Ask a Tradesman
Is 6mm cable with a 40 amp mcb sufficient for a 8.5 kw electric shower?
I was told by a BG engineer that my 6mm cable to my 8.5 KW electric shower needs to be replaced by 10mm cable because it runs through wall insulation. I was also told that the 40 amp MCB is too high and needs to be replaced. I have done the calculation of 8500W divided by 240V, I get 35.4 amps so surely a 32amp MCB would be too low and keep tripping. I understand the next one up is the 40 amp MCB which is the one I already have! Is the engineer trying to get a job to replace the 6mm cable when it is not necessary, or is he correct in saying that I need to have a 10mm cable?
You have to divide it by 230 volts which is nearly 37 amps, insulation reduces the current carrying capacity of the cable ( which you haven't taken into consideration) so, without dragging the regs book out and applying the formula I would say he is correct-a 10mm cable on a 40 amp MCB for your current 8.5 kw shower,as the MCB size protects the cable not the shower. the new cable should be installed in ' free air' away from any insulation etc. Bear in mind that a higher powered shower up to 10.8 kw can be installed on this cable in the future and the MCB swapped for a 50 amp.
Answered 4th Aug 2017
6mm twin and earth cable (which this almost certainly is) can be backed up by a maximum of a 32a MCB, even lower if other factors are involved. You'll need that 10mm cable and a 40a MCB unfortunately as your shower is going to draw over 32a. Whoever fitted the 40a MCB onto the 6mm cable originally was in the wrong.
Answered 8th Aug 2017