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.

 

[James]

9Kw max on a 40A supply in my book.
customer needs a new cable or a new shower.
Alternatively, a "mate down the pub" to install what he has got instead of a professional electrician.

We all know it would work, but you cant document it or defend yourself it it all goes wrong.

 

[davesparks]

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.

It will almost certain ly be 10.5kW at 240V, so at the UK nominal 230V that will be 9.6kW which works out at 42A so still non-compliant.

230V is not a declared voltage, it is the nominal voltage.

 

[davesparks]

10,500/40 = 262.5V

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

Power is not constant so that calculation doesn't work, it is the resistance which remains constant.

 

[brianmoooore]

Resistance works out as 5.486 ohms, which gives 41.9A at 230V. Now factor in a compliant 5% voltage drop to the shower and we're down to 218.5V, dropping the current to 39.8A.
I'm in no way suggesting that a 40A breaker should be used with a 10.5kW shower, but just illustrating that 1) the 10.5kW figure is just advertising for the consumption of Joe Public, who just thinks that bigger is better, and 2) there's a lot more to the calculation than appears at first sight.

 

[davesparks]

Resistance works out as 5.486 ohms, which gives 41.9A at 230V. Now factor in a compliant 5% voltage drop to the shower and we're down to 218.5V, dropping the current to 39.8A.
I'm in no way suggesting that a 40A breaker should be used with a 10.5kW shower, but just illustrating that 1) the 10.5kW figure is just advertising for the consumption of Joe Public, who just thinks that bigger is better, and 2) there's a lot more to the calculation than appears at first sight.

Yes, and the nominal voltage has a tolerance of -6%\+10%

Plus of course the vast majority of substations are still outputting the same 433V/250V that they have been since the national grid was created.

There's no need to over-complicate this, it's 10.5kW at 240V so work out the current from that and design the circuit accordingly.