Voltage Drop Calculator

Compute single-phase voltage drop in mV, V and as a percentage of 230 V from the BS 7671 Appendix 4 mV/A/m value, design current and circuit length.

Source of mV/A/m

Cable CSA (T&E, ref method C)

Design current (A)

Circuit length (m)

Nominal voltage (V)

Enter current and length to calculate voltage drop.

Formula

Voltage drop (mV) = (mV/A/m) × current (A) × length (m). Pick a T&E cable size from BS 7671 Appendix 4 Table 4D2B, or enter your own mV/A/m for other cable types.

Limits

BS 7671 Appendix 4 limits voltage drop to 3% of nominal for lighting and 5% for other circuits supplied directly from a public LV network. Higher limits apply where supply is via a private transformer.

Used in

EICR — checked when verifying circuit suitability.
EIC — required at design stage on new installations.

Reference

BS 7671:2018+A2:2022 Appendix 4, Table 4D2B

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