Ducting first fix wiring

[PeterW]

I'm looking at first fix wiring at the moment and three of the walls that will have sockets on are external existing walls that are being lined with 25mm Kooltherm prior to being boarded. I've managed so far to keep the sockets on either internal walls or stud walls to keep chasing out to a minimum. 

 

I'd considered using battens to hold the board and run a couple where the sockets are so that there is a defined "run" to the back boxes however I'm now considering using a 25mm duct to the back boxes (it's as cheap as batten..!) and adding two for the doubles; this would then be foamed into place and cut flush to hold it in. 

 

Questions are :

 

- does this count as "insulation" and does it de-rate the circuits..?

- are 35mm back boxes deep enough for decent double sockets...?

- is it really worth doing this and would I be creating work for myself ..??

 

Thoughts please ..!

[Nickfromwales]

You'll get 2x 2.5mm T&E's down a 25mm tube without any trouble so why fit two?

If the tube is open to the joist voids then I doubt very much if you'd need to consider grouping or insulation factors, with it being so close to the surface and the sockets not being airtight. You'll get a little convection airflow through there too so that'll help keep things at ambient. 

Oh, and 35mm is a good size in case any are USB. 

[PeterW]

Thanks @Nickfromwales

 

I was only thinking 2 just in case I wanted to run a spur off but tbh I think it will be fine. 

 

Not it sure about those USB ones yet - haven't found one I like that doesn't look dodgy .... 

[Onoff]

Got a sketch of what you're thinking?

 

Not sure what installation method this would equate to. Enclosing the conduit in foam with obviously de-rate it's heat dissipation ability. Obviously vertical runs / ducts are more likely to suffer higher ambient temperatures at the top of the run. A horizontal run like yours doesn't sound like it's going to be particularly airtight unless I'm wrong so it'll have "gaps" at the face plates. 

 

You want a 35mm box if considering flat plate accessories though some with go in a standard back box. Usually says in the accessory spec.

 

I don't think the regs have really "caught" up with all Passive type wall build up variations.

[ProDave]

Do you even need duct? Surface mount the cables on a tiny recess cut in the insulation.  Note to anyone reading, do NOT do that if using EPS or XPS, pvc cable and polystyrene do not mix.

 

+1 to 35mm boxes. Don't be fooled by some flat plate accessories that claim to "fit" 25mm back boxes. I found some that just came with a spacer for use with a 25mm box. Hardly the flush look you were hoping for,

[PeterW]

4 hours ago, Onoff said:

Got a sketch of what you're thinking?

 

Not sure what installation method this would equate to. Enclosing the conduit in foam with obviously de-rate it's heat dissipation ability. Obviously vertical runs / ducts are more likely to suffer higher ambient temperatures at the top of the run. A horizontal run like yours doesn't sound like it's going to be particularly airtight unless I'm wrong so it'll have "gaps" at the face plates. 

 

You want a 35mm box if considering flat plate accessories though some with go in a standard back box. Usually says in the accessory spec.

 

I don't think the regs have really "caught" up with all Passive type wall build up variations.

 

All I'm looking to do is run vertical ducts from the ceiling void to the back box between the edges of two bits of 25mm PUR foam board. To fill the gaps and keep it all tight I will then foam over the top - pretty much like you've done with your soil pipe in the wall. 

 

Was thinking the duct would give decent mechanical protection for the cables - other option is surface mount and cover with steel cover strip and then foam over. 

[Nickfromwales]

@ProDave

Where does this stand with de-rating? ( if vertically run in conduit ).  

[ProDave]

If you can (in effect) run the cables on the surface, between the insulation behind them, and the plasterboard in front of them, then I don't believe they need derating. I stand to be corrected as the regs are ambiguous, but derating applies when IN insulation, not when ON insulation.

 

For the sake of making a narrow service void, you can totally avoid derating, and I waa suggesting a way to achieve that may be in effect to chase a channel into the insulation for the cables to sit on. imho if you sink a conduit into the insulation then foam over that, then the cables are IN insulation.

[PeterW]

Hmmm ... and if the conduit and the insulation are the same thickness ..??

 

How do they cope with insulated plasterboard then as that is in use a lot these days. 

[Onoff]

How about 25mm deep x 38mm wide mini trunking. You could butt your 25mm thick insulation up either side. You'd have plenty of space, air surrounding the cables for some cooling etc. What are the inside & outside faces of the 25mm insulation touching?

 

[PeterW]

Insulation is between inner blockwork skin and plasterboard. 

 

I suppose this is why I struggle with the derating issue as I can sandwich the cable between insulation and plasterboard and have no problems but when I put it into a duct that should allow airflow it doesn't count .... aaagghhh !!!

 

[Onoff]

15 hours ago, PeterW said:

Insulation is between inner blockwork skin and plasterboard. 

 

I suppose this is why I struggle with the derating issue as I can sandwich the cable between insulation and plasterboard and have no problems but when I put it into a duct that should allow airflow it doesn't count .... aaagghhh !!!

 

 

Don't forget conduit is really meant for singles historically which is why you wont find much definitive & discussed on T&E in conduit in books etc.

 

I reckon Installation Method 2, Reference Method A "enclosed in conduit in an insulated wall" is nearest if we equate T&E to a multicore. In fact the T&E table points to that diagram (Method 2) in very small print at the bottom.

 

http://www.ti-soft.com/en/support/help/electrical/reference/bs7671/tables/table-4a2-schedule-of-installation-methods-of-cables

 

Playing Devil's Advocate here: 

 

For 2.5mm T&E, Reference Method A you're looking at 20A current carrying capacity. (Quite a drop from 27A Clipped Direct).

 

Looking at the derating tables then for 2 multicore cables, Ref Methods A to F it's 0.8 ("Bunched in air, on a surface, embedded or enclosed"). For one cable there is no derating factor (well it's "1").

 

Maybe some mitigation as you're touching plasterboard one side and block the other. So the 2.5mm capacity drops to 16A worst case. I'm sure it'll be better than that as in reality you're not actually covering the conduit in the true sense of the word just foaming up, in ever decreasing thickness to the apex of the tube both sides.

 

This is a ring circuit btw is it?

 

 

 

 

[PeterW]

Correct - a ring circuit. 

 

Confused..?? Moi..?? You bet ..!

[Onoff]

39 minutes ago, PeterW said:

Correct - a ring circuit. 

 

Confused..?? Moi..?? You bet ..!

 

Why?

 

 

[Onoff]

Being pedantic but might as well have a crack (until I get bored )Can you take a stab at the thermal resistivity at the point where the conduit is, i.e. pb front, block back and 25m insulation either side? 

 

(There is no precedent for EXACTLY this detail that I know of).

 

EDIT: With that Inst. Method 2 it says "Multicore cable in conduit in a thermally insulated wall with an inner skin having a thermal conductance of not less than 10 W/m²K". So the inner skin is the pb. On the British Gypsum site I note for 12.5mm it says R (m²K/W) = 0.05. How the two figures correlate I haven't a clue!

Edited August 7, 2017 by Onoff

[PeterW]

Plasterboard is 20 then based on that as it's 1/R

 

Block ..? Will need to check ..!