Replacing an electric shower questions

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Hi all, been sifting through the pages here and seems like an excellent website.

Anyhow, I'm about to change the shower and have a few questions.

We've recently moved here and the old shower is pretty rubbish, plus we want to move it. It used to be in a cubicle but I threw that out and am going to put in the new shower above the bath.

I've cut a single channel in the concrete and plaster wall to take the cable and flexi-pipe for where the shower will be mounted (This will be tiled over - around half a metre at most - the rest of the cable is in open air). Currently the existing shower is an 8.5kW unit which is wired using what looks like 5mm cable? (could be 4 or 6? although I don't think it looks quite heavy enough to be 6). There are no markings on the cable, hence I am estimating. The cable is wired to a 40 amp breaker in an old Dorman fuseboard. This cable then runs 9 metres directly to the shower unit! The unit works and I've checked bothe ends of the cable and all is fine.

What I want to replace this with is this:
Cable from 40 Amp breaker to remote box containing RCD. Cable from RCD up to bathroom and wire to isolating pull-switch (rated at 50 Amps). Then cabe from pull-switch and terminate at shower which is 9.5kW.

Question 1 - Will the existing 40 Amp breaker be alright with the new set up?

Qustion 2 - Can I re-use the existing cable? I read on this site that as the length is under 13 metres 4mm is okay?

Question 3 - Is there any way of telling exactly what cable I have? As there are no markings I'm guessing not? (existing T&E - 7 strands measuring 0.7mm each and the earth measures 1.3mm - don't know if this is any help?)

Question 4 - is there anything else I should be aware of?
 
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Niall76 said:
Question 1 - Will the existing 40 Amp breaker be alright with the new set up?
9500/240 = 39.6A so the breaker is fine, but the question is is the cable adequate

Qustion 2 - Can I re-use the existing cable? I read on this site that as the length is under 13 metres 4mm is okay?
Ah, there is a lot more to sizing cables than just volt drop, you have got to pick the correct referenece method, and then apply factors for ambient temperature, cable grouping and additional insulation, then you have to check the Z's will be ok... it is very unlikely that 4mm² cable will be good for more than 32A in most situations in a house and may well be quite a bit less

Question 3 - Is there any way of telling exactly what cable I have? As there are no markings I'm guessing not? (existing T&E - 7 strands measuring 0.7mm each and the earth measures 1.3mm - don't know if this is any help?)

The mm² of cable is the cross sectional area of conductors, so quick calculation from lower high school maths:

(0.7/2)² x pi x x 7 = 2.7mm²

Ah we have a problem here, its come up as 2.7mm² which isn't a proper cable size, the nearest is 2.5mm² which isn't stranded in T&E (but is in conduit singles) so what we have here is the old imperial equivelent of 2.5mm² ... 7/.029 (thats seven stands of .029 inch diameter[0.74mm]) so what you have is not only grossly undersized, it is also a fair few years old... can you still get 5kw showers? :LOL: ... rip it out in re-cable in 10mm² T&E ;)


Question 4 - is there anything else I should be aware of?
Part P of the building regs?
 
Niall76 said:
Question 1 - Will the existing 40 Amp breaker be alright with the new set up?
9500/240 = 39.6A so a 40A MCB is adequate.
Qustion 2 - Can I re-use the existing cable? I read on this site that as the length is under 13 metres 4mm is okay?
4mm is too small for use on a 40A breaker. 6mm at a push depending on how it's installed, I'd recommend 10mm as a minimum if you are replacing it, then again this also depends on how its installed.
Question 3 - Is there any way of telling exactly what cable I have? As there are no markings I'm guessing not? (existing T&E - 7 strands measuring 0.7mm each and the earth measures 1.3mm - don't know if this is any help?)
//www.diynot.com/forums/viewtopic.php?t=7553&start=40
Question 4 - is there anything else I should be aware of?
Part P of the building regs as replacing a cable with one going a different route and of a different current carrying capacity is notifiable (daft imo though as we are talking larger!!) & supplementary bonding are a couple of issues which spring to mind.


Bah, beaten by adam :LOL:
 
Thanks guys. I'll pick up 10mm cable tomorrow. Good news that the breaker is suitable though. The cable is taking exactly the same route - just the end point is 1 metre to the side of where it is now.

Supplementary bonding? I'm not sure what this is?

The way I saw it was that as I'm uprating the cable, installing an RCD and installing a pull switch the whole set up is a lot safer. And of course correct.

I shall browse the pages to see if I can find out about supplementary bonding. Although a laymans terms explanation would be much appreciated if anyone has the time to explain.

Many thanks for your help guys - very much appreciated.
 
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Supplementary equipotential bonding is the bonding together of all extraneous conductive parts in a bathroom such as metal water pipes, central heatingpipes, building structure and bonding across to the CPC of circuit supplying class 1 or 2 equipment in zones 1,2,3 of the bathroom. In practice this normally means cross bonding the light ckt, shaver socket ckt (if not on light ckt), shower ckt, electric heater ckt, hot water pipe, cold water pipe, central heating pipes etc. This may be a good time to point out that as you don't understand it is important to get a spark in to advise you on this. If you are missing supplementary bonding there is a good chance your main bonding will be missing or undersized.
 

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