MCB for replacement shower

[Dawniechic]

Hi

Can't find the answer to this in my DIY bible. Would be very grateful of your expert knowledge.

My shower has died, and I will head out tomorrow to purchase a replacement. The existing shower is 7kW, but I would like to replace it with maybe an 8.5kW. The existing cable is 10mm and runs aprox 4m to the consumer unit (2m of that is in stud wall with insulation). The existing MCB is B 32, with the main RCD being 80A, 30mA. I believe that for an 8.5kW shower I would need to upgrade to a 40 amp MCB? However, I have a nagging suspicion that I once read the maximum on an 80A RCD is an MCB of 32A.

Could anyone confirm if this is correct? Or if Im being dumb.

Many thanks

 

[hairyben]

40A MCB will be fine.

when you replace it, check the tightness of ALL the busbar connections, sometimes adjacent ones can loosen.

 

[sparkybird]

I would suggest that it would be prudent to test the cable before the work to check that it is in good order. If you don't have the equipment to do this, ask an electrician. It's not a big job and they can change the mcb at the same time.

(Apologies, if you have a tester and know your way around a Consumer Unit)

 

[Dawniechic]

Thank you hairyben for your answer, and handy tip - could save me a lot of time and head scratching!

Sparkybird - is there any reason why the cable would not be sound? Or is it just a precaution? I only installed it just over 2 years ago.

many thanks

 

[GaryMo]

The existing cable is 10mm and runs aprox 4m to the consumer unit (2m of that is in stud wall with insulation). The existing MCB is B 32, with the main RCD being 80A, 30mA. I believe that for an 8.5kW shower I would need to upgrade to a 40 amp MCB?

Method 102# - 10mm twin and earth in a stud wall with thermal insulation with the cable TOUCHING the inner wall surface.

Method 103# - 10mm twin and earth in a stud wall with thermal insulation with the cable NOT TOUCHING the inner wall surface.

If the cable is clipped down the studwork and is not touching either side of the plasterboard then apply method 103#.

Current carrying capacity for 10mm twin and earth:

Method 102# - 47A
Method 103# - 32A

So, the cable needs to be touching either side of the plasterboard to carry the required current for a 8.5kW shower (~37A @ 230v).

All the above assumes that the thermal insualtion has a minimum U value of 0.1 W/m²K

 

[Dawniechic]

Hi Garymo

Thanks very much for your answer.

This throws me a little. Im trying to remember how I ran the cable. Im pretty sure I would have clipped it down the middle of the studwork, but I cant be 100%. Also, I'm sure it will be touching the inner wall surface for a short while where it changed direction to pass through the adjoining brick wall. Does this mean I will need the 47A - and if so, is my RCD OK to take an amp of that size.

Thanks again
Dawn

 

[GaryMo]

When I mentioned inner wall surface, I meant the plasterboard wall.
If the cable is clipped in the middle of the studwork and covered with thermal insulation with a minimum U value of 0.1 W/m²K then you need to use method 103# and a shower rated at less than 32A.

Thermal insulation causes chaos where cables are involved. It's always best to keep the two as far apart as possible.

 

[Dawniechic]

Hi again GaryMo

Yes, sorry, I realised you meant plasterboard - so did I, I meant it will be touching the plasterboard where it changed direction before it went through the brick wall.

Anyway that short amount is irrelevant by the looks of it, if the rest of the cable is clipped to the middle of the stud work - which I think it probably will be. I will be cautious, and replace the shower with another 7kW, and stick with the 32A.

Thanks so much for your help everyone.

Another tip for the future as well, I wont insulate around cables again!