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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

    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.



    In practice there's a bit more to consider. Since it's difficult to guarantee precise sharing of the current around a ring, the regs generally require a minimum cable rating closer to two-thirds the circuit rating, rather than half, for any ring (e.g. 20A cable for a 30A ring) - so any potential saving might not be as great as foreseen. Indeed the regs also impose a minimum conductor size for reasons of physical robustness and soundness of connections - which for lighting circuits is 1.0mm² - more than likely the size that would be used anyway - so in practice no reduction is cable size at all is likely. Indeed the extra leg needed to complete the ring will actually increase the length of cable used and the extra testing needed for a ring compared with a simple straight radial will add to the labour costs so overall increasing costs not reducing them.


       - Andy.
  • 0

    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.



    In practice there's a bit more to consider. Since it's difficult to guarantee precise sharing of the current around a ring, the regs generally require a minimum cable rating closer to two-thirds the circuit rating, rather than half, for any ring (e.g. 20A cable for a 30A ring) - so any potential saving might not be as great as foreseen. Indeed the regs also impose a minimum conductor size for reasons of physical robustness and soundness of connections - which for lighting circuits is 1.0mm² - more than likely the size that would be used anyway - so in practice no reduction is cable size at all is likely. Indeed the extra leg needed to complete the ring will actually increase the length of cable used and the extra testing needed for a ring compared with a simple straight radial will add to the labour costs so overall increasing costs not reducing them.


       - Andy.
  • 0

    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.



    In practice there's a bit more to consider. Since it's difficult to guarantee precise sharing of the current around a ring, the regs generally require a minimum cable rating closer to two-thirds the circuit rating, rather than half, for any ring (e.g. 20A cable for a 30A ring) - so any potential saving might not be as great as foreseen. Indeed the regs also impose a minimum conductor size for reasons of physical robustness and soundness of connections - which for lighting circuits is 1.0mm² - more than likely the size that would be used anyway - so in practice no reduction is cable size at all is likely. Indeed the extra leg needed to complete the ring will actually increase the length of cable used and the extra testing needed for a ring compared with a simple straight radial will add to the labour costs so overall increasing costs not reducing them.


       - Andy.
  • 0
    A ring circuit would indeed reduce losses, but this this is most unlikely to be of any significance on a domestic or similar size circuit.

    Saving energy that would otherwise be lost in cable losses is of increasing importance, but even I am not going to worry about reducing losses from 0.1% down to 0.05%.


    And reducing the cable size required in theory from say 0.2mm down to 0.1mm is pointless when nothing smaller than 1.0mm is permitted in any case.


    A more important saving is from dividing the lamps over several switches, not to reduce cable losses, but to allow greater flexibility in use and save energy by turning on say 6 lamps instead of all say 25 lamps.
  • 0
    A ring circuit would indeed reduce losses, but this this is most unlikely to be of any significance on a domestic or similar size circuit.

    Saving energy that would otherwise be lost in cable losses is of increasing importance, but even I am not going to worry about reducing losses from 0.1% down to 0.05%.


    And reducing the cable size required in theory from say 0.2mm down to 0.1mm is pointless when nothing smaller than 1.0mm is permitted in any case.


    A more important saving is from dividing the lamps over several switches, not to reduce cable losses, but to allow greater flexibility in use and save energy by turning on say 6 lamps instead of all say 25 lamps.
  • 0
    A ring circuit would indeed reduce losses, but this this is most unlikely to be of any significance on a domestic or similar size circuit.

    Saving energy that would otherwise be lost in cable losses is of increasing importance, but even I am not going to worry about reducing losses from 0.1% down to 0.05%.


    And reducing the cable size required in theory from say 0.2mm down to 0.1mm is pointless when nothing smaller than 1.0mm is permitted in any case.


    A more important saving is from dividing the lamps over several switches, not to reduce cable losses, but to allow greater flexibility in use and save energy by turning on say 6 lamps instead of all say 25 lamps.
  • 0
    As Chris pointed out I forgot to mention the bridging running across the centre of the joist span.


    In an existing ceiling I drill a hole in the plaster board at the centre of marked positions then stick a long Pozi screwdriver up through it to check for obstructions that could be joists, bridging, pipes and cables. Then if it is possible yo get a board up in the room above feed the cables in to the joist runs from one end and that determines the cable runs.


    In kitchens the perimeter lights are spaced around 600 to 750mm from the wall so when someone is stood in front of a countertop they don’t throw a shadow on the work surface in front of them


    If you are dining in the kitchen you then want to turn these lights off to take the illumination off anything on the countertops whilst you eat.


    When I was a kid my Dad did some building work for a Lord and Lady, the Lady said the Architect was wrong to put glazed doors between the dining room and kitchen as she and her guests did not want to see the caterer preparing the dinner party or the washing up left on the side for the cleaning lady to sort out the next morning, I know that’s a bit extreme and we have a glazed door on our kitchen, but we do at least turn the lights  off in the kitchen to hide the washing up.


    Andy
  • 0
    As Chris pointed out I forgot to mention the bridging running across the centre of the joist span.


    In an existing ceiling I drill a hole in the plaster board at the centre of marked positions then stick a long Pozi screwdriver up through it to check for obstructions that could be joists, bridging, pipes and cables. Then if it is possible yo get a board up in the room above feed the cables in to the joist runs from one end and that determines the cable runs.


    In kitchens the perimeter lights are spaced around 600 to 750mm from the wall so when someone is stood in front of a countertop they don’t throw a shadow on the work surface in front of them


    If you are dining in the kitchen you then want to turn these lights off to take the illumination off anything on the countertops whilst you eat.


    When I was a kid my Dad did some building work for a Lord and Lady, the Lady said the Architect was wrong to put glazed doors between the dining room and kitchen as she and her guests did not want to see the caterer preparing the dinner party or the washing up left on the side for the cleaning lady to sort out the next morning, I know that’s a bit extreme and we have a glazed door on our kitchen, but we do at least turn the lights  off in the kitchen to hide the washing up.


    Andy
  • 0
    As Chris pointed out I forgot to mention the bridging running across the centre of the joist span.


    In an existing ceiling I drill a hole in the plaster board at the centre of marked positions then stick a long Pozi screwdriver up through it to check for obstructions that could be joists, bridging, pipes and cables. Then if it is possible yo get a board up in the room above feed the cables in to the joist runs from one end and that determines the cable runs.


    In kitchens the perimeter lights are spaced around 600 to 750mm from the wall so when someone is stood in front of a countertop they don’t throw a shadow on the work surface in front of them


    If you are dining in the kitchen you then want to turn these lights off to take the illumination off anything on the countertops whilst you eat.


    When I was a kid my Dad did some building work for a Lord and Lady, the Lady said the Architect was wrong to put glazed doors between the dining room and kitchen as she and her guests did not want to see the caterer preparing the dinner party or the washing up left on the side for the cleaning lady to sort out the next morning, I know that’s a bit extreme and we have a glazed door on our kitchen, but we do at least turn the lights  off in the kitchen to hide the washing up.


    Andy
  • 0
    One or more of these Surewire Pre-wired maintenance free Junction Boxes . Surewire

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