Voltage Drop Calculator

About this calculator

This calculator works out the voltage drop along a cable run from its length, current, cross-sectional area, and the conductor material (copper or aluminum), returning the drop in volts, the drop as a percentage of supply voltage, and the actual voltage reaching the load.

Electricians and electrical engineers use it to check that a cable run stays within code — most standards call for under 3% drop on a branch circuit and under 5% total — before the wire is pulled, since undersized conductors on a long run waste energy as heat and can starve equipment of the voltage it needs.