new lighting circuit


Postby nick H » Tue Mar 18, 2008 5:58 pm

Hi,

I am installing some low voltage lighting in my bathroom. However installing the new lights on the existing lighting circuit would potentially exceed the 1200w limit.

I have recently removed an electric shower and the circuit for this is now redundant. If I were to replace the 20 Amp fuse for this circuit with a 5 amp fuse in the consumer unit can I then use this as a new lighting circuit? Or is the 2.5mm cable unsuitable and will I have to have an electrician fit a new lighting circuit for me with new 1mm cabling?

thanks

Nick
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Postby sparx » Tue Mar 18, 2008 11:15 pm

Hi Nick, you say "would exceed 1200W limit" but how do you get that figure?, it's not written anywhere.
Don't just add up all light wattages , you can allow some diversity for not all being on together for long periods {unless you got teenage kids}!
5A X 230V = 1150W total but allowing for some low energy lamps may never be reached. also if runs fairly short ie not a mansion house 1.0mm rated at 13A & 1.5mm @16A you may be able to go up a fuse size if volt drop calcs allow, regs only a guide and if happy to show alternative calcs it's wide open for alternative designs
regards SPARX
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Postby nick H » Wed Mar 19, 2008 8:53 am

Hi Sparx

thanks of that. True I am unlikely to have all lights on at once and I'm using some low energy bulbs in some rooms so in reality not likely to reach 1150W. But I thought regs were set up so if you sell your house and some dunce moves in and puts 150W bulbs in every socket and turns them all on the fuse won't blow. Can you clarify.

Also reckon I only have about 50m of cable in the circuit, the house certainly isn't a mansion

thanks

Nick
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Postby kuzz » Wed Mar 19, 2008 4:56 pm

Sparx is completely right. If we had to worry about "dunce's" and not apply diversity things would get quite silly. for example a ring main with 8 twin sockets, not unreasonable, but you could in theory plug in 16 3kw electric fires. this would exceed 200A. Without diversity we'd be using huge cables huge fuses and 3phase supplys.
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Postby sparx » Wed Mar 19, 2008 10:08 pm

Hi Nick,
a) not your problem, worst case fuse blows - circuit protected from overload!
b) lamps over 100W no longer legally sold in UK.
c) regs are 'for the prevention of fire' & 'protection from electric shock',
all taken care of by correct over current device & installation methods.
I am afraid having read many of the well intentioned, in depth replies on this and other forums we all seem to be 'over-egging the pudding' somewhat. Basically is it safe, does it work, is there a reg says It can't be done, am I allowed to do it, can I back up my design with calcs/regs?
These cover the legalities, all books, including IET regs, On site guide,
guidance notes have same legal standing as Highway code, ie None!
but if followed should keep us out of jail!
regards SPARX
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