Wiring safety with loft insulation

[stuffandnonsense]

I've stripped out the wafer-thin insulation from my loft and am replacing with 70mm between joists then 200 mm over the top. Given the limited slack of my existing wires (and that I can't afford to rewire), I think running the wiring over the top of the insulation is not an option. There are two oldish junction boxes and then the general tangle of flat cable. There's no electric shower, and I presume (is this likely to be correct?) that socket wiring for appliances wouldn't go through the loft.
From other posts, it looks like it is OK to run lighting cables under the insulation (against joist or foil-backed plasterboard in this case), given that it is only a 5A circuit and hence running well below the max cable load. as long as it is only the 5A lighting circuit. Can anyone confirm this?

 

[PrenticeBoyofDerry]

Normally you will find lighting circuits are installed in 1.00mm cable on a 5A fuse or 6A MCB.
1.00mm cable can carry a maximum of 16A.
Factors for cable routed within thermal insulation can lead to a 50% de-rating of the cable, thus a 1.0mm (16A) will only safely carry 8A.
The circuit is protected by 5A so it's safe.
This is why lighting circuits are generally okay and safe.
With power/socket circuits the cable is rated much closer to the rating of the protective device and gets a bit tricky.

 

[EFLImpudence]

There are two oldish junction boxes

These must remain accessible.

I presume (is this likely to be correct?) that socket wiring for appliances wouldn't go through the loft.

It is unlikely except in bungalows.

From other posts, it looks like it is OK to run lighting cables under the insulation (against joist or foil-backed plasterboard in this case), given that it is only a 5A circuit and hence running well below the max cable load. as long as it is only the 5A lighting circuit. Can anyone confirm this?

Yes, that is correct.

 

[stuffandnonsense]

Hmm - it is a dormer bungalow. But I don't think there are sufficient wires going down for the sockets too - just the lights and light switches.

For the junctions, by 'accessible' do you mean exposed, or just can be got at through a join/cut in the insulation?

Wires are flat grey with old-school black/red current-carrying inners (each comprising three copper cores) and unsheathed earth.

You can tell I don't know much about electrics can't you......

 

[holmslaw]

..

 

[PrenticeBoyofDerry]

If you have several junction boxes and making them accessible is a problem? You could consider replacing the existing length of cable between boxes, for longer lengths that could then help you achieve accessible joints and have the cable laid over the top of the insulation.

 

[wingcoax]

The OP will also have to affix warning notices to the CU regarding different standards of cable in use.

 

[stuffandnonsense]

The OP will also have to affix warning notices to the CU regarding different standards of cable in use.

erm - what is the CU? Consumer Unit? Is this the main fuse board? What do you mean about 'different standards'? As far as I know, all the cable is the same.

 

[PrenticeBoyofDerry]

Two versions of BS7671 wiring colours, notice. Assuming they are different, that is.
The colour change means that new colours on live conductors Brown & Blue, old version were Red & Black.
This will also apply to the three core colours: New Brown, Black, Grey and old red, yellow, blue.
A notice like this should be placed on your board, if both versions of the colours are present on your system.

Untitled

• PrenticeBoyofDerry
• 14 Oct 2011
• 4

CU is short for Consumer Unit, which means in your case the fuse board.