Lighting circuit design

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Hi

I’ve been asked to fit some downlights to a very large kitchen/diner and adjoining conservatory.

In total there will be 35 lights. Allowing 100w per outlet this is around 15amps.

As I see it there are 3 options:

1) Split the lights between 3 standard 6A circuits using 1.5mm T&E.
2) Split the lights between 2 circuits each protected with a 10A MCB and using 1.5mm T&E
3) Use 2.5mm cable protected by 20A MCB (obvious problems with fitting cables into accessories)

I’m relatively new to the job and would be grateful to hear if anyone can see any advantage/disadvantages with the various options.

Thanks in advance.
 
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You can use either (1) or (2), but if you go for (1), I would be inclined to use 6A MCB 'C' instead of the usual 'B'.
 
are they single 50W downlights?

If so, 35 X 50W = 1750W / 230V = 7.6A

This would be OK for a single 10A circuit. This could be wired in 1.5mm² cable providing there are minimal derating factors.

I assume you have worked to the 100W per point figure as per the diversity tables in the OSG?

Can you not persuade your customer to fit something better than downlights, or perhaps to use CFLs?
 
Thanks for the replies.

Yes, I used 100w as per the OSG. I know it's highly unlikely that anyone will ever change the downlights for 35 100w tungsten lamps(!) but I thought I should do it by the book.

CFL's is a good idea especially considering the cost of running halogens; but wouldn't I still have to use the "100watts per outlet" scenario or have I misinterpreted the regs? - wouldn't be the first time!

Thanks again.
 
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the 100W per point is used as a guide for working diversity for an entire lighting circuit. (i.e. when the circuit supplies an entire floor of a domestic dwelling)

It uses the 100W figure as this is the largest lamp you can use in a standard T2 pendant set.

You can't apply diversity to your proposed circuit as there is a very good chance that all the lights will be on at the same time for long periods.

Diversity assumes that not all the lamps for a circuit will all be switched on at any one time.


As you are installing downlighters, the maximum lamp that can be fitted into them is 50W, so it is safe to use 50W as the figure to calculate the circuit load.


If someone wanted to replace the fittings for something which could acommodate 100W lamps in the future, it is upto them to make sure the circuit has sufficient capacity to take the additional load.
 
Thanks RFLighting.

So I DID misinterpret the regs! I assumed (wrongly) that it was my responsibility to ensure that the circuit was future proof in the event that someone would change the fittings, hence my concern to get it right.

Thanks a lot.
 

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