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?
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.
Best Answer
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.