An EV charger is an unusual domestic load. It draws its full rated current for hours at a time, often overnight. The garage it feeds sits at the far end of the house. Both of those facts push the cable size up.
The question
A 7 kW wall charger sits in a detached garage. It draws 32 A at 230 V on a dedicated circuit. The cable runs 30 m from the switchboard, enclosed in air, at a 30 °C ambient. A 32 A curve C breaker protects it, and the external loop impedance at the board is 0.35 Ω. The design needs the smallest conductor that passes capacity, drop, device rating and loop impedance.
The inputs
- Phases: 1
- Voltage: 230 V
- Design current: 32 A
- Route length: 30 m
- Installation method: enclosed in air
- Ambient temperature: 30 °C
- Protective device: 32 A, curve C
- External loop impedance Ze: 0.35 Ω
The clause
Cl 7.9.3 of AS/NZS 3000:2018 covers electric vehicle charging installations. It sets the requirements for the supply arrangement and the additional protection at the charging point. Cl 3.4 sets the current-carrying capacity requirement and Cl 3.6.2 the voltage drop limit. Ratings come from AS/NZS 3008.1.2:2017 Tables 4 to 21.
The steps
At 30 °C, enclosed in air, with no other circuit on the route, no derating applies. The ambient factor is 1 and the grouping factor is 1. The total derating factor is 1.
The required tabulated current is therefore the full 32 A. The tool then tests each standard size against all four checks.
4 mm² carries a tabulated 29 A on this method. That is below the 32 A the charger draws, so it fails on capacity. Its voltage drop of 4.25 % would have passed, and its loop impedance of 0.79 Ω would have passed. The capacity check alone stops it.
6 mm² carries a tabulated 38 A, which clears 32 A with room to spare.
The result
The answer is 6 mm², with a 2.5 mm² earth conductor.
- Derated capacity: 38 A
- Voltage drop: 2.85 %
- Device rating: 32 A
- Earth fault loop impedance: 0.73 Ω
Every check passes. The tightest of them is the voltage drop at 2.85 %, and that still leaves headroom against the default limit.
Why 30 m changes the shape of the job
Drop is the check that moves fastest with length on this circuit. At 30 m the 6 mm² cable sits at 2.85 %. Push the same cable to a garage at the back of a long section. The drop climbs in proportion. Run the length up in the tool before committing to the cable on the van.
The continuous nature of the load matters too. A charger holds 32 A for several hours, so the conductor reaches its steady temperature and stays there. A cooktop or a shower never does. Treat the full 32 A as the design current with no diversity on a dedicated charging circuit.
Loop impedance is the check to watch on a long run with a small earth. Here Zs lands at 0.73 Ω with a 0.35 Ω external figure. A site with a higher Ze, or a longer run, can fail that check while the capacity check still passes. Measure Ze rather than assuming the default.
For the additional protection at the charging point, read Cl 7.9.3 with the charger manufacturer's instructions. Some chargers include DC fault detection internally and change what the upstream device has to provide. The clause number is the place to start, not the product brochure.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Cable size for a 32 A single‑phase 25 m run, Voltage drop with the mV/A·m method and Current-carrying capacity and derating.