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

On 7 Jan, 15:08, "Graham Jones" wrote:
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

Load too small to consider.

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

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.

Correct.

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.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

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

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.

Correct.

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.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Thanks, that puts my mind at rest!

Graham

Graham Jones wrote:

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?

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

Have a look at the table he

http://wiki.diyfaq.org.uk/index.php?...es#Cable_Sizes

Compare 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.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 7 Jan, 15:08, "Graham Jones" wrote:
Hello,

Just been topping up my loft insulation and in one place I have buried a
short section of 1.0sqmm of T&E

..///snip


I don't know how it works,

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.

what happens to cable as it heats up?

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
you can have a short section of cable buried, but does this work through
junction boxes? Am I making sense?

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