Voltage drop calculator

Enter the design current, run length and conductor size to get the volt drop in volts and as a percentage of nominal voltage, using Appendix 4 mV/A/m values for copper conductors. The BS 7671 limits are 3% for lighting and 5% for other uses, measured from the origin of the installation.

Inputs

Design current Ib (A)

Run length (m)

Conductor size (mm²)

Supply

Insulation

Permitted drop (%)

Result

9.00 V

Voltage drop

3.91%

of 230 V nominal

mV/A/m for 2.5 mm²

Exceeds the 3% limit of 6.90 V.Smallest conductor that meets this limit at 20 A over 25 m: 4 mm².

How it is calculated

Vd = (mV/A/m × Ib × L) ÷ 1000

mV/A/m comes from the Appendix 4 tables for copper conductors; three-phase values are line-to-line and compared against 400 V. Voltage drop is only one of the constraints on a cable — current-carrying capacity after correction factors, earth fault loop impedance and the adiabatic check all have to pass too.

Run the full cable sizing check

A design aid only — results must be checked by a competent person against the current edition of the wiring regulations. See the terms & engineering disclaimer.