Insulation and thermal bridging and an EV charger

Personally I prefer not to run cables within thermal insulation, but pick a side (inside or outside) and run them to that side as far as possible. A trick I've used with 75mm celotex is to have "chases" where strips of 50mm rather than 75mm board is used - which gives a 25mm void behind the plasterboard (but still "inside" of the insulation) for cables and back boxes. Foil tape over the insulation board joints maintains the vapour barrier.

I haven't quite pictured the insulation board/studs/battens arrangement - I'm guessing the 70mm studs will be packed off the walls by 5-10mm and insulation boards between? (Trying to squeeze 75mm celotex/kingspan board into a 70mm gap by screwing the plasterboard down on top of them would be a disaster - PIR/PUR boards might feel a bit soft when pressure is concentrated over a small area like a finger, but it takes an awful lot of force to squeeze them over larger areas - and if there's a gap between the studs and plasterboard, the screws heads will pop though the plaster when the studs shrink and then expand with the seasons.

I'm not sure I'm making sense of the EVSE cable route - without anything visible to mark its route it looks like it would be outside zones - which would be a no-no unless you can ensure its more than 50mm deep, regardless of 30mA RCD protection.

   - Andy.

A few more points from Andys reply. I dont thnk metal trunking has enough mechanical protection to allow the cables contained within it to be non RCD protected. the usual answer is 2mm of metal for mechanical protection, though I dont think 20mm conduit has a wall thickness of 2mm, but it is certainly a lot thicker than metal trunking.

Building Regs. I also dont think 75mm of insulation is enough to comply with curent building regs.

As for the rating factors, just use the tables in BS7671 for an approximate level of the derating required.

Could the "tails" be moved to the outer face of the wall in suitably weather-tight containment?

Thanks for the reply Andy
I  persuaded the owner to let me run the EV cable external.
I just didn't like it in the insulated stud.

Im not sure what he is planing with the insulation boards being proud of the stud.
As you say it will cause problems with the plaster board.

So just to recap a garage is being converted into a TV room
so what was surface mounted in now going to be hidden in stud.
Which he wants to insulate.

One thing I can't move is the meter tails
These tails are going to be in the stud wall. I have placed these tails into 50x50 mm earthed metal trunking.
as they are outside the zone of the CU . OK when surface ran in a garage, but now its being boarded over.
So that would be ref method B so for 25mm tails 101A

But if that wall is insulated it comes down to 80A  -Table 4D1A
Would that mean the insulation touching the trunking and covering it?

What if the insulation touched the sides but not the front and back

What if you left a gap say 25mm between the trunking and the insulation at the sides and non on top?

I'm not sure how this wall can contain insulation and tails, and the tails will not be moved?

JohnE said:

But if that wall is insulated it comes down to 80A  -Table 4D1A

Agreed, so now the DNO's fuse needs to be downgraded to suit.

Thanks vey much for the comments
I am leaning towards suggesting running an external cable.
Run 25mm swa  into suitable external enclosures and feed the conductors through the wall
 and into the box and CU. Add a couple of layers of heat shrink on to act as a second layer of insulation and feed into the board?



Just on the subject of cable in an insulated wall
Thanks very much. If more stud was added  so as much of the wall was insulated as possible 
but we has a small section left un insulated. would those cables still be considered in an insulated wall?
Would that be acceptable?

JohnE said:

Run 25mm swa  into suitable external enclosures and feed the conductors through the wall
 and into the box and CU. Add a couple of layers of heat shrink on to act as a second layer of insulation and feed into the board?

SWA should be properly terminated at both ends, although we have discussed in the past whether a meter cupboard is an enclosure and whether the insulation of the cores is adequate for such a location.

I really do not think that heat shrink around the cores would give adequate mechanical protection where it goes through the wall, and in any event, how would you terminate the SWA?

That meter cupboard looks terribly tight, but the original meter was probably significantly smaller and there may have been no isolator.

I dont thnk metal trunking has enough mechanical protection to allow the cables contained within it to be non RCD protected. the usual answer is 2mm of metal for mechanical protection, though I dont think 20mm conduit has a wall thickness of 2mm, but it is certainly a lot thicker than metal trunking.

I reckon earthed  steel trunking is OK in that respect - it doesn't, as you correctly say, provide adequate mechanical protection, but it does provide protection by guaranteeing ADS should anything metallic pierce through to a line conductor (in much the same way as for BS 8436 cables, or indeed SWA - as it's dead easy to force something sharp between the strands of the armour).

Regs wise, it meets 522.6.204 (iii), rather than (iv).

   - Andy. 

One thing I can't move is the meter tails
These tails are going to be in the stud wall. I have placed these tails into 50x50 mm earthed metal trunking.
as they are outside the zone of the CU .

For myself I'm a bit uneasy about the steel trunking inside the insulated wall - whatever you do almost all the depth of the thermal insulation will be lost, which may not be that significant for overall heat loss, but will be significant locally - likely leading to condensation and related problems. If it were mine I'd probably run the tails on the surface in PVC trunking inside then dive through the insulated wall for the shortest distance to the meter box. Circuits after the CU (and so with 30mA RCD protection) can go, within zones, behind the plasterboard (with strips of 50mm rather than 75mm insulation behind).

  - Andy.

My thinking was to terminate the SWA in  enclosures, so properly glanded off.
Then install some plastic conduit through the wall and run the cores of the SWA  through the
conduit and into the board. A few layers of heat shrink also to act like a sheath?