Dropping cable down cavity.

My Mum has decided to have an electric shower fitted.
I had previously, perhaps in error, told her that it would involve a lot of mess and re-decoration for the cable to be chased into the wall. Either that or she would have to suffer trunking attached to the wall.
However, she has had an estimate for the job and mentioned my concerns. The plumber guy said that his sparky mate could drop the cable down the cavity and straight into the back of the consumer unit... No mess! I assume that the electrician would be able to self-certify his work under Part P.
Now, somewhere inside me is the notion that the dropping of cables down a cavity is considered taboo.
There is, at present, no cavity insulation so it would be a relatively straightforward job and I can't really think of many reasons against it except that any future cavity insulation would degrade the current carrying rating of the cable. Also the vertical drop would be unsupported, but at about 3m, I don't think that this would be a problem for 6 or 10mm T/E.
Perhaps I just have this misguided idea that it is a no-no!
Any advice appreciated.
Thanks
Steve.
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The cable can be dropped down the cavity no problem. As to whether the electrician is correctly qualified, well that's another matter. Don't forget it will need some form of isolation switch between the shower and the consumer unit. Do your mum a favour and watch she isn't ripped off by cowboys. They usually lie and say the consumer unit can't accommodate the cable or there isn't enough space. They will offer to fit a very expensive one. Then they will try other tricks of the trade to get more money. Cowboy plumbers are bad. I have inspected work they have done. One charged £200 to fit a cold water pipe through a wall from an airing cupboard to a shower - all of 18 inches! Check the company details with the online Companies House website to see if they are really a company. Never use one with a mobile phone number only, then check the registered office is correctly stated by using Google and/or a business directory.
You really do have to watch out. Usually cowboys rip people off with fascias, guttering, roof parts and TV aerials!
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Norm wrote:

An electrician who runs twin & earth cable in wall cavities *is* a cowboy, IMHO. It certainly isn't a recommended practice:
- the cable could be damaged by sharp intrusions into the cavity: mortar snots, wall ties and so on, also by the fauna and flora that could be present in the wall cavities of older buildings, also by some wall thermal insulation materials - particularly polystyrene, although I don't think that's used for cavity wall insulation (CWI) any more;
- the cable cannot be secured against one leaf of the wall. If CWI is subsequently introduced you'd have to assume that the cable is completely surrounded, so the current rating is reduced by 50%. Thus even with 16 mm^2 cable for your shower (that's the largest T&E made) you would be limited to 42.5 A (9.7 kW). Moreover that size cable is likely to be unmanageable in the wiring accessories that you would normally use in conjunction with a shower;
- the cable cannot be secured in place and must support its own weight. This can lead to damage, particularly for long vertical drops;
- the cable could easily touch both leaves of the wall, thus bridging the cavity and leading to penetration of damp.
--
Andy

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Andy Wade wrote:

Thanks Andy.
This more or less confirms my feelings and you make a couple of additional very valid points against it that I hadn't considered.
I think Mum will either have to have walls chased or surface mounted trunking after all!
Cheers
Steve
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You could start off with the trunking and chase in as you decorate. Not ideal I know.
Adam
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ARWadsworth wrote:

Indeed, but better than a cowboy job.
Steve
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Steve,
Be careful, a shower unit can draw up to 80 amps and needs to be wired directly to a properly fused consumer unit with an unused fuse rated for 80 amps
I suggest you visit this site if you want to understand the pitfalls
www.focusdiy.co.uk/stry/diy19&bklist=pcat,3,advicecentrecategory,diycategory
You can copy and paste it into your browser. Though I probalbly shouldn't advertise commercial outlets.
Steve wrote:

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80 amps, you sure about that - I haven't seen a domestic shower that draws anything like that!
40A maybe, 80A no way!
An 80A shower would give a great flow though!
Sparks...
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road_dog snipped-for-privacy@msn.com wrote:

It might be. One has to consider the strain on the cable at the point at the top of the wall where it is anchored, as T&E of any size is not designed to be self-supporting.
> Be careful, a shower unit can draw up to 80 amps and needs to be wired > directly to a properly fused consumer unit with an unused fuse rated > for 80 amps
URL for a 19 kW shower available in the UK please?
> I suggest you visit this site if you want to understand the pitfalls www.focusdiy.co.uk/stry/diy19&bklist=pcat,3,advicecentrecategory,diycategory > You can copy and paste it into your browser. > Though I probalbly shouldn't advertise commercial outlets.
You shouldn't top post.
Owain
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road_dog snipped-for-privacy@msn.com wrote:

That might actually be a quite decent shower at getting on for 20kW.
The reality is that most electric showers you can buy for domestic use consume no more than 12kW - and most somewhat less than that (7 - 11kW being most common). So say upto 45A is a more realistic limit.
--
Cheers,

John.

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On 3 Nov 2006 14:46:01 -0800 someone who may be road_dog snipped-for-privacy@msn.com wrote this:-

I would be interested in a domestic consumer unit that can take an 80A protective device. There may be one, but I suspect there are very few. It is more the territory of industrial/commercial boards.
The cabling to the shower would be interesting, as would the size of the supply cable to the house.
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
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You should assume the cavity will become insulated at some time, so ask the electrician to pick a cable size suitable for the current load when buried in insulation, which means derating by 50%. 16mm² T&E will derate to 42.5A, which is about a 10kW shower. Might want to knock down to 10mm² from the switch to the shower as it's easier to handle, if not buried in insulation.
--
Andrew Gabriel

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Better still do away with that piece of pox known as an Electric "shower" which it is, and get a power shower much, much, better :))
--
Tony Sayer


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Andrew Gabriel wrote:

Thanks Andrew
I am going to persuade Mum against the idea... If I can ;-)
Steve
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On Sat, 04 Nov 2006 13:16:02 GMT someone who may be Steve
Assuming she has mains pressure cold water and gravity pressure hot water then perhaps you could persuade her to install a venturi shower instead. Not many types are available but http://www.screwfix.com/app/sfd/cat/pro.jsp?id —330&tsI804 is one of them.
It only needs two pipe connections and may well be rather less disruptive, as the only work will be in the bathroom and perhaps the loft.
--
David Hansen, Edinburgh
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Are these things any good ? My gut reaction is to think that they aren't such a good idea.
Do they regulate temperature properly ? The manufacturers claim that they do, but they also mention that certain types of poor operation can only be sorted by tweaking the temperature of the hot water tank. To me, that suggests that the range of temperature adjustment that's possible is very limited.
Is the flow good and strong ?
I'm sceptical, but if they work well in reality, it could be a useful option.
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On Mon, 06 Nov 2006 01:03:24 GMT someone who may be snipped-for-privacy@Tesco.net (Roly) wrote this:-

Within limits. As the cold supply fluctuates so will the amount of hot. However, they will not compensate for a cold cylinder.

As strong as the mains is.

Provided they are installed in accordance with the manufacturers instructions they seem to do well. I'm sure those with problems with them would be complaining loudly if there was a major flaw.
--
David Hansen, Edinburgh
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Andrew Gabriel wrote: /snip/

is that correct?
IIRC the relevant IEE regs table (regrettably not to hand at the moment) only gives derating factors up to 500mm. Inspection of the factors & consideration of the physics behind them suggests that extrapolation to longer runs would imply much larger derating factors.
The problem surely is that if the cable is embedded in (good) thermal insulation then there is nowhere for heat to escape except along the length of the cable & there's clearly going to be a limiting point beyond which the cable heats up to >70deg C (the limiting operational temperatiure for normal FTE).
IOW 500mm is the maximum length the IEE reg authors thought worth specifying factors for, not the maximum derating factor for longer cable runs.
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From memory, yes -- I don't have the regs on me.

IIRC, the table is for lengths up to 500mm. I think the text says 50% derating for anything longer, but again, that's from memory.

Insulation isn't a perfect insulator, and with the power dissipated being the square of the current, 50% derating is just 25% of the power.

I'll check on Monday unless anyone beats me to it.
--
Andrew Gabriel

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Andrew Gabriel wrote:

Yes, that's correct:
"For a single cable likely to be totally surrounded by thermally insulating material over a length of more than 0.5 m the current carrying capacity shall be taken, in the absence of more precise information, as 0.5 times the current carrying capacity for that cable clipped direct to a surface and open (Reference Method 1)." [523-04-01 (part of)]
The same regulation first requires that "the cable shall wherever practicable be fixed in a position such that it will not be covered by the thermal insulation."
--
Andy

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