Cloakroom water heater.

[TicklyT]

I am considering putting a 4.2 KW instantaneous water heater in a cloakroom re-using an existing spur about 4 Metres total from the CU to where the previous 3 KW over basin heater was fitted, and replacing the MCB in the CU.

By my calculation 20 Amp sounds about right for 4.2KW (18.26A), as opposed to the 16 Amp MCB currently fitted in the CU.

I will be replacing the cable, as it will probably be a bit short to reach the position of the new heater under the basin. I don't see much point in joining a cable on such a short run.

Given the installation method (2 Metres in mini trunking + 1 metre through ceiling void), will re-running 2.5 mm cable be adequate, or should I consider running a heavier cable?

 

[Spark123]

Providing the cable is away from thermal insulation, not grouped, ambient temp not over 30ºC, no rewirable fuse etc, 2.5mm is rated for Method 3 @ 23A, over 3m I can't see volt drop being a problem.
Please note that adding a new circuit is notifiable to your LABC under part P of the building regs.

 

[RF Lighting]

I'm not sure what you plan, but you must provide a double pole switch local to the heater.