8.5 Kw shower on 6mm tripping MCB

[olsonn]

Went to a job today where a lovely new bathroom has just been fitted upstairs with a new 8.5 Kw shower replacing a 7.2kw one. After a few minutes in the shower the MCB trips. The shower was fully turned up and when I asked the customer they said that they always ran it like this. On inspecting the cable size it was 6mm. The cable is run up an internal concrete wall and appears to have been plastered in and not run in conduit The run is about 12 metres. It also runs through insulation in the loft As a matter of course I would run this size shower in 10mm. It is possible to run a replacement 10 mm cable up from the CU using trunking, but the cable from the switch which is also in 6mm and about 1.5 metres long, is buried behind a lovely expensive tiled wall and doesnt want to come out. Would it be acceptable to leave this short run in 6mm or should I tell the customer that we have to remove the tiles and replace the cable.

 

[JamesBrownLive]

i always thought that increased resistance meant lower current. i agree that problems will arise if cable overheats, but 8.5kw ( manufacturers optimistic rating ) is only 35A. get the candy floss away and 6mm is good for 45A.

You've confused me now Tel...

I always believed that the current drawn would stay the same, but to the detriment of the cable? Will the shower still not draw what it wants to draw, regardless of the whether or not the cable CSA is appropriate? With the consequence being that ultimately the cable would be destroyed if this was way off the mark?

Sorry if that is a stupid question :6:

Cheers