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Recommended wiring topology for multiple lamps on one switch?

Nicholas Lee
As having an array of mains downlighters or spotlights in a ceiling is now fashionable, (rather than a single central luminaire), this begs the question of how to optimally and correctly wire them all up to the light switch.

In the case where one light switch operates multiple lights (as powered by from the domestic lighting circuit), there are several possible ways to wire from the switch to each of the lamps.

[It is taken as obvious that each lamp needs to be wired in parallel, with connections to the mains neutral and the switched-live wires.]


The (twin-and-earth) cable routing from the switch to all the lights and could be one of:

1) Daisy-chain   (the cable goes from the switch to one lamp, and then on to the next lamp, until all lamps are connected)

2) Ring    (like the daisy-chain wiring, but the final connection then links back to the switch to create a "ring main")

3) Star   (separate cables go from the switch to each lamp)

4) Tree   (i.e. a balanced spanning-tree, where the cable goes from the switch to two lamps, and each of them connect to two lamps, branching until all lamps are connected)

Each topology has different implications regarding the current distribution, and its associated heating losses in each segment of cable.

For example, a Daisy-chain topology has the highest current loading in the cable segment closest to the switch, but it uses the least cable; whereas the star topology has equal current loading in all its cable segments, but it uses the most cable.


Do the IET regulations recommend a particular wiring topology for particular scenarios, and what does everyone use in practice?

Thank you
  • 0
    Hi,

    Please bear in mind that I am not a electrician. I'm just a chartered engineer (my degree was in control engineering with mathematics).

    This is why I came to this forum to ask my question about what the IET regulations had to say on the matter.


    I'm certainly not going to tell anyone here how they should wire up anything.


    I'm happy to discuss theoretical optimisation techniques, on the understanding that nobody will mistakes that for being told how they should do their everyday job.

    723e25921523b7dc71e3c2e25f9b37b4-huge-sp


    Where the lamps to be joined are randomly distributed across the plane (ceiling), then a spanning-tree like the one shown above would link all the lamps using the shortest amount of wire.


    f24bfae69943a1066483b1a1d92a05f6-huge-ce



    Where the lamps are mostly in straight lines, then the spanning-tree optimisation has less branching and more longer daisy-chain strings, like the above image I doodled of the ceiling with the 20 lamps.


    Naturally, these are highly theoretical discussions, and I appreciate that in practice a professional electrician would implement whatever was the most pragmatic solution.


  • 0
    One thing that is being overlooked is that the spanning tree would look to an electrician like 'a mess' (although it's optimal from a cable length point of view) and many (most) electricians are kind of proud of their work and want it to be neat looking. Thanks for the clarification of the direction the question was coming from. There's a tendency hereabouts to assume everyone's an electrician :)


    I have to admit I'd never considered  it from anything more than a pragmatic 'run the cables alongside the joists as it minimizes drilling' view.


    Your second diagram looks very much like the actual topology we used for a recent job with 3 rows of 4 lights. I called it a 'fork'.  <grin>
  • 0
    One thing that is being overlooked is that the spanning tree would look to an electrician like 'a mess' (although it's optimal from a cable length point of view) and many (most) electricians are kind of proud of their work and want it to be neat looking. Thanks for the clarification of the direction the question was coming from. There's a tendency hereabouts to assume everyone's an electrician :)


    I have to admit I'd never considered  it from anything more than a pragmatic 'run the cables alongside the joists as it minimizes drilling' view.


    Your second diagram looks very much like the actual topology we used for a recent job with 3 rows of 4 lights. I called it a 'fork'.  <grin>
  • 0
    One thing that is being overlooked is that the spanning tree would look to an electrician like 'a mess' (although it's optimal from a cable length point of view) and many (most) electricians are kind of proud of their work and want it to be neat looking. Thanks for the clarification of the direction the question was coming from. There's a tendency hereabouts to assume everyone's an electrician :)


    I have to admit I'd never considered  it from anything more than a pragmatic 'run the cables alongside the joists as it minimizes drilling' view.


    Your second diagram looks very much like the actual topology we used for a recent job with 3 rows of 4 lights. I called it a 'fork'.  <grin>
  • 0
    Dear MHRestorations ,

    Thank you. You make a good point.

    In the context of domestic wiring, I would imagine that the visual neatness of the wiring layout and its associated maintainability, and the need to follow the lines of the joists, would in most cases outweigh the desire to optimally minimise the wiring costs.


    NB: For my drawing of the ceiling , the "root" of the tree on the right is where the switch would be. (Mathematically speaking, for a true minimum spanning tree, the switch input could be applied to any node.)

  • 0
    Dear MHRestorations ,

    Thank you. You make a good point.

    In the context of domestic wiring, I would imagine that the visual neatness of the wiring layout and its associated maintainability, and the need to follow the lines of the joists, would in most cases outweigh the desire to optimally minimise the wiring costs.


    NB: For my drawing of the ceiling , the "root" of the tree on the right is where the switch would be. (Mathematically speaking, for a true minimum spanning tree, the switch input could be applied to any node.)

  • 0
    Dear MHRestorations ,

    Thank you. You make a good point.

    In the context of domestic wiring, I would imagine that the visual neatness of the wiring layout and its associated maintainability, and the need to follow the lines of the joists, would in most cases outweigh the desire to optimally minimise the wiring costs.


    NB: For my drawing of the ceiling , the "root" of the tree on the right is where the switch would be. (Mathematically speaking, for a true minimum spanning tree, the switch input could be applied to any node.)

  • 0

    Sparkingchip:

    A 3 x 3 grid is probably not workable if the joists are at even centres as you will hit one or more of them on the crossing run, an even number of lights usually work better on that run. 




    Let's take a 12 ft x 12 ft kitchen. Are you going to use 4, 9, or 16 lamps? It depends upon the ceiling height, but 4 are unlikely to give even illumination over the counters and may cast shadows. 16 may be too bright and the more you have, the more difficult it is to fit around the joists. However a 3 x 3 grid does place one in the middle. When I first did this, I had very carefully set out the position of the joists and there wasn't one on the centre line, but the cross-bracing took me by surprise. Fortunately, it was just off the centre line, so a near miss rather than a c***-up. ?

  • 0

    Sparkingchip:

    A 3 x 3 grid is probably not workable if the joists are at even centres as you will hit one or more of them on the crossing run, an even number of lights usually work better on that run. 




    Let's take a 12 ft x 12 ft kitchen. Are you going to use 4, 9, or 16 lamps? It depends upon the ceiling height, but 4 are unlikely to give even illumination over the counters and may cast shadows. 16 may be too bright and the more you have, the more difficult it is to fit around the joists. However a 3 x 3 grid does place one in the middle. When I first did this, I had very carefully set out the position of the joists and there wasn't one on the centre line, but the cross-bracing took me by surprise. Fortunately, it was just off the centre line, so a near miss rather than a c***-up. ?

  • 0

    Sparkingchip:

    A 3 x 3 grid is probably not workable if the joists are at even centres as you will hit one or more of them on the crossing run, an even number of lights usually work better on that run. 




    Let's take a 12 ft x 12 ft kitchen. Are you going to use 4, 9, or 16 lamps? It depends upon the ceiling height, but 4 are unlikely to give even illumination over the counters and may cast shadows. 16 may be too bright and the more you have, the more difficult it is to fit around the joists. However a 3 x 3 grid does place one in the middle. When I first did this, I had very carefully set out the position of the joists and there wasn't one on the centre line, but the cross-bracing took me by surprise. Fortunately, it was just off the centre line, so a near miss rather than a c***-up. ?

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