Electric Shower wiring help

[Ross]

Dear DIYers,

I have recently bought my first house and I am wiring an electric shower (9.5kw) into the bathroom. The house has an en-suite bathroom in an extension next to the main bathroom and I am going to take the existing electric shower circuit from the en-suite and re-use it in the main bathroom. The existing circuit is made up of 10mm twin & earth cable, a 45amp double-pole switch and a 45amp MCB at the consumer unit, the consumer unit also has a RCD device built into it.

I understand that I can only have one shower unit using the circuit and that is not a problem since I have removed the electric shower from the en-suite and chucked it and I intend to put a mixer shower in there later on.

The problem is that the cable (twin & earth) which used to go to the double pole switch in the old bathroom does not reach the position where the new double pole switch is going to be, so I was wondering how I can safely connect some new cable to the old cable in the loft above the bathroom bearing in mind that it is going to be pulling approx 40 amps and if connected incorrectly would get pretty hot. I had thought of using the old double pole switch positioned in the attic and left on which seems safe because I will have a second double pole switch between that and the shower. I do not want to run a new cable back to the consumer unit since I already have a perfectly good one which comes 9/10s of the way!

Any advice will be very much appreciated, thanks in advance,

Ross.

[stefek.zaba]

"Normal" junction boxes are available in ratings up to only 30A, so (as you've already realised) you don't want to be using one of they. Rather than using the DP switch, a "widely available" way to get a

40/45A junction box would be to use a cooker final connection unit - not the gert big thing with a DP switch and optional 13A socket, but the accessory plate which goes on the wall, mounts on a deep galvanised box or (in your application) deep surface-mount box, and expects 6mm or 10mm sq cable both in (in original application, fixed wiring embedded in plaster) and out (in original application, final cable to the cooker which once a year after the marmalade-making season is over gets pulled out from the wall ;-). Mounted in the loft counts as an "accessible" location for fault-finding/repair purposes, assuming you don't board over it later!

HTH - Stefek

[usenet]

A 'Henley box' as used for making a junction to two CUs would also be suitable.

You should also be able to get junction boxes for 10sq mm cable, try an electrical wholesaler rather than a DIY place.

[Grunff]

I'd really be tempted to solder this up. That's what I did when moving our CU recently, and it's by far the neatest and lowest-resistance joint you can make.

[Dave Jones]

As you Joining 10 mm cable you really need to use a connection block of greater than 65 Amps, i.e. same rating as the cable.

Try these

[Ross]

Thanks for everyones advice on this. I called up my local electrical wholesalers (Newey & Eyre) and they recommended the cooker connection option too, so I will go out and get one of those later on today. I am a bit nervous about the earthing though. The hot & cold pipes are cross-bonded at the base of the bath and I routed a cold supply pipe under the bath and inside the opposite wall where the shower will be, my understanding is that I just need to connect the earth terminal in the shower to an earthing clamp on the new supply pipe. Is that alright? Or am I going to cook myself (or the bird) when I first have a shower? Also I had to use a little PTFE tape on some of the compression joints because of some pesky leaks (under the bath - nightmare), is that going to break the earth connection? Should I get my hands on a multimeter and do a continuity test?

Thanks in advace,

Ross.

[Ross]

Thanks for everyones advice on this. I called up my local electrical wholesalers (Newey & Eyre) and they recommended the cooker connection option too, so I will go out and get one of those later on today. I am a bit nervous about the earthing though. The hot & cold pipes are cross-bonded at the base of the bath and I routed a cold supply pipe under the bath and inside the opposite wall where the shower will be, my understanding is that I just need to connect the earth terminal in the shower to an earthing clamp on the new supply pipe. Is that alright? Or am I going to cook myself (or the bird) when I first have a shower? Also I had to use a little PTFE tape on some of the compression joints because of some pesky leaks (under the bath - nightmare), is that going to break the earth connection? Should I get my hands on a multimeter and do a continuity test?

Thanks in advace,

Ross.

[stefek.zaba]

Really? I Think You'll Find that sizing according to the overcurrent device - a 45A presumably-type-B MCB, in this case - is the relevant design procedure. One could plausibly argue that the lower load current of 40A (9.6kW/240V - yes, 240V not 230V since the 230 figure is a regulatory fiction and should the supply drop to 230V in some distant future the load drawn by the heater elements will also drop) for the fixed-equipment shower, which has no way of producing an overload, could be used as the circuit design current: but that's a bit of margin-shaving in which the sensible person would not indulge. But sizing for a 65A current throughout, when the 10mmsq cable has been selected to give a decent margin for voltage-drop/earth-loop-impedance/passing-thru-thermal- insulation or whatever, rather than the "might just do it on a good day with the wind in the right direction" use of 6mmsq, is over-egging in the other direction.

Stefek

[Lurch]

On 16 Aug 2004 06:01:39 -0700, [e-mail] (Ross) strung together this:

You want one of these.

[Dave Jones]

My understanding was that if you have a any cable which requires a join, that join should be of equivalent rating to the cable. Other wise if Ross sells up in 2 years, job bloggs moves in, sees 10mm in consumer unit, upgrades the fuse and installs a 12kw shower, not knowing about the under rated junction box in the loft. Get's up in the morn, he has shower, wife has shower, kids have shower, come home house burned down!

Dave

[stefek.zaba]

While it's good belt-n-braces practice to follow the guideline you suggest, it's not required: it's the responsibility of each "joe bloggs" making new use of existing cabling to satisfy themselves as to its length, routing, condition, and so on. It's not using a 45A-rated connector in a cable feeding a non-overloading design load drawing 40A which is the negligent action: it's seeing a 9k6 shower on the end of a 10mmsq cable and blithely sticking on a 12k unit. No-one reusing the 10mmsq cable can know without inspection whether it was put in at that size because (at one end of the possibilities, meaning simply well overrated for the job) it's what the sparks had on the van at the time, having just run fresh out of 6mmsq and not having time to pop down the road to WF before the end of the day, or (at the other end of the spectrum, where 6mmsq would've been well underspec and even 10mmsq is only just OK) because 200mm of it runs through thermal insulation (through the insulation in IMM's loft, maybe? ;-) and its capacity is therefore reduced to 43A.

Stefek