Voltage drop calculator
Check a cable run against the 3 % limit for lighting and 5 % for other circuits on a 230 V supply.
From the voltage drop column of the cable's table in BS 7671 Appendix 4.
One-way length of the run, not there and back.
Voltage drop
Values are for 70 °C thermoplastic flat twin and earth cable at full operating temperature. Check against your copy of BS 7671 or the On-Site Guide before relying on a result.
The formula
Vd = (mV/A/m × Ib × L) ÷ 1000
mV/A/m is the cable's voltage drop per amp per metre, from BS 7671 Appendix 4. Ib is the design current and L the one-way length in metres. The tabulated figure already allows for both the line and neutral conductors.
The limits
For an installation supplied from the public low-voltage network, BS 7671 (Regulation 525 and its voltage drop guidance) gives 3 % for lighting and 5 % for other uses. On 230 V that is 6.9 V and 11.5 V.
Worked example
A 2.5 mm² twin and earth radial carrying 20 A over 30 m: (18 × 20 × 30) ÷ 1000 = 10.8 V, which is 4.7 % and inside the 5 % limit.