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Mike Kapusniak
Joined: 21 May 2007
Posts: 2
Location: Shetland
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 Posted: Mon May 21, 2007 6:14 pm Post subject: Cable size and thermal insulation |
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| Am remodelling parts of my single storey 1980's kit house (timber frame). Ring mains are 2.5mm twin and earth, in contact with plasterboard one side, 100mm mineral wool the other, walls and ceiling. However, in the ceiling the cable is frequently taken over the wooden ceiling rafters of the engineered roof trusses (approx 95mm deep and 35mm thick). I will be adding another 200mm insulation on top of the existing. Where the cable leaves the plasterboard to run over the ceiling rafters there is timber/cable/thermal insulation instead of plasterboard/cable/thermal insulation. Is there a derating issue here due to the more limited heat transmission through the timber when compared with plasterboard? If so what are the cabling/re-cabling solutions? |
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vince knight
Moderator
Joined: 04 Mar 2004
Posts: 1328
Location: Essex/East London
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| Posted: Tue May 22, 2007 9:44 pm Post subject: |
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If the cable is enclosed in insulation for more that 3 meters of its length then the cable current capacity must be halfed. So a 2.5 will become 4mm.
Vince |
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Mike Kapusniak
Joined: 21 May 2007
Posts: 2
Location: Shetland
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| Posted: Wed May 23, 2007 6:08 pm Post subject: |
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| vince knight wrote: | If the cable is enclosed in insulation for more that 3 meters of its length then the cable current capacity must be halfed...
Vince |
My understanding of IEE Wiring Regs 16th edition 523-04-01 is that where cable is completely enclosed in insulation for 0.5 metres or more the current-carrying capacity should be halved. Lower derating factors apply for thermally insulated runs betwen 50mm and 400mm. In the situation I'm dealing with the cable is installed as per Reference Method 4, plasterboard one side and insulation the other. For the 2.5mm 70C twin and earth installed the rating from table 4D2A is 18.5 amps, which I've always understood to be appropriate for a 30 or 32amp ring-main where current can be drawn from both directions (please correct me if I'm wrong).
The question is -- where the cable leaves the ceiling plasterboard to run over a timber ceiling tie and back down to the plasterboard, (remaining underneath the insulation the whole time and not running through it), should the timber of the ceiling tie be treated as insulation, or should it be treated as an extension of the plasterboard?
Put another way, does plasterboard/timber/cable/insulation equate to plasterboard/cable/insulation as per Method 4? Or does it equate to being enclosed by insulation on all sides?
One set of accredited electricians tell me there is no need to increase cable size to bring the installation up to date. Another says different. I don't feel confident in employing one until the issue is resolved.
A side issue, but nevertheless a very important one, is that companies contacted to give quotes for installing extra insulation in the roof at ceiling level generally have no idea that doing so can, in some circumstances, derate even relatively recently installed wiring where it passes completely through the insulation. They also have no idea that failing to notify the client of this fact is an offence!
Mike |
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