VERY IMPORTANT! Before working on any electrical modifications of your home read the first article in this series: Electrical Safety.
A three way switch circuit is used for turning one or more lights on from two different places. For example, turning the light on from the top of the stairs and turning it off at the bottom of the stairs.
Notice in the diagram: How to Wire a Three Way Switch Power to Light 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 shown connected to the three way switch is connected at the common terminal on the three way switch. This terminal usually has a red screw and the word COMMON stamped on the back.
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.
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