Cable Runs In Loft

[Invisible-Man]

Hi,

If numerous cables have been laid in loft above ceiling and is not practical or very easy to lift them before fitting insulation what are the remedies, or is it not that important or serious matter to leave them under insulation.
Any information would be most helpful

Thanks !!!

 

[ban-all-sheds]

If numerous cables have been laid in loft above ceiling and is not practical or very easy to lift them before fitting insulation what are the remedies,

Slide the insulation under the cables?

or is it not that important or serious matter to leave them under insulation.

Can be very important and serious - it depends on the size of cable and how much current it carries.

See here for details of how capacities can vary according to how cables are installed.

 

[Invisible-Man]

Thanks for all your comments, it has erged me to lift all the wiring today (took about four hours though), the problem that I had was that every bit of wiring is run over the ceiling in loft then down to sockets etc, so you can imagine the entry point from the consumer unit.

Good Job Done !!!

 

[Invisible-Man]

I forgot to add that I now intending boarding the loft, I assume that this would be fine now !!!

 

[ban-all-sheds]

Surely the cables are on top of the joists?

 

[jimbox]

in our loft some of the cables are in metal trunking but many of them just trail all over the place, this at least made it easy to move them out the way when I was adding extra insulation

 

[Invisible-Man]

The cables run through the joists, which is why it was not easy lifting them all, one advantage is that the boarding is straight forward, no wires on top of joists, these were laid with the view of boarding out the loft

 

[ban-all-sheds]

So is there an air-gap above the cables?

i.e. they are lying on top of insulation, then there's a gap of about half a joist depth before you get to the boards?

 

[Invisible-Man]

unfortunetly not, do you think this would be ok?

 

[ban-all-sheds]

So - cables through holes in joists, and no gap above the insulation means cables completely surrounded by insulation.

Cables buried in insulation need to be derated by 50%.

Your lighting circuits will be fine, assuming they are on 6A breakers, as the nominal capacity of 1.0mm² is 16A, but if you've got socket circuits, or shower cables etc up there then you are in trouble.

 

[Invisible-Man]

Any suggestions, could I for example lay something flat under the cables in order to stop insulation creeping round each side of cable, bearing in mind that the cables are about 1.5" down

 

[Invisible-Man]

So - cables through holes in joists, and no gap above the insulation means cables completely surrounded by insulation.

Cables buried in insulation need to be derated by 50%.

Your lighting circuits will be fine, assuming they are on 6A breakers, as the nominal capacity of 1.0mm² is 16A, but if you've got socket circuits, or shower cables etc up there then you are in trouble.

I would have thought that if you had wires running through a cavity for instance, then have it filled with insulation, this would be classed as buried in insulation! (meaning a 50% derating), surely what I have done is no different from passing wires through studwork and then insulate with rockwall, I would have thought that wires laid within a modern floor would have insulation on one side, as in many walls (with no air circulation). I did not think that my situation would be classed as (buried in insulation)!

If however, this would be classed as buried in insulation or at least derated in some way, what are the consequencies, is it dangerous, for instance not throwing the typical MCB for that size wire, ie 32amp for 2x2.5mm ring main, causing possible fire hazard etc, or would it mean that the MCB would just keep throwing when loading or trying to run apperatus.

Many thanks

 

[ban-all-sheds]

I would have thought that if you had wires running through a cavity for instance, then have it filled with insulation, this would be classed as buried in insulation! (meaning a 50% derating),

That's one of the two reasons why you're not supposed to run cables in cavities....

surely what I have done is no different from passing wires through studwork and then insulate with rockwall,

Passing through is not too bad - 50mm of insulation means a 0.89 derating, and 100mm means 0.81.

I would have thought that wires laid within a modern floor would have insulation on one side, as in many walls (with no air circulation).

Insulation on one side is not so bad - if the cables are in contact with a thermally conductive surface on one side that is Installation Method 15, and 2.5mm² T/E is derated to 21A, which means it's OK for a 32A ring final.

I did not think that my situation would be classed as (buried in insulation)!

But they are actually buried in insulation, are they not?

If however, this would be classed as buried in insulation or at least derated in some way, what are the consequencies, is it dangerous, for instance not throwing the typical MCB for that size wire, ie 32amp for 2x2.5mm ring main, causing possible fire hazard etc, or would it mean that the MCB would just keep throwing when loading or trying to run apperatus.

If we assume a free-air rating for 2.5mm² T/E of 30A, or if you want to be stricter, the highest rating quoted in BS7671 which is 27A for Reference Method 1 (clipped to a wall), then 50% derating gets you 13.5 - 15A, so it no longer complies with the regulations for ring-finals, which are specifically written to allow the peculiarly British ring-final circuit of 2.5mm² with an assumed worst-case cable rating of 20A.

As a guide you might revert to the pre Amendment 1 regulation 433-02-04 which required that the “minimum current carrying capacity Iz of the cable be not less than 0.67 x the rated current setting In of the protective device”, and figure that a 20A MCB would make your circuit safe, but you would still be in contravention of the wiring regulations, and you would have to show that you'd calculated things properly, and had taken into account the accelerated aging of the cables to determine their lifespan.

But MCBs just detect the current flowing - they have no idea of how hot the cables are getting, so if you stick with a 32A MCB the answer is.....

....fire hazard.

 

[Invisible-Man]

Hi,
Well I would not say that they are buried in insulation, as I have lifted all wiring (sitting on top of insulation), I was then going to board the loft with chipboard, therefore the wiring would be touching the chipboard along with the insulation.

What do you think!

And Thanks

 

[mottsy]

Hi,

I'm not an electrician, but I am planning to board my loft.

I intend to use 3" x 2" battons running at right angles to the joists and then put the boards on top of these. (The reason for this was to allow air circulation between board and insulation and prevent damp, it also means that where cables run over the joists I can move the battons slightly to miss them.)

Just a thought in case it may help. It would give you a good 2" of clearance over the cable.