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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
    Dave Stevenson:


    I think that the requirement for fireproof downlighters or fire hoods is driven not so much by the risk of the luminaire catching fire, it is driven by the need to protect the integrity of the ceiling, or rather, the floor above it, where there is a "habitable space" above the room, i.e., to maintain the fire rating of the ceiling. Where there is no habitable space above, neither are mandated. The floor/ceiling construction in modern homes is scary! - Do a search for photos of engineered joists after a fire - you might get a shock - I did! 


    I think in response to these modern construction methods, the Technical Handbook that support Scottish building regulations now require fire protection for "elements of structure", i.e., the floor joists. If, for example, the fire protection (plasterboard) is applied to the bottom joist flange, then any ceiling below them need not have a fire rating, hence fireproof downlighters or fire hoods are not required. Most new builds won't have that void space, mine does. (There may be a requirement for fire detection in the void space, but that is under discussion with Building Control at the moment) 


    Wot, this sort of consideration?

    The NHBC burns a hole in fire rated downlight certification with i joists - Bing video


    Z.


Reply
  • 0
    Dave Stevenson:


    I think that the requirement for fireproof downlighters or fire hoods is driven not so much by the risk of the luminaire catching fire, it is driven by the need to protect the integrity of the ceiling, or rather, the floor above it, where there is a "habitable space" above the room, i.e., to maintain the fire rating of the ceiling. Where there is no habitable space above, neither are mandated. The floor/ceiling construction in modern homes is scary! - Do a search for photos of engineered joists after a fire - you might get a shock - I did! 


    I think in response to these modern construction methods, the Technical Handbook that support Scottish building regulations now require fire protection for "elements of structure", i.e., the floor joists. If, for example, the fire protection (plasterboard) is applied to the bottom joist flange, then any ceiling below them need not have a fire rating, hence fireproof downlighters or fire hoods are not required. Most new builds won't have that void space, mine does. (There may be a requirement for fire detection in the void space, but that is under discussion with Building Control at the moment) 


    Wot, this sort of consideration?

    The NHBC burns a hole in fire rated downlight certification with i joists - Bing video


    Z.


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