I am installing a 8.5 kW shower as an upgrade to a 8.0 kW shower. I am upgrading the MCB from 32 to 40A. I have 6mm cable supplying the 8.0 kW shower, would it be ok for the 8.5 kW shower ?
Cable route? Does it pass through an thermal insulation? Is it in trunking or conduit for any part of its length? Length of run? Will it have 30mA RCD protection?
The consumer unit is in the downstairs WC at the front of the house and the shower is in the en suite on the 2nd floor at the back of the house ( 3 storey townhouse) The shower cable length is approximately 12 metres and it passes the insulated loft and down to the shower unit. The cable will be in between plasterboard walls. It does have 30 mA RCD protection. The upgrade is only 500 watts ( an extra 2 Amps or so) so I would be surprised if I needed to replace the wiring.
Reference method 101 - above a plasterboard ceiling covered by thermal insulation exceeding 100mm in thickness, 6mm t&e has a tabulated current carrying capacity of 27A. If insulation is less than 100mm then it's reference method 100 meaning 6mm t&e will carry 34A.
I make 8.5kw 35A at 240 volts, the 32A mcb might be okay as its a small overload for only as long as you take a shower. Having said that you shouldnt really run a circuit like this.
i wouldnt upgrade to a 40A mcb before checking that the 6mm cable is good for this (40A with 6mm is very borderline, oh and you also mention thermal insulation).
my guess, without knowing all the facts is that you ought to be upgrading the wiring to 10mm really.
You may be only adding a couple of amps, but the existing (8kW) shower was already overloading the circuit design, so 8.5 kW just makes things worse. Unfortunately it looks like you need 10mm cable since you are running through walls and insulated loft spaces. As GaryMo says, 6mm run in this way will only safely carry 34A which is not enough.
Or 27A if there's more than 100mm of insulation. I'm guessing a 3 storey townhouse would be a recent(ish) build and will have 270mm of loft insulation.
I assumed that I have 6mm cable because the overall diameter of the cable is 13mm. I have measured the diameter of the live cable as between 4 to 5 mm . The neutral is the same. Both live and neutral have 7 strands. The earth only has only one strand. They are 40 homes on my estate which are wired the same and I am surprised they havent burnt down with what you guys are saying.
We have never had any problems with the shower and the MCB has never tripped.
I assumed that I have 6mm cable because the overall diameter of the cable is 13mm. I have measured the diameter of the live cable as between 4 to 5 mm . The neutral is the same. Both live and neutral have 7 strands. The earth only has only one strand. They are 40 homes on my estate which are wired the same and I am surprised they havent burnt down with what you guys are saying.
We have never had any problems with the shower and the MCB has never tripped.
As this work requires an Electrical Installation Certificate, I would suggest that BS7671 is adhered to regarding csa calculations and not ignored, as TopTrumper is suggesting.
As this work requires an Electrical Installation Certificate, I would suggest that BS7671 is adhered to regarding csa calculations and not ignored, as TopTrumper is suggesting.
As the other SS says, quite rightly, 8kW is already exceeding the capacity of the MCB and quite possibly (from what has been written) the cable too. It may only be 500W, but that is an increase on what is already an overload situation.
You are increasing the load on an existing circuit & should be making every effort to ensure that every aspect of that circuit complies with the regulations, which means providing a new circuit & consequently, an EIC.
The OP will be changing the cable and the breaker as well as the shower- a complete new circuit, which requires an Installation Cert according to 631.1.
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