My bedroom light is controlled by only one switch (as far as I'm aware of...), so I assumed my switch would only have 3 wires (current in, current out, ground). However I opened my switch and I saw 4 wires (see picture below). What is the 4th wire for?

enter image description here

UPDATE: If I reverse the two "push to release" wires, this switch seems to end up turning on/off a completely other switch in my bathroom. For example, after reversing the wires, in order to put on my bathroom light, I have to put on this light. So I guess this switch is connected to my bathroom somehow, and that is related to the 4th wire. But why, and how?

BTW, I am asking all of these question because I want to install a dimmer switch.

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    The answers below work. If you are adding a dimmer, be sure it is not a type that needs a neutral (white wire). The setup you show does not seem to have a neutral in the box (unless it is hidden). – bib Sep 21 '15 at 21:43

Vivek, as you said in your answer, the other black wire feeds the remaining devices on the circuit. When you switched the connections around, you changed it so the additional devices in the circuit would be interrupted.

I'm not well versed on the code, so I can't tell you if it is code, but this is something that should never be done in practice. The only wires that should ever be hooked to a switch are the ones being switched. If there are additional devices that are being fed down the line, then they should be connected with a pigtail.

Since you are replacing this switch, you will need to get a new piece of wire that is the same gauge and pigtail off of the always hot wire. The new wire should be black, or marked as black per code requirements. One branch will go to the switch, and the other will feed the rest of the circuit.

If you have a choice to use a push in terminal or a screw terminal on the new switch, you should use the screw one instead. It provides a stronger connection that is less likely to fail.

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    The switch should be rated to carry the full load of the circuit, so it shouldn't be a problem connecting it in that way. I'm not recommending it be wired that way, just pointing out that it should not be dangerous to do do. Secondly, there's no way based on that photo that you can tell which wire is which, you're simply guessing. Lastly, to create the pigtail, you'll only need one additional piece of wire. – Tester101 Sep 21 '15 at 18:50
  • @Tester101 I made edits per your suggestions, thanks! – Jason Hutchinson Sep 21 '15 at 19:33

What you have there is a single pole switch that has been tapped to continue the permanent hot to another source. Using the screw terminal and the backstab connector avoids the use of a separate pigtail to join the two wires together

  • It is my opinion that backstab connectors should be banned. These connections are always inferior to screw terminals. – Jason Hutchinson Sep 21 '15 at 17:07

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