Owain wrote:
Staffbull wrote:
On reading the plans it states wiring should not come into contact with
insulation, but requires the first floor to be insulated between joists
with rockwool !!

As suggested in responses to your original post, please pause & take
stock. Do a spot of reading and google on this group on the subject of
domestic wiring & IEE regs. By reading a few dozen threads, I'd guess
you'd form quite a good appreciation of how to handle this as I'd be
certain this problem has been covered. You would also become much
better informed as to how to tackle your project, and perhaps find an
answer to your next problem after this one..

(a) wiring must not come into contact with *polystyrene* insulation
because the polystyrene chemically reacts with the PVC sheath.


AND rots the cable which becomes hard and then you get faults. Also
verboten are tar products such as roofing felt adhesive - it doesn't
harm the cable but makes hard tar go soft. So is exposure to UV light
(eg sunlight) which makes the cable leech plasticiser.

Standard UK cable (PVC FTE) cable is rated to run at 70 deg C in ideal
ventilation. If run continuously at that temperature it has *perhaps*
a life of 20 years. Cable deteriorates over time due to leeching of
the plasticiser which makes it flexible. As the cable gets hotter,
plasticizer leeches ever faster. Over 70deg C cable life is
considerably shortened, even down to a year or so in even modest
overrated use. Run at more modest currents, perhaps not continuously,
life might extend to 60 years or more.

Which basically is why there is the instruction about your cable not
touching insulation - unless installed correctly in such an
environment, it will overheat at standard rating.

There is no 'one' current rating for a particular size of cable, it all
depends on how well the cable can radiate the heat it self-generates
from I^2R. The IEE regs (BS7671) include tables . which give current
ratings based on 'installation methods' for a large number of standard
cables. The main one you want was originally Table 4D2A but other more
recently issued tables may apply especially where insulatated building
materials are in use. I'd be the first to admit these tables are not
the easiest read, but once you've worked through an example or 2 the
mud becomes much less thick. Post here for help.

As far as I'm aware the installation methods are only set out in Table
4A, with diagrams, of the regs - they are not in the On-Site Guide.

Bearing above in mind the basic thing that needs to be done is obvious:
allow the cable to radiate heat - one method is to ensure at least one
wide side is in contact with a 'heat sink': usually resting on ceiling
plasterboard or pinned to a timber joist. So fas the IEE installation
methods go, both plasterboard and timber are considered adequate
heatsinks compared with thermal insulation. Even so you're better
advised to lift the insulation clear and allow some free air around the
cable - 100mm if you can achieve that..

At the other extreme you can derate the cable according to regs Table
52A, which is quite drastic and is only valid up to 400mm of cable -
above that length you're on your own, but extrapolating the stated
derating factors = don't waste your time. Once the cable is in the
insulation there's only 2 places for its heat to go - through the
insulation (not much heat leakage) or along its length, ie conducted
along the PVC sheath/insulation (not much) or along the Cu conductors
(which is just not what you want).

BTW you mentioned use of 4mm2 cable - beware its CPC is undersized:
don't use unless you appreciate the need for CPCs of certain sizes.
Ask first - eg post here. If you look in the On-Site Guide you will
not see 4mm2 cable circuits.

(b) you can run the wiring in plastic conduit

Perhaps, but more likely it won't solve the problem as cable contact
with the ceiling is reduced, and although the cable then runs through
free air in a tube, the air - unless the tube is of wide diameter and
has open ends - has nowhere to dissipate its heat as the conduit is
surrounded by insulation. NB even without the insulation problem,
running cable in conduit derates FTE cable by around 25% as shown in
the IEE tables.


(c) if the wiring is run in conduit or insulation there is a derating
factor you must apply to the cable; this isn't usually a problem for
lighting circuits, it may be a problem for ring circuits, and if you
have an electric cooker or shower those cables will need careful
calculation.

I should like to see experimental data for FTE cable in conduit to
justify that. My suspicion is that there wuld be very little
improvement, if any, in cable operating temperature over what it would
be in direct contact with the insulation..

Any route wins or loses by its ability to dissapate the I^2R losses.
Ask the Q 'where is the heat going to be lost?' for every route you
choose.

My personal preference would be to run the cables over the insulation
if at all feasible - but they really ought to be run at least 50mm
below the underside of floorboards to avoid nail damage. Or fins a
different route - up to the attic & down?? Visible cables are at much
less risk of inadvertant damage & are easily inspected.

Perhaps you should enquire why the insulation is specified and whether
there's an alternative. 2 possible reasons are- 1. to prevent air
currents circulating in the floor void cooling the room below 2. sound
insulation. The last you might be able to use different flooring and
ceiling materials to improve.

HTH