Running cables through thermal insulation.

[BW2]

In my loft conversion I need to run power & lighting under the loft floor that’s going to be insulated. Usual construction – old lathe & plaster ceiling held up by old joists, topped with new independent joists holding up a wooden floor, and all to be stuffed with rockwool mineral fibre.

What's the rules / good practice cabling through this lot? Do I need more than the usual 2.5 T&E … ?

 

[davy_owen_88]

Best practise is to keep the cables out of the insulation entirely. Can't you clip them to the roof rafters instead of under the floor?

Lighting circuits are run in cables suitable for much higher loads than are actually carried so if they must be run under the floor you should be ok.

Ring circuits require both conductors to be rated at 20A or more. Above a plasterboard ceiling covered with >100mm of insulation gives a CCC of 17A for 2.5mm² t&e so they are not suitable and you'll need to reduce the MCB or increase the cable size to 4mm².

Never run shower/cooker cables under the insulation, they are run too close to their limits to remain safe.

 

[BW2]

Thanks for your input davy_owen_88.

Best practise is to keep the cables out of the insulation entirely. Can't you clip them to the roof rafters instead of under the floor?

Some yes, but unfortunately mostly not so easy for where I want to get to.

Lighting circuits are run in cables suitable for much higher loads than are actually carried so if they must be run under the floor you should be ok.

Good to know.

Ring circuits require both conductors to be rated at 20A or more. Above a plasterboard ceiling covered with >100mm of insulation gives a CCC of 17A for 2.5mm² t&e so they are not suitable and you'll need to reduce the MCB or increase the cable size to 4mm².

So I can just run my power ring as normal, replacing 2.5 with 4mm T&E – that’s great – really easy solution.

Actually I should have been clearer – I envisage the cables clipped along a joist, and then the voids between the joists all it packed in with insulation, so the cable has joist on one side & insulation surrounding the rest. Depth of the joists are approx 225mm.

Never run shower/cooker cables under the insulation, they are run too close to their limits to remain safe.

Ok – makes good sense

 

[Adam_151]

Actually I should have been clearer – I envisage the cables clipped along a joist, and then the voids between the joists all it packed in with insulation, so the cable has joist on one side & insulation surrounding the rest. Depth of the joists are approx 225mm.

Thats what Davy was working with to get 17A as far as I know, if it was juts run through the middle of the insulation with no joist or ceiling in sight then it would be nearer 13.5A (it would be 27A without insulation and we de-rate it to a half) and you'd probably need 6mm² cable which would be impossible to terminate


Oh, on the note of terminating, BS1363 means the sockets will accept 3x2.5mm² cables or 2x4mm² cables, some makes might be designed to accept more/larger but they don't have to, so when wiring a ring in 4mm² try and not have more than 2 cables at each socket by not having any spurs (its bad practice to install spurs from the off on a new circuit anyway unless building layout gives you a good reason to do so!)

 

[Adam_151]

Actually, now I have consulted BS7671 it looks like 2.5mm² may just be sufficent!

Its instalation method 15: "Sheathed cables installed directly in a thermally insulating wall or above a thermally insulating ceiling, the cable being in contact with a thermally condcutive surface on one side (otherwise as Reference Method 4)"

And sure enough if you look up ref method 4 in table 4D2A you get 18.5A (I assume Davy was reading the three phase column), but 4D2A is actually based on round cables, if you consult 4D5A which is a new table which was inserted recently for flat cables, you find that 2.5 and install method 15 gives 21A

Don't take it as gospel yet though, I'm still half asleep, hopefully someone else will give it a 'sanity check' while I'm at work!

EDIT: I should point that that the paper from the IET which stated that install meth 15 could be used for this does say its based on 100mm of installation, and as expected 270mm is worse in terms of current carrying capacity, how much insulation do you have?

 

[JohnD]

It seems to me that if you have to go to all the effort of taking out the old cable, and re-running new bigger stuff, you might as well do about the same amount of work, and re-route in 2.5mm, out of the insulation, at less than half the material cost.

No?

 

[BW2]

how much insulation do you have?

I’m afraid I’ve done a bit of a red herring here – for which I apologise.

My old loft floor was stuffed completely full of insulation, and I was assuming it had to be the same for the new one. In fact the new void is 225mm deep, but only 100mm of rockwool is deemed necessary, so I’ve got 125mm of air gap to play with.

So I guess it’s back to 2.5 T&E ? - and what was all the fuss about.

But it was interesting & thanks for everybody’s inputs.

 

[RF Lighting]

I saw this in a domestic loft recently.

It's one way, if a bit overkill of keeping cables out of thermal insulation

 

[markie]

Does it matter that much if the cables have abit of insulation on them if it's only lighting/socket cables, as long as you dont over load the cables with ten washing machines on the go

 

[Adam_151]

Does it matter that much if the cables have abit of insulation on them if it's only lighting/socket cables, as long as you dont over load the cables with ten washing machines on the go

Lighting cables, no in most cases you can even get away with totally surrounded by insulation, nevermind clipped to a joist under it

Sockets, no, you can just scape by if the cable is clipped to a joist and there is no more than 100mm of insulation, but if there is more than that, or if heaven forbid it runs straight through the middle of it, then going up a cable size (or more) is going to be necessary

@OP, 2.5mm² is going to be tight I make no bones about that, and it relies on it being clipped to the joist as it stops the cable being totally surrounded by insulation and acts as a heatsink to dispate some of the heat. You may want to consider pricing up 4mm cable and unless there is a massive amount of it it shouldn't cost the earth and may be worth doing just to make sure you pass by a good margin (still needs to be clipped to joist etc)

 

[bernardgreen]

And if that cable tray is earthed then any rodent with a craving for PVC is going to get the full 230v at near zero impedance up his teeth.

 

[davy_owen_88]

Actually, now I have consulted BS7671 it looks like 2.5mm² may just be sufficent!

Its instalation method 15: "Sheathed cables installed directly in a thermally insulating wall or above a thermally insulating ceiling, the cable being in contact with a thermally condcutive surface on one side (otherwise as Reference Method 4)"

And sure enough if you look up ref method 4 in table 4D2A you get 18.5A (I assume Davy was reading the three phase column), but 4D2A is actually based on round cables, if you consult 4D5A which is a new table which was inserted recently for flat cables, you find that 2.5 and install method 15 gives 21A

I was looking at the draft of the 17th edition because I was reading through it at at the time

Table 4D5A - Ref. 101 (Twin flat and earth cable clipped direct to a wooden joist above a plasterboard ceiling and with thermal insulation exceeding 100mm in thickness) gives 2.5mm² at 17A.

Sorry for the confusion, Adam is of course correct in saying 21A is the figure to use assuming 100mm of insulation.

 

[ColJack]

the cable being in contact with a thermally condcutive surface on one side

would you consider wood to be a good thermal conductor???

and doesn't the 100mm of insulation depth depend on where on the jist you clip it?

if you clip at the middle then you have 50mm above and below, but if you clip at the top you have 10mm above and 90 below.. since heat rises???

do these "top up joists" run parallel or perpendicular to the existing joists?