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?
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.
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.