Voltage drop calculator

About this calculator

Enter the supply voltage, the current, the one-way cable length and the wire size. The calculator finds the cable resistance and the volts lost, and the voltage left at the far end.

A drop of 3% or less is a common target for a branch circuit. This is a guide, and wiring should be designed and installed by a qualified electrician.

Measurements and terms explained

One-way length

The distance from the supply to the load, not the round trip. The calculator doubles it for a two-wire circuit.

Wire size

The conductor's cross-section, in mm² or AWG. Bigger means less resistance.

AWG

American Wire Gauge. A smaller AWG number is a thicker wire.

Resistivity

How strongly the metal resists current. Copper is about 1.7 × 10⁻⁸ Ω·m and aluminium about 2.8 × 10⁻⁸ Ω·m.

Common questions

Why does voltage drop matter?

Too much drop makes lights dim and motors run hot. It also wastes power as heat in the cable.

How do I reduce it?

Use a thicker cable, shorten the run, or raise the supply voltage.