Voltage Drop Calculator

This calculator works out the voltage drop along an electrical cable, a key check in any wiring design, from the current, the length of the run and the resistance of the conductor. Whenever current flows through a cable, the conductor's own resistance causes some voltage to be lost along the way, so the voltage arriving at the appliance is a little lower than the voltage at the supply. Over short runs this is negligible, but over long cable runs, or at high currents, the drop can become significant, dimming lights, slowing motors and wasting energy as heat. New Zealand wiring rules limit voltage drop to keep equipment working properly, commonly around five percent from the point of supply to the load, so estimating it is part of choosing the right cable size. This tool gives a clear estimate for a single-phase, two-wire circuit. You enter the current in amps, the one-way length of the cable run in metres, the resistance of the conductor in ohms per metre for the cable size you are considering, and the supply voltage. The calculator returns the voltage drop in volts, accounting for the full there-and-back length of the circuit, the drop as a percentage of the supply, and the voltage that actually reaches the load. The results update as you type, so you can compare cable sizes or run lengths quickly. Use it as a planning aid for circuits, to check whether a cable is adequate, or to understand why a long run underperforms. This is an estimate for guidance: always follow the wiring rules and consult a registered electrician for actual installations. For three-phase circuits the calculation differs. Results are rounded for display.

Single-phase, two-wire: drop = 2 x length x current x resistance per metre. NZ rules commonly limit total drop to about 5%. An estimate only; consult a registered electrician.