3-way switch wiring diagrams and explanation
A 3-way switch circuit consists of power in to the common terminal ( odd colored screw ) of one of the switches, two traveler wires between the switches ( brass colored screws ) and a switch leg to the light from the common terminal of the second switch. There are numerous ways to route the wiring but the connections and functionality remain the same. Below are some single line diagrams of how things work.
The 4 diagrams below illustrate the correct wiring and how they work.
A 3-way switch connects the common terminal ( C ) to either one or the other of the traveler terminals, ( A or B ) depending on the position of the switch. A and B never connect to each other inside a 3-way switch.
When the common and one of the travelers at one switch ( X for example ) get reversed the circuit is completed only when both switches are in a position that connects the common at both switches with the traveler that is mixed up . For example you can turn the light on and off at switch Y but only if switch X is in one particular position (C to B ). You can turn the light on and off from switch X but only if switch Y is in a particular position ( C to B ).
When the same traveler ( B for example ) gets connected to the common ( C ) terminal at both switches the light will only work if both switches are in the position that connects the common to the mixed up traveler ( B to C position for example ).
When one traveler gets connected to the common at one switch and the other traveler gets connected to the common at the second switch, the light will never come on because terminals A and B never connect together inside a 3-way switch.
