Why is my 4mm cable getting hot when running my EV charger at full power?
Quick answer
Your 4mm cable is getting hot because it is undersized for the continuous load of a full-power EV charger. A 7.4kW EV charger draws around 32 amps continuously, and 4mm twin and earth cable is typically rated to carry only 27-32 amps depending on its installation method and any derating factors. Overloaded cables are a serious fire risk. Stop using the charger at full power immediately and have a qualified electrician assess the installation. The cable almost certainly needs replacing with 6mm or 10mm before the charger is used again.
🏠Homeowner view
Finding that your EV charger cable is getting warm or hot to the touch is a warning sign you should not ignore. Heat in a cable means it is working harder than it should, and in the worst cases this can lead to insulation damage, a fire inside the wall or conduit, or a dangerous fault developing over time.
The most common reason this happens with EV chargers is that the installer used 4mm twin and earth cable when the load actually required a larger size. A 7.4kW home EV charger running at full power draws around 32 amps. That is a continuous load, meaning the cable carries that current for potentially many hours every time you charge. Most 4mm cable in a typical domestic installation can only reliably carry somewhere between 25 and 32 amps once you account for how it is installed, whether it runs through insulation, and how long the cable run is.
Other things that can make the problem worse include:
A long cable run from your consumer unit to the charger, which increases resistance and heat.
The cable being clipped inside a wall cavity with insulation around it, which traps heat instead of letting it escape.
The cable passing through a loft space that already gets warm in summer.
Poor or loose connections at either end, which create extra resistance exactly where heat is least welcome.
What you can do safely right now:
Reduce the charge rate in your EV charger app or settings to 16 amps or lower as a temporary measure.
Check whether the cable feels hot along its entire length or just at one end, and let your electrician know.
Do not leave the charger unattended on a full charge schedule overnight until the issue is resolved.
Do not open any consumer unit, junction box, or charger enclosure yourself.
This is not a problem you should try to diagnose or fix yourself. Call a Part P registered or OZEV-authorised electrician who works with EV charge point installations. They will measure the cable's current-carrying capacity against the actual load, check the installation method, and advise whether the cable needs to be replaced or re-routed.