Extending ring mains

One of the sockets that I need is now hidden behind a chest of drawers, so I want to have a new one only about 1m away.
My plan is to disconnect one of the two cables going into the socket (it is definitely part of the upstairs ring, not a spur), extending it to reach the new socket, and run the return back to the original socket.
I intend to use crimps + heat shrinks to extend the cable, and run the new cable at the back of the wall, under the eaves - there is a small gap behind the wall that I can just about manage to squeeze through.
The wall both sockets are/will be fixed to is made up of plasterboard with 100mm celotex insulation behind it. I'd like to leave at least some of the celotex behind the backbox. Would it be enough to simply drill a hole through the celotex, just large enough for the two cables to go through, or does the hole need to be much bigger (the socket is likely to be used with a hair dryer)?
Is there anything wrong with my plan?
TIA
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On 07/12/2015 15:29, JoeJoe wrote:

PS: I want to extend the cable rather than run a new one as both cables connected to the inaccessible socket are fed from other sockets across the room, running under the floor, with carpet and chipboard flooring over (glued and screwed).
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Why not simply spur from the original?
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*I started out with nothing, and I still have most of it*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

Because electricians and DIYers keep banging on about how much better it is to extend the ring?
Where in the real world a spur is fine for almost every application.
--
Adam


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On 07/12/2015 18:34, ARW wrote:

First time I heard that from an electrician!
What about running the cable(s) through the celotex? Are there any rules?
Just wondering as it is likely to carry relatively high current. Would making the hole(s) slightly larger be enough?
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wrote:

It is not going to be carrying a relative high current. It will carry a short term 10A load when your wife dries her hair

It makes no difference if you extend the ring or use a spur. You should ensure sure that the current carrying capacity of the cable does not drop below 20A.
Appendix 6, table 6B of the OSG covers your proposed installation very nicely.
The CCC of 2.5mm T&E without any derating is 27A.
Passing the 2.5mm T&E through insulation gives a derating factor (determined by the length of insulation) as
100mm = 0.78 derating factor so your CCC is 21A.
--
Adam


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On 2015-12-07, ARW wrote:

The thing I find curious is that (IIRC!) the limit on the number of unfused spurs is the number of non-spur sockets. So if you hit the limit & need another spur, you just add another socket to the ring & take the spur off that.

There's nothing to stop a future occupier from plugging two kettles in, though (I'm assuming it's a double socket). ;-)
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In most cases it is all bollocks.
Apart from a kitchen what is the most likely demand on a ring circuit in 2015?
10A for a short while while the OH buggers around with her hair or 13A for a little longer when you are stripping wallpaper when redecorating?
The final ring circuit can easily cope with the hair drying and the wallpaper stripping at the same time.
It can even manage to boil two kettles from a double socket that are fed from an unfused spur in a bedroom if you wish to.
--
Adam


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On 2015-12-07, ARW wrote:

Especially since kettles only run for a few minutes. I guess the only real potential problem is from high-power portable electric heaters (which aren't used so much now)?
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You are looking at a 20A load on a double socket for one hour before you get problems. The same problem occurs if the socket is on the ring or a spur.
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On 2015-12-08, ARW wrote:

I remember that. How long at 26 A?
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What do you plan to run at 26A 92 x 13A) for an hour?
The worst case scenario is still a buggered double socket.
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On 2015-12-10, ARW wrote:

Me? Nothing. But there's nothing to stop someone from plugging two 3 kW heaters in. I agree with you that it's unlikely.
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Have you ever encountered caterers? They don't understand the laws of physics, or electrics, and are quite content to plug in half a dozen tea urns, water boilers, at 2+KW each and then moan about the crappy electrics. I've experienced them on outside events too many times to count.
--
Bill

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There's nothing to stop someone plugging a 13 amp load into every socket in the house.
Except, perhaps, the maximum load the feeder into the house can supply.
So I assume those who continually come up with a 'what if' as regards final ring circuits and just how horrible they are have taken this into account? And limited the number of sockets in their house to about 7?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

I think its more likely that they just accept that its unlikely that many will ever have than may 13 amp load appliances available and that the main breaker/fuse will handle the worst case where someone does, fine.
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Modern kettles are generally 10 amps maximum.
--
*The first rule of holes: If you are in one, stop digging!

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 08/12/2015 00:21, Dave Plowman (News) wrote:

Most, yes. You don't have to look too hard for a 3kw kettle: http://www.argos.co.uk/static/Search/searchTerm/kettle+3kw.htm#pdpFullProductInformation
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On 2015-12-08, Dave Plowman (News) wrote:

Really? Ours isn't more than a couple of years old & it's the full 13 A.

What if you're trying to make a foundation?
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The vast majority use the standard kettle connector. Rated at 10 amps.
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*Many people quit looking for work when they find a job *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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