T&E cables and expanding foams.

[nostos156]

Hi all,

Have a relatively simple question. Doing a rewire to make a circuit in a single room. My father is a retired spark, so no worries about a DIYer doing electrics or anything, he just isn't sure about this particular question.

I'm aware modern PU foams like Soudal have no adverse affects on PVC cable, so no worries there. My question is, for one particular part of the run, I had to drill a hole through the bottom of the frame/floorboard into the joist spacing; to fish a couple of 2.5+1.5 300/500v T&E cables though to reach a socket on one wall.

As this is in an attic space, under the floorboards can get draughty. To prevent those draughts reaching up in between the wall (which had recently been redone with 50mm PIR + 25mm on top of the studs); and getting in between the two PIR sheets (thus ruining their insulation ability a bit), I foamed up that hole. Sealing it with mastic would've been difficult as reaching it properly around the back wasn't happening.

However, I am now wondering if that has had an adverse effect on the rating of the cable. From what I am gathering, such a short section of foam (probably no more than 5cm) shouldn't really do anything major, as the heat will just conduct to the rest of the cable. Just wondering if I am overthinking it or not.

As it's a bedroom, there won't be any significant loads—just things like a TV and gaming PC would be the biggest constant power draws in the room when they're on.

 

[littlespark]

Right on both counts.

Not very big load to generate heat, and length of cable covered by insulation negligible.

Was there a draught?
Cables are run in voids all the time, and holes can be filled in more accessible positions if needed.

 

[MJPD29]

I doubt it will make any difference in this situation as it is over such a short length of the cable

 

[nostos156]

Thanks guys, that's what I had hoped for. I had ran some quick calcs and even a top end gaming PC would be maxing out at 1kW, TV would be 300w, etc. Even running an oil heater (which I used to do but no need since central heating now) should be more than fine.

My next question alongside that is, some of the roof facing sockets are in the attic space (so I can access the rear of the back boxes), so when redoing those I was thinking about foaming around the side/top/bottom of them, as it'll get very cold in there and I imagine the metal will condense something fierce. I'll be mindful about covering the cables at the bottom too much, but is the effect of insulation cumulative across the entire ring circuit, or does it only matter per cable?

I imagine any derating of any single cable would derate the entire ring circuit, but if every cable only has a short section of foam on it, would that be okay?

Right on both counts.

Not very big load to generate heat, and length of cable covered by insulation negligible.

Was there a draught?
Cables are run in voids all the time, and holes can be filled in more accessible positions if needed.

Yeah, I could feel one when it was windy. The attic has glass wool in it, but glass wool isn't the best at stopping air movement entirely, I have found. At least in ancient 150+ year old attics.

 

[DPG]

Thanks guys, that's what I had hoped for. I had ran some quick calcs and even a top end gaming PC would be maxing out at 1kW, TV would be 300w, etc. Even running an oil heater (which I used to do but no need since central heating now) should be more than fine.

My next question alongside that is, some of the roof facing sockets are in the attic space (so I can access the rear of the back boxes), so when redoing those I was thinking about foaming around the side/top/bottom of them, as it'll get very cold in there and I imagine the metal will condense something fierce. I'll be mindful about covering the cables at the bottom too much, but is the effect of insulation cumulative across the entire ring circuit, or does it only matter per cable?

I imagine any derating of any single cable would derate the entire ring circuit, but if every cable only has a short section of foam on it, would that be okay?



Yeah, I could feel one when it was windy. The attic has glass wool in it, but glass wool isn't the best at stopping air movement entirely, I have found. At least in ancient 150+ year old attics.

I think adequate ventilation would be the best answer to sort any condensation.

 

[nostos156]

I think adequate ventilation would be the best answer to sort any condensation.

Maybe, but I think its undeniable that the box basically placed out in a draughty attic space will end up becoming rusty over time, ventilation or not. Which is why I'm more curious on my question about the foam.

 

[davesparks]

Maybe, but I think its undeniable that the box basically placed out in a draughty attic space will end up becoming rusty over time, ventilation or not. Which is why I'm more curious on my question about the foam.

Why not use plastic boxes?

 

[nostos156]

Why not use plastic boxes?

Because those walls are crumbly lath and plaster.