Electric Shower Cable question

[Bing]

Hi all.

I have just purchased a 10.5kW to replace my broken 8.5kW Shower. Whilst removing the unit I noticed that there was a discrepancy in the supply wiring. I was expecting to find 10mm square L, N & E but found a Twin & Earth, L & N are both 6mm square with a single strand 1.7mm Earth. Then there is a separate 10mm square 7 strand Earth wire. So the main terminal block is as follows L = 6mm, N = 6mm E = 10mm & on the other side of the terminal block going to the Element is L = 6mm, N = 6mm E = 1.7mm single strand. The Circuit is protected with a 40A MCB.
Basically, what I want clarifying is would this be a safe installation if I exchanged it for 8.5kW similar to what I am removing?
Your thoughts please.

 

[riveralt]

The manufacturers instructions will give the specifics but for the shower you have described you will need a 10mm² T&E cable probably to a 45A MCB and additionally protected by an RCD.

 

[PrenticeBoyofDerry]

Basically, what I want clarifying is would this be a safe installation if I exchanged it for 8.5kW similar to what I am removing?
Your thoughts please.

It would be no more dangerous!
As far as safety is concerned that would depend on whether the existing circuit is safe for continued service.
But a better option than replacing with a higher output than the circuit can deal with.
The MI would recommend 30mA RCD protection, so that would be worth considering if not existing.

 

[Bing]

Thanks for your thoughts guys. My Consumer unit is protected with a 30mA trip. I would have liked to have kept the 10.5kW & replaced the wiring for 10mm but I don't think I could pull it through, it appears to be quite tight @ 6mm. I don't want to have to break the Tiles in the Bathroom.
But just to be clear, are you saying 6mm would be fine for 8.5kW?
Cheers.

 

[PrenticeBoyofDerry]

8.5 or even 9kW should be okay, but being remote of your installtion I would be taking that on assumptions, as I do not know the route of your cable nor the method it has been installed, as this can effect the current carrying capacity. Once you get over 9kW I would personally recommend upgrading cable size.

 

[Bing]

Thanks Prenticeboy I appreciate your advise.
You know what its like, you start a simple replacement job assuming everything was correct before it failed, until you spot something & start asking questions. Once I take responsibility for changing it over, I have a duty of care to my family or anybody else who might use the Shower to ensure it is done correctly.
Thank you.

Bing

 

[EFLImpudence]

10500/240 = 43.75A

Maximum Current Carrying Capacity of 6mm² cable if clipped direct or in a non-insulated wall is 47A.

 

[PrenticeBoyofDerry]

The problem being the availability of 45A MCBS, and we wouldn't want to have 50 amper on a cable that is only designed to carry a max of 47A.
Then if we look in to ref methods of cable routed within floors, it could well be ref meth B, which would produce a derating.

 

[EFLImpudence]

The problem being the availability of 45A MCBS,

True, but they may be available for the OP's.

and we wouldn't want to have 50 amper on a cable that is only designed to carry a max of 47A.

433.3.1(ii)

Then if we look in to ref methods of cable routed within floors, it could well be ref meth B, which would produce a derating.

True again but I did write "if clipped direct or in a non-insulated wall" which it may be.

 

[Bing]

A big thank you to all who have posted replies.
A new 8.5kW shower is now up & running & I feel confident that it is a good installation which isn't pushing any load boundaries.
Yes the 10.5kW would have been nice. But it wasn't worth trying to replace the supply wire & MCB.
Cheers guys.

Bing.