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I have this bathroom light switch, when I turned it on, I tested power between red and black wire, there is no power

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If I turned it off, there is power red/black line

How does it work exactly? I don’t understand this

UPDATE 1

According to answers below, this is a 3-way switch but it unnecessary because there is only one switch for the bathroom light. My goal is to move this switch to a different location. Right now it is OUTSIDE the bathroom. I want to turn it inside the bathroom. So should I continue to use this same switch and the way it is routed? Or should I take advantage of this and re-wire it?

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    How do you know what "on" and "off" is? The switch has no labels to that effect. (because it's a 3-way) – Harper - Reinstate Monica Dec 8 '19 at 17:06
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What you're trying to do isn't going to work

Judging by the extra Romex freshly brought into this box, it looks like you're trying to power an additional lamp or outlet from here, and specifically, from this here switch.

Not gonna happen. At least not the way you intend.

However, a new thing changes the picture: Smart switches. There's good chance we can save this.

If you can pull the other switch out, and get us some photos of what's going on in the back of the box, we will be able to advise.

Why you can't just pull wire to any old place

In the future, don't buy anything or pull the wire until you've figured out the circuit :)

Your new branch needs "always-hot" and "neutral" if it's an outlet, or "switched-hot" and "neutral" if it's a lamp or fan. You can't just assume those are going to exist in any given location.

That switch is a 3-way switch, as HP discusses. In 3-way circuits, the downside is they almost never have the 2 wires you need in any given switch location. The upside is, they have extra wires, and with smart switches, we can take advantage of that.

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  • I was able to relocate the switch with 2-way version and it works fine. I connected the new one exactly as the old one. Is there any concern doing that? Please see my update above. – HP. Dec 9 '19 at 23:00
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If I’m understanding correctly, you’re connecting a voltmeter (or similar device) to the two terminals of the switch.

When the switch is “on”, it is effectively a short circuit between the terminals (allowing current to flow to the load). Because there is no (or very little) resistance there, the voltage is the same on both terminals and your meter reads zero.

When the switch is “off”, it is (very near) infinite resistance between the terminals. Your voltmeter draws very little current so it is seeing full mains voltage, conducted through the load.

If you were to disconnect the load (e.g. unscrew the light bulb) then you’d measure zero volts in both cases. (It might not be quite zero when off due to capacitance between wires.)

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That is a three way switch. The black screw is a common terminal that switches to two different terminals on the switch. If you look at the other side of the switch, you'll see another brass screw. The black terminal will switch between the two other terminals depending on the toggle position. This and one other switch should be operating whatever is being switched. Hope this helps.

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