2.5mm T+E and more than 100mm roof insulation?

[skyblob]

Hi. A questions for the forum

I'll be installing a ring in a new extension. (So far I'm taking lots of care with IEE wiring regs. I need to get it right, and anyway my Building Control officer will need to organize my Building Completion Certificate under Part P sections 1.24..1.27).

So here I have a shallow pitch roof (open to the room) with 150mm PU insulation between rafters and another 25mm PU+plasterboard under. Insulation is not yet in.

BS7671/Table 7.3 in on-site guide says that 2.5mm T+E cannot be used at all for thermal reasons in a ring circuit in a ceiling with >100mm insulation. (method 101). It does not seem to me to distinguish whether the cable is beneath, above or inside the insulation. It seems that I just cant do it. I could do it with 4mm cable, or with 2.5mm used as separate (shorter) radial circuits.

I have asked on electrician who claims he's install it anyway as a ring, and put it above the insulation. (It's OK for partP competent person eh?- but the spotlight would be on me). So is Is this strictly allowed? I don't read that from my understanding of the regulations.

I do see elsewhere on this forum that someone suggests a plastic conduit to bring it through the foam from top to bottom where it needs to penetrate, and provide some ventilation, but again this isn't spelled out in IEE regs as acceptable.

So bottom line, if I install 2.5mm ring ABOVE 175mm insulation with occasional runs THROUGH it, would I get pulled up at inspection time?

Or do I really use 4mm? - Is that really what all you competent persons do? - I don't think there is a new roof anywhere which meets insulation regs with less than 100mm these days.

 

[ban-all-sheds]

my Building Control officer will need to organize my Building Completion Certificate under Part P sections 1.24..1.27).

Does he know about that?

Is he prepared to do it?

I have asked on electrician who claims he's install it anyway as a ring, and put it above the insulation. (It's OK for partP competent person eh?-

No, it isn't alright, but sadly there are quite a few electricians out there who don't move with the times.

So is Is this strictly allowed?

No.

I do see elsewhere on this forum that someone suggests a plastic conduit to bring it through the foam from top to bottom where it needs to penetrate, and provide some ventilation, but again this isn't spelled out in IEE regs as acceptable.

I'd say it isn't - if you look at installation methods 1, 2 & 3, conduit makes no difference.

So bottom line, if I install 2.5mm ring ABOVE 175mm insulation with occasional runs THROUGH it, would I get pulled up at inspection time?

Hopefully.

Why are you running a socket circuit in the roof, anyway?

Where's the vapour control layer?

Or do I really use 4mm?

Yes.

Is that really what all you competent persons do?

It should be.

I don't think there is a new roof anywhere which meets insulation regs with less than 100mm these days.

There aren't many roofs with socket circuits in the insulation....

 

[ColJack]

Table 4A2 (continued - Installation methods specifically for flat twin and earth cables in thermal insulation)

the operative word there is IN.. if it's above the insulation then it's not method 100, 101, 102 or 103.. it's free air..or clipped direct but as they are very close in the current ratings ( with clipped direct being the lower of the two ), and table 4D5 doesn't mention free air, take it as clipped direct.

you should be aware of the interaction between PVC cable and the foam insulation products.. the wires tend to melt into / through the foam, so putting it on boards or cable tray where it crosses the insulation, and in conduits where it passes through the insulation ( recommend 25mm for air space.. ) will prevent this.

EDIT: sorry, my bad, I thought this was in a loft space above the ceiling, I dodn't read it right.. there is no loft space?

then BAS is probably right.. it would be classed as a partition so would be ref 103? perhaps?
or not as if it's on the tile side of the insulation then it's going to be in a very narrow "loft space" where the tiles are on cross battens allowing some air flow????
it's late and my head hurts..

 

[LoveRocket]

Do you really need a 32a ring circuit? Just because these have always been installed, doesn't been they're needed nor the best.

 

[skyblob]

Thank you guys for taking me seriously, especially BOS.

One or two points:

Building Completion Certificate under Part P sections 1.24..1.27: I have asked BCO (he hasn't replied yet). He has been pretty good though before, (he didn't grumble when he inspected my part J flue install), and did mention possibility on his visits schedule, so I hope to organize that one OK without having to wave the book at him.

Why am I running sockets through roof? - Well I'm looking at the possibilities. It could have helped shorten some of the wiring from the consumer unit, because it's open plan to a low pitched roof, but I have now concluded, especially after this conv that it's still a silly idea, and as you say it doesn't obviously meet regs anyway without other compromises. QED.

Re 4mm: "Is it really what competent persons do" - As you say, let's hope so

Where's the vapour control layer? - Kingspan K7 is foil lined and I also have big rolls of foil tape at the ready.


And for ColJack:
PVC and foam insulation: I believe this problem applies only to polystyrene foam? (Please correct me if you have found otherwise). Kingspan do say,"Electrical cables and the PVC sheathing remain unaffected when positioned next to either rigid phenolic or urethane insulation".

"the operative word there is IN.. ". Yes I see your point. There is a 25mm air gap between the top of the foam and the underlay. So technically using this, one could run as clipped direct. I'd still need to bring it through though, and an air gap/conduit would compromise insulation. It's probably easier for me to forget the idea for the 2.5mm.
After our little talk I shall now run only the lighting up there, and lighting is allowed to be IN under method 101 (or even 103), I don't need to worry about air-space conduits.

And for LoveRocket: It is a kitchen so I'll stick with the ring, - lots going on.

Regards,

 

[ban-all-sheds]

Where's the vapour control layer? - Kingspan K7 is foil lined and I also have big rolls of foil tape at the ready.

So when you poke holes in that to allow cables to pass through it.....

It is a kitchen so I'll stick with the ring

Kitchens are often characterised by a number of large loads close together - W/M, T/D, D/W etc, and therefore a ring is a poor choice.

 

[oharaf]

I fully understand what you're asking here. I recently had to carryout a similar installation, bungalow with vaulted ceiling. I was on site from start to finish. TO satisfy LABC the cabling has been run against the joist work through 2 routes only.

The cables are upto the lat work for the roof. Cellotex and then Kingspan has been used to insulate the roof.

The 2 voids containing cable contain kingspan but a void has been left around the cable. We also just happenened to use LSX cable rather than T&E.

Work with LABC then there is no heart ache later on

 

[plugwash]

Kitchens are often characterised by a number of large loads close together - W/M, T/D, D/W etc, and therefore a ring is a poor choice.

Depends really. If the runs at the end of the ring are long and balanced with each other a ring is fine. OTOH a ring in a kitchen very close to the fusebox requires more care.