One-way run length, not the return path.
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
What voltage drop actually means on site
Every metre of conductor has a bit of resistance, and current flowing through that resistance loses a bit of voltage along the way. By the time it reaches the load, the appliance isn't seeing the full 230V or 400V you started with at the switchboard - it's seeing that minus whatever the cable ate on the way. Volt drop is just that loss, expressed as a voltage or a percentage of nominal supply.
It matters because too much drop means motors run hotter and less efficiently, lights dim and flicker, and electronic gear can behave oddly or throw faults under load. The 5% limit isn't an arbitrary number - it's the rough point where those problems start showing up in real installations.
In practice, volt drop is the thing that decides cable size on long runs far more often than current-carrying capacity does. A circuit to a shed 40m from the house will usually fail on volt drop long before it fails on ampacity, which is why sparkies size up on distance even when the actual load is small.
Worked examples
Garage submain, 35m run
A single-phase submain feeding a detached garage, 35m from the main board, carrying 20A. On 4mm² copper this is a common spec - but check it against the 5% limit before you commit to that size, because a lot of garage submains fail volt drop at this kind of distance even though the load current is modest.
Heat pump circuit, short run
A single-phase heat pump circuit, 12m from the switchboard, drawing 16A on 2.5mm². Short runs like this almost always pass comfortably on volt drop - the cable size here is set by current-carrying capacity and breaker coordination, not volt drop, which is a useful sanity check when you're deciding where to spend your cable budget.
Three-phase workshop submain, 60m run
A three-phase submain to a workshop 60m from the house, carrying 32A per phase. Three-phase circuits generally handle distance better than single-phase for the same current, but 60m is far enough that it's worth checking rather than assuming - particularly if the workshop has its own switchboard and you're deciding what size feeder to run to it.
Frequently asked questions
What's an acceptable voltage drop in NZ?
The general rule electricians work to is a maximum of 5% total volt drop from the point of supply to the load. Some circuits (lighting on long runs, sensitive equipment) are often designed tighter than that in practice, but 5% is the standard reference limit this calculator checks against.
Do I use the single-phase or three-phase mV/A/m value?
Cable tables are usually published as three-phase mV/A/m values. For a single-phase circuit, that value gets adjusted by a conversion factor to reflect the actual current path. This calculator handles that conversion for you when you select single phase, so you just enter the circuit as it actually is.
Does length mean one-way or there-and-back?
One-way run length - the distance from the switchboard to the load. Don't double it yourself; the calculator and the underlying method already account for the return conductor.
Why does the same cable pass on one job and fail on another?
Volt drop is a function of current, length and cable size together, not any one of those alone. A 2.5mm² circuit might comfortably pass at 10m but fail at 40m for the same load, which is exactly why a long run to a shed or a garage often needs a size up even when the load current is modest.
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