Check voltage drop using the mV/A/m method.
Enter cable data from manufacturer tables or BS 7671 appendix.
For design guidance only. Always verify with a qualified engineer.
About this voltage drop calculator
This free voltage drop calculator checks whether a cable run keeps the volt drop within the limit allowed by BS 7671. It is aimed at UK building-services engineers, electrical designers and contractors who need a quick check on a circuit during design or verification. You enter the system type (single-phase or three-phase), supply voltage, design current, one-way cable length and the cable’s mV/A/m figure, and it returns the drop in volts, the drop as a percentage of the supply voltage, and a clear PASS or FAIL against your chosen limit.
How the voltage drop is calculated
The tool uses the mV/A/m method from BS 7671 Appendix 4. The drop is found as dV = mV/A/m × Ib × L ÷ 1000, where Ib is the design current and L is the one-way length. That voltage is then expressed as a percentage of the supply voltage and compared with the maximum allowable drop, 3 % (typical lighting), 5 % (typical other circuits) or a custom value. A single mV/A/m scalar is assumed; the abridged cable presets give the single-phase (two-core) figure and are automatically scaled by √3/2 (≈0.866) when three-phase is selected. The result is indicative design guidance. Always verify against manufacturer data and a qualified engineer.
Frequently asked questions
Where do I get the mV/A/m value?
From the cable manufacturer’s data or the BS 7671 tables. You can also pick one of the built-in presets (T&E and SWA copper from 1.5 mm² upwards), which fills the mV/A/m field for you. Verify the figure against the actual cable being used.
What length should I enter?
Enter the one-way length of the cable run. The mV/A/m method already accounts for the return path, so you do not double the length.
What limit should I use, 3 % or 5 %?
The calculator offers 3 % (typical for lighting), 5 % (typical for other circuits) or a custom percentage. Choose the limit appropriate to the circuit and project specification; the result passes when the calculated drop is at or below it.