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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
    Let's take a 3 x 3 grid with joists running left to right.


    "tree": go up the right hand side - JB (MF of course) in the first space with its cables supplying the first three lamps to the left, and going onwards; drill through joist, another JB and supply the 4th to 6th lamps; drill through joist, supply 7th to 9th lamps.


    "daisy chain": start on the right hand side and supply lamps 1 - 3; drill through joist and supply 6th - 4th lamps; drill through joist, supply 7th to 9th lamps.


    One method uses two junction boxes, the other doesn't. However, I do accept, as Andy said, that it may not always be possible to lift boards as you would wish.
  • 0
    Let's take a 3 x 3 grid with joists running left to right.


    "tree": go up the right hand side - JB (MF of course) in the first space with its cables supplying the first three lamps to the left, and going onwards; drill through joist, another JB and supply the 4th to 6th lamps; drill through joist, supply 7th to 9th lamps.


    "daisy chain": start on the right hand side and supply lamps 1 - 3; drill through joist and supply 6th - 4th lamps; drill through joist, supply 7th to 9th lamps.


    One method uses two junction boxes, the other doesn't. However, I do accept, as Andy said, that it may not always be possible to lift boards as you would wish.
  • 0
    Okay, I think this question is daft, but let’s run with it.


    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.


    I will typically put three lines of lights on the narrow run, joists permitting and switch the outer runs together and the centre run separately. At home I have six lights on the two outer lines and two in the centre, staggered. This allows the lighting level to be adjusted without dimmers for energy efficiency.


    This complies with the Building Regulations that have been in force for the last five years, a sngle light switch should normally operate no more than six light fittings with a maximum load of 100 circuit Watts.


    So I would take the circuit feed into the light switch back box then run a radial our to each group of lights with the lights daisy chained, if there is a junction box make sure if can be pulled out through a down light hole.


    What possible reason could there be for installing a ring, “tree” or “star”?


    Andy 




  • 0
    Okay, I think this question is daft, but let’s run with it.


    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.


    I will typically put three lines of lights on the narrow run, joists permitting and switch the outer runs together and the centre run separately. At home I have six lights on the two outer lines and two in the centre, staggered. This allows the lighting level to be adjusted without dimmers for energy efficiency.


    This complies with the Building Regulations that have been in force for the last five years, a sngle light switch should normally operate no more than six light fittings with a maximum load of 100 circuit Watts.


    So I would take the circuit feed into the light switch back box then run a radial our to each group of lights with the lights daisy chained, if there is a junction box make sure if can be pulled out through a down light hole.


    What possible reason could there be for installing a ring, “tree” or “star”?


    Andy 




  • 0
    Okay, I think this question is daft, but let’s run with it.


    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.


    I will typically put three lines of lights on the narrow run, joists permitting and switch the outer runs together and the centre run separately. At home I have six lights on the two outer lines and two in the centre, staggered. This allows the lighting level to be adjusted without dimmers for energy efficiency.


    This complies with the Building Regulations that have been in force for the last five years, a sngle light switch should normally operate no more than six light fittings with a maximum load of 100 circuit Watts.


    So I would take the circuit feed into the light switch back box then run a radial our to each group of lights with the lights daisy chained, if there is a junction box make sure if can be pulled out through a down light hole.


    What possible reason could there be for installing a ring, “tree” or “star”?


    Andy 




  • 0
    Dear Sparkingchip : I'm sorry to hear you think my question is daft.

    To answer your question: "What possible reason could there be for installing a ring, “tree” or “star”? "...


    These topologies have different advantages when large numbers of (low-wattage) lamps need to be connected, optimising for efficiency and wiring costs.

    1) A ring topology balances the currents in the ring such that each half caries half the current. This halves the wire thickness needed, reducing costs. This is why this it is the topology used for mains sockets.


    2) A "minimum spanning tree" topology would link a set of lights randomly distributed across an area using the shortest total amount of wire, of minimum current-carrying capacity.

    More formally:  A minimum spanning tree or minimum weight spanning tree is a subset of the edges of a connected, edge-weighted directed graph that connects all the vertices together, without any cycles and with the minimum possible total edge weight."

    As an optimal solution it might well have been recommended by the IET. This topology is already used for computer networks to minimise cabling costs.

    If you need to wire up (say) twenty-five 3.5W GU10 LED bulbs, then it might save you some cable, and copper is expensive these days.


    3) A Star network is useful as each segment of the star caries the exactly same current    , so the heating losses in the cable would be minimised in the Star topology.


  • 0
    Dear Sparkingchip : I'm sorry to hear you think my question is daft.

    To answer your question: "What possible reason could there be for installing a ring, “tree” or “star”? "...


    These topologies have different advantages when large numbers of (low-wattage) lamps need to be connected, optimising for efficiency and wiring costs.

    1) A ring topology balances the currents in the ring such that each half caries half the current. This halves the wire thickness needed, reducing costs. This is why this it is the topology used for mains sockets.


    2) A "minimum spanning tree" topology would link a set of lights randomly distributed across an area using the shortest total amount of wire, of minimum current-carrying capacity.

    More formally:  A minimum spanning tree or minimum weight spanning tree is a subset of the edges of a connected, edge-weighted directed graph that connects all the vertices together, without any cycles and with the minimum possible total edge weight."

    As an optimal solution it might well have been recommended by the IET. This topology is already used for computer networks to minimise cabling costs.

    If you need to wire up (say) twenty-five 3.5W GU10 LED bulbs, then it might save you some cable, and copper is expensive these days.


    3) A Star network is useful as each segment of the star caries the exactly same current    , so the heating losses in the cable would be minimised in the Star topology.


  • 0
    Dear Sparkingchip : I'm sorry to hear you think my question is daft.

    To answer your question: "What possible reason could there be for installing a ring, “tree” or “star”? "...


    These topologies have different advantages when large numbers of (low-wattage) lamps need to be connected, optimising for efficiency and wiring costs.

    1) A ring topology balances the currents in the ring such that each half caries half the current. This halves the wire thickness needed, reducing costs. This is why this it is the topology used for mains sockets.


    2) A "minimum spanning tree" topology would link a set of lights randomly distributed across an area using the shortest total amount of wire, of minimum current-carrying capacity.

    More formally:  A minimum spanning tree or minimum weight spanning tree is a subset of the edges of a connected, edge-weighted directed graph that connects all the vertices together, without any cycles and with the minimum possible total edge weight."

    As an optimal solution it might well have been recommended by the IET. This topology is already used for computer networks to minimise cabling costs.

    If you need to wire up (say) twenty-five 3.5W GU10 LED bulbs, then it might save you some cable, and copper is expensive these days.


    3) A Star network is useful as each segment of the star caries the exactly same current    , so the heating losses in the cable would be minimised in the Star topology.


  • 0
    Really?
  • 0
    Really?

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