Lighting cables in the loft
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Dazzman
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Joined: Thu Jul 12, 2007 6:12 pm

Lighting cables in the loft

by Dazzman » Thu Jul 12, 2007 6:19 pm

When laying cables in the loft should they be clipped to the top or side of the joists.
I always thought that the side was correct but in my new house they are on top of the joists and are getting in the way of me putting loft boards down.
Would someone please let me know what correct method should be.
With thanks.

BLAKEY1963
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Re: Lighting cables in the loft

by BLAKEY1963 » Fri Jul 13, 2007 1:25 am

[quote="Dazzman"]When laying cables in the loft should they be clipped to the top or side of the joists.
I always thought that the side was correct but in my new house they are on top of the joists and are getting in the way of me putting loft boards down.
Would someone please let me know what correct method should be.
With thanks.[/quote]

DAZZMAN
NORMALY CABLES WOULD BE RUN THROUGH JOISTS
WITH THE HOLE BEING DRILLED IN JOIST NO LARGER THAN 0.25D
OF THE WIDTH OF THE JOIST ITSLEF.
I ASSUME THAT CABLES ARE ALREADY RUN ON TOP OF JOISTS .
IF THIS IS THE CASE THEN JOISTS CAN BE NOTCHED OUT INSTEAD OF
DRILLED. NOTCHES TO BE NO DEEPER THAN O.125 D OF THE JOIST
WIDTH. IF U NOTCH JOISTS THEN TO AVOID PIERCING CABLES WHEN BOARDING WITH SCREW FIXINGS, THE NOTCHES SHOULD BE COVERED
WITH A METAL PLATE AS MECHANICAL PROTECTION TO THE CABLES .
YOUR PART P SPARK CAN ADVISE ON COVER PLATES FOR THIS
PRACTISE AVAILABLE IN BOXES OF 100 FROM UR ELEC WHOLESALER.
( I GOT A BOX LAST WEEK FROM DENMAMS )
BLAKEY1963

oldie
Apprentice
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by oldie » Fri Jul 13, 2007 10:25 am

The big problem with cables in lofts these days is the presence of vast amounts of insulation.

All cables produce a small amount of heat (often not very much but can be up to 3watts/metre).

Usually, if the cable is burried in a wall or clipped to a wall or joist etc, this is disipated quite easily. Thats's why the regs say a clipped cable can carry a higher maximum current than one passing through insulation.

If however a cable becomes embedded in insulation it could get hot and might need to be derated. If pvc cables run much above 60 degrees for an extended period it can shorten their lifespan.

If you are considering re-running the cables and covering them up with with floorboards and insulation it might well be worth replacing them with higher rated cables. i.e. replace 1.0mm with 1.5mm, 1.5mm with 2.5mm etc.

Similar problems can occur with lighting. 100 watt recessed lights under thick wads of insulation are very good at melting and burning wiring connected connected to them - same goes for 50 watt mini spots.

Another good reason for switching to low energy bulbs using about 20% of the power.

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