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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

 


  • 0
    Hi Folks


    thanks a lot for the replies, lots of food for thought there! Picking up on a couple of points . . . 


    Lighting in general - I agree, that down lighters are not appropriate for all areas. I kind of simplified my original question, there will be a large quantity of downlighters, but also other LED luminaries where they are better suited, e.g., pendants, wall lights etc.


    I will give some more thought to inrush currents, but other than perhaps on recovering from a trip if a large number of luminaires had previously been switched on, the number of devices being powered on simultaneously is going to be limited the number powered from a single switch - it's not like the whole circuit will be energised at one time. As I said earlier, the final design will be done by the Main Contractor's electrical designer, and it may be, for example, that type C RCD/RCBOs are a good way of mitigating higher inrush current while still providing adequate protection for the circuit.


    External lights - I had not considered the greater chance of damage or faults in the external lighting due to their environment and hence the greater risk of them taking out the internal lights. I have changed that and the external lights will be separated from the internal ones


    regards

    Dave
  • 0
    it's not like the whole circuit will be energised at one time

    Unless there's been a power cut - even a short blip of a few seconds will mean everything coming back on together - and most people don't really want to be wondering around the house in the dark trying to turn things off (remembering that it's not trivial to work out whether a 2-way switched light is on or off by the switch position) before resetting the MCB.


       - Andy.
  • 0
    Yes, fair point. but the whole house wouldn't be in darkness. On both floors there are a number of separately powered "smart" lighting circuits that would provide sufficient light to locate switches and there is only a single two way switch circuit in the whole house :-)

    regards

    Dave

    Edit - before someone makes the comment, obviously, the "single two way switch" has two switching points :-)
  • 0
    In my view, all but very small houses need three lighting circuits.

    1) Hall, stairs, landing, and any enclosed porch.

    2) Upstairs.

    3) Downstairs.

    This will unsure some lighting in most areas in the event of any trip.


    A single good quality and well installed external light may be connected to the "hall/stairs/landing" circuit with minimum risk of RCD tripping.


    Any more than that needs a fourth circuit for the outside lights.
  • 0
    broadgage:

    In my view, all but very small houses need three lighting circuits.

    1) Hall, stairs, landing, and any enclosed porch.

    2) Upstairs.

    3) Downstairs.

    This will unsure some lighting in most areas in the event of any trip.


    Interesting! I would include hall with downstairs, but there would be a 2-way switch from hall to landing. Similarly, landing would be upstairs with a 2-way to hall.


    This sub-division into lots of circuits begins to irritate me. If an installation has been designed and erected properly, (1) there will be no nuisance tripping; and (2) there will be no faults.


    I have lived here for 24 years and never had to reset an MCB.


  • 0
    wallywombat:

    Just as an aside, I'd recommend not having lots of downlights. They are a very inefficient way of lighting a room and give a poor quality distribution of light to a room - i.e. most light coming from above, giving a harsh effect. You want light coming from all around, e.g. the light bouncing off the walls from a traditional pendant, or directly from the wall by wall-mounted fittings or strips.


    Agreed.  The very bright point sources can cause visual discomfort. I find that in my kitchen which has GU19 downlighters on a track, the bright light reflects from polished items like my kettle, large bread knives and other shiny mirrorlike surfaces. It is unpleasant. I prefer indirect rather than direct light.


    Z.


  • 0
    Copy my bathroom then Z! A bit tricky to install but shadowless light everywhere and only about 400 sources. The LED string inside a PVC tube came from CPC a couple of years ago. The whole lot is about 20W.
  • Former Community Member
    0 Former Community Member
    Chris Pearson:
    broadgage:

    In my view, all but very small houses need three lighting circuits.

    1) Hall, stairs, landing, and any enclosed porch.

    2) Upstairs.

    3) Downstairs.

    This will unsure some lighting in most areas in the event of any trip.


    Interesting! I would include hall with downstairs, but there would be a 2-way switch from hall to landing. Similarly, landing would be upstairs with a 2-way to hall.


    This sub-division into lots of circuits begins to irritate me. If an installation has been designed and erected properly, (1) there will be no nuisance tripping; and (2) there will be no faults.


    I have lived here for 24 years and never had to reset an MCB.




    There maybe no faults or nuisance tripping from the fixed wiring side of the system, but what about faulty appliances or fittings that will most likely cause the trip. Surely it’s far better to have those extra circuits. 


  • 0
    Nikp:
    Chris Pearson:
    broadgage:

    In my view, all but very small houses need three lighting circuits.

    1) Hall, stairs, landing, and any enclosed porch.

    2) Upstairs.

    3) Downstairs.

    This will unsure some lighting in most areas in the event of any trip.


    Interesting! I would include hall with downstairs, but there would be a 2-way switch from hall to landing. Similarly, landing would be upstairs with a 2-way to hall.


    This sub-division into lots of circuits begins to irritate me. If an installation has been designed and erected properly, (1) there will be no nuisance tripping; and (2) there will be no faults.


    I have lived here for 24 years and never had to reset an MCB.




    There maybe no faults or nuisance tripping from the fixed wiring side of the system, but what about faulty appliances or fittings that will most likely cause the trip. Surely it’s far better to have those extra circuits. 




    Oh yes I agree. Do not rely upon the minimums in life. I am a belt and braces person.


    "The standard contains the rules for the design, erection and verification of electrical installations as to provide for safety and proper functioning for the intended use." 120.1.


    Why do motor cars have dual breaking systems and a hand brake as well, the Americans call the latter an emergency brake.


    Z.


  • 0
    Zoomup:


    Why do motor cars have dual breaking systems and a hand brake as well, the Americans call the latter an emergency brake.


    Z.



    Because there would be an immediate danger to life if the brake circuit failed - I would suggest that's hardly the same as a lighting circuit going out


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