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I have a switch box with 3 switches in it, each powering a different light. All three switches are supplied by pigtails from one incoming cable.

Two switches are simple single pole switches and when the breaker is turned off those lights stop working.

The third is a 3-way switch and when the breaker is turned off, the light continues to work.

All four combinations of both 3-way switches work as expected, with the breaker off.

When the 3-way switched light is lit, it does NOT back-feed the other two lights.

I read online that there used to be a technique used where two breakers/fuses on the same phase were connected through some black magic to power a light, but I can't envision how it works.

Sadly half of the wires are knob and tube so I can't see any neutrals.

  • The incoming supply cable. Does it have 3 conductors black red white? – Harper Apr 24 '18 at 1:04
  • Can you turn off a different breaker along with the first one and make the 3-way controlled light turn off? – ThreePhaseEel Apr 24 '18 at 1:30
  • The supply to the box is a single wire, pigtailed to all 3 switches. On the 3-way switch it is connected to the Common terminal. Yes, I've identified another breaker that cuts power to the light. – Franq Apr 24 '18 at 1:44
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It looks like a Carter-style 3-way (as you state that have knob&tube). In my opinion you have a 3-way on a circuit and the other on another circuit with a shared neutral.

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  • I agree that it sounds like the Carter system, amd that explains the comment I saw about same-phase hots. Wikipedia's diagram of the Carter system seems to show that there is one traveler wire, but the common connection on my 3-way switch is connected to a cable known to go back to a breaker, and not to the light fixture. – Franq Apr 24 '18 at 0:04
  • You can verify: get a test-light (not the touch-less model), with all breakers on, switch the light off, unscrew the bulb and test both connectors, if you have both cold flip both switches (light should still be off but switches should be in the opposite position than in the first test) and test again, if you got 2 hots on the bulb contacts it's the proof you're on carter system – DDS Apr 24 '18 at 0:22
  • Another hypotesis is that your 3-way is connected to another fuse and the only thing that your switches share is the wall box – DDS Apr 24 '18 at 0:27
  • Thank you, I will test the fixture to see if there is a combination that results in the socket being Hot-Hot while off. – Franq Apr 24 '18 at 1:50

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