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Number of luminaires on a lighting circuit – new house

Dave Stevenson

Hi,


I am doing a provisional lighting layout for a new house which will use LED lamps throughout. (The final electrical design will be done by the building contractor, but I want to get things right in the design brief  that I give him and want to make sure that I’m not giving him “guidelines” that are not possible to meet.)


The intent is that there will be separate circuits for the ground and first floors, but that additional circuits should not be required. As all of the luminaires will be using low power, mains voltage, LEDs, predominantly downlights, I don’t think there is any concern around exceeding the current capacity of the (1.0 or 1.5mm2) cable and a 6A RCD.


The only issue that I could see is the electrical designer raising is the guidelines in the 2018 On-Site guide in Appendix on Maximum Demand and Diversity. The Appendix is marked as “Guidance Only” but I am concerned that it would be easy for the contactor to just take compliance with it as the easy way out.


Table A1 gives the assumed current demand for a lighting circuit to be 100W per lighting outlet (I guess from when 100W incandescents were the norm), that would be around 12 luminaires or 18 allowing for the diversity in Table A2. (The section does say that “The values given in Table A2, therefore, may be increased or decreased as decided by the installation designer concerned.”)  With LED downlighters everywhere, there will be around 60 luminaires with a load around 300W. (One could take the view that a new owner could replace all the LEDs with 100W fittings, but I think that would be an unreasonable argument.)


Other than arguments around inconvenience should the circuit trip, would it be unreasonable to ask the designer to implement a singe circuit for these loads? (Assuming of course, that the voltage drop was within spec.)


As an aside, is there any reason that external lights could not be on the same RCD protected circuit as the rest of the floor?


Regards

Dave

 


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  • 0
    In theory you could have one circuit, but from a practical point of view, not really


     60 downlighters surely would be more convenient split up into at least two ( ground and first?) with maybe an additional for the kitchen..


    You really need to separate the lighting circuits for safety and common sense not  just tripping. There won't be emergency lights, so if they do trip in the dark its a major problem. If you are sharing circuits on an RCD, its usual to mix them up at bit anyway: ie downstairs lights and upstairs power. in this way if an rcd wont re-energise, the householder can plug a lamp in in that room.

     I would also put the linked smoke alarms onto one of the lighting circuits. In my experience as a fire officer, faulty smoke alarms on dedicated circuits just end up with the breaker turned off, cause " the customer is going to sort it out tomorrow". However if that means turning your kitchen lights off it get fixed. 

    The same would apply to sharing too much stuff on an RCD: your iron or kettle can trip the whole house lighting.  I personally only use Rcds for fixed appliances, cooker, boiler, Shower: if there's a tripping  problem with any of these you turn the appropriate DP isolator off, and you can still  use the rest. I would then use an RCBO for each of the lighting and final socket  circuits. However, you don't really need to specify Rcds for a provisional design: the contractor will sort that out, as he/she will have their own way of working.

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  • 0
    In theory you could have one circuit, but from a practical point of view, not really


     60 downlighters surely would be more convenient split up into at least two ( ground and first?) with maybe an additional for the kitchen..


    You really need to separate the lighting circuits for safety and common sense not  just tripping. There won't be emergency lights, so if they do trip in the dark its a major problem. If you are sharing circuits on an RCD, its usual to mix them up at bit anyway: ie downstairs lights and upstairs power. in this way if an rcd wont re-energise, the householder can plug a lamp in in that room.

     I would also put the linked smoke alarms onto one of the lighting circuits. In my experience as a fire officer, faulty smoke alarms on dedicated circuits just end up with the breaker turned off, cause " the customer is going to sort it out tomorrow". However if that means turning your kitchen lights off it get fixed. 

    The same would apply to sharing too much stuff on an RCD: your iron or kettle can trip the whole house lighting.  I personally only use Rcds for fixed appliances, cooker, boiler, Shower: if there's a tripping  problem with any of these you turn the appropriate DP isolator off, and you can still  use the rest. I would then use an RCBO for each of the lighting and final socket  circuits. However, you don't really need to specify Rcds for a provisional design: the contractor will sort that out, as he/she will have their own way of working.

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