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Twin immersion heaters

Former Community Member
Former Community Member
I am in the process of going unvented.  I have to use immersion heaters, and the new cylinder has two.  I have one feed to the existing immersion.  It is on 2.5mm2 T&E and connects to a 32A MCB on the CU.  I'm adding a DP isolating switch in the cylinder cupboard.  Will that be complaint for one of the immersion heaters?

To fully install the cylinder (ie the second immersion), I plan ask an electrician to either
(1) run an additional 2.5mm2 T&E cable back to the CU, connect it to the existing 32A immersion heater MCB at the CU. So the MCB protects two separate cables and the 2.5 T&E is within its capacities. 

Or
(2) run a new 6mm2 T&E cable back to the CU, connect to the existing MCB and in the cylinder cupboard, split the feed to two DP isolators, one for each immersion and its timer.


I'd be grateful for advice - are both approaches compliant?  Is one better than the other?  The amount of work will be similar.
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  • 0
    Grahams fault sounds very unlikely to me as the element is very tightly packed around with aluminium oxide or similar like pyro. I suppose if one split and the packing washed out there could be a contact to the case but it sounds very unlikely. In principle, the same could happen to cooker and oven elements, but I have never seen one which failed any way except open circuit. The position of such a fault in an element would have to be just right too, to take a dangerously high current and yet not trip the OPD. With a 2.5mm cable in the usual household installation methods, the current would have to be about 40A, and even after a very long time the cable might get to 100C, unless completely surrounded by insulation, Any more current would trip the OPD at some point (all using a B32). The risk is very minimal. The two thermostats should still keep the water from boiling, and such a connection would probably not last long if wet. With a B16, there is only any risk from 1.5mm cable, again very small.
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  • 0
    Grahams fault sounds very unlikely to me as the element is very tightly packed around with aluminium oxide or similar like pyro. I suppose if one split and the packing washed out there could be a contact to the case but it sounds very unlikely. In principle, the same could happen to cooker and oven elements, but I have never seen one which failed any way except open circuit. The position of such a fault in an element would have to be just right too, to take a dangerously high current and yet not trip the OPD. With a 2.5mm cable in the usual household installation methods, the current would have to be about 40A, and even after a very long time the cable might get to 100C, unless completely surrounded by insulation, Any more current would trip the OPD at some point (all using a B32). The risk is very minimal. The two thermostats should still keep the water from boiling, and such a connection would probably not last long if wet. With a B16, there is only any risk from 1.5mm cable, again very small.
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