loft insulation

[davetherave1979]

Hi Guys rewiring a flat 2 bed small flat the sockets in the bedrooms lounge cables will go through the roof which will get covered in hairy mearey if I deceide to ring main it whill I be able to use a 20mcb or shall i do a 4mm radial on a 20mcb cannot rember the derating factor for cables covered in insulation thanks

 

[stuart569]

Look in the brb/bgb and do the calculations properly then!And since when do you put ring mains on 20amp?

 

[trev]

Why? what's your reasoning for a 20A RFC?

 

[davetherave1979]

only need 20amp supply for tv small gear kitchen will be 32 ring thats not through the loft

 

[Rockingit]

If it's covered for more than a metre (in total) then you derate by 50%, so a 2.5mm T&E normally good for 27ish amps becomes 13ish. Derate protection devices accordingly.

Remember though that methods 100-104 allow for insulation and if the cable is laying on plasterboard ceiling/clipped to stud/joist and just covered (on one side) in insulation then it's still method C and you don't have to de-rate.

 

[davetherave1979]

Ok cool owner says he might be getting loft insulated after we leave so for the job we will take this as a yes havent rewired a flat were all cables run through loft for years so had to get my thinking cap on and its late we just going to fit a 20amp radial for small power in 4mm

 

[danzor]

Stick a 32a on it, you won't know what they're gonna be plugging in-save yourself a trip back to upgrade the breaker

 

[Richard Burns]

if the cable is laying on plasterboard ceiling/clipped to stud/joist and just covered (on one side) in insulation then it's still method C and you don't have to de-rate.

Is this not method 101 or 100 and is derated to 21 or 17 amps respectively for 2.5?
This would mean that insulation covered 4mm with cable touching the inner wall and >100mm insulation would be derated to 22A and you could not use a 32A breaker on such a radial.

 

[Deleted member 9648]

Use running boards and keep the majority of the wiring clear of the stuff....doesnt take much longer,just spike some 6x2's or something above the level of any insulation.

 

[Engineer54]

Agree with wirepuller here, keep the cables above the insulation level and you'll be fine. You can even use a small sized wire-basket to span the joists etc!!! That stuffs cheap enough, and makes the job look a little more professional too, ...Perfect for taking the scam assessor to see...lol!!!

 

[Rampage]

According to BGB:

Length going through insulation up to 200mm is Cr of 0.63
Up to 400mm is 0.51

 

[stuart569]

I must of been up hundreds of lofts that have 2.5mm ring finals absolutely covered in insulation and been like that for many years.Never heard of or came across any melted cables or shorts yet!

 

[davetherave1979]

Same hear 2.5 rings every wear covered in insulation that I have seen also

 

[Deleted member 19807]

I have always been a bit confused about loft insulation and the regs.

the regas state that table 52.2 is based on a value of 0.04 Wm-1 K-1, basicly 1 m2 of insulation will let 0.04w/degK through it.

loft insulation as detailed below has a value of 0.16Wm-1 K-1 so that will let 4 time the amount of heat through that the regs are calculated on, so i'm not sure "loft insulation" cannot be called "thermal insulation" with regards to the regs.

Loft Roll 270mm

I am happy to be put right on this if the above is wrong.