Another garden room wiring question

The derating factor of 6 inches of insulation is NOT the tabulated factor for an insulated wall. Given a reasonable air space around the cable, the factor is 1.0, because it is simply not possible for a short length to get very hot, there is a very significant conductive transfer along the cable, and with some air circulation the conditions are entirely different to a cable encased in insulation running along a wall as in the reference method diagrams.

I would run the SWA into the building, then convert to T&E if you want, it will be much easier. Why not run the SWA to the local CU? The pipe can then have a space around the cable, vent to the outside, and have a simple silicone seal on the inside. A 50mm pipe would make life much easier to thread and install and the heat loss difference is negligible.

Thanks David. I have two concerns here. One is maintaining the integrity of the Tyvek membrane on the outside of the SIP, which is why I want to prepare the hole myself. The other is accommodating the bend radius of the SWA (90-100mm?) if it were to be routed through the wall. I don't imagine drilling a SIP is easy and drilling it at an angle would be even harder. Hence the reason for converting from SWA to T+E outside the building.

Mike

What size SWA is it? I suppose you are looking at a 4mm2 or 6mm2 size for some heating etc. This will easily travel through a 35mm sweep elbow (plumbing) which you can seal through the Tyvek with silicone. It is easy to drill these panels, use a long drill to make a centre hole right through. Use a suitable hole saw on each side to drill the wafer board and into the PU. Finish the PU hole with a chisel or gouge to just fit the pipe. At the inside end of the pipe fit a cap with a hole to fit the SWA, and seal with silicone to keep insects out. The elbow prevents any water ingress and looks quite tidy. Do not seal the outside end of the pipe, you want the ventilation. You could run pipe right down the wall if you prefer. I assume you are tiling the outside, or perhaps a brick skin. This can easily be built around the pipe which will then be hidden  or run the SWA behind the tiles or bricks.

What size SWA is it? I suppose you are looking at a 4mm2 or 6mm2 size for some heating etc. This will easily travel through a 35mm sweep elbow (plumbing) which you can seal through the Tyvek with silicone. It is easy to drill these panels, use a long drill to make a centre hole right through. Use a suitable hole saw on each side to drill the wafer board and into the PU. Finish the PU hole with a chisel or gouge to just fit the pipe. At the inside end of the pipe fit a cap with a hole to fit the SWA, and seal with silicone to keep insects out. The elbow prevents any water ingress and looks quite tidy. Do not seal the outside end of the pipe, you want the ventilation. You could run pipe right down the wall if you prefer. I assume you are tiling the outside, or perhaps a brick skin. This can easily be built around the pipe which will then be hidden  or run the SWA behind the tiles or bricks.

David

It's a 32 A circiuit and I think the electrician advised 6mm2 SWA.

The building will be clad with tongue & groove cedar. The spacing between the cladding and the SIP wall is only 25mm so the SWA will be run on the surface of the cladding. It's the back of the building so can't be seen.

I was planning to use a hole cutter with an extension bar to go through the SIP wall, insert and seal PVC conduit then fit the cladding over that.

Mike