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How do I change a light switch when I took switch out and it has three wires all the same color - red?

Which wire is which??

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    You move the wires from the old switch to the new switch one at a time so that you don't mix them up.
    – brhans
    Apr 19, 2017 at 18:17
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    Is it a 3-way switch? That is, is it one of a pair of switches which operates one light or one set of lights which go on and off together? Apr 19, 2017 at 19:04
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    @RickKapsch Take a picture and post it here . That way we know if it is a 3 way or what kind of switch we are assuming you mean a wall switch for a light. Please clarify - pictures are better..
    – Ken
    Apr 19, 2017 at 20:35

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While annoying, it is entirely legal in the USA to use all same-color wires for a 3-way switch setup. This implies wires in conduit, where any wire color can be chosen... rather than multiconductor cable, where the factory decides the colors and they'd never put 2 reds in a cable.

Two of the screws are the same color. These are the messengers. One screw is a different color. This is the common. If the screws are not different colors, look at the markings on the switch to figure out which is which.

The wires enter the box in one or more cables or conduits. The messengers will always be together. Depending on topology, there's probably a 70% chance that the messengers exit via a different conduit than the common. The fact that the installer used the same colors makes this even more likely... if they were all in the same conduit, he'd confuse himself! So most likely the common goes into a different conduit/destination, and the installer used the same color because it was in his hand.

Assuming we're dealing with a 3-way switch, there is no harm in trial and error; worst case the circuit won't work fully. However, do not connect a ground wire anywhere except the ground terminal.

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  • Your suggested approach doesn't work if the OP didn't mark the wires to begin with. We don't even know (not your fault) whether it's a 3-way or just controls two separate fixtures. Apr 19, 2017 at 19:58
  • I for one reserve downvoting for answers which are seriously or dangerously wrong. @CarlWitthoft good point. I had earlier thought of adding what I now added, but thought it would be confusing or redundant. Also, you can often figure out where the wires went based on how they are bent/fall in the box. I added some methods to detect the wires. Apr 19, 2017 at 20:50
  • SInce the intent of up and down votes is to distinguish helpful and valuable answers from everything else, I believe it's not only allowable but beneficial to the site as a whole to downvote incomplete or inaccurate answers. Apr 20, 2017 at 11:05
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    @CarlWitthoft I edited my answer to address your concerns, Apr 20, 2017 at 13:22

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