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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.

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  • The incoming supply cable. Does it have 3 conductors black red white?
    – Harper - Reinstate Monica
    Commented Apr 24, 2018 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
    Commented Apr 24, 2018 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
    Commented Apr 24, 2018 at 1:44

1 Answer 1

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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.

enter image description here

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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
    Commented Apr 24, 2018 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
    Commented Apr 24, 2018 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
    Commented Apr 24, 2018 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
    Commented Apr 24, 2018 at 1:50

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