3Kw Immersion Heater circuit using trunking

olsonn · Nov 17, 2011

Made the fatal mistake of offering to do a friend a favour which always comes back to bite you. Its a one bedroom flat with concrete floor and ceiling and only one ring final circuit for sockets. Wants panel heaters fitting in the hall 1.25Kw and bathroom 0.75 kw so i said i would run a new circuit to accomodate this using 2.5mm T&E in surface mounting mini trunking. At the same time I was also going to put in another circuit to take the 3Kw immersion heater which currently resides on the socket circuit. I had hoped to run this circuit in the same trunking as the other new circuit using 2.5mm T&E as it is such a bugger to drill through the concrete ceiling but having done a few calculations it looks like if I use the same trunking I would have to up the cable size to at least 4mm (the run is about 12m). Is this correct? Another sparky seems to think that there is a reg which says that cable for an immersion heater has to be in trunking by itself, but I couldn't find this anywhere in the regs

MarkieSparkie · Nov 17, 2011

The space heating and immersion heater need to be on separate final circuits as you propose but the cables can be run in the same trunking, your sparky mate is wrong.
The two cables in the same trunking, installation methods 8&9 (reference method B), have a de-rating factor of 0.8 for the cable grouping. 2.5mm[SUP]2[/SUP] T&E would still be rated at 18.4A (Table 4D2A, 23Ax0.8) assuming you do not have any other de-rating factors, so would appear to be suitable for both final circuits.

olsonn · Nov 17, 2011

Thanks Markie that will solve the trunking problem, just need to check now if I need to up the cable size for the immersion final circuit

flyingsparky · Nov 17, 2011

2.5 t&e on 16a will be fine.

MarkieSparkie · Nov 17, 2011

olsonn said:
Thanks Markie that will solve the trunking problem, just need to check now if I need to up the cable size for the immersion final circuit

See my edited post #2.