wiring woes part two

[Staffbull]

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 !!

help!

[Owain]

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

(b) you can run the wiring in plastic conduit

(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.

Owain

[Colin Wilson]

Cheap plastic conduit ?

[John Rumm]

There are two issues here...

You don't want wires in contact with polystyrene insulation since the plasticisers leach out of the PVC leaving a nice gooey mess and a cable with defective insulation. The second issue is that cables in any other form of insulation will probably need their correct rating de rated to allow for the extra difficulty getting rid of heat. The first problem is a no no, the second is acceptable so long as you check the design is adequate in the circumstances.

[TheScullster]

"John Rumm" wrote

John

Is this also true of expanding foam type filler/insulation? I have some central heating cables which have been clarted with this stuff where they pass through a cavity wall.

Phil

[chris French]

In message , TheScullster writes

No, that is something different.

[Staffbull]

Cheers, this would mean using 4mm for the ring mains i take it !! seems an overkill. or is there enough redundancy in 2.5mm to allow for installation in rockwool insulation. I'd prefer to not put any insulation in the floor at all having an uninsulated middle floor in a build that is otherwise completley insulated with no cold bridges does not seem an issue! I have thought of using an alternative insulation such as the foil backed bubble stuff this would leave the cable in free air! more expensive i know but would it be acceptable?

[Christian McArdle]

It depends on the exact installation method. If the cable is only passing through a layer of insulation, it is not so bad as if the cable runs under the insulation, or is attached to a joist (which can absorb the heat). If the cable runs for a long way right through the middle of the insulation, it is worst. We need to know the exact intended installation method and the length that the cable will be in that condition for.

Even if cable derating drops you under 20A for 2.5mm cable, this doesn't mean the entire circuit must be done in thicker cable. Only that bit that runs through the insulation needs to be done (although it is better to do the entire leg, rather than crimp up and down sizes).

Christian.

[Staffbull]

Hi and thanks, theere will be two ring mains running through the floorspace. two 5 metre lengths would be run across the joists and therefore would be in the centre of the joists and resting on top of the rockwool. Another solution i have thought about is to use the foil bubble insulation along where the cables run and rockwool for the rest, leave a 6-8" channel in the rockwool for the cables this would allow them to run in free air between joists (400mm centres)

[John Rumm]

No, that is a PU foam again, and seems to be safe (same applies to celotex type products)

Shoud not be a problem.

[John Rumm]

On top of the insulation is in no way as bad as "in insulation" (i.e. surrounded on all sides) most of the cable surface is still free to dissapate heat.

If the cables can go on top of the rockwool rather than be wrapped up in the stuff then there is probably nothing to worry about really.

[Christian McArdle]

Running across the top of the insulation, not within it, is absolutely fine.

Christian.

[Staffbull]

Nice one!! thanks :-)

[Staffbull]

Nice one!! thanks :-)

[jim]

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..

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.

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.

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