Maximum permissible voltage drop

Photo credit: Blue Diamond Machinery 

NECA Technical frequently receive queries regarding maximum permissible volt drop for varying types of installations and scenarios. 

In Western Australia, the nominal single phase supply voltage is 240 volts plus or minus 6%, this represents a range between 225.6 and 254.4 volts. Clause 1.6.2 in AS/NZS 3000:2018 refers to a nominal voltage of 230 volts plus 10% or minus 6%, which ranges between 216.2 and 253 volts.

On standard installations the maximum permissible volt drop between the point of supply and any point of the installation is 5%, as specified in clause 3.6.2 of AS/NZS 3000:2018. Therefore on a 240 volt supply, a 5% volt drop would be 12 volts or 228 volts at the equipment or appliance.

There are some exceptions to this clause, one being where the installation is supplied from a sole use transformer. In this case, the maximum volt drop can be 7% across the installation.

Another exception is for stand-alone systems (off-grid generation) where it is permissible for the volt drop to be as high as 11% below the nominal supply voltage of the source, providing the voltage at the terminals of equipment and appliance is not below the operating voltage of the equipment or appliance. If the source of supply voltage is 240 volts, then an 11% volt drop would result in a voltage of 213.6 volts which would be outside the design parameters of 220-240 volts for most equipment and appliances and therefore not an acceptable voltage. This issue could be overcome by either increasing the cable size to reduce the volt drop, or by increasing the voltage at the source of supply.