Voltage Drop
Drop (V) = (mV/A/m × Ib × L) / 1000
Work out the voltage drop along a circuit from its tabulated mV/A/m figure, design current and length, and express it as a percentage of the nominal supply voltage.
Reference
BS 7671 Appendix 4 (mV/A/m tables)
How this works
Appendix 4 of BS 7671 tabulates voltage drop per ampere per metre — mV/A/m — for each cable size, construction and installation method. Multiply that figure by the design current and the circuit length, then divide by 1000 to get volts:
Voltage drop (V) = (mV/A/m × Ib × L) / 1000
The tabulated figure already accounts for the go and return path, so the length you enter is the route length of the circuit, not double it.
The guideline limits
Appendix 12 gives 3% of the nominal voltage for lighting circuits and 5% for other uses, measured from the origin of the installation. At 230 V single phase that works out at 6.9 V and 11.5 V.
These are guidance, not absolute limits. What Regulation 525.1 actually requires is that the voltage at equipment terminals is no less than the lowest that equipment is designed to work at. Meeting a percentage does not by itself prove that, and exceeding one is not automatically a fault — but it is where you start looking.
Where a circuit runs through a distribution circuit as well, the drops are cumulative: the allowance is for the whole route from the origin, not each leg separately.
Limitations
This calculator is a revision and cross-checking aid. It does not replace the published tables in BS 7671, the specific requirements of the installation in front of you, or the judgement of a competent person. Always verify against the current edition of the standard before relying on a result.