Wiring Three Way Switches
The difference between a regular switch and a three way switch is one additional terminal,or connection. A 3 way switch has 2 terminals which might be either silver or brass coloured and one terminal that is dark coloured. The silver or brass terminals are called travellers or messengers and the dark coloured terminal is called the common. If all three terminals appear similar in colour, the common terminal will be identified by the word “common” stamped on the back of the switch.
The arrangement of those terminals on the switch can vary depending upon the manufacturer, so check carefully to make sure proper identification of the common terminal. In a 3 way switch circuit there will probably be two 3 way switches and a light or series of lights which can be controlled by the switches. As each switch can turn the lights on or off,there needs to be a way by which one switch can communicate this back to the opposite switch. That is the function of the 2 travellers or messengers and you’ll notice in the wiring diagrams that regardless of the configuration, the 2 travellers or messenger terminals on each switch are always interconnected (trace the red and white conductors of the 3 conductor wire).
Because the travellers or messenger terminals are always interconnected, the common terminal is the just one left. To understand this terminal, consider for a moment a normal switch that has only 2 terminals. When wiring an ordinary switch the incoming power feed (hot) conductor is connected to one terminal (usually bottom right) and the outgoing power feed to the lights is connected to the other terminal (usually top right). This is the aim of the common terminals in a 3 way switch circuit, on one in every of the two 3 way switches the incoming power feed (hot) conductor is connected to its common terminal and on the other of the two 3 way switches the outgoing feed to the lights is connected to its common terminal.
It is crucial to notice that the white conductor in the 3 conductor wire (red, black and white) connected between the two switches is not a neutral. Depending on the switches on/off position either the red or white conductor can be hot.