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I'm trying to figure out why my hallway was wired in such as strange fashion:

I have two hallway lights and three switches. Switches 1 and 2 appear to be 3-way switches; if they are both ON or OFF, light 1 is on.

Switch 3 appears to control Light 2 if and only if Light 1 is ON, otherwise it has no effect.

The wires in the junction box for Switch 3 are illustrated below.

When Light 1 is on: When Switch 3 is OFF, Terminal 2 = 120V, T1=0v. When Switch 3 is ON, T2= 120V, T2=120v

Things get confusing when light 1 is off: When Switch 3 is off, the voltage at Terminal 2 is about 12 (not 120), and when Switch 3 is on, it is about 2.

My guess is that the bottom of Switch 3 leads to Light 2, and that the top left of the box for Switch 3 is the return, while top right goes to Light 2.

Is this plausible? Correct? Common?

Hallway lights

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  • You say that your picture illustrates the wires of the Switch 2 and yet on the picture itself the wire details are labeled Switch 3. You are confusing us in addition to your messed up wiring.
    – Michael Karas
    Nov 9 '15 at 6:51
  • 120 volts to what, ground, neutral? How are you measuring voltage?
    – Tester101
    Nov 9 '15 at 11:56
  • Can you get us photos of the insides of the boxes? Nov 9 '15 at 12:45
  • Yes, though they aren't very clear -- there are 2 wire nuts each with three wires. One wire nut has all three white wires, white wires, other wire nut has the wires from the top two inputs pigtailed to the switch, and from the switch there's a wire to the bottom input.
    – gbronner
    Nov 9 '15 at 13:56
  • @Tester101 Using a multimeter measured from the terminal on the switch to the case of the metal box, which (in theory) should be ground, but might be off a little from neutral.
    – gbronner
    Nov 9 '15 at 13:58
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It sounds like the original intent was that Switch 1 and Switch 2 were supposed to be 3-way hookups to allow Light 1 to be controlled from two locations. It sounds like this part is working just fine.

My guess is that Light 2 was meant to be operated by Switch 3 from a single location. Whoever wired in Switch 3 thought they could tap into the power circuit someplace related to the Light 1 setup. In the process they did not get the power tap correctly connected from an always connected power source and instead either connected it to the Light 1 load side or into one of the travelers that connect the two 3-way switches.

It is difficult to fully visualize just how this may be miss wired without knowing which other boxes the switch 3 actually feeds from and what the configuration of the wiring there is in each of the Switch 1, Switch 2, Light 1 and Light 2 boxes. It will be important to ascertain just which of the boxes of the 3-way light circuit has the power feed to it. And then if the Switch 3 is not directly wired into that same box it may be necessary to re-wire the Switch 3 box to a valid power feed source.

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    It sounds from his description like the Light 1 load side was tapped for Light 3. Nov 9 '15 at 12:45

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