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Voltage Drop Calculator
Check a cable run against the 5% voltage drop limit in AS/NZS 3000 — for consumer mains, sub-mains to granny flats and sheds, EV charger circuits and long commercial runs.
Formula: voltage drop = mV/A/m × current × length ÷ 1000, using typical copper conductor values in the style of AS/NZS 3008.1.1. Route length is the one-way cable run distance.
Why voltage drop matters
AS/NZS 3000 limits voltage drop to 5% from the point of supply to any point in the installation. Long runs to granny flats, sheds, pool equipment and EV chargers regularly exceed the limit when cable is sized only for current-carrying capacity — the cable passes on amps but fails on volts.
If your consumer mains are the problem, that's Level 2 work: Sydney Power Group upgrades overhead and underground consumer mains across the Ausgrid, Endeavour Energy and Essential Energy networks. Sizing a new supply? Start with our maximum demand calculator.
Voltage drop FAQs
What is the maximum allowable voltage drop in Australia?
AS/NZS 3000 limits total voltage drop between the point of supply and any point in the installation to 5% — about 11.5 V on a 230 V single-phase supply or 20 V on a 400 V three-phase supply. Long consumer mains runs and undersized cables are the usual causes of excessive drop.
How do I calculate voltage drop for a cable run?
Multiply the cable's millivolt-per-amp-metre (mV/A/m) value from AS/NZS 3008 by the load current and the route length, then divide by 1000. Compare the result against the 5% limit for your supply voltage.
What happens if voltage drop is too high?
Lights dim, motors run hot, EV chargers derate and sensitive equipment can fault. Excessive voltage drop also fails inspection — the fix is a larger cable size, a shorter route, or upgrading the supply, which for consumer mains is Level 2 ASP work.
Why does voltage drop matter for granny flats and sheds?
Long sub-mains runs to granny flats, sheds and pool areas commonly exceed the limit if sized only for current-carrying capacity. The cable often needs to be upsized for voltage drop, especially with air conditioning or EV charging at the far end.