Shower on 4mm ring final circuit.

[Bobble23]

Hi,

I've got a customer who wants a 10.5 kW shower installed in his bathroom. As it happens he already has two unused 4mm T&E cables run from the CU cupboard to the loft. I know it's unconventional but is there any reason I can't use these cables to make a dedicated RFC for just the shower?

Thanks.

 

[davesparks]

That's not a ring circuit, that would be 2 cables in parallel.
There's an appendix in the regs which sets out the additional requirements for cables in parallel and correct protection of them.

Are you certain that these 2 cables are the same length and follow the exact same route?

 

[Dustydazzler]

When you say 'Un-fused' what is protecting this 4mm cable ? are you sure these cables haven't been ear marked for something else like future solar PV install

 

[Bobble23]

Thanks Dave, thanks Dusty.

I thought I said un-used, not unfused - sorry of there was a typo. They were run a while ago as future-proofing for a loft conversion that didn't happen (I've no idea why he ran two of them, but yes they run along exactly the same route). If i was to use them there's space for a 40A RCBO in the board.

Thanks for the heads up about the appendix in the regs. I'll dig it out and have a look. Suppose my main question is whether it's OK to double up the current carrying capacity of a circuit by using two cables instead of one thicker one.

 

[davesparks]

Suppose my main question is whether it's OK to double up the current carrying capacity of a circuit by using two cables instead of one thicker one.

It's perfectly normal for higher current circuits but very unusual to do it for a domestic installation.
You will have to make the assessment as to whether this is a safe and acceptable solution for this installation.

 

[Risteard]

Thanks Dave, thanks Dusty.

I thought I said un-used, not unfused - sorry of there was a typo. They were run a while ago as future-proofing for a loft conversion that didn't happen (I've no idea why he ran two of them, but yes they run along exactly the same route). If i was to use them there's space for a 40A RCBO in the board.

Thanks for the heads up about the appendix in the regs. I'll dig it out and have a look. Suppose my main question is whether it's OK to double up the current carrying capacity of a circuit by using two cables instead of one thicker one.

10.5kW exceeds 40A.

 

[Dustydazzler]

Yes its a tad unconventional in a domestic setting to have parella cables and yes 10.5kw is a snidge over 40a, but in reality your set up will work and do the job

 

[davesparks]

but in reality your set up will work and do the job

And be non-compliant

 

[brianmoooore]

And be non-compliant

Not necessarily. The rated power of an electric shower is at a stated voltage, and is for comparison/marketing purposes. If it can be shown that the available supply voltage is sufficiently less than that stated voltage* to reduce the current to or below 40A, then it will be compliant.

*Especially if the stated voltage is 240 rather than the UK declared voltage of 230.

 

[nicebutdim]

Not necessarily. The rated power of an electric shower is at a stated voltage, and is for comparison/marketing purposes. If it can be shown that the available supply voltage is sufficiently less than that stated voltage* to reduce the current to or below 40A, then it will be compliant.

*Especially if the stated voltage is 240 rather than the UK declared voltage of 230.

10,500/40 = 262.5V

I don't see how a 10.5kW shower could be considered complaint on a 40A breaker.