Electrical wire over or under loft insulation??????? Please help!

My builder states he will be using 8 inches of "rockwool" type insulation - four inches running with the beams four running over.
I had thought it was good practice to
a: Cleat the cable to the ceiling joists for support, and b. Ensure the cable runs over the insulation to prevent overheating of the cable (related to fault current calculations).
In my situation above the two (a and b) seem to be contradictory as four inches of insulation will be over all the joists. What do you guys do in this situation???????
Many thanks for all your help
Laurie
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On Sun, 24 Aug 2003 21:21:38 +0100, "Laurie"

It is reasonable to clip the cables to the ceiling joists, the important thing is to size the cable adequately for the load.
For the most part, cabling in a roof space will be for a lighting circuit; however watch out in case power ring circuit, immersion heater or electric shower circuits run through the space.
A good book for working this all out is the Electrician's Guide to the 16th Edition of the Wiring Regulations by Whitfield.
It's worth buying, although conveniently almost all of it is reproduced on TLC's web site in the technical section.
www.tlc-direct.co.uk
If you refer to Chapter 4, all is revealed. There are tables of current carring capacity for various scenarios, including insulation around the cable.
The worked example 4.1 covers the deratings for insulation and high ambient temperature in a loft for an immersion heater cable. You can easily adapt it for other types of circuit.
I suspect that most electricians would just ignore all of this and just go ahead and run 1.0 or 1.5 sqmm cable for lighting anyway, but it is a good idea to check.
.andy
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You would have problems if you put the cables on top of the first layer and then put another layer on top of that, but if you go for option a: you should be ok. Personally I'd tack the cable to one side of the joists rather than on top, just in case I wanted to put some boarding down at a later stage.
As I understand the regulations, so long as the cables are "in contact with a thermally conducting surface on one side" you can cover the rest in insulation and then apply the deratings as in table 4D5A (regs) or 6F (On Site Guide). For the purpose of the regulations, the wood of the joists should be sufficiently conductive (see regulations table 4A1, note following method 4) supposing that the ceiling side is just a layer of plasterboard or similar.
If you've followed all that you'll be pleased to hear that 1mm cable commonly used for lighting and usually fused at 6A is rated to be able to carry 12A under these conditions, and for all normal purposes should be fine - when normal loads are connected there's plenty of "headroom" and under large overloads (such as the "shorts" experienced when some types of bulbs blow) an MCB will cut the power well before the cable is damaged.
On the other hand, from my reading of fig. 3.4 in the regulations, it might take 3 minutes for a type B MCB to trip if you or someone else should happen to install enough lamps to take 12A (that's nearly 3kW of lighting). There's probably someone else here who can tell you what 3 minutes at 12A will do to 1mm cable under such circumstances. It's not very likely to happen, and you may not consider it neccessary to design for the situation.
I'd probably play it safe though and install 1.5mm instead, fused at 6A. This cable is hardly any more expensive and can carry 15A under the circumstances described. 15A should trip a 6A MCB in well under a minute.
No doubt if there's something glaringly obviously wrong in the above (I don't *think* there is) someone will correct me, so perhaps you'd better wait a day or two before acting on my advice!
Hwyl!
M.
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Wow,
Thank you both for your answers, the roof space in question is only ever going to carry lighting and the max loading at present is 800w therefore 3and a bit Amps max under normal conditions, so will apply regs/OSG as suggested, but expect you are both right and I can put cables under insulation. Will probably tack to side of joists as well.
Happiness is a bank holiday spent wiring!!!!!
Laurie
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In my loft there wasn't too much cable leeway so I insulated between the joists, then insulated over the top with a second layer leaving a small channel for the cable runs to go along. Where there were bathroom halogen lights, I left this bit uncovered. It's now pretty obvious where the cable runs are if I need to trace one.
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Good idea, thanks
Laurie

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