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I am trying to piece together the 1960s-era wiring decisions in my home and am a bit stuck. I discovered a multiwire branch circuit at the panel (wide 2P 15A Siemens breaker, correctly spanning both phases and mechanically connected handles).

Altogether, when I flip the large handle on the 15A breaker it kills 13 receptacles and 6 overhead lights. I haven't removed every cover, but I couldn't see any 3-wire Romex anywhere and everything has just one hot and one neutral. I'm assuming that the neutral is split in a junction box somewhere that I also haven't found.

Is there a way (ie a tool) to test the voltage between receptacles to see if I get 0 or 240V (indicating they are on the same or different leg), or a different way to detect the phase of each receptacle? This will help me figure out if/where I can add more receptacles to avoid overloading the circuit.

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  • You should not have 240 Volt on lights
    – asinine
    Nov 30, 2022 at 3:17
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    @Ruskes the US uses split-phase power, so you will have a 240v difference with the opposite phase’s hot
    – Jacob Krall
    Nov 30, 2022 at 15:09
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    Regarding "I couldn't see any 3-wire" ... in my house I have some MWBCs where the /3 goes from the basement panel to the far opposite side of the house where a junction box in the basement ceiling is used to split the circuit into various directions with /2. So the only place you would see the /3 is in the basement ceiling junction box. Look for box on the opposite side of your house from the panel that will have a blank cover and/or be larger than most boxes.
    – jay613
    Nov 30, 2022 at 15:11
  • What does the wiring look like inside the panel? Is there definitely a shared neutral? Perhaps it’s not a MWBC at all - could be just an unnecessary use of a 2 pole breaker.
    – pericynthion
    Nov 30, 2022 at 21:24
  • It’s definitely a MWBC, there’s a black + red + white exiting the breaker (I pulled off the panel cover to look).
    – topher
    Dec 1, 2022 at 17:02

2 Answers 2

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To find out if two outlets are on the same leg or different legs on a multiwire branch circuit, plug an extension cord into one outlet and take the female end of the extension cord to the other outlet. Use the pointy probes of an AC voltmeter to measure the voltage between the hot slot of the extension cord and the hot slot of the outlet. A reading near 240V means different legs. A reading near 0 volts means they are on the same leg.

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  • I’m gonna try this one since it doesn’t involve taking off the panel cover - had to buy a new multimeter (mine was stolen out of my car) and as soon as it arrives I’ll report back.
    – topher
    Dec 2, 2022 at 3:37
  • @topher With '60s era wiring you may have some non-polarized outlets. If you have trouble determining which slot is hot and which is neutral with a digital voltmeter because the readings don't make sense, search for posts here that include the term "phantom voltage". Digital AC voltmeters with a low impedance scale or "Lo-Z" switch can eliminate phantom voltage readings and make it clear which slot is neutral and which is hot.
    – MTA
    Dec 2, 2022 at 12:45
  • Update - I was able to figure which leg was which using the extension cord and a multi-meter.
    – topher
    Mar 3 at 1:59
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Extremely simple:

  • Turn off the pair of breakers
  • Remove one hot and cap it with a wire nut for safety
  • Turn on the pair of breakers
  • See what does/doesn't work
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    Since this involves pulling the cover plate off the panel, it may also reveal distinctions in the wiring connected to each side of the breaker that may also provide a clue, since the OP has been into most of the outlet/light junction boxes.
    – FreeMan
    Nov 30, 2022 at 14:11

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