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I have a four way switch. I pigtailed two wires with the source power wire. One pigtailed wire is connected to the common screw terminal for power to the four way switch which is a three screw terminals. The other is connected to give power to another switch for the outside light. And all neutral wires are connected together.

These two switches are side by side in one double-gang box. The four way switches have no problem to turn on and off at three different location. But the outside light could not work independently by itself. It only works when I turn on the four way switch. What have i done wrong?

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I don't believe you have four way switches. Four way switches do not have commons as they do not control the light directly, they simply break the path of two other three ways. Three ways have 3 screws, four ways have 4. (And the ground)

Next, (to answer your question) since the exterior light only works when one of the switches are on, most likely you've connected to the final wire feeding from the switches to the light. I believe this imagine below will help you out:

Four way switch wiring

Basically, you use 3 way switches coming from the power source and then leading to the light. Then you can use an infinite amount of four way switches between these two.

Since you can control your light from 3 locations, but this location has 3 screws - this switch is a 3 way. Now it's just finding out if it's either the first or the last. If it were the first, connecting to the common would give you what you want. However, you've most likely tied off from the last switch leading to the light and so it only works when the overhead light works. There is no way to split off power from the current location and work as you desire. You'll have to run another wire.

  • Thank you so much. I do have exactly what your drawing is because i did have a drawing for the wires i ran. In that case, I might have made a mistake some where. I do have one 4 screws terminals switch and two 3 terminals to control the light. I do not understand why the 4 way switches work if i did mistakenly connect the wrong wire. Once again thank so much to have answered my question. – tomlee Feb 9 '16 at 5:09
  • I rewrote it again to be a bit more clear.Thank you so much. I do have exactly what your drawing is. Before i ran the wires, I drew them first in case I made a mistake. I do have one 4 screws terminals switch and two 3 terminals switches to control the light. I might most likely have made a mistake somewhere. But I do not understand why the 4 way switches still work if i did mistakenly connect the wrong wire? Can you explain it to me? Thanks. Once again thank so much to have answered my question. – tomlee Feb 9 '16 at 5:18
  • According to your drawing, pigtails was done on the first switch where the power source comes. – tomlee Feb 9 '16 at 5:20

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