Rerouting/replacing light wiring in loft


Postby K.o.R » Thu Oct 02, 2008 5:02 pm

I'm planning on putting down additional insulation and boarding in our loft, however I have run into the problem of the current wiring for the first floor lights, which is currently at a level that would be covered by the new insulation (obviously a no-no) and which doesn't appear to have enough slack to simply run on top of the new level of insulation. In any case I wouldn't want the wires to be hidden under the boarding and so I was considering running new wiring along the loft rafters so that it would be well out of the way and using vertical lengths of wood over where the wires need to drop to ceiling level for the light fixtures and switches.

A couple of things are concerning me and any advice would be greatly appreciated.

1) The current wiring appears to be in a loop - I can find one point where two wires come up from inside a wall and go off in opposite directions. Is it necessary for it to be in a loop through each light fitting or could I have a smaller loop with a spur off to each fitting?

2) The wiring only appears to be 2-core (ie un-earthed). Presumably any new wires would have to be earthed, though this would mean new wires down to the consumer box?
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Postby ericmark » Thu Oct 02, 2008 10:26 pm

You would need to add an earth which has to take same route as power so in reality this means new cables to the consumer unit.
Cable length for radial feed with 1.5mm twin and earth on a 6 amp MCB is around 40 meters which when looping around to each lamp is easy to exceed.
Using junction boxes may be one way to reduce cable length but it does have other draw backs with extra screws which can become lose etc.
Also using a ring main can help. I think to try to plan a part rewire on a forum is not really on and I would suspect you will have bathroom lights somewhere and likely to be Part P so I would consider getting in an electrician is really order of the day.
Eric
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Postby K.o.R » Fri Oct 03, 2008 10:22 am

I figured as much. Next question then: Is it fire-safe to run cable over notches in joists so that the cable is above the insulation but below the boarding? On a closer inspection I think there's enough slack to go the extra couple of inches.
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Postby ericmark » Sat Oct 04, 2008 8:39 am

Touching something is what is required. See http://www.ericpalmer.fsnet.co.uk/Table4D5.html this has a picture of both wall and loft installation and gives the different current carrying capacity only rough but will give you an idea of what is required. Main problem is 50mm rule if a cable that was viable is buried in a wall or ceiling at less than 50mm from surface then to protect people from shock at a latter date either one of a list of cables which if a nail is put through them will produce a short to earth so fail safe or a 30ma RCD is required. My loft done before the rule has drawing showing where cables are below boards and everything in my house is RCD protected anyway has been for many years but this is not the norm.
How much notice you want to take of the 2008 regulations I don't know yours is the first where non electrical work will contravene the regulations I nearly missed it.
Eric
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Postby K.o.R » Sat Oct 04, 2008 6:02 pm

The upstairs lighting circuit powers 10 bulbs at maximum, and since they are all 20W energy saving the maximum wattage will be ~200W - less than 1 amp at full load. Would this mean that there probably wouldn't be an overheating risk in any case?
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Postby ericmark » Sun Oct 05, 2008 12:05 am

I see my spell checker changed some words viable instead of visible sorry. But to your answer what you use does not matter it is what you are fused to that matters i.e. what someone in the future may use. I would not expect any problems with lights. Power either sockets, shower, or immersion heater is another thing. Although I did not expect a problem I can't just say don't worry as some people do go daft on lighting.
Eric
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