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The hot wire and traveler wire behave oddly in a three-way switch in a 1950s house. The metal box has three wires - red, white and yellow.

The red-color wire measured 116V, so I assumed it's the hot wire. The white and yellow wires measured 4-6V when not connected to anything, so I assumed they are the traveler wires. I connected the red wire to the black common screw, and connected the white and yellow to brass traveler screws.

The switch worked, except that the light turned ON when I flipped the switch OFF and turned OFF when I flipped the switch ON. I measured the voltage on traveler wires - the ON wire fluctuated between 25 and 30V, the OFF wire had 0V. I measured with a regular multimeter, with its black lead grounded to the metal circuit box.

Then I re-connected the white and yellow wires to the opposite brass screws. Now the light worked normally - turned on when I flipped on the switch and turned off when I flipped off the switch.

Can someone help with these three puzzles:

1) Why is the hot wire red instead of white or black?

2) Is white wire really a traveler wire?

2) Why does flipping white and yellow wires to the opposite brass traveler screws change the switch behavior?

  • If this is a three-way switch, the red wire may actually be the hot traveler coming from the other three-way switch location. I'm confused when you say ON and OFF, because a three-way doesn't have that insignia. Do you mean UP and DOWN, instead? It is possible you have a bad 3-way switch somewhere, also. Try finding the other location and then do the same tests at it and describe. Also, pictures or diagrams can help. Thanks for the question, it sound interesting. One more thing, are the boxes wired using conduit or cable? – Nicknamednick Aug 20 '18 at 2:59
  • Can you post photos of the insides of the boxes? – ThreePhaseEel Aug 20 '18 at 3:07
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1) As many answers and many more diagrams discuss here, color-coding is absolutely meaningless in 3-way circuits. The only exceptions are if

  • someone deliberately marks the wires with colored tape (I use yellow for both travelers, no need to distinguish them from each other) or
  • the work is in conduit and someone used two oddball colors (I use yellow-yellow) for the travelers.

2) throws up hands Aww, why not? 3-ways are a morass of senseless color coding and absolute lack of marking.

3) because that's the way 3-ways work. Seriously.

enter image description here

And this seems to be a confusion: expecting "up/down" to correspond to "on/off". They won't. Go get a 2-way switch and hold it in your hand. See where it says "On" and "Off" on the lever? Now look at a 3-way switch. No markings.

Note that in the above diagram, the left switch is "up for off, down for on".

In the 3-way world, there are only 2 states: the state the light is in; and the other state. Throwing the switch once changes the state. That's it.

So if you want "Up" at this 3-way location to be "on", you can swap wires all day (it's actually easier to just flip the switch over). However this effort will be wasted the moment someone throws the other switch in the 3-way circuit; now this switch will be "upside down" again.

Suppose you flipped the left switch above; then somebody throws the right switch. Now you have to start over.

That's how 3-ways work.

If you just can't stand it, try looking for a 3-way switch which is a pushbutton instead of a toggle. Making them toggles like they did really pains my OCD.

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