VERY IMPORTANT! Before working on any electrical modifications of your home read the first article in this series: Electrical Safety.
A three-way switch is used for turning one or more lights on or off from two different places. For example, turning the light on or off from the top of the stairs and turning it off or on at the bottom of the stairs.
The trick with wiring switches is that you only switch the black or hot wire. You'll notice that in both diagrams, the white or neutral wire goes through the switch boxes and connects to the light - it is not switched. On the 3-way switch, the white wire is used as an extra switch wire and is said to be colored black. Whenever an electrician uses a white wire as a hot wire, he/she will put black tape or colorant on it to distinguish it as a hot wire, rather than a neutral.
Notice in both diagrams that the black wire is used for switching purposes in combination with the red and white wires. The white wire connected to the three-way switch is colored black to show it is not a neutral wire. The black wire from the power source is shown connected to the three-way switch at the common terminal of both switches. This common terminal usually has a red, copper or black screw and the word COMMON stamped on the back of the switch. The other terminals usually have brass screws.
The first drawing shows the wiring of a three way switch with the power source entering the light box.
This second drawing: How to Wire a Three Way Switch Power to Switch shows the wiring for a three way switch when the power source enters one of the three way switch boxes.
Dave(Ask Dave) (About Dave)
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