Replacing a light switch with three black wires

Question

I am trying to replace a Skylink Home switch with a regular switch to control the fan with ceiling light in my room.

The switch box has three sets of black, white and copper wires coming.

• All the white wires are connected together.
• All the copper wired are connected together.

I now have three black wires to connect.

The switch has three screws one of which is black.

The issue I am facing is that, my bathroom which is behind this wall looks to be affected by how I connect the wires. The bathroom has its own switches inside.

I have tried to connect the three black wires in all possible combinations with the three screws. In all of my tests, the bathroom lights are either controlled by this switch, or they don't turn on at all. The same happens with the fan.

Ideally, the bathroom lights should not be controlled by this switch, and should always receive power. I am guessing one set of wires should not be connected to this switch, but I am not sure how and in what combination.

I have put the pictures of the switch and the box and am looking for guidance.

Answer 1

What I think is happening is:

• Your original switch had two wires together and one wire separate.

One of the two wires together was incoming power from the breaker. The other supplied power to the bathroom lights. The separate wire was switched hot to your fan/light.

The two wires together could have been: Two wires in a wire nut with a third short wire (pigtail) going to the switch, two wires under one screw or one wire on a screw and one wire on a "backstab" connection. I suspect the last possibility because the first two are fairly obvious to most people as being "together" but the third one is not.

• Your new switch is a 3-way switch with one commons crew (black) and two traveler screws (brass).

This type of switch is used when you have two separate switches operating one light - e.g., switches at the ends of a hallway or at different entrances to a room. It is possible to use a 3-way switch in place of an ordinary switch by only using one of the brass screws. In this case that would generally mean connecting two wires to the common screw using a wire nut and a pigtail (because you don't want to use backstabs and you can't normally put two wires under one screw) and the third wire to one of the brass screws. (Which brass screw you use will determine whether on is the up or down switch position). Or you can swap it for an ordinary switch - but still a wire nut and a pigtail.

So now you just need to figure out which wire is which:

• Turn off the breaker, disconnect all the black wires and spread them apart for safety.
• Turn on the breaker and use a non-contact voltage tester to determine which wire is hot.
• Turn off the breaker and connect the hot wire to the black screw.
• Connect one of the other wires to a brass screw.
• Turn on the breaker and see if the switch operates the light/fan or the bathroom lights. If it operates the light/fan then the wire on the brass screw stays there and the remaining wire gets connected to the wire on the black screw. If it operates the bathroom lights then the wire on the brass screw gets connected to the wire on the black screw and the remaining wire gets connected to a brass screw.