Cables in loft

[Taffyo]

Hi all, I was wondering if the cables in my loft are running safely. The shower cable is 10mm, this runs above the insulation all the way and is clipped to the joists, the only other cable up the is 1.0mm lighting circuit for the upstairs which feeds 2 bedrooms (1 light each), a landing and the bathroom light and fan, these cables are clipped to the joists but half way below the level of the insulation, the insulation is about 100 - 125mm thick.

Thanks
Taff

 

[Chri5]

Cables under insulation will get warmer, and have to be derated re: load capacity. They shouldn't be under the insulation.

Could you pop the clips and lay the cable over the insulation to allow some air flow around the cable ?

Most lighting circuits are 1.0mm or 1.5mm TE wired to a 6A fuse at the board.

If you use LE 21w lamps then the load over 3 lights (up hall, bed 1 and bed 2) would be a modest 63w, which is a tad over 0.25A.

This means that even with 600% derating the 1.0m cable could handle 1.0A

Even though the load could be very small, it's still not acceptable (in my book) to ignore the problem on the basis of low load. At some time in the future 2 x 10 arm 600w chandeliers could be installed. So I'd suggest you unclip the cables, and have the insulation under the cable. A nominal 125-150mm in insulation (between top of insulation and ceiling) is ok.

 

[davelx]

1.0 mm cable clipped direct will handle 15A. So, worst case - surrounded by insulation but with no other derating factors that's 7.5A (16ed tables - don't have 17th to hand)

Assuming that your 6 amp "fuse" in in fact an MCB, you should be fine.

If it's actually a BS3036 rewireable fuse (it would be 5A), then it's still just inside (7.5*0.725 = 5.4 >5)

Unless I'm missing something?

 

[ban-all-sheds]

Even though the load could be very small, it's still not acceptable (in my book) to ignore the problem on the basis of low load.

It's not acceptable in the IET's book either...

 

[scousespark]

Even though the load could be very small, it's still not acceptable (in my book) to ignore the problem on the basis of low load.

It's not acceptable in the IET's book either...

It is acceptable. If not, there would not be a correction factor.
The rules for cable calcs would state 'Do not cover with insulation'.
The present load does not exceed the cable carrying capacity.
We don't design based on some unknown possible future load.

FWIW, the cable is safer where it is than running along the top of the joist.

 

[davelx]

It is acceptable. If not, there would not be a correction factor.
The rules for cable calcs would state 'Do not cover with insulation'.
The present load does not exceed the cable carrying capacity.
We don't design based on some unknown possible future load.

FWIW, the cable is safer where it is than running along the top of the joist.

Surely you meant " It is not acceptable. If it were, there would not be a correction factor".

The existence of the correction factor means you are not ignoring the issue!

(edited for lousy typing)

 

[scousespark]

I mean, It is acceptable to install where insulation is present, providing the cable used is big enough to carry the laod, after correction factors have been applied. In this instance it is within tolerance.

The OP doesn't have a problem. IMO.

 

[ban-all-sheds]

Even though the load could be very small, it's still not acceptable (in my book) to ignore the problem on the basis of low load.

It's not acceptable in the IET's book either...

It is acceptable. If not, there would not be a correction factor.

Chri5 was clearly talking about the situation where the corrected de-rated capacity of the cable was OK for the actual load current but not OK for the maximum current that the breaker would pass....

 

[scousespark]

I don't know what you mean BAS. Dave has done the calcs and show the values for 5A fuse and 6A breaker.

The calcs are done for the present load, not an unknown future load.

 

[ban-all-sheds]

Ib <= In <= Iz.

The current load may satisfy the first half of that, but I took "Even though the load could be very small, it's still not acceptable (in my book) to ignore the problem on the basis of low load. At some time in the future 2 x 10 arm 600w chandeliers could be installed." to mean that it wasn't acceptable in his book to ignore In not being <= Iz just because, for now, Ib <= In.

All I was doing was pointing out that it's not acceptable in the Wiring Regulations either.

 

[Taffyo]

Thanks everyone, dont start chucking buns at each other The circuit is wired to a 6 amp mcb, and I think i will unclip the cables and put it on top of the inso. .... just to keep you all happy

 

[davelx]

BAS: Ah, I see what you were referring to.

Taffyo - we all agree that you don't have a problem, you can leave the cables where they are.

 

[Taffyo]

Cheers davelx, if I do any more exploratory work in the loft I will fall through the bugger!!

 

[Taffyo]

Hey, I didnt realise this board put *** in when you tried to write bugger.
Taff

 

[ban-all-sheds]

Then you're a very silly bugger...