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Topic Title: Insulated walls - derating factors?
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Created On: 11 October 2017 09:15 PM
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 11 October 2017 09:15 PM
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dinth

Posts: 5
Joined: 09 October 2017

Hi. Due to internal insulatation being installed in my home, i will need to move some sockets and im also thinking about adding a couple more. As far as i understand this is non-notificable work and as i had some electrical education (long time ago and in different country though!) i decided i will do that myself. Ive got a couple of scenarios with which i need help.
Sockets in insulated stud walls, the void between plasterboards is 3cm thick, filled with Wickes Acustic Insulation (0.037 W/mk). The cables will only run in the stud wall for 20cm each, 2x2.5T+E as this is the main ring enclosed in 20mm "oval tube" conduit. I cannot guarantee that the conduit will be touching the plasterboard. Alternatively, i could run the cables without using conduits, but still couldnt guarantee that they will be touching plasterboard. Ive been thinking about using underfloor connection boxes and running just one cable up the wall, but i would have more spurs than sockets connected directly to the ring then and as far as i understand Building regs dont allow that.
Sockets in an external wall (just one as it's end of terrace). Walls are insulated using Celotex 30mm (0.021 W/mK, flammable), the conduits will be placed in a small service cavity between insulation and plasterboard, touching both. Instead of running two T+E cables in one 20mm oval conduit i can run them in separate 16mm oval conduits. Those cables will be longer, circa 2m.
On the top of that im worried about old cables which are already in the wall: one 4mm2 radial and a couple of 2.5mm2 spurs. Those cables are running in oval conduits chased into brick walls. Im not going to touch them, but im little bit worried because they will be covered with Celotex insulation. Could you please advise how much what are derating factors for each of those scenarios?
 12 October 2017 08:57 AM
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lyledunn

Posts: 1112
Joined: 13 August 2003

I really doubt that you have anything to worry about in terms of de-rating.

-------------------------
Regards,

Lyle Dunn
 12 October 2017 10:43 AM
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AJJewsbury

Posts: 16114
Joined: 13 August 2003

I would tend to agree - but it can be nice to have some numbers to backup a gut feeling sometimes...

Your installation method in stud walls seems to be method 2 (ref method A) - (I presume the cable/conduit won't be surrounded by insulation even if it's not touching the plasterboard - i.e. it'll be in a gap between the insulation and plasterboard) and 2.5mm2 T&E cable method A is rated at 20A - which is that you'd need for a 32A ring - so that should be fine.

On the external wall - with the cable in a gap between insulation and plasterboard - it's somewhere between method A and B - method B has an even higher rating than A, so again that should be fine. Existing cables chased into the original wall should be fine - as they'll be able to loose heat to the outside. My only worry there is that if the existing back-boxes form a cold bridge through the insulation - the front of sockets & switches aren't air-tight and so there might be a condensation risk there.

On my re-furb I created a 25mm cavity between insulation & plasterboard - that both allowed me to claim method B at worst throughout, more importantly, allowed for 35mm flush back boxes - often needed for the more 'decorative' sockets that 'the customer' preferred.

- Andy.
 12 October 2017 12:45 PM
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OlympusMons

Posts: 71
Joined: 05 February 2016

What is the 4mm radial for and what is the rating of its protective device?
I am thinking it is method B and looking at 4D2A (no method B in 4D5) the rating is 30A.
 12 October 2017 01:26 PM
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geoffsd

Posts: 1786
Joined: 15 June 2010

Originally posted by: dinth
Ive been thinking about using underfloor connection boxes and running just one cable up the wall,
but i would have more spurs than sockets connected directly to the ring then and as far as i understand Building regs dont allow that.

Not so as far as the Electrical Regulations are concerned, but all those junction boxes under the floor is probably not a good idea.
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