There are three basic types of light switches. The first is a simple, single light switch. The second is a 3-way switch that controls one lighting fixture from two locations. The third is a 4-way switch that controls one lighting fixture from three or more locations.
The easiest way for three-way switches is to place the light between the two switches. You have to look closely at an electrical wiring diagram to understand.
The cable should be 12.2 Romex right up to the first switch, then run 12.3 Romex to the light and then on to the second switch.
The 12.3 Romex has an extra wire called a traveler (usually red), which is necessary for 3-way switches.
At the first switch, the 12.2 Romex from the circuit run enters the bottom of the switch box and the hot (black) cable connects to the bottom right terminal on the switch.
The 12.3 Romex enters through the top of the switch box. The hot traveler (red) wire connects to top left terminal on the switch. The black (hot) wire connects to the top right terminal on the switch.
The 12.3 neutral (white) wire connects directly to the 12.2 neutral wire with wire nuts and the copper ground wires are also connected together with wire nuts.
At the light box, you should have two ends of 12.3 Romex hanging out. One end comes from the first switch the other end comes from the second switch.
First connect the copper ground wires together with a wire nut to get them out of the way, and then connect the two red traveler wires together with a wire nut as well.
Now find the black wire coming from the first box and the white wire coming from the second box and connect them together with wire nuts. Now, this is important, you need to mark each end of the white wire by wrapping it with black electrical tape. This means you have made it a hot wire. This is a code requirement! It is common practice for 3 and 4-way switches.
Now, at the second switch, you will connect the red traveler wire to the top left terminal. The white, wire now labeled as black with tape, connects to the upper right terminal. The black wire will connect to the lower right terminal screw. The copper wire will connect to the box if it's a metal box or the green ground screw on the switch if the box is plastic.
The light box should now have a black wire and a white wire sticking out which is what you will need to connect the light to.
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