Insulation

[philrosenberg]

hi all
I hope this doesn't cause as much controversey as the last insulation post

We are about to have the insulation redone in our loft. As we are in a concrete floored bungalow all the cable for the house goes through the loft.
The current set up is cables draped on top of insulation, layed on fibreboard, nailed to joists.
The fibreboard is at best useless and at worst a hazzard. It isn't strong enough to support a persons weight and because it hides the joists i've already put my foot through it once (putting a hole in the ceiling below )

The new setup will be 100 mm of insulation between the joists plus 160 mm of insulation on top of the joists. My question is what do I do with the cables? I don't like the way they are just draped all over the place, but what do you pros normally do? Have I any other sensible options?

Also what about where the cables go down through the ceiling, does anyone know how much space i need around the cable to avoid the derating by going through the 260 mm of insulation, would running it through trunking do?

Cheers for any advice

Phil

 

[Big_Spark]

Phil...run in insulation is run in insulation...however in metalic protection, such as conduit or steel trunking, then the effects are ignored as largely irrelevent..not sure if this is the case with plastic trunking/tube though..

If you have a copy of the Regs book, then I would guide your attention to Page 206: Appendix 4: 6.1.1 which deals with this issue and the derating factors for different cable types in Table 4C1.

To be honest, it is difficult to give diffinitive advice on this issue as no-one here can see your installation. We do not know the condition of the cables, their age, the size of the conductors, the loading on the circuits or the overcurrent protective devices installed..these all have a bearing on the subject.

Apologies if this sounds like a cop-out, I am sure the Guys here that undertake mostly or at least a lot of domestic work may have encountered your situation and be in a better position to give perhaps more helpful advice..that is the peril of doing new installs in commercial and industrial environments

 

[ltheb]

Hi, Phil & Big Spark, I'm new to the forum but hope I can add my few pence worth - I've just been looking at another topic regarding insulating a shed...

As BS says insulation is insulation, HOWEVER, scratching my head a bit i re-read the items in building and wiring regs. The de-rating appears to apply to cables "surrounded" (ie in full contact) with insulation. If 0.5m or more of the cable is surrouned then the derating factor is 50%!
However, having cross-referenced this with the building regs they say if the cable is in contact with a thermally conductive surface (eg a wall) on one side or inconduit or trunking fixed to same then this appears to be ok (Reference Method 6 for the techies)

 

[jj4091]

I don't like the way they are just draped all over the place, but what do you pros normally do?

Phil

I would guess some "pro" wired it in the first place but if you are talking about professional loft insulators then they don't think twice about just rolling another 6" of insulation on top of everything. I have never had a problem with cables over heating except where they are close to flush ceiling fittings which the insulation should be kept well clear of. I would reccomend you check this before they leave if you have any light fittings of that type

 

[JohnD]

Lighting circuits are not really a problem with loft insulation. For example if they are run in 1.5mm, then even when they have been derated, they can still carry 6A with ease, which is what they are fused at.

It is more likely to be a problem with the cables for electric showers, which can get quite warm, and immersion heaters. These are better clipped to the loft timbers above the insulation, and it is worth calculating their rating to make sure.

I agree with jj4091 about lighting fittings, especially those horrible downlighters. Some people put a large terracotta flowerpot (yes, really!) over the lamp to prevent it being swamped in insulation. I am hoping that the wider use of low energy lamps (plus people realising how horrible downlighters are) will eventually reduce this problem.

 

[philrosenberg]

Cheers for the input
The wiring is not new, but it is pvc and I did an insulation check on it after I added some sockets and it was almost off the scale of my meter (at the good end )
I only have 3 circuits. Lighting (1.5 T+E 6A), ring main (2.5 T+E 30 A) and a radial for the cooker (sorry can't remember the cabe csa or fuse).
All fed by rewirable fuses on a TT supply with a single rcd covering the lot - not ideal I know but I . (actually I have an underground supply with lead sheathed cables that are not earthed and I have been wondering if it's worth a call to the DNO to PME it, but we've been there in another thread)

I've just pulled my calculator out to work out cable ratings. Am I right in saying that cable in a ring with a 30/32A fuse should be rated at 20A? Am I also right in saying that rewirable fuses are also known as semi-enclosed fuses and have a derating factor of 0.725?
In this case my 2.5 mmsq ring main is already underated before I apply any other correction factors - 27 A rating (2.5 mmsq clipped direct) x 0.725 = 19.6 A.
Add to that the loft temperature is probably 40 degrees in summer, another derating of 0.94 gives 18.4 A

I'm hoping someone is going to correct me here and tell me different

If anyone has any further advice it would be very welcome right now

 

[philrosenberg]

I would guess some "pro" wired it in the first place

Yes, well, I wouldn't necessarilly agree. I've found numerous loose (or not done up at all ) connections in fixtures, and if my numbers are right in my last post, then it again does not bode too well.
He might have been a spark for a living, but that's not the same as being a pro

 

[JohnD]

You've calculated it as 18.4A after derating? And it's a ring with two parallel (ish) legs, and reasonably distributed load?

Then the 30A rated fuse (rewirables are not 32A) spread between the two legs sounds fine to me (18.4A per leg = 36.8A capacity). Rings are wired in 2.5mm so that they are usually OK after typical de-rating. It is rare to find a ring that needs to be run in 4mm.

If the cables have a bit of slack in them you can run some timbers on top of the joists, deep enough to clear the insulation, and clip the cables to that. It will probably only be the ring final and immersion heater that need it.

p.s. calculate it again for an MCB. If it's a Wylex I can send you some at modest cost

 

[philrosenberg]

Cheers John
I was looking at reg 433-02-04 for the value of 20A

An MCB protected 2.5mmsq clipped direct cable at 40 degrees would be rated at 23.5, so that would at least sort that out. Still wouldn't fix my loft insulation problems though. I guess I have to round up the thickness passing through insulation for table 52A so that meand my 260mm of insulation has a derating factor of 0.55, This takes the cable on an MCB down to 12.9 A.

I guess I'll have to put my thinking cap on to work out how best to sort it.

What's that modest price by the way? not sure how personal messages work on this board but my email is if you want to drop me a mail.

Just found this document on the iee page, makes an interesting read
http://www.iee.org/Publish/WireRegs...rent_ratings_cables_in_thermal_insulation.pdf

 

[JohnD]

I have mailed you

 

[philrosenberg]

Cheers for all the advice. I'll be trying to raise the cable above the insulation where possible as John mentioned. I'll put it up against joists where it comes up through the insulation and I will also try putting something like 4" tubing cut down its length round it so it can breathe. Where this is not practical I'll run it allong the bottom of joists, which gives it the highest rating when in insulation.

Cheers again

Phil