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Voltage Drop Calculator

Work out how much voltage is lost along a copper or aluminium cable, single-phase, three-phase or DC, and the smallest standard cable size that keeps the drop within your limit.

Rules last checked 25 October 2026Every step shown

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V

230 V for single-phase in Pakistan, the UK and the Gulf; 400 V between lines for three-phase; 12, 24 or 48 V for most solar batteries.

A
m

From the supply to the load. The return path is counted for you.

%

UK wiring rules allow 3% for lighting and 5% for other circuits; solar installers often aim for 3% or less.

Voltage drop

9.888 V (4.3%)

Drop as a share of supply
4.3%
Voltage at the load
220.112 V
Power lost in the cable
197.76 W
Smallest size within 5%
2.5 mm²

A 30 m run of 2.5 mm² copper at 20 A drops 9.89 V (4.3%), within your 5% limit.

How it's worked out

  1. Voltage drop2 × 30 m × 20 A × 0.0206 Ω·mm²/m ÷ 2.5 mm²9.888 V
  2. Share of supply9.888 ÷ 230 V4.3%
  • Uses conductor resistance at 70 °C and ignores reactance, which is fine for cables up to about 25 mm². The factor 2 counts the cable out and back.
  • Also check the cable's current rating for how it is installed, and follow your local wiring rules. Get a qualified electrician for mains work.

The formula

Single-phase and DC: Vd = 2 × L × I × ρ ÷ A · Three-phase: Vd = √3 × L × I × ρ ÷ A

ρ is the resistivity at a working temperature of 70 °C: 0.0206 Ω·mm²/m for copper and 0.0339 for aluminium. L is the one-way length in metres and A the cable size in mm².

Example

A 30 m run of 2.5 mm² copper carrying 20 A on 230 V drops 9.89 V, which is 4.3%, leaving 220.1 V at the load and wasting about 198 W as heat. That is within 5%; to stay within 3% you would need 4 mm².

Questions people ask

What voltage drop is acceptable?

UK wiring rules allow 3% for lighting and 5% for other circuits from the supply. For solar DC cables, installers often aim for 3% or less to save energy.

How is voltage drop calculated?

For single-phase and DC: 2 × length × current × resistivity ÷ cable area. For three-phase, use √3 instead of 2, which gives the drop between lines.

Why does a long cable need a bigger size?

Resistance grows with length, so a long run loses more voltage and wastes power as heat. A thicker cable has less resistance.

Is this enough to choose a cable?

No. Also check the cable's current rating for the way it is installed and follow your local wiring rules. Mains work should be done by a qualified electrician.

Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem