Electrical Question

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?

2 Answers

Best Answer

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


Member since 13 Jan 2015

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

AMG Electrical

Member since 4 Aug 2017

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