Electrical wiring immersion

To do the job properly you need to wire two separate 20amp circuits using 2.5mm cable from each 20amp double pole switch (one for top element and one for bottom element) all the way back to the consumer unit.

Hi An immersion heater must not be supplied by a ring final circuit. Anything rated over 2KW should have its own dedicated circuit.

Whilst I agree partially with the above answers, Yes the installation does require two designated circuits but you cannot just assume that 2.5mm cable will be of sufficient size ! A detailed cable calculation should be carried out by a qualified engineer. In laymans terms, various things effect the carrying capacity of any given cable : Type of protective device fitted, I.E. RCD/MCB, HRC fuse , rewireable fuse. Also the method of installation of the cable I.E. buried in plaster / enclosed in conduit / clipped direct to building material. Also relevant as the installation is in a loft-space, the effect of any thermal insulation material, (has the cable or will it be covered in loft insulation). Finally Volt drop, The longer / smaller diameter of the cable then the more the voltage will drop and the more the current will rise in proportion. All the above can have a dramatic effect on ability of the selected cable to withstand the expected current demand. In conclusion, I feel to just say use 2.5mm is woefully misguided, it may be the case many times but certainly not in all !!! Adrian, southsea plumbing & heating