Voltage Drop Calculator

Written byOleh Soloninupdated 20 August 2026

Load current, A

Run length, m

Conductor area, mm²

Supply voltage, V

Conductor

Supply

Result

Voltage drop, V11.2
Drop, %5.09%
Voltage at the load, V208.8
VerdictToo much — go up a cable size
Power lost, W179.2
Area for 3%, mm²4.24

Why the far end of a long run is dimmer

Computes the voltage lost along a cable from length, cross-section and current, checks it against the three percent limit and suggests a size.

Every conductor has resistance, so some voltage is lost on the way. Over a short run it hardly registers; over a hundred metres to an outbuilding or a distant light it becomes real — equipment underperforms, motors run hot, lamps sit at half brightness.

The rule is simple: up to three percent for lighting circuits and five for power. Exceed that and the only practical cure is a larger cross-section, because the length is rarely negotiable.

What this calculator shows

Voltage drop in volts and as a percentage
The voltage that actually arrives at the load
The cross-section that would bring the loss inside three percent

What to keep in mind

Resistivity is taken at 20 °C: 0.0175 for copper and 0.0282 for aluminium in Ω·mm²/m. A hot cable loses slightly more.
Single-phase current travels out and back, so the length doubles in the formula; three-phase uses root three instead.

FAQs

How much voltage drop is acceptable?

Three percent for lighting and up to five for power circuits. For sensitive electronics, keeping it under two is wiser.

Why does single-phase double the length?

Current flows out along the live conductor and back along the neutral, picking up resistance on both. The formula therefore uses twice the run length.

Is aluminium much worse than copper?

Its resistivity is about 1.6 times higher, so at equal cross-section the losses are 1.6 times greater. The usual answer is to go one size up.

Where does the lost voltage go?

Into heat in the cable itself. An 11 volt drop at 16 amps means the run is dissipating nearly 180 watts along its length.

Worked example

A run to the garage

Input: 16 A, 50 m, 2.5 mm² copper, 220 V

Output: 11.2 V dropped, which is 5.09 percent — too much

Note: Staying within three percent needs 4.25 mm², so 6 mm² off the standard range. Dearer cable, but otherwise the far end sits at 209 volts.

Useful links

All tools in Electrical

Voltage Drop Calculator

Result

Related tools

Wire Size Calculator

Watts Amps Volts Calculator

Ohm's Law Calculator

Useful links