Loft insulation and wiring

[ajp]

The instructions with the loft insulation say that electric cables should go above the insulation - presumably in case there's any build up of heat.

Is this an absolute because in places there's not enough slack in the wiring and there are connection boxes which are screwed to the joists.

Alan

[Toby]

Not answering your question, just adding to it...

I was also wondering this, I plan to board my loft, so when I rewire I was planning on clipping all the cables to the sides of the joists, is this going to be an issue when I then insulate it?

Toby...

[David Hansen]

On Fri, 28 Nov 2008 15:00:27 +0000 someone who may be ajp wrote this:-

Because the heat can't get away from the electric cables so easily when they are covered in insulation. If the heat can't get away then the cables may at best just melt and at worst set the house on fire.

Mostly, yes.

Lighting cables are probably alright if covered with insulation, but you need to do the calculations to check this. Other cables are probably not alright, though you need to check this in the same way.

You don't say what sort of house you have or what sort of cables are in the loft, so there is almost nothing else which can usefully be said other than in a "typical" two storey house the only circuits in the loft tend to be lighting circuits. We are not remote viewers and so we cannot see what is in your loft.

[jal]

Ditto. I put my cables over the insulation when I rewired my house 12 or so years ago. This summer we had the (now-advised extra 6" of insulation put in, and the cables are all too tight to go over that.

I am figuring: (1) I'll keep a weather eye out for fires starting in the loft (don't ask me how).

and

(2) All legislation and "advisories" are based not so much upon practical risks, as upon protecting insurance companies from claims by tradesmen / contractors / householders, i.e. a 1 in a million chance[1]. In other words, one in a million lofts may catch fire -- so make all

1,000,000 of them install things "properly".

-- [1] It's still a chance, of course.

[Owain]

This is not usually a problem for 6A lighting circuits where the cable is already veryt generously over-sized.

It may be a problem for immersion heaters, especially if on 1.5mm fused spurs, or for electric showers.

The issues are heat disspiation from cables, and the possiblity of physical damage if the cables are trodden on unwittingly.

Owain

[ajp]

We're in a bungalow so there is power circuitry in the loft.

If I'm not allowed to cover them then fitting insulation is going to be a problem. So far I've managed to fit the insulation under the wiring but I'm reaching an area where this may not be possible - or at least not without some tugging on the cables :-)

Alan

[John Rumm]

With lighting circuits this is mostly a non issue if they are protected at 5 or 6A as is normal. The smallest cable typically used is 1.0mm^2 T&E and this is good for 11A when clipped direct to a surface or in free air. So even if you do cover it with insulation you are unlikely to de-rate it enough to be a problem unless there are other factors to take into account (high ambient temperatures, cable grouping etc).

Clipping them to a joist is worthwhile, since the wood will conduct heat away more heat from a cable when compared to one completely surrounded by insulation.

Power circuits (sockets etc) however are another issue. These you will need to take far more care with since you probably don't have the spare capacity to play with.

[Toby]

Thanks John,

I will use 1.5mm cable to give a little more headroom.

I will have an outside lighting circuit, connected to a 10A RCBO, but I think this will be all in the 1st floor floor, so that shouldn't be an issue.

In the bedroom, on the 1st floor (top floor) there are already a load of GU10 down lighters installed, I will be keeping them (mainly to save having to fill the plasterboard!) and will also install some wall lights, so they won't be used much, I assume I need to install fire-hoods over them - if so, is it OK to put insulation over this, or would it be better to leave holes over each one?

The same with the boarding I will then put down, can I just board over them, or do I need to cut vent holes in!?

Thanks!

Toby...

[John Rumm]

Not really practical is it? You need to make sure it is not going to be a problem by taking the required action when insulating. That may require rerouting, or extending cables, it may even require you substitute larger cables in some cases (e.g. upgrading 2.5mm^2 T&E on a ring circuit to 4mm^2).

No offence, but this is bollocks. The reason they warn you of the risks may well come down to disclaiming responsibility etc, however the actual risks themselves just come down to basic physics.

well you did ask ;-)

Remember that the current carrying capacity of cables is dictated by the maximum sustained temperature that they can endure without suffering damage. Most PVC cables with be rated for operation at upto 70 deg C. Hence if you live in a single story property, and your kitchen ring circuit is fairly fully loaded (not uncommon), then you will routinely be running the cables quite hot and close to their design limit. If you now cover those with insulation you are likely to get cable damage as a result. You will also de-rate the cable to a point where its current carrying capacity is unacceptably low for a ring circuit. In a modern installation this is unlikely to result in a fire directly, but it may well result in nuisance trips, failed insulation, and the need to replace cables before you can restore power to the effected circuits.

(2.5mm^2 T&E can carry 27A clipped to a surface, but that can fall to

17A when sat on a plasterboard ceiling and the covered with more than 100mm of loft insulation)

[Tim S]

John Rumm coughed up some electrons that declared:

And if I might add to John's excellent post, the correct design for any leg on a 32A ring final circuit such that the current carrying capacity of the cable be 20A or greater, after all derating and grouping corrections are applied (17th, regulation 433.1.5).

So 20A is the magic number to aim for. Obviously with any other type of circuit, the cable must be capable of carrying at least what the breaker/fuse is rated at irrespective of the anticipated actual load on the cable. So a 7.6kW shower fed by a 40A breaker needs a 40A cable, not a 33A one.

Taken from the OnSite guide, I've copied out some key numbers for 1.5mm2,

2.5mm2 and 4mm2 T+E cable which may prove useful:

But you should check these again before actually using, typos etc..

Method C, "Clipped Direct"

1.5mm2 19.5A 2.5mm2 27A 4mm2 36A

Method B, "Conduit in/on non insulating wall"

1.5mm2 16.5A 2.5mm2 23A 4mm2 30A

Method 100, "Above plasterboard ceiling covered by no more than 100mm thickness of insulation"

1.5mm2 16A 2.5mm2 21A 4mm2 27A

Method 101, "Above plasterboard ceiling covered by more than 100mm thickness of insulation"

1.5mm2 13A 2.5mm2 17A 4mm2 22A

As you can see, Method 101 kills 2.5mm2 as a 32A ring cable, but Method 100 is OK, just, as long as you don't further derate the cables due to grouping factors etc.

Going up to 4mm2 is a pain, as you are only allowed 2 4mm2 wires in the back of a BS standard 13A socket terminal, so no spurs.

Cheers

Tim

[John Rumm]

Indeed...

I will add a slight complication to that one...

The cable needs to match or exceed the overcurrent protection. While that is traditionally provided by the circuits main fuse/breaker, it does not *have* to be at the head end (as might be assumed by some reading the "breaker/fuse" bit). The obvious example is a socket on an unfused spur. The single run of 2.5mm^2 T&E is given adequate fault protection by the 32A breaker, but overcurrent protection is down to the fusing in the plugs.

There is also a table here if needed:

[John Rumm]

Oops, better correct that. That should read 16A clipped direct. The 11A (well 10.5) is actually the case when covered in >=100mm insulation.

From the PoV of current carrying capacity there is no need on a standard circuit really - you are still well clear in most cases. If you were protecting them at 10A and wanted to bury them in insulation, and apply other de-ratings, *then* it might be worth using 1.5mm^2.

The time 1.5mm^2 becomes handy is for dealing with voltage drop. The

17th edition limit is now 3% (so 6.9V) on a lighting circuit. With a 6A MCB and 1.0mm cable, with a full load distributed around the circuit, that results in a maximum cable length of 59m (Table 17.1 in the OSG). Falling to 35m for a 10A protected circuit. Using 1.5mm^2 raises these limits to 90m and 52m respectively.

The main problem with downlights is if you let them get too hot, bulb life suffers even more (especially with GU10s). Sometimes a flower pot (real one, not plastic!) is suggested as a way of keeping some air space round them. Fire hoods are handy if you need to maintain the fire integrity of the ceiling.

Much depends on the joist depth. On slim loft joists you may even find the boards are prevented from seating correctly by the upstanding lights.