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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
    I know you are always very aware of fire Z, but this is ridiculous. You need to realise that LED power supplies are inherently fire resistant in several ways. The components used are very small, they are often the fastest fuses known to man, and cannot dissipate many watts without vapourising. To start a fire one needs significant power dissipation but this is close to impossible with these items, at perhaps 5W maximum, and if anything goes short circuit (which it almost certainly will) the current will trip the circuit OPD. Followed logically your idea of close fusing for everything leads to a large number of final circuits, perhaps 50 for a houseful of downlighters, which is quite ridiculous and is an increased risk due to other factors as well as being enormously expensive, perhaps 50 AFDDs needed! Electrical fires are a tiny risk in most modern properties and are quite rare, that is why buildings insurance is very cheap (£100-150) for reasonably sized properties. A bigger electrical installation is offset by the risk, which is much reduced with LEDs anyway as they do not produce much heat compared to incandescent lamps.


    The backlights caught fire is not in the order you say, it is PSU fault damages backlights, not the other way around. LEDs often fail open circuit anyway, again the power is high compared to the semiconductor size, in other words, similar to a fuse. Poor quality imports should not be used as examples, they are probably inherently unsatisfactory, and illegally CE and other standards marked.
Reply
  • 0
    I know you are always very aware of fire Z, but this is ridiculous. You need to realise that LED power supplies are inherently fire resistant in several ways. The components used are very small, they are often the fastest fuses known to man, and cannot dissipate many watts without vapourising. To start a fire one needs significant power dissipation but this is close to impossible with these items, at perhaps 5W maximum, and if anything goes short circuit (which it almost certainly will) the current will trip the circuit OPD. Followed logically your idea of close fusing for everything leads to a large number of final circuits, perhaps 50 for a houseful of downlighters, which is quite ridiculous and is an increased risk due to other factors as well as being enormously expensive, perhaps 50 AFDDs needed! Electrical fires are a tiny risk in most modern properties and are quite rare, that is why buildings insurance is very cheap (£100-150) for reasonably sized properties. A bigger electrical installation is offset by the risk, which is much reduced with LEDs anyway as they do not produce much heat compared to incandescent lamps.


    The backlights caught fire is not in the order you say, it is PSU fault damages backlights, not the other way around. LEDs often fail open circuit anyway, again the power is high compared to the semiconductor size, in other words, similar to a fuse. Poor quality imports should not be used as examples, they are probably inherently unsatisfactory, and illegally CE and other standards marked.
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