet, per the NEC's informational notes. In the UK, BS 7671 sets maximum drops of 3% for lighting and 5% for other uses on installations supplied from the public network.
• Motors: large drops reduce starting torque and raise running current and temperature. Manufacturers often require supply voltage within ±10% of nameplate.
• Low-voltage DC (12/24 V): ABYC marine practice uses 3% for critical circuits such as navigation lights and electronics and 10% for less critical loads.
• Long runs at 120 V: if a run is long, moving the load to 240 V halves the current and quarters the percentage drop for the same power.

The three levers are thicker wire, a shorter run or a higher voltage. Each step down of three AWG numbers roughly halves resistance, so going from 14 AWG to 10 AWG cuts the drop by about 60%. The wire gauge calculator can suggest a gauge directly from length and allowed drop. Remember that ampacity and voltage drop are separate checks: a wire can be safe for its current and still lose too much voltage over a long run.