Loft Insulation and Electric Cable

[Graham Jones]

Hello,

Just been topping up my loft insulation and in one place I have buried a short section of 1.0sqmm of T&E. The section of cable in question is about

4ft long and goes from a junction box to a light fitting (the junction box also supplies two more light fittings). Half of the cable is buried, so about 2ft is visible. The supply cable to the junction box is completely visible.

I think as the junction box supplies three light fittings the total load on the supply cable is 180w, the total load on the buried cable is 60w.

What I am confused about is because the section of cable in question is only

4ft long will half of it being buried cause an overheating problem? Or does the fact that the supply cable is completely uncovered compensate?

I don't know how it works, what happens to cable as it heats up? I know that you can have a short section of cable buried, but does this work through junction boxes? Am I making sense?

Also from a theoretical point of view would 1.0sqmm T&E carrying say a 60w load overheat under 370mm of insulation anyway? Is the load to small to worry about?

I hope this make sense!

Thanks,

Graham

[Phil]

Load too small to consider.

[David Hansen]

On Wed, 7 Jan 2009 15:08:09 -0000 someone who may be "Graham Jones" wrote this:-

It is usual to allow 100W per light fitting. That is less than half an amp, in a cable which in open air is rated at something like 14A (can't be bothered to look it up). Such a low load isn't going to cause the cable to heat up to any measurable extent and so the insulation can be neglected. That is generally the case with lighting cables, though this should always be checked.

I'm glad you asked, lots of people don't.

[Graham Jones]

Graham

[John Rumm]

Nothing to worry about - there is plenty of headroom on 1mm^2 cable in this application.

Have a look at the table here:

the "clipped direct" column (C) with the "above plasterboard ceiling covered by thermal insulation, insulation thickness >100 mm" column (E).

Its more of a problem if you cover cables feeding sockets or showers etc where the design load is much closer to the full rating of the cable.

[jim]

.///snip

Cu conductor has resistance & self generates heat via I^2R. If this heat isn't radiated or conducted away then the cable heats up. Usually a bit of both occur.

Normal PVC FTE cable has an acceptable temperature design limit of

70deg C. Beyond that the PVC sheath and/or insulation will deteriorate rapidly & may even melt: conductors may short together.

Even below 70degC a warmed FTE cable sheath & insulation deteriorates from self-heat though more slowly, reducing design life from 60 years to just a few years near the design limit. The cable goes stiff & has the flexibility of lead sheet rather than the springiness of new cable. The stiffness is due to leeching of the plasticiser in the PVC.

This is the reasoning behind specified bend radii for cables: - bends which are too tight can allow adjacent conductors to short through warmed PVC.

The more completely insulation surrounds cable, the more of a problem cable self-heating is. The problem is often discounted for cable operating 30% or more below its full load current carrying capacity. However it would be wise to consult the new tables in 17th wiring regs & check requirements against your actual installation. The regs also include a simple deregulation % table for cables run in thermal insulation. Also see the On-Site Guide.

If possible always allow at least one side of the FTE to rest against a thermally conducting surface - timber & plaster board, unlikely as it may seem, are generally considered sufficiently conductive for this purpose.

I know that

It is wise to make the same allowance for junction boxes as for the surrounding cables (the plastic boxes are made of somewhat stouter stuff than pvc insulation, but don't count on it). In any case you want to be certain the whole lot is kept as far below 70deg as possible.

HTH