Cable Size Voltage Drop Calculator

About this calculator

Voltage drop is the loss of voltage along a cable run caused by the conductor's own resistance, calculated from Ohm's law as current multiplied by the cable's total resistance (which depends on the conductor's cross-sectional area, length, and material — usually copper), doubled for a single-phase circuit to account for both the outgoing and return conductors. Electrical codes such as the UK's BS 7671 and the US NEC recommend keeping voltage drop under roughly 3-5% of the supply voltage, since excessive drop causes dimming lights, underperforming motors, and equipment that fails to operate within its rated tolerance at the far end of a long cable run.

Electricians calculate this before wiring a run — particularly long runs to outbuildings, workshops, or EV chargers — because a cable that's perfectly adequate for the current-carrying (ampacity) rating can still fail the voltage-drop check if the run is long enough, meaning you sometimes need a thicker conductor than the basic ampacity table would suggest, purely to keep the drop within limits.

This calculator takes your cable length, conductor size, load current, and supply voltage and returns the voltage drop and whether you need a larger conductor to stay within your drop limit.