loft insulation over cables

19 Nov 2006
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United Kingdom
I have read that it is better not to lay loft insulation directly over power cable as the cable may become too warm.

So what are the best methods for routing power cables when insulating lofts? Is it to use some sort of ducting to run the cables through?

(I plan to lay rockwool between the joists and then lay Knauff Supadeck over that)
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In practice no gives a hoot. The cables should really be in 'free air' but when you consider that they run alongside your central heating pipes under the floorboards all over the house - the question becomes meaningless.
running the cables over the insulation will help. ignore joe-90, as far as i know, he's not a spark. (nor am i, but i have knowledge on the matter)

I believe its also acceptable to nail or screw wood across the joists and run cables on top of them, to keep it off the insulation.

If you have any shower cables up there, DO NOT bury these. They are frequently run at close to their capacity. Insulate them and they will get very HOT. Its not so much of an issue with lighting cables, but still bad practise.
OK then Crafty. Is it OK to run cables in the same void as central heating pipes?
Answer please all knowing one.

Oh and I am qualified in the 16th edition regs. Are you?
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What if the cables rest on top of the insulation and then loft flooring boards are placed over the top?


What if the cables are fixed to the side of the joists and a small gap is left between the joists and the insulation?
Was looking this up recently myself, dont know if it helps but BS7671 says when a cable is installed under a floor or above a ceiling it shall be run in such a position that it is not liable to damage by contact with the floor or ceiling or there fixings. unarmoured cables passing through a joist shall be at least 50mm from the top or bottom as appropiate or enclosed in earthed steel conduit. alternatively the cables can be provided with mechanical protection sufficient to prevent penetration of the cables by nails, screws etc. note the requirement to prevent penetration is difficult to meet.

page 52 thats on...

also re the pipes point, it says on page 55 about that "for example , cables should not be in contact or run alongside hot water pipes"

although not qualified myself all the lofts i have seen have the cables clipped to joists where possible, its looks tidy and its visiable when anyone lifts the boards up.
The issue of physical damage is important, but separate.

The issue of insulation is that if cable is surrounded by insulation than it's current capacity is significantly reduced. If you have access to the IEE On-Site Guide there are tables which explain. Put simply, if a cable is enclosed for 500mm or more then it is down-rated by 50%.

If the cable is covered or in contact with insulation on one side but clipped to a "thermally conductive surface" eg a wall or plasterboard celing OR in trunking or conduit clipped to same it is ok. It's just about heat dissipation. This is referred to as Reference Method 6 and according to the building regs should be achievable vene in a moderately well insulated dwelling.
joe-90 said:
I am qualified in the 16th edition regs. Are you?

[Sigh] And what qualifications are those? And how much experience do you have of working as a professional electrician?

And we've been here before: I'm not going over it all again: you know EXACTLY how us professional sparks feel about this subject. Why are you attempting to spark yet another debate?

The OP mentioned Power Cables.

Lighting cables are always found in lofts, and frequently buried under insulation. However, most domestic UK lighting circuits are run in 1.5mm T&E, which is loaded a long way under capacity with a 5A or 6A fuse or MCB. Lighting cables are not usually a problem, because even when de-rated for insulation, they are generally still safe.

However, cables for Electric Showers and Immersion Heaters are usually run near their maximum capacity, and when buried in insulation are likely to overheat. This shortens their life and can be dangerous. Upgrading them to a larger size of cable is expensive and a fair amount of work, so it is important to put these cables where they will not be buried in insulation. This can usually be achieved by clipping then to the roof timbers.

edited: Sorry, Crafty has covered this.
JohnD, thanks for your post on this. You also provided me with some information on a similar topic in the 'lofts' forum, however I don't think a conclusion was reached.

To recap, the house was re-wired in Oct '05, with 1.5mm T&E wired to the upstairs lighting which is clipped rather tightly to the ceiling joists. There is not sufficient free play in this cable to enable me to free it up and put it over the thicker insulation I plan to lay. The wiring is on a 6A RCD in the CU.

Part of the loft will be simply re-insulated, i.e. out with the old (75mm) and in with new (170mm) stuff. This should render the upper surface of the ceiling joists 'open' and thus allow whatever small quantity of heat there is (if any) in the wiring to be dissipated into the loft air.

However I plan to board out the central 8' of loft space by putting new 100mm insulation between the current joists, then laying 4x2's on top running perpendicular and putting more 100mm insulation between these. This insulation will cover some (a total of three 8' lengths) of the lighting wiring.

The feeling I am getting from many posts here is that this will be OK (but bad practice according to Crafty). Am I right to think it's OK? I plan to complete this project this weekend, any guidance will be much appreciated, thanks.
Lighting wiring is usually run in 1.5mm T&E. This has a rating of 20A single phase clipped direct.

Buried in insulation you halve it, giving a capacity of 10A.
If it is protected by a rewirable fuse (ugh) you derate it to 7.25A

It is not usually bunched so no derating there.

(still safe so far ;) )

If the loft gets very hot in summer you may have to apply another derating factor. E.g. 35°C is 0.91, 40°C is 0.82, 45°C is 0.71
So at 45°C it is derated to 7.1A if protected by an MCB or a cartridge fuse, but only 5.15A on a rewirable. However a rewirable would only be 5A, so still OK. That would be pretty hot!

If the cable is not surrounded by insulation, but covered one side only. the derating is less, but for a lighting circuit, you can see it isn't usually necessary to allow for that.

This is why lighting circuits are not usually considered a problem, and lots of people wouldn't bother doing the calculation unless they knew of other factors (like long runs, or bunched in ducts) to include. Shower and especially Immersion Heater cables, which may be loaded for long periods, usually need a proper calculation if they are in hot or insulated positions.
When I bought a new house I thought the builders must have had a game of catch with the cable drums as the loft had all the cables going anywhich way but the correct way. When it came to boarding the loft I had to also re-lay all the insulation - as this was just thrown up there - and carefully run the cables as best I could to avoid being buried in fibreglass or trapped under the boarding. They even had the cables running over the tops of the rafters stretched really tight!

Couldn't understand how any of these ever passed inspection.
yes, i have the same thing- wires run tight across joists and I'm trying to floor it. I'm simply using a hole cutter to cut the new flooring where the cables run over the joists rather than mess around taking notches out the joists to run the cable through.. (I'm only using it for storage anyway)

Question is - cables not buried in insulation but sitting against the underside of my chipboard flooring will be fine as the chipboard can conduct the heat away- yes?

obviously as discussed, lighting isn't a problem, but I also have the shower wiring..


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