Another garden room wiring question

20 mm PVC electrical conduit? Be aware that significant de-rating may be required.

Table 7.1(ii) of the OSG derates 6.0mm² in conduit in an insulated wall to 32 A and this is 32 A circuit. Should be OK?

Is there anything special about 20mm PVC conduit? I happen to have a lot of 22mm plastic water pipe!

Mike

22mm plumbing fittings will not fit electrical boxes so well...  Standard hole sizes are 20mm or 25mm  Apart from that a pipe is a pipe, the wire will not care. A short hop in any sort of tubing will not cause as much derating as a long length. Less than a few inches can be safely neglected,

Mike

Thanks Mike, good point about the fitting. Total length of conduit will only be 150mm so should be OK then.

Mike

How are you going to join a piece of water pipe into the back of an IP rated enclosure mounted on the outer face of the exterior wall and maintain the IP rating?

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.

Good point, I won't be using water pipe then (c:

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.

Eeh.

I`ll rest as happy with OSG, SWA, CU, T & E and PVC but please can you confirm the meaning of SIP, OSB & PU please fellahs? If I don`t get an answer I`ll not ask my MP (Member of Parliament not Military Policeman by the way!) 

Thanks