The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 42 replies
  • Subscribers 31 subscribers
  • Views 6017 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
    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.

  • 0
    Thanks a lot for the reply, as I said in the original post, there will be separate circuits for ground and first floors.


    The "60" was the ground floor only :-) It doesn't take long to get to that kind of number when you consider that, for example, the utility room will have 4, the staircase ~12, etc. etc. The staircase is a good case in point, there are likely to be lots of very low power devices on a single "string" (for want of a better description), so assuming 100W per "lighting outlet" just makes no sense, so, hopefully, common sense will prevail and rigid adherence to advisory appendices in the regs won't be a problem. I'm probably over thinking this, but didn't want a nasty surprise when the Contractor comes back with his proposed design.


    (The smoke alarm requirement is in hand, but we are awaiting a response from the fire brigade to a building control officer's question because the first floor area is quite large. Thoughts are radio linked detectors on a lighting circuit, but number, grade, positions, etc. awaiting FB response.)


    I was working on the principle that each circuit would have its own RCD, but the designer can specify that as he sees fit. I was lazy and said RCD, but yes, RCBO is likely. The requirement for RCD (/RCBO) is driven by the fact that most/all of the cables will run behind the plasterboard and will probably have less than 50mm protection unless they are buried in insulation, so I think that the RCD type protection is mandatory.


    Yes, as I said, the electrical contractor would do the final design, but I have some preferences that I'd like taken into account.


    (I have not mentioned it, because I didn't think it relevant to the question, but there will be separately powered "smart" lighting in some areas, including kitchen and lounge)

  • 0
    Modern L.E.D. lights often cause a large current surge (inrush current)  when first turned on. This has to be considered.


    Outside lights connected to internal lighting circuits could potentially cause trouble after a year or two. Outside lights are the ones that have to tolerate extremes of temperature, rain, wind and snow. If they become faulty and they trip off a protective device, then that can cause inconvenience. They can be a nuisance when fault finding after you have spent ages looking for an internal fault.


    Lighting does not have to lower and upper lighting circuits. The lighting could be divided to left side of house and right side of house to allow lights to work if one circuit fails, thus allowing some light to each floor.


    I used to supply a ground floor light near to the ground floor consumer unit on the upper floor circuit so that the user could change a fuse or reset a breaker with some helpful light.


    If outside lights are on their own independent dedicated circuit then villains or vandals that damage the outside lights will not trip off the inside lights as well. So safety and security are maintained.


    Z.
  • Former Community Member
    0 Former Community Member
    Personally I'd keep outside lights on a separate circuit/breaker/RCD/RCBO - better for maintenance as even the best seem to suffer effects from the weather/insects.

    Some LED suppliers limit the number on a circuit to cope with the inrush current if all switched together.  Halers used to specify a max of I think 16 per MCB.  Personally I wouldn't switch 16 downlighters together but many do.
  • 0
    Removing a defective lamp and putting the power back on was easy in the days of the 100W bayonet bulb.

    Modern fittings are 'sealed for life'  and often filrmly attached to the building, so as and when they do fail the duration of being out of order may be quite a  bit longer That is OK if it just goes off and everything carries on as before. Less helpful if it means the trip cannot be reset until the offending fitting has been removed.

    By all means have a modest no of circuits, based on real loads not the 100w per fitting, but do have plenty of light switches.

    Mike.
  • 0
    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.
  • Former Community Member
    0 Former Community Member
    Zoomup:

    Modern L.E.D. lights often cause a large current surge (inrush current)  when first turned on. This has to be considered.


     


    Hi zoomup, is that theoretical heresay or do you know of any actual cases where is has been a problem?


  • 0
    Weirdbeard:
    Zoomup:

    Modern L.E.D. lights often cause a large current surge (inrush current)  when first turned on. This has to be considered.


     


    Hi zoomup, is that theoretical heresay or do you know of any actual cases where is has been a problem?




    Hello, it is a well established fact Weirdbeard.


    Z.


  • 0
    Don't be boring! something like this in my bathroom is very good, shadowless and IP68 if anyone asks!

     
    9d455dfde8741c376fc609b513883123-original-20210210_162605.jpg

  • 0
    The switch on surge depends on the design Z, some it is zero, some quite bad. As they say, you pay your money and make your choice. Lights do not have to have a type B RCBO or whatever, a 10A type C is equally good, and a 16A if you use 1.5 cable. No problem there then.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines