Insulation Around Cables in Joists

[PolarNorth78]

All,

I have a bathroom above an unheated integral garage and I plan to insulate the floor (about 2.5M x 1.5M) The primary issue is all of the house electrics are bunched together in the middle of the joists (the house was built like this in the 1970s). The joists are 170mm thick (I am aware of the methods, so closest to Method 103 in this case) and the bunching of cables (cooker circuit, ring main, 2 x lighting x circuits). While the house will be rewired, I have to deal with the immediate issue.

The question: what would be acceptable to prevent overheating? e.g would some form of box around the cables (akin to Method A) be acceptable and if so, how large?

Other considerations:

I would like to insulate the entire floor. I have considered Kingspan/Celotex but I also have water and heating pipes running on the top of the joists. Plus I also have to branch off the pipes for other supplies meaning a stiff board construction becomes a little complex - thus I am moving towards rockwool or similar.

When the floor is insulated, I will be lowering the ceiling to create a service void to run cables. This means when any cables that do go through the insulation, it is less than 0.5M to maximise current carrying capability.

I plan to run at least a 10mm cable for an electric shower ready for connection at a later date (ideally to take a 50A), most likely in the service void (as running along the top means it touches the insulation and de-rates to 44A) then up through the floor alongside the foul pipe. For belt and braces, I am considering using XLPE steel wire armour as I understand this has a higher permitted operating temperature. Linked, is there any distance a cable needs to be from water pipes?

The bathroom floor is likely to be covered in luxury vinyl (sub floor is 18mm plywood) and the garage ceiling in fire resistance plasterboard.

Thanks all in advance.

 

[blup]

The service void seems like an answer, 0.5 m through the insulation isn't too bad in that context though at the cost of compromising the insulation you could run them through a pipe. Use 16mm for the shower cable if concerned and are planning a high current model. Assume you have rcd/rcbo's fitted.

 

[PolarNorth78]

Thanks. I will have a look at 16mm and see if it works (main issue will be the bending radius if I use XLPE SWA) but worst case scenario, it will take a 40A which works. Linked, are there any documents that refer to XLPE SWA touching insulation? I know there are methods for burying in the ground as thats its primary use. My main thought here is the higher rating as space is at a big premium.

Main issue at the moment is the bunched cables. Its an old fuseboard and while it will be replaced, it's not an immediate job (for the avoidance of doubt, the shower cable is prep - it wont be connected to anything yet as I am aware it needs an RCD/RCBO, I just dont want to take up the floor again in a few months time). Yes I could remove the existing cables and run them in the service void (as it's 6 cables) but it means lengthening them and a load of mess in the process until the place is re-wired. If moving them and splicing in a new section with some wagos is the best option then so be it but am interested in other options too.

 

[PolarNorth78]

I've done some calculations and if I treat it as Reference Method A (cable in conduit in insulated wall), a 10mm might work. Future proofing a 16mm has other benefits though.

Has anyone wired a shower in 16mm (I.e. Will the shower and the pull cord isolator take the cable)?

 

[flameport]

I am considering using XLPE steel wire armour as I understand this has a higher permitted operating temperature.

Only valid if the items it connects to are also rated at that higher temperature, which in a domestic situation they will not be.

16mm (I.e. Will the shower and the pull cord isolator take the cable)?

Some might, plenty will not. 10mm² maximum is typical.

I plan to run at least a 10mm cable for an electric shower ready for connection at a later date

Why bother? Electric showers are last resort items for situations where there is no other hot water supply available.
They apply significant peak loads to the electrical supply which some supplies won't be suitable for, and have zero compatibility with solar, battery storage, time of use tariffs and so on.

 

[PolarNorth78]

Thanks. I will use 10mm T+E in that case.

Theory on an electric shower is I am gravity bath and hot water tank (traditional boiler) so seemed more logical and easier. House is still on an old fuse board so will be replaced with RCBOs (and perhaps AFDDs) at some point. DNO cut out is 100A.

Side note, I found your article on Y System Heading super useful last year. Helped when I needed to replace the motor in the valve!

 

[flameport]

so seemed more logical and easier.

Not really.
A mixer shower will work from the stored hot and cold, although the pressure and flow will be low.
That can be fixed with a pump for both the hot and cold supplies to the shower, or a shower with a pump in it. Either option won't be much different in cost to an electric one.