shower plumbing & electrics

Hi,
My mother would like her electric shower replaced. The hot water header tank is in the airing cupboard, not the loft, so I think the shower is higher than the tank meaning negative head, which may make a pumped shower expensive. Unless anyone knows better?
It may also confuse her having to wait for a tank of hot water rather than shower as and when she likes, so for both reasons we may stick with what she knows and do a like for like swap.
I haven't looked at the cable yet. Hopefully the CSA will be stamped on the side. I had a feeling it was quite meaty but the MCB is only 32A, which limits us to a 7.5kW shower. Is there a reason an electrician would fit 40A capable cable but fit a 32A MCB?
I haven't looked at the old shower to see its rating but it is very, very old. Perhaps it is <7.5kW and since 32A was all it needed, that's all that was fitted? I have seen one 7.7kW shower in the Wickes catalogue but if the MCB could be upgraded that would allow more options.
On my to do list is to see where the cable goes. If it runs under insulation I guess the cable will have to be derated and a bigger MCB may not be allowed.
My next problem is the plumbing. I could run the pipe up or down. I would prefer down. If it goes up, it would go into the loft which is pretty inaccessible and liable to freeze.
If the pipe goes down, do I chase it into the wall or surface mount chrome pipe? The advantage of chasing is it hides the pipe, which appeals to me but perhaps there is an argument for the pipe being accessible?
If I run chrome down the wall, it saves the mess of chasing and making good. I'd prefer not to create more jobs to do! Can chrome pipe be bent or will I have to use chrome plated elbows to get round corners?
The shower is in a cubicle with walls around three sides. The next problem is getting the pipe through the wall. That's another thing on the to-do list: find out whether they are brick walls.
It is the quickest way, perhaps not the most elegant, to take the pipe through the wall. On the other side of the wall is the wash basin fed from the rising main and I could tee into that.
Would there be a problem taking the pipe through a sleeve in the wall and using sanitary sealant to make it watertight?
Thanks in advance.
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On 18/12/2010 17:29, Fred wrote:

If you're doing a like for like replacement, why is there any plumbing to do? Can't you use the existing pipework?
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Roger
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wrote:

The existing pipe goes up into the loft into the most inaccessible corner (behind a chimney) and it has recently frozen, which is what had prompted the decision to upgrade everything. Since the basin is next to the shower cubicle, I could run a new pipe from that and there would be no danger of anything freezing and not having to crawl through the loft. Really it was the pipe that needs changing but I thought I may as well change the medieval shower it is fitted to at the same time.
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You will be lucky to have the CSA stamped on the cable. You will probably have to measure the cable.
With 4mm cable you are stuck with the 7.5kW shower.
As for the plumbing, then anyway you want it. Through the wall or buried are both fine.
--
Adam

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On Sun, 19 Dec 2010 03:15:05 -0800 (PST), Adam Wadsworth

Thanks. I thought I had seen T&E with 1.5 or 2.5 printed on the sides but perhaps I have imagined that or perhaps I have just been lucky with the brands I have seen. Before Harry chips in, of course I would not dream of using such small cables for an electric shower!
I've been and had a look at the cable to the shower and as you expected, the grey insulation (where it enters the isolation pull switch) has nothing printed on it. Is there a foolproof way to tell 4mm^2 and 6mm^2 apart? Stick a vernier caliper across the conductor with the power OFF?
The CU is in the room beneath the bathroom so the cable run is quite short (so possibly could be changed without too much hassle) but all the cables run up the wall in some sort of trunking and then under the floorboards together so I guess I need to check about grouping derating too.
Thanks.
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http://wiki.diyfaq.org.uk/index.php?title=Cables#Cable_Sizes
gives a good guide for the external diameter of the outer PVC sheath.
Adam
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On Sun, 19 Dec 2010 07:24:31 -0800 (PST), Adam Wadsworth

Thanks. I'm reading using agent and whenever anyone posts a link to the faq, I always see a "3D" after the equals sign. Have I got a setting wrong somewhere?
I think in my earlier posts I was worrying whether it was 6mm^2 or 10mm^2. It seems more likely that the choice is now between 4mm^2 or 6mm^2. If it is 4mm^2 then that would explain the 32A MCB because isn't that the limit for 4mm^2?
I've never seen 4mm^2 T&E. From what I have read here, people don't like it because the cpc is too small. Perhaps that will be another clue to help identify it?
Are 7.5kW showers worth bothering with? Most seem to start at 8.5kW, with 9.5kW being a common option. I worry that putting in another 7.5kW model might be disappointing. I think it might be best to change the cable. The CU is "squared D". I see their mcbs are available from toolstation. Who are square D; are they a good make?
I would think if you are fitting an electric shower it is best to go for the most power-hungry one you can find? A 10kW model could be ruin at 8kW but an 8kW model could never be turned up to 10kW. That's my reasoning; is it flawed?
TIA
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Fred wrote:

The post you replied to is using Quoted Printable encoding:
Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding
This allows special characters to be posted by encoding them as a sequence beginning with "=". This then requires an actual "=" to be encoded (so it's not mistaken for the start of an encoded character), and an encoded "=" is "=". Possibly Agent doesn't understand quoted-printable encoding and shows you the encoded version.
Mike
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If the CU is a new one and match Toolstations MCBs (an slightly older CU may have different MCBs) then that is fine.

Yes, get the highest powered one that you can that is suitable for your cable.
The difference between 4mm and 6mm cable is easy to check if you have a bit of 2.5 T&E knocking about. The cpc(earth) of 4mm T&E is 1.5^2 the same as the cpc of the 2.5 T&E. The cpc of 6mm T&E is 2.5^2 - the same size as the 2.5 oddly enough:-)
Cheers
--
Adam



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On Sun, 19 Dec 2010 18:03:03 -0000, "ARWadsworth"

That was my worry, that they may have changed the design. I can't remember when the house was rewired but it is definitely in red and black, so how long ago is that? 2006? I think it must have been rewired in the early 2000s but that's 10 years ago. Will they have changed in that time?
If so, can you buy "old" mcbs or do you have to look on ebay?

Thanks. That's clever.
I have a horrible feeling it will be 4mm^2, otherwise why use a 32A mcb?
I know fat T&E is expensive but I'd only need what, 5 metres ?, at most so if I had to put 6mm^2 in, it wouldn't be the end of the world. I suppose you'll say that if I am changing the wire to go to 10mm^2?
Re. the other reply about venturis. They are a good idea, thanks, but the URL you gave said they need a 30cm head. I think the header tank is more or less at shower height; in fact the shower is probably higher than the bottom of the tank, so this rules out venturis. I think it rules out power showers because all the neg. head pumps I have seen are in the region of 400
The wall is hollow, so that's some good news but with my luck, I'll want to drill through a stud. Assuming it's got some studs. I have a zircon detector but IME it is useless.
Thanks again.
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If its along length of cable they could use 6 mm^2 to avoid voltage drop. You would still fuse it at 32A if that's what the fitting instructions said.

Probably, its hard to make cable too big, it just gets harder to fit in the terminals and to bend. ;-)
.
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On Sun, 19 Dec 2010 20:33:55 -0000, "dennis@home"

Thanks. I'll have to measure the conductors to see. Let's hope I am lucky and they rewired in 6mm^2 but used a 32A MCB because that was all that was needed by the (presumably) 7kW shower.
Since the wall is hollow, could I thread plastic pipe inside it or would it need supporting at regular intervals? I don't want it falling off and flooding the place!
Thanks
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You can get coils of plastic pipe so there wouldn't be anything to fall off inside the wall. I have had plastic pipe in my system for 30 years and nothing bad has happened yet. I do know of copper pipe that has failed in less time (but there is a lot of copper pipe that old and not much plastic that old).
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On Mon, 20 Dec 2010 13:03:43 -0000, "dennis@home"

Yes, that's what I was thinking of. I just wondered whether it needed clipping to a stud at certain intervals. That would be easy to do if you were building the wall from scratch but since the wall is already there, I will not be able to do this. I wasn't sure whether the pipe might "kick" when switched on and off and whether over time this might make it loosen any compression joints. Hopefully I am being far too pessimistic. Thanks.
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On Mon, 20 Dec 2010 08:37:05 +0000, Fred wrote:

That's what I did, 30 years ago when 7kW was the norm., then fitted a 40A MCB for the 8.5kW unit - both on 6mm cable.
--
Peter.
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On Mon, 20 Dec 2010 16:16:57 +0000, PeterC

Thanks to the table in the UK.DIY FAQ, and thanks to ARW for posting the link, I think it is 6mm^2 cable that is in place.
It is red and black cable. Was that fatter than the blue and brown? I had a small off cut of blue and brown 6mm^2 with me and it did look smaller overall. Perhaps the insulation is thinner on the new coloured stuff?
I measured the overall size as being 13.4mm x 7.5mm. The FAQ says 6mm^2 cable is 13.1mm x 6.8 mm. So this one is slightly thicker than expected. Do the cables vary by manufacturer? If so, this might explain the difference.
The table says that a 2.5mm^2 CPC would have a diameter of 1.78mm, so let's call that 1.8mm to one decimal place. I measured 1.9mm on a vernier calliper, so that's pretty close to the expected result. So far so good.
I measured the diameter of the neutral (once the circuit had been disconnected). I measured an overall diameter of 3.0mm but I am not sure how accurate this was as I guess it depends how the individual cores lie. I measured a single core and it was 1.0mm. The table says it should be 1.04 but I would not have been able to see the 0.04, so I guess my result matches the table.
I am pretty confident this is 6mm^2 T&E. Is there any reason to doubt this now?
The table gives much bigger values for the various dimensions of 10mm^2 T&E so I am sure it is not that.
The only thing that concerns me now is the path of the cable. All the cables leave the CU and go up the wall in sort of trunking. Does this count as clipped direct to the wall? I was worrying that the trunking makes it count as being in conduit. I'll have to take a closer look at what it actually is and how the cables are grouped inside it. I suspect they are just all thrown in together.
The cables then appear under the bathroom floorboards. They then run 3' or so all next to each other and where there are joists in the way, the cables go through one big hole in the middle of the joist. I thought you were supposed to have a separate hole for each cable and space the holes by three times their diameter?
But since this is "red and black" wiring, perhaps the regs. were more relaxed then? Were they? It was rewired by a pro, so I hope it was done properly at the time.
It's crazy really because the cable is not far from the corner with the shower cubicle and all that needed to be done was for the cable to go up the wall to a pull switch on the bathroom ceiling. For reasons I cannot understand (perhaps he didn't want to chaser the wall nor have cable clipped or trunked or otherwise visible), the cable does not do this; instead it turns and runs away from the cubicle!
The cable runs to the airing cupboard in the opposite corner of the room and then runs up the inside of the cupboard and into the loft. I don't know whether the cable is buried under any insulation because that part of the loft is inaccessible. Because it is inaccessible, there shouldn't be any insulation there but who knows?!
It is a little old lady living on her own, so I don't expect the wiring ever gets near full load but all the same, should I derate the shower cable on the basis that it runs close to everything else and may or may not be under insulation in the loft?
If so, what should I derate it to?
I think, but will check, that there is another 6mm^2 cable for the electric cooker and there is an immersion heater (2.5mm^2 ?). Quite often she has an electric fire plugged in to a socket downstairs. These are the only big loads I can think of that exist in the house.
That said, the cooker is only on for short periods of time at meal times and the boiler rather than the immersion is used for HW, and a ring main can cope comfortably with an electric heater, so I don't think any of these would be on at the same time as the shower and I doubt any of the loads would be on long enough to cause any heating of the cables. That only leave the insulation to worry about.
What do you all think? I could lay the cable in a more sensible route but I'd prefer to avoid the hassle if necessary.
TIA
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I am:-) And for only a few pennies more.

Not my link mate.

Of course there will be studs in just the wrnong place! Measure the existing cable first and satart from there
--
Adam



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I hadn't noticed that. I posted it because it explains the principle well. The other 2 brands I've seen (Trevi Boost and Newteam) both handle small negative heads. Replacing the electric does sound like the simplest option though.
A
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On 19/12/2010 15:12, Fred wrote:

Assuming you're familiar with 2.5 T&E, or have got a bit to hand, look at the earth conductor in the 4 or 6 mm^2 cable. Then:
- if it's 6 mm^2 the earth will be the same size as the L & N conductors in the 2.5 sample;
- if it's 4 mm^2 the earth will be the same size as the earth conductor in the 2.5 sample.
--
Andy

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He was discussing the idea of either keeping an electric shower of some kind, or of fitting an ordinary shower instead. Though maybe his post wasn't quite as clear as he could have made it.
--
Windmill, Use t m i l l
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