Voltage Drop Calculator

Voltage drop is the silent tax on long cable runs. BS 7671 sets indicative limits (3% for lighting, 5% for other circuits, measured from the origin of the installation), and a run that looks fine on the design spreadsheet at 50 metres often falls outside spec at 150 metres unless the conductor steps up. The fallout shows up at the appliance: motors that run hot and shorten brushgear life, LED arrays that flicker on inrush, EV chargers that derate themselves, and process equipment that fails CE-marked input voltage tolerances. Each of those reads back as unplanned downtime or premature replacement on the maintenance ledger.

Voltage drop matters most on the runs Purely Energy clients are now installing in volume: EV charger circuits in car parks, long submains in warehouses and distribution centres, BESS and solar AC interconnects on industrial roofs, and the new dedicated heat-pump circuits replacing gas plant in commercial buildings. Each project tends to push the cable run length, and each one increases the chance that the day-one design margin is consumed by a future capacity uplift. A first-pass voltage-drop check at the layout stage is far cheaper than re-pulling cable after the EV bays sit dormant for a month while the issue is diagnosed.

This calculator is a BS 7671 indicative reference and not a substitute for a competent electrical designer signing off the final cable schedule. The drop numbers it returns depend on assumed conductor temperature, installation method (free air, conduit, ladder, buried), grouping factors and the actual installed circuit length including diversions. Treat the output as a sanity check on a proposed design, not the design itself. Once the numbers look workable, hand them to a designer to run against the full BS 7671 calculation and to sign off the project at install completion.