Mains & Submain Cable Selection Calculator
Find the smallest compliant cable for consumer mains or a submain.
This cable's share of the 5% total allowed from the point of supply (Clause 3.6.2). Mains, submain and final subcircuit drops all add up against that one 5%.
NZ domestic: AS/NZS 3000 Clause 3.4.1 requires wiring to assume ceiling, wall and underfloor insulation will be installed in the future. Rate cable in those spaces as at least partially surrounded by thermal insulation, and completely surrounded where it passes through more than 400 mm of insulation (Clause 3.3.2.13).
Multicore 4 to 10 mm² is rated as PVC (75°C); 16 mm² and up as XLPE (90°C). 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 runs (consumer mains from the meter box to a main board close by) tend to be decided by current-carrying capacity. Long runs at moderate current (a submain across a section to a garage, shed or sleepout) tend to be decided by volt drop instead. This calculator checks both and gives you the smallest size that clears both bars.
What this calculator covers
This calculator sizes consumer mains and submains, including mains on a 32A supply. It recommends multicore copper from 4 mm² up, neutral screen copper from 10 mm² up, single-core copper from 16 mm² up, and aluminium from 70 mm² up. Multicore and neutral screen up to 10 mm² is rated as PVC insulated (75°C), which is what small submains and internal runs usually are. Everything from 16 mm² up is rated as XLPE insulated (90°C). Each result is labelled PVC or XLPE. Because multicore goes down to 4 mm², it also suits larger dedicated circuits, like a 7 kW EV charger on 32A. Each result shows the cable's current-carrying capacity for your installation method and its approximate conductor operating temperature at your load, so you can see how hard the cable is working, not just whether it passes. For everyday final subcircuits like lighting and sockets, the Voltage Drop calculator will check a chosen cable size against the run.
The volt drop box is the share of the 5% total that you're giving to this one cable. AS/NZS 3000 Clause 3.6.2 allows 5% from the point of supply to any point in the installation, so the mains, any submain and the final subcircuit all add up against that one 5%. The 2.5% default is a common design split, not a separate limit.
AS/NZS 3000 also notes that the size of unprotected consumer mains should be coordinated with the electricity distributor (Table 3.3, Note), so check your local network's requirements for mains.
Worked examples
Consumer mains on a 60A supply, short run
Mains from the meter box to a main board a few metres away, on a 60A fuse. A short run like this is usually decided by current-carrying capacity, so the installation method matters most. If any part of the run goes through a ceiling or wall space in a house, rate it as at least partially surrounded by thermal insulation (Clause 3.4.1), not as open air.
Submain buried to a sleepout, long run
A 32A submain, buried direct, running 30m to a sleepout at the back of a section. Burial isn't the problem here: cable buried direct is rated the same as or higher than the same cable in open air, so current-carrying capacity is easy to meet. The length is what decides it. Volt drop grows with every metre, which is why a buried run to an outbuilding often needs a bigger cable than the load current alone would suggest.
Three-phase workshop submain, balanced or unbalanced
A three-phase submain at 32A per phase, run 40m in underground duct to a workshop board. If the board feeds a mix of single-phase circuits (lights, sockets, a heat pump), the load is unbalanced: the busiest phase behaves like a single-phase circuit and volt drop is checked against 230 V, not 400 V. Run the same job as balanced and unbalanced and you'll see the unbalanced case can need a bigger cable. Only choose balanced when the load really is balanced, such as a three-phase motor.
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?
Heat is what limits a cable's current rating, so the rating depends on how easily the cable can shed heat where it's installed. Thermal insulation is the harshest condition: a cable surrounded by it has to be rated well below the same cable in open air. Direct burial works the other way. Soil draws heat away well and sits cooler than air, so a cable buried direct is rated the same as or higher than the same cable in open air across the common sizes (AS/NZS 3000 Table C6 shows this, and Note 3(a) treats direct buried cable as enclosed in ground). That's why the method you pick changes which size comes out as compliant.
Which installation method should I use in a house?
In New Zealand, AS/NZS 3000 Clause 3.4.1 requires wiring in domestic installations to be installed on the assumption that thermal insulation in ceilings, walls and under floors will be installed in the future, even if it isn't there yet. In practice that means rating cable in those spaces as at least partially surrounded by thermal insulation (Note 3(c) to Tables C6 and C7 treats cable lying on insulation this way), and as completely surrounded where it passes through more than 400 mm of insulation (Clause 3.3.2.13).
Should I choose balanced or unbalanced for a three-phase mains or submain?
Balanced means the load draws the same current on all three phases, like a three-phase motor. Volt drop is then measured line to line against 400 V, and the neutral carries little or no current. Unbalanced means the phases are loaded differently, which is normal for a board feeding a mix of single-phase circuits. The neutral carries current, and the most heavily loaded phase behaves like a single-phase circuit, so volt drop is checked on that phase against 230 V. For the same load and run, an unbalanced load can need a bigger cable, so pick unbalanced unless the load really is balanced.
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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