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Can I tap a wire to the switch 1 to add a light? the power source is to the light first. I can't run the wire from the ceiling light.

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This diagram belongs to do-it-yourself-help.com. The original image can be found here: https://www.do-it-yourself-help.com/3-way-switch-wiring-diagrams.html

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Looking at your diagram. If you are going to parallel a light by tapping the black conductor and you want it switched with the ceiling light. Then yes you can do that. The problem is where are you going to get a neutral? Does the box for SW1 have a neutral in it?

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    It is a old house wiring. The black and white wires come in the SW1 box, the black goes to the switch,(as shown) the white connect to the 3 wire cable with the black. – Jemilianl Oct 19 '18 at 17:26
  • Neither of those are the neutral for this subcircuit. You can't just grab any random neutral you find. Not only does it need to be the neutral for this 3-way circuit, but also it needs to be carried in the same cable or conduot. – Harper Oct 19 '18 at 17:52
  • so how can i add light on it? – Jemilianl Oct 20 '18 at 0:01
  • @Jemilianl -- you'll have to tap it at the lamp, the previous installers boxed you into a corner unfortunately – ThreePhaseEel Oct 20 '18 at 3:54
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Tap it at the lamp

For the lamp, you need switched-hot and neutral.

The designers of this 3-way circuit wired it as a switch loop without neutral. Neutral cannot be found anywhere in the 3-way network, so you have no option to tap there.

You are not allowed to tap one wire in one place and another wire in another place. That would make currents unequal in cables (causing undesired cable heating, vibration and EMF) and risk overloading tapped neutrals (neutrals don't have breakers).

Therefore the only place you can tap to add lights in this lamp arrangement is the existing lamp.

If you want the new light to be separately switched, the answer remains the same, just in that case you want always-hot and neutral.

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