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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
    Three gang switch by the kitchen door with two 2-way switched to beside the patio doors.


    A group of four over the centre of the dining table,  the perimeter lights around the dining area and the lights of the seating area where there will be a TV.


    There is a similar arrangement in the kitchen where there are thirty two lights in four groups. 


    Not within the six lights per switch limit,  but we'll within the 100 watts per switch and allowing the lighting levels to be adjusted with lights that are not required being easy to turn off.


    Having all the lights in the room on one switch would not permit energy efficiency,   mood or accent lighting. 


    The building control would have something to say as well.

    Andy
  • 0
    Three gang switch by the kitchen door with two 2-way switched to beside the patio doors.


    A group of four over the centre of the dining table,  the perimeter lights around the dining area and the lights of the seating area where there will be a TV.


    There is a similar arrangement in the kitchen where there are thirty two lights in four groups. 


    Not within the six lights per switch limit,  but we'll within the 100 watts per switch and allowing the lighting levels to be adjusted with lights that are not required being easy to turn off.


    Having all the lights in the room on one switch would not permit energy efficiency,   mood or accent lighting. 


    The building control would have something to say as well.

    Andy
  • 0
    Three gang switch by the kitchen door with two 2-way switched to beside the patio doors.


    A group of four over the centre of the dining table,  the perimeter lights around the dining area and the lights of the seating area where there will be a TV.


    There is a similar arrangement in the kitchen where there are thirty two lights in four groups. 


    Not within the six lights per switch limit,  but we'll within the 100 watts per switch and allowing the lighting levels to be adjusted with lights that are not required being easy to turn off.


    Having all the lights in the room on one switch would not permit energy efficiency,   mood or accent lighting. 


    The building control would have something to say as well.

    Andy
  • 0
    If access from above is available, and cable no object, a simple daisy chain works for me.  It has the advantage of only 2 cables in any fitting (except for the one used for the switch drop, if that is the case) and even under the worst case, 1.0mm is capable of 9 amps. A 6amp lighting circuit will carry many dozens of LED downlighters with ease and most LED lights have a constant current driver, so even quite major (well beyond what BS7671 allows) voltage drops don't affect brightness*.  My larger concern if there are a big number of lights would be inrush current to the drivers, not the topology of the wiring system.


    *Of course dimmable lights will react differently to changes in voltage.


    I believe you're overthinking the problem unless this is a HUGE installation with many dozens of lights.


    [edit: replying to the original question, not the discussion about how the skylight install would be wired, sorry ]
  • 0
    If access from above is available, and cable no object, a simple daisy chain works for me.  It has the advantage of only 2 cables in any fitting (except for the one used for the switch drop, if that is the case) and even under the worst case, 1.0mm is capable of 9 amps. A 6amp lighting circuit will carry many dozens of LED downlighters with ease and most LED lights have a constant current driver, so even quite major (well beyond what BS7671 allows) voltage drops don't affect brightness*.  My larger concern if there are a big number of lights would be inrush current to the drivers, not the topology of the wiring system.


    *Of course dimmable lights will react differently to changes in voltage.


    I believe you're overthinking the problem unless this is a HUGE installation with many dozens of lights.


    [edit: replying to the original question, not the discussion about how the skylight install would be wired, sorry ]
  • 0
    If access from above is available, and cable no object, a simple daisy chain works for me.  It has the advantage of only 2 cables in any fitting (except for the one used for the switch drop, if that is the case) and even under the worst case, 1.0mm is capable of 9 amps. A 6amp lighting circuit will carry many dozens of LED downlighters with ease and most LED lights have a constant current driver, so even quite major (well beyond what BS7671 allows) voltage drops don't affect brightness*.  My larger concern if there are a big number of lights would be inrush current to the drivers, not the topology of the wiring system.


    *Of course dimmable lights will react differently to changes in voltage.


    I believe you're overthinking the problem unless this is a HUGE installation with many dozens of lights.


    [edit: replying to the original question, not the discussion about how the skylight install would be wired, sorry ]
  • 0
    Ok, not sure where the kitchen is, assume the table goes under the larger of the 2 skylights.


    If fed from above, then two runs of three and earth between the switch locations, and one T and E to the kitchen and two to the one by the patio - I don't like shared lives, but it would be possible at a push to away with less 3 core.


    If fed from above the lines from the switches become T and E switch lines. Steady live feed to the lights nearest these . Then centre fed chain from the patio doors for the perimiter and  either star or chain the 4 to minimise joist crossing.

    If fed via switch box, then it gets quite full at the 3 way switch, as you need an L + N and the three cores becomes L1.L2 and N from the supply end and L1.L2 and switched live to lamp at the slave end, and need cutting in part way along, and you end up with a black switched live and a black neutral at that point, assuming black is the slider core and Brown and Grey are the L1 L2 'strappers' between the fixed contacts.

    I'm sure there is an infinite sea of alternative right answers.



  • 0
    The actual skylights  are supplied from SFCU on the socket circuit and operated with remote controls.


    The grouping of the lights is adjusted to work around the ceiling joists as well as being determined by the use of the floor space below. 


    Andy
  • 0
    Ok, not sure where the kitchen is, assume the table goes under the larger of the 2 skylights.


    If fed from above, then two runs of three and earth between the switch locations, and one T and E to the kitchen and two to the one by the patio - I don't like shared lives, but it would be possible at a push to away with less 3 core.


    If fed from above the lines from the switches become T and E switch lines. Steady live feed to the lights nearest these . Then centre fed chain from the patio doors for the perimiter and  either star or chain the 4 to minimise joist crossing.

    If fed via switch box, then it gets quite full at the 3 way switch, as you need an L + N and the three cores becomes L1.L2 and N from the supply end and L1.L2 and switched live to lamp at the slave end, and need cutting in part way along, and you end up with a black switched live and a black neutral at that point, assuming black is the slider core and Brown and Grey are the L1 L2 'strappers' between the fixed contacts.

    I'm sure there is an infinite sea of alternative right answers.



  • 0
    The actual skylights  are supplied from SFCU on the socket circuit and operated with remote controls.


    The grouping of the lights is adjusted to work around the ceiling joists as well as being determined by the use of the floor space below. 


    Andy

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