I am going to fit a new electric shower, 8.5 or 9kw, not bought it yet. I am going to use 10mm T&E cable. Do I have to use 10mm earth cable as well or would 6mm earth cable be ok.
The earth cable would just be for the shower. The earth on the 10mm T&E cable is not the same size as the live & neutral cores. The rest of the bathroom is already bonded.
The earth in the T&E cable is sufficient without any addition.
This is because the 10.0mm² conductors will be carrying the 40A+ which your shower requires for a sustained period.
The earth will only carry a current in the event of a fault, and at the worst the fault will clear in a maximum of 5 seconds, (although is usually far quicker than this) so a smaller CPC (earth) is sufficient.
I know not quite why, but in the 70's and 80's (and obviously still today!) it was often found that a 6/10mm² earth would be run in along with the t&e for the shower.
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