Junction boxes, cables and 250mm Rockwool
 

Hi

I'm busy rolling out Rockwool as per the BCO's instructions, 100mm
between the joists and then another 150mm transversely over the top.
God what a job. Haven't done this in 20 years, but it's as bad as I
remember it. No, I'm lying, it's worse, because (a) my joints are 20
years older and (b) back in them days you only needed 50mm insulation
or something.

So I'm now building up a veritable sea of glass fibre up there. No
way of telling where the joists are now, so heaven help me if I ever
need to access it again.

The Rockwool instructions helpfully tell you to avoid covering
electrical fittings, junction boxes etc - but how the hell are you
supposed to do that? I've been attaching my cables and any jct boxes
to joists, but these have now vanished. I have visions of attaching
little timber masts to the joists, to attach the jct boxes etc just so
I can see them above the Rockwool! This can't be an unusual
problem...

David

Junction boxes, cables and 250mm Rockwool
 

In article ,
(Lobster) writes:

The Rockwool instructions helpfully tell you to avoid covering
electrical fittings, junction boxes etc - but how the hell are you
supposed to do that? I've been attaching my cables and any jct boxes
to joists, but these have now vanished. I have visions of attaching
little timber masts to the joists, to attach the jct boxes etc just so
I can see them above the Rockwool! This can't be an unusual
problem...

When running cable inside thermal insulation, you have to derate it
by 50%. The free air rating of 1.0mm² is around 14A. In lighting
circuits, it's normally protected at 5A or 6A (and the design loading
maybe even less). This means it's already derated by more than 50%,
so you can cover it safely. Another issue is damaging a covered
cable by stepping on it and not noticing, or something similar.

If you have any other circuits up there, you will have do do a similar
calculation, and you might have to reroute the cable or increase the
conductor size. (Electric shower would be a particular concern,
and ring circuit would need sorting too.)

--
Andrew Gabriel

Junction boxes, cables and 250mm Rockwool
 

In message ,
(Andrew Gabriel) wrote:

In article ,
(Lobster) writes:

The Rockwool instructions helpfully tell you to avoid covering
electrical fittings, junction boxes etc - but how the hell are you
supposed to do that? I've been attaching my cables and any jct boxes
to joists, but these have now vanished. I have visions of attaching
little timber masts to the joists, to attach the jct boxes etc just so
I can see them above the Rockwool! This can't be an unusual
problem...

When running cable inside thermal insulation, you have to derate it
by 50%. The free air rating of 1.0mm² is around 14A. In lighting
circuits, it's normally protected at 5A or 6A (and the design loading
maybe even less). This means it's already derated by more than 50%,
so you can cover it safely. Another issue is damaging a covered
cable by stepping on it and not noticing, or something similar.

If you have any other circuits up there, you will have do do a similar
calculation, and you might have to reroute the cable or increase the
conductor size. (Electric shower would be a particular concern,
and ring circuit would need sorting too.)


The shower, quite possibly, the ring, maybe not. I don't have the book
to hand (it's in the car and it's peeing down outside) but IIRC method
15 rates 2.5mm2 T&E cable at 21A which is fine ("free air" rating is
27A). Method 15 is "in conduit, surrounded by insulation" which, when
you study the detail, basically means that if your cables are tacked to
the joists (for these purposes wood is considered to be thermally
conductive) or are generally in contact with the ceiling boards (again
thermally conductive) then you're ok.

Shower cable is usually run much closer to its maximum rating -
particularly 6mm2 cable running 8.5kW of shower - and so has less
leeway. Installing 10mm2 cable can be a right pig, so if this is a
problem it may be worthwhile investigating other solutions.

As you say though, 1mm2 cable should be fine, even if it isn't in
contact with a thermally conductive surface on one side.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
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