Loft Wires

[Richard13]

I'm installing a permanent light in the loft so that I don't need to get the plug down and plug it in, I've found a ceiling rose above one of the bedrooms I'm going to connect it to, I'm going to run it from there to the roof beams, up to another ceiling rose and then down one of the beams to a switch at the other side of the room next to the hatch.
All this is fine but what should I do about the wire which comes up from the ceiling rose through the insulation? Is there anything special I need to put it in, like some trunking, incase the wires overheat or something and light the insulation. Ha, wouldn't want that to happen and I think it's probably pretty flammable!
Do I need to cover it or is it ok just to clip it to the wooden beams? Cheers for any help.

 

[JohnD]

The thing to look out for, is that you don't put the cable on top of the timbers so that you trample it with your great heavy hobnailed boots. You can use mini-trunking if you like, the current will be very slight so it won't overheat.

Also do not put any loops or loose wire that can trip you. It is usual to clip cable to the sides of the timbers.

Put a downwards loop before any switches or lamp fitttings to that any water from a future leak that runs along the cable will drip off and can't run into the fitting.

You may find a pull cord that hangs over the loft hatch handier than a wall switch. I have a big curtain ring on mine to make it easy to grab and pull in the dark.

 

[philrosenberg]

You can just clip to wooden beams.
However if it passes through insulation you need to derate the cable by an appropriate amount. I think most insulation is about 300mm which means the cable can only carry 55% of the current it is normally rated to. The derating factors for various insulation thicknesses can be found at http://www.tlc-direct.co.uk/Book/4.3.6.htm

I'm not sure what size hole you need to avoid this derating effect I'm afraid.

 

[JohnD]

I wouldn't bother calculating the derating.

Lets say, a 100W bulb and a 1.5 mm cable

0.5Amp load on a cable that starts at 19.5Amp capacity...You could bunch it in a conduit, bury it in insulation and run it through a sauna and still have heaps of spare.

 

[philrosenberg]

But shouldn't all lighting be rated to at least the value of the fuse that is protecting it? So that would be maybe 6A not 0.5

In this case 1.5mmsq is rataed at 20A derate by o factor of 55% for insulation gives 11A. If you have a semi-enclosed (rewirable) fuse then you derate by 72.5% giving about 8A. In a loft in summer it can probably reach 50 degrees which adds a further derating of 87% giving a final rating of 6.9A

So in the end it is only clear by 0.9A, not exactly loads.

 

[JohnD]

You're right