Cables, insulation and separation

[DJM]

We have a room which is half over an unheated garage (which is really just a storage dump and not used for a car) and I want to insulate under the floor (ie between the garage ceiling and room floor) to keep the upstairs room as warm as possible.

The problem is that the T+E for the upstairs ring, lighting etc are running along the garage ceiling between the joists. Yes i know they shouldn't be there and should be clipped. But as they were like that when I got the house and they have been cut too short to move them on to the joist now they are staying pretty much where they are.

However, I obviously cant just lay insulation over them, so I am looking for a solution. The idea I came up with was square guttering to lay the T+E in.

I cut a channel in the insulation and lay the plastic gutter in it. The T+E is in contact with the plastic gutter and has a large volume of free air above to prevent overheating. It has the added advantage of moving the cable off the garage ceiling, but still keeping it remote from the upstairs floor.

So is the idea barking?

 

[holmslaw]

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[JohnW2]

If its 1mm cable it will be ok on a 5A cartridge fuse or mcb, but it is just under the limit for a 6A.

holmslaw, I'm really not trying to be clever or awkward, but I'm struggling to find where that comes from (in any Table of the regs); the lowest CCC I can find for 1.0mm² T&E with any installation method in any table is 8A Could you possibly explain?

Kind Regards, John.

 

[holmslaw]

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[DJM]

Always happy to learn and if the idea really doesn't fly then OK, but I'm struggling to see why. I understand the issues of cables in insulation, but that is in covered cables with no route for wild heat evacuation.

The IEE report I read had 3 scenarios for cable under insulation (flat on plasterboard, clipped to bottom joist, clipped to top joist) and for 100mm or less it basically said it could be regarded as the same as conduit in walls. For greater than 100mm insulation temp increrase could be an issue.

The gutter is 114mm wide and completely open at the top giving a significant volume of air for heat dispersal. Certainly more than a 20mm or 25mm pipe in a buried in a wall.

So why is this such an issue?

The lighting is 1.5mm2 and the ring final is 2.5mm2. They do not all run in the same joist void and so would be in seperate gutters. There are no shower or other cables.

 

[PrenticeBoyofDerry]

The other alternative would be to split the ring final circuit in to two radials, down fuse the two circuits to 20A MCBs or RCBOs if not protected by RCD.

 

[JohnW2]

I made a mistake, thanks for alerting me (waking me up)

No problem. As I'm always saying, we all make mistakes - and I'm at least as guity as everyone else!

Kind Regards, John.

 

[holmslaw]

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[holmslaw]

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[DJM]

Always happy to learn and if the idea really doesn't fly then OK.

Your gutter idea will still leave bits of the cable buried in insulation, thats why it doesn't work. It also adversley effects the performance of the insulation.

Incorrectly sized cable can overheat and cause fires.

I understand the overheating risks, which is why I am trying to resolve them innovatively as I don't want to damage the cables or the house.

At no point would the cables be buried in insulation, they would always be in either the gutter or square downpipe connected to the gutter.

Due to the joist size and pipework in other joist cavities I would only use 100mm insulation which whilst far from perfect is better than none at all. And, therefore the gutter would at be sitting in on about 25mm with 75mm up the sides. The cable would effectively be in contact with plastic on between 25mm and 50mm insulation and at least 50mm free space either side and at least 75mm free air above.

So whilst I can see that there is the potential for the cable to run a little warmer than previously under full load, I struggle to see with that level of heat sink how it is a real issue. The standard derating installation methods do not seem to be entirely appropriate as they all cover fully covered cables.

I'm not going to do anything stupid and simpy ignore advice, but I would like to understand the reasons why such a configuration would not be suitable.

 

[PrenticeBoyofDerry]

I was thinking along the lines of splitting the ring, so it did not need to run through the insulation within the ceiling void. But saying that you could also do that with the RFC.
ie bring cables out of ceiling void and re routing, within garage.

 

[DJM]

I was thinking along the lines of splitting the ring, so it did not need to run through the insulation within the ceiling void. But saying that you could also do that with the RFC.
ie bring cables out of ceiling void and re routing, within garage.

The upstairs bedroom is only half across the garage so already done in effect. Along side is the hall way so can't run them through there. The cables enter the bedroom floor space approx halfway across.

 

[holmslaw]

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[DJM]

Well you call it Heath Robinson and I'd call it an engineering solution. Either way if it works the name would be irrelevant. As for starting a fire I doubt it as the IEE tests showed cable fully covered in insulation and at high load only runs at a max of 90 degrees so uncovered cable will inevitably run lower. But again irrelevant in this context.

Anyway you really don't get it do you? This was an idea to improve and if it was a non starter to accept the status quo. But as an engineer I see nothing wrong with asking for an explanation why the idea doesn't work before dismissing it. So far nobody has given a reason and the alternatives suggested have not been suitable. Perhaps next time I am up at Savoy Place I'll pop in the library and do some research.

In the meantime I'll leave the void as is.

As for it being a big job to lift the floor - not at the moment. After a dry rot incident the room has nothing in and very little plaster so flooring is the least of my concerns. Certainly I have never spoilt a ship for a pond of tar let alone a half pence

 

[holmslaw]

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