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Lighting circuit - Ring circuit??

Postby kingrolo » Fri Jun 27, 2008 9:46 am

My understanding is that lighting circuits do not have sufficient power demands to warrent the use of a ring circuit. Is this correct? I have been studying the existing wiring in my house and am convinced that the ground floor lighting circuit has been made in a ring configuration. Is this overkill and not required?? I need to make modifications ( a wall is being removed along with a couple of wall-lights).

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Postby ericmark » Fri Jun 27, 2008 6:41 pm

What you say is in general correct but to turn a ring into a spur needs some careful testing. We have a very limited volt drop on lighting 3% thats about 26 meters of 1mm and 39 meters of 1.5mm but with a ring 104 meters of 1mm cable could be used. So without doing some measurements one can't really work out if a ring is required or not.
It does not need to be with tape measure either R1 + R2 or earth loop impedance can be used to calculate the volt drop. Although I would not expect any DIYer to work this out.
Often electricians will realize by general look of a house if they need a ring or not as they will have tested many houses before but with the limit reducing from 4% to 3% not so easy. Energy saving bulbs are more critical to volt drop than old tungsten type so volt drop has been reduced.
I have worked on 6 amp MCB as that is maximum for most ceiling roses if no ceiling roses were used then it is allowed to go to 16 amp and with 10 or 16 amp MCB a ring main would likely be required.

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