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I have 3 lights in my house which are controlled by 3 individual or single pole switches. Now I want to add another 4th light which should be controlled by these 3 switches meaning if at least one out of these 3 switches is turned on, then 4th light should also turn on but the 3 lights that are wired directly to their individual switches should only turn on when their respective switch is turned on. How do wire all 4 lights with 3 switches to produce above outcome?

Below is rough diagram. enter image description here

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  • 1
    Can you run new wiring to the switch locations? Are all of these switches on the same circuit? Nov 21, 2021 at 4:47
  • Any chance of using "smart switches"? The cost may be quite a bit less than running new cables. Nov 21, 2021 at 4:52
  • @ThreePhaseEel I have updated the question with rough diagram. For blue light that I want to added I only have one live & neutral wire running from light to my switch box. Nov 21, 2021 at 8:32
  • @manassehkatz-Moving2Codidact I will check that out as well but I do not think smart switch will fit into my electrical box. Nov 21, 2021 at 8:36
  • You could use three relays. Each existing switch is connected to the control side of its own relay. The contact sides of the three relays are all in parallel with the fourth light. You'll need access to run cables between each existing switch and the fourth light. If this is all in. unfinished space or can be run on the surface that'll work. Smart devices are another approach. –
    – jay613
    Nov 22, 2021 at 13:32

2 Answers 2

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You can't - without some active component or spare poles on the switches.

  • If you have spare poles on your switches, simply connect all three switches in parallel and to the 4. light.
  • If you don't, you'll have to either change switches, or install a relay in parallel with each of the three light bulbs, and connect the output of those relays in parallel to control the 4. light.

Example wiring diagram: enter image description here

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  • Thank you again for the diagram. It is very easy to understand but looking at the diagram it seems that if all the switch are in up position or in one direction then light number 4 will stay on and if all switches are in down or other direction than light number 4 will not turn on. Is there any way to avoid that as well? Nov 22, 2021 at 11:34
  • Avoid what exactly?
    – vidarlo
    Nov 22, 2021 at 11:37
  • The normal 2 way switch with 3 terminals either stays up or in down position so If I use that switch with the diagram that you provided, if all 3 switches are in up position (in your diagram right switch) then light 4 will always stay on and if all the switches are in down position (in your diagram left switch) then light 4 will not turn on even if all 3 lights are on. The outcome needed is that if light 1,2,3 are ALL off then 4 should also be off and if any of the light 1,2,3 including all of them is turn on then light 4 should turn on. Nov 22, 2021 at 11:47
  • #4 will be on if any of the three switches is in the on position.
    – vidarlo
    Nov 22, 2021 at 11:50
  • Yes right but if all of 3 are ON, then 4 will be off right? And if all 3 are OFF then 4 will be on where it should also turn off. Nov 22, 2021 at 11:54
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This is a common demand in "flats" - buildings with one common entryway serving 2-4 apartments. Each flat wants a switch which operates both its private stairway and a light in the shared entry way.

The common answer is to use light fixtures which contain 3 bulbs, with the fixture designed so each bulb gets its own live and neutral wire.

This works because of the way light works -- on a decibel (exponential) scale. Doubling the lumens does not double the light -- it only increases the light by 3 dB.

Each bulb is wired in parallel with the stairway lights for each flat. They are wired with independent neutrals. Why does neutral need to be separate?

Because this is not DC power in a vehicle. This is AC power. AC power throws a varying electromagnetic field - that is how transformers work, after all. This magnetic field will cause vibration in the wires (due to magnetic reaction) and eddy current heating in nearby metallic (not even ferrous) objects. Effectively, if AC power travels in a big loop, the interior of the loop is the core of a transformer.

The cure for this is to bundle all related conductors, so that all current going "out" in one wire, comes "back" in another wire in that same cable or conduit. The currents sum to zero (if we take polarity into account), and the magnetic fields cancel each other out.

When drawing a wiring diagram (which is not the same as a schematic), the cables or conduits follow a "tree topology" - they spur off any number of branches, but never, ever loop back upon themselves. If you draw one in a paint program and use the "paint bucket" on open space, the entire background fills.

in practical construction, this is made easy by the fact that all cables have 2 or more conductors. You have the return conductor anyway, so you might as well use it.

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  • Thank you for the detailed answer. You are right, my actual wires does follow inverted tree like topology. I have one live and neutral along with one ground coming to my electrical box and all the switches share same 3 wires. Regarding light with 3 bulb, I can not use it because all of my 4 lights in 4 different physical location. Nov 23, 2021 at 10:08
  • The three-bulb solution is appropriate for this application. The three bulbs all reside at location #4, but one bulb is wired in parallel with lamp #1, another bulb is wired in parallel with lamp #2, and the third is wired in parallel with lamp #3. Nov 23, 2021 at 17:30

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