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I've replaced the two three-way switches of a hall light and now the second switch has to be in the on position in order for the first switch to turn the light on or off. The hot source is in the first box and connected to the black screw. The wire with continuity in the second box is connected to the black screw. What have I done wrong and how do I fix it?

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    Which wire does the wire connected to the black screw in the second box have continuity to? Jul 24 '18 at 1:51
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The source goes to the black screw, yes. But both brass screws are travelers. What's always true with travelers is

  • There are two of them
  • They are interchangeable
  • They are in the same cable on both ends
  • They are the same colors on both ends (obviously)
  • On both 3-ways, the two travelers go to the two brass screws

So for instance the travelers might be the red and white. It really depends on what your installer picked for each segment of cable run, and some installers choose different colors for each segment.

Now, I greatly simplify my life by marking both travelers with yellow tape. And anytime you mark a wire with tape, mark both ends of the wire.

Now the first 3-way is super simple, both "yellow" wires are landed on both brass terminals. Assuming the cable run is continuous between the 3-way switches, the other two travelers are also marked yellow and go on the brass screws there.

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Look for an unhooked traveller (wire that goes to brass screw)

It sounds like one of the traveller wires between the two switches was left unconnected at one or both ends, leading to a situation akin to connecting two ordinary light switches in series, where they both need to be in the ON position for the light to turn on. Turning the breaker off and making sure that the two brass screws both have wires on them should fix it.

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