Why cable size isn't a fixed number
There's no single correct cable size for a 7kW EV charger, despite what a quick search might suggest. The right size depends on your specific installation, primarily how far the cable needs to run from your switchboard to the charger. A longer run needs a larger cable to carry the same current without excessive voltage drop, even though the charger's rating hasn't changed.
The two things that actually determine it
Current rating comes first, a 7kW single-phase charger draws roughly 32 amps, which sets the minimum starting point for cable size. Distance comes second, and it's the variable that actually changes the answer property to property. AS/NZS 3008 sets the standards electricians calculate against to keep voltage drop within safe limits over that specific distance, which is why the same charger can need different cable sizes at two different homes.
What happens if the cable's undersized
Undersized cable for the distance involved causes excessive voltage drop, meaning the charger doesn't receive full, stable voltage, which can affect charging performance and, over time, cause the cable itself to run hotter than it should under sustained load. This is a genuine fire risk over the years the circuit's in use, not just a performance inconvenience. This is also why a cheap, rushed EV charger installation quote should raise questions, if the cable size hasn't been discussed or calculated at all, there's a real chance it's been guessed at rather than properly worked out.
Why installation method matters too
The way the cable's installed, whether it's run through open air in a roof space, enclosed in conduit, or buried underground, changes how well it dissipates heat under sustained load, which is another factor in the sizing calculation. A cable that's perfectly sized for one installation method can be undersized for another over the same distance, which is why the whole route needs to be considered together, not just the total length. See our guide on running a dedicated circuit to a garage for how different routes affect this.
Why this is a calculation, not a guess
A properly calculated cable size accounts for the charger's actual rating, the exact length of the run, and the installation method, whether it's in conduit, buried, or run through a roof space, since each affects how the cable performs under sustained load. This is why the electrician needs to walk the actual route before finalising cable size, not just quote based on the charger's kW rating alone.