# Cable derating-insulated walls

Morning all,

Mulling over options for rewiring sockets in a room where the 2 external walls will be internally insulated (100mm solid stuff). This isn't a new circuit so (unless I've misread the regs) not notifiable- however I would like to do it correctly.

At the moment, 2.5mm T & E cables are under the timber floor then chased 300mm up the wall into backboxes. Underfloor is accessible so I can replace cable where necessary rather than joint it (to get the extra length since the sockets are going to be 125mm at least away from the masonry wall)

Question is how exactly to apply the derating factors in table F2 (Onsite Guide BS 7621) if I just push the cables through the insulation. There will be 4 sockets in that part of the ring where cabling would pass through insulation- does that mean I need to consider the ring having passed through 800mm of insulation (so derate to 50%) or do I instead consider that each single cable has passed through 100mm of insulation so derate that single cable to 78%?

I'd prefer to use the above method (one big advantage is it keeps the cables at least 100mm away from the wall surface so pretty safe from future fixing attempts) but not if it means using 6mm T & E to overcome the derating effect of the insulation.

Plan B does exist- bring cables up through floor just behind skirting board and up the front face of the insulation so that it is in contact with the plasterboard (Method 102).

And Plan C also exists- use 4mm T & E in conduit through the insulation for those sockets affected, reduce back down to 2.5mm at sockets not affected by insulation (on internal walls)

All advice appreciated (except the one about insulate externally- can't in these 2 rooms, only 100mm between my place & next door!). Oh yes, the CU is fairly modern split load with RCD on both sides so that bit is covered.

Question is how exactly to apply the derating factors in table F2 (Onsite Guide BS 7621) if I just push the cables through the insulation. There will be 4 sockets in that part of the ring where cabling would pass through insulation- does that mean I need to consider the ring having passed through 800mm of insulation (so derate to 50%) or do I instead consider that each single cable has passed through 100mm of insulation so derate that single cable to 78%?
Common sense suggests the latter, but I'll be interested to hear other views. That (if anything!) is certainly what I would personally do.

Kind Regards, John

There will be 4 sockets in that part of the ring where cabling would pass through insulation- does that mean I need to consider the ring having passed through 800mm of insulation (so derate to 50%)
No.
or do I instead consider that each single cable has passed through 100mm of insulation so derate that single cable to 78%?
Yes.
27A x 0.78 = 21A

Plan B does exist- bring cables up through floor just behind skirting board and up the front face of the insulation so that it is in contact with the plasterboard (Method 102).
Method 102. 2.5mm² 21A.
That would seem the easiest method, surely.
And Plan C also exists- use 4mm T & E in conduit through the insulation for those sockets affected,
Method A. 2.5mm² 20A.
reduce back down to 2.5mm at sockets not affected by insulation (on internal walls)
Why bother reducing? 4mm² is nice.

Ta both

..........

Method 102. 2.5mm² 21A.
That would seem the easiest method, surely.

It is definitely the easiest but inelegant and means I'll have a gap between plasterboard and insulation which will give me that nasty hollow partition wall sound to the wall (if that makes sense- experiments in the shower room downstairs gave a much more solid sound to the wall with no gap between plasterboard and Kingspan or whatever- def be solid, depends what's cheap when I get to that stage ). Don't really want to groove the insulation either- its a right messy job

Method A. 2.5mm² 20A.

Why bother reducing? 4mm² is nice.

4mm² is about 175% of the price of 2.5mm² (£58/100 instead of £33/100) so that's a bit of a factor. And I got that 20A figure for 2.5 as well, which searching on here tells me means I can't use it as part of a ring final with a 32A breaker on it (sure I saw somewhere on here that each leg needed capacity of 21A under those circs). Am I wrong (as in can I use 2.5mm Method A as part of a ring circuit with 32A breaker at the CU)?

... And I got that 20A figure for 2.5 as well, which searching on here tells me means I can't use it as part of a ring final with a 32A breaker on it (sure I saw somewhere on here that each leg needed capacity of 21A under those circs). Am I wrong ...?
You are wrong The requirement for cable for a 32A ring final is that it should have a CCC (after de-rating etc.) no less than 20A.

Kind Regards, John

You are wrong The requirement for cable for a 32A ring final is that it should have a CCC (after de-rating etc.) no less than 20A.

Kind Regards, John
Awesome- cheers chap, that'll do nicely.Conduit here we go......... (2 x 20mm I think, easy life )

Have you thought about how you will maintain the absolutely essential vapour seal on the warm side of the insulation when you've got back-box recesses cut into it?

Have you thought about how you will maintain the absolutely essential vapour seal on the warm side of the insulation when you've got back-box recesses cut into it?
I have thought about it- not sure exactly how effective it'll be or how much of a pain in the a*se it'll be.
• Run cables up through floorboards/gap between boards and wall/behind the sole plate anyway
• Cut insulation to size (to fit between noggins & verticals).
• Make 2 x 20mm holes in insulation for the conduit stubs (not going to try bends in solid insulation, might try the first ones with flexible conduit from floorboards, we'll see), bring cables through conduits, fold slack flat against the insulation
• Mark exactly where the conduit holes are (might try using my shoddy laser pointing at them from directly across the room)
• Cut and part fix plasterboard to that bit of the wall
• Mark and make socket box cutout in plasterboard- go a bit deep to ensure the insulation is also marked/scored but not scoring through the cable which is just behind the plasterboard
• Take the plasterboard off
• Dig out about 25mm of insulation where the backbox marking is (and that's going to be a messy job, need to invent a Tool to try and do it neatly. Does hot wire cut Kingspan?)
• Push conduit back so the ends are just proud of the back of the hole
• Tape hole with the shiny foil tape overlapped onto the (foil faced) insulation board and round the conduit, seal the cable into the conduit ends (silicon)
• Final fit the plasterboard
• Once wall skim is done, fit backbox, socket etc
I'm fairly sure that this procedure won't last past fitting the first socket but it is a start

Not sure why you think making the cutouts for the backbox in Kingspan/Celotex/other make is difficult or messy. I'll assume you use all the correct ppe to do this and cover merely the process which works well for cuts up to about 2/3 depth of insulation.

- Mark the shape.
- Take a snapoff or retractable knife with a new or sharp blade and set at the depth you want in the insulation plus 2mm.
- Use a straight edge, preferably metal and preferably with finger guards to cut the edges of the shape, penetrating to the depth of the exposed blade.
- The place metal edge again bottom cut. Insert knife at top edge of cutout and cut down to metal edge. Do this in approx. 1 to 2 cm spacing across area.
- Repeat going horizontal.
- Carefully snap each of the standing cut cubes out
- Might need a little tidying up, but mostly they snap out cleanly at the depth you have cut to without lots of dust.

And why does the Thanks button disappear once you've Liked a post? Ta for that

And why does the Thanks button disappear once you've Liked a post?
So that one can't both Like and Thank the same post! You can 'undo' your Like and Thank the post instead, if you want.

Kind Regards, John

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