Wiring for instant electric water heater

A 12,000 watt load = 52 amps so depending on the length of cable it would need to be 10mm or even 16mm. The cable will need to be installed in a safe zone and if the cable is to be concealed below plaster depth on the wall and/or under the floor void it will need full RCD protection. This may require you having your existing fusebox upgraded. Also as the work is part P building regulation notifiable, you will need to register the work with building control and pay the relevant fees (approx. £200 - £300). Probably cheaper to hire a part p registered electrician to do the job in the long run.

This is a 50A appliance that would require a 10mm supply. So a lot of other things need to be taken into consideration such as maximum load on your main supply, and RCD protection. A new circuit would require installation and be signed off/certificated by a registered electrician and notified to local authority borough council with a compliance certificate obtained. Kevin.