Voltage Drop Calculator
Enter the cable mV/A/m, the design current and the run length to get the voltage drop in volts and as a percentage of the supply, checked against the usual limits.
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How voltage drop is calculated
Voltage drop along a cable is worked out from its tabulated millivolt-per-amp-per-metre figure, the current the circuit is designed to carry, and how far the cable runs:
The result is then expressed as a percentage of the nominal supply voltage. For single-phase work use the single-phase mV/A/m value; for three-phase, use the three-phase value from the cable table, which already accounts for line voltage.
The limits
BS 7671 recommends that voltage drop from the origin of the installation to the load stays within about 3% for lighting and 5% for other uses on a 230 V supply. Those percentages are measured from the origin, so on a longer installation you need to account for drop in the submains as well as the final circuit.
About the presets
The dropdown holds commonly quoted mV/A/m figures for 70°C thermoplastic insulated copper conductors, rounded for convenience. They are there to save time, not to replace the table. For anything you are going to certify, read the exact value for your conductor size and installation method straight from BS 7671.