Mains Cable Selection Calculator
Find the smallest compliant mains cable for your load and run.
For typical New Zealand installation conditions only. Based on 30°C ambient air temperature, 15°C ambient soil temperature and 1.2°C.m/W thermal resistivity. No other heat sources (eg: loaded cables) in thermal proximity.
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.
Two checks, one answer
Picking a cable size is really two separate questions that both need a "yes": can this cable carry the load current continuously without overheating, and will the voltage at the far end still be within an acceptable range after the run. Either one can be the limiting factor depending on the job.
Short, high-current circuits (a range, a hot water cylinder close to the board) tend to be decided by current-carrying capacity. Long runs at moderate current (a shed, a garage, a submain across a section) tend to be decided by volt drop instead. This calculator checks both and gives you the smallest size that clears both bars.
Worked examples
EV charger circuit, mid-length run
A 32A single-phase EV charger circuit run 25m from the board, clipped direct. EV chargers pull their full rated current continuously for hours at a time, which makes this exactly the kind of load where cutting the cable size too fine on capacity alone catches people out.
Range circuit, short run
A range circuit at 32A, only 8m from the board, enclosed in conduit. Short runs like this are almost always decided by current-carrying capacity rather than volt drop - a good example of when the "obvious" larger cable for a high-current appliance isn't actually needed.
Submain buried to a sleepout, long run
A 20A submain, direct buried, running 45m to a sleepout at the back of a section. Direct burial derates the cable's current rating compared to clipping it in open air, and the length pushes volt drop up at the same time - the two effects compound, which is why buried runs to outbuildings often need a noticeably bigger cable than the load current alone would suggest.
Frequently asked questions
Why isn't current-carrying capacity enough on its own?
A cable can be rated to carry a certain current all day without overheating, and still fail the job because too much voltage is lost along a long run. Sizing needs to satisfy both checks - capacity and volt drop - and whichever one is stricter for your particular run decides the final size.
Why does installation method change the answer?
A cable clipped to a wall with air around it sheds heat far more easily than the same cable buried underground or bundled inside wall insulation. The rated current for the same physical cable size drops as the installation gets less forgiving, which is why the method you pick changes which size comes out as compliant.
Should I size for the breaker rating or the actual load?
Size against the actual load current for the circuit, then make sure the breaker you choose sits between the load and the cable's rating - that coordination between load, breaker and cable capacity is what keeps a circuit properly protected.
What if no standard cable size passes both checks?
It happens on long, high-current runs more often than people expect - the load needs a bigger cable for volt drop than for capacity, right past the largest size in the checked range. At that point the practical options are usually reducing the run length, splitting the load, or stepping up to a larger cable than this tool's default range covers.
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Once the cable's sized, CertMate handles the paperwork - full electrical test sheets and CoCs, generated on site.
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