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If either of these breakers are on, both of these wires have power. I believe that makes this a multi-wire branch circuit and that, configured thusly, this is a multifeed and that that is dangerous. Is that right?

Background:

I bought this house three years ago and am trying to untangle some electrical problems that came with it. Learning as I go. I did not wire this panel -- I bought it this way.

Explanation:

Using my voltage detector pen, I can determine that if either of these breakers is on, both of these wires have power. Also, I traced the wires up to where they exit the box in a cable, and although it's hard to see so I can't be 100% sure, I'm 99% sure that they are part of the same 3-wire cable. Using the pen again, if either of these breakers is on, the cable registers voltage.

My research suggests that this is a multi-wire branch circuit and would be OK except that the two breakers should be on different buses, which these are not. I'm just hoping to confirm that understanding so I can see about fixing it.

Thank you for any insights!

electrical panel

Alaska Man
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J. Matthew Turmer
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    Are there receptacles on either or both of these circuits? – ThreePhaseEel Jan 04 '21 at 00:48
  • Yes. There are 7x 15A receptacles and 3x light fixtures between the two breakers. But I don't know what is on which breaker since I can't test without them both on at the same time. – J. Matthew Turmer Jan 04 '21 at 01:30
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    Try using a radio/boombox as a test load to see which receptacles *actually* die when you turn one breaker or the other off – ThreePhaseEel Jan 04 '21 at 05:16
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    There's another one on the right side - the red wire on a lone 20A goes back to a 12/3 cable with a black on another 20A single. – J... Jan 04 '21 at 15:24
  • @ThreePhaseEel Thanks for your reply! I used my outlet tester, and every receptacle (and all lights) have power no matter which breaker is on. Is there something special about a radio that would give me different results? – J. Matthew Turmer Jan 04 '21 at 22:11
  • @J... Thanks for your reply, and thanks for pointing out the other potential problem. :) I didn't mention it b/c I didn't want to confuse the issue, and b/c I thought it was safe (although I'm learning I may be wrong). That one is a 3-wire cable, as you said, but it's connected with the breaker immediately above it. I thought adjacent breakers were the correct way to do this. Am I wrong? Should I be concerned about that one, too? – J. Matthew Turmer Jan 04 '21 at 22:15
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    @J.MatthewTurmer Yes, but for a 240V cable the breaker needs to be a two-pole breaker with common trip (like the two-pole 30A breaker bottom left). For a MWBC it at least needs a handle tie so that both poles switch and break together. In neither case are two independent single-pole breakers acceptable. – J... Jan 04 '21 at 22:31
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    @J.MatthewTurmer -- a 3-light tester probably draws enough current that it's not going to light up off phantom voltages...so yeah, the answers here are correct – ThreePhaseEel Jan 04 '21 at 23:46
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    Related, if not duplicate https://diy.stackexchange.com/q/209697/55930 – Machavity Jan 05 '21 at 19:58
  • @Machavity There's some good information in that discussion, too. Thanks! – J. Matthew Turmer Jan 05 '21 at 21:45

2 Answers2

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The evidence would suggest that it is an improperly wired multi-wire branch circuit. The two legs of the circuit are erroneously and dangerously shorted together somewhere. When a MWBC is properly wired, the only path from one leg to the other is through the loads.

The reason the two breakers are on the same pole is some delirious spark monkey discovered that if installed properly they trip instantly when main power is restored. He was unable to discover the real problem but by moving stuff around at random he effected a miracle cure.

The quickest test for a real short circuit, as opposed to a phantom voltage transmitted through the loads, is to properly position the breakers on opposite poles and see if they trip immediately.

A. I. Breveleri
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    I suspect they're likely picking up phantom voltage on the turned-off leg... – ThreePhaseEel Jan 04 '21 at 00:48
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    ...or there are loads connected on the circuit. – Ecnerwal Jan 04 '21 at 00:56
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    yeah some forgotten 240V load could be interesting... – Jasen Jan 04 '21 at 05:46
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    Or disconnect the wires and use an ohmmeter to check for continuity. – Hot Licks Jan 04 '21 at 16:17
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    Thanks for your reply! If I understand you correctly, if this were properly wired/installed, the two breakers would be adjacent to each other AND if one were switched off, the other would not register voltage. Is that right? And I'm just guessing that the improper part of the wiring is that there are two hots (one from each breaker) touching somewhere. Is that right? – J. Matthew Turmer Jan 04 '21 at 22:53
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    Yes that is exactly what I mean. – A. I. Breveleri Jan 05 '21 at 02:01
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    @J.MatthewTurmer `AND if one were switched off, the other would not register voltage.` If you have a MWBC or a 240V circuit it should always be impossible to turn off only one leg. They cannot be fed by independent single-pole breakers - this is a code violation. – J... Jan 05 '21 at 10:59
  • @J... Gotcha. This is definitely not supposed to be a 240V circuit. It's nothing but 15A receptacles and light fixtures up there. But wait -- I think I understand: it is a code violation to switch off one breaker at a time in a MWBC, b/c that would be dangerous. And any fixture in the circuit would still be energized by the other. But if you did it anyway, in my panel, _would the hot wire coming out of the disabled breaker still register voltage?_ I'm just making sure I understand what is supposed to be happening / how may problems I actually have. :-/ Thanks! – J. Matthew Turmer Jan 05 '21 at 12:57
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    @J.MatthewTurmer No, a wire leaving any breaker that is turned off should never have voltage. – J... Jan 05 '21 at 13:34
  • @J... Ok, great -- that's what I thought. You were just saying that the breakers should be physically connected so they cannot be independently flipped. Got it. Thank you! (PS - I want you to know I appreciate all of this advice and take it very seriously. By the time I'm ready to switch these back on, I will buy some handle ties and have the breakers connected as per your advice/code.) – J. Matthew Turmer Jan 05 '21 at 13:48
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    Just a quick update with good news: I found the problem! @NoSparksPlease called it -- there was a duplex receptacle that was supposed to be split, with a dedicated A/C circuit. I broke the tab and now I only get voltage in one circuit at a time, as expected. I suspect A. I. Breveleri is right in his prediction of how it came to be this way. For now the breakers are off until I can rearrange the circuits and install handle ties. This thing won't let me @ everyone, but I so appreciate everyone in this thread -- every comment got me a little closer to understanding. Thanks everyone! – J. Matthew Turmer Jan 06 '21 at 01:53
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Just to be clear it sounds like turning on either of these breakers energizes the receptacles, if this is correct then you should leave one off until you correct the actual problem. Actually you should leave one breaker off until resolved even if they aren't joined, having both of those on the same leg you could get 40 amps on the "neutral" without tripping and it could melt the neutral insulation.

You are right, the two wires of a MWBC do need to be on separate legs, when properly wired the neutral would only carry the difference between the two legs, but as wired it will carry the sum of the two.

The requirement for the breakers being handle tied only dates back to he 2008 version of the NEC. Before that usually electricians tried to put the circuits on adjacent breakers, but it wasn't required and rarely did we use handle ties.

If either of those breakers energizes all the same stuff then my first step would be to turn off both breakers and take apart every involved outlet box to locate where the two legs are joined, and separate them. Then I would rearrange the circuits so they would be adjacent and install handle ties.

NoSparksPlease
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    they may not be joined it may just be phantom voltage. – Jasen Jan 04 '21 at 05:44
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    @Jasen Could be, but from comments he left below the question he can't tell which of the lights or receptacles are connected to which. If it was just induced voltage then something wouldn't work when one is off. – NoSparksPlease Jan 04 '21 at 06:10
  • NoSparksPlease and @Jasen, thanks for your replies! Yes, every receptacle/fixture in question is energized when either of those breakers is switched on. That said, I thought that was the point of a MWBC -- that they shared both breakers. Am I wrong? (Again, I only even know that term from googling to figure out what was happening in this panel...) – J. Matthew Turmer Jan 04 '21 at 22:22
  • "take apart every involved outlet box to locate where the two legs are joined" Would you mind elaborating on this? How would they become joined? Hot to hot? – J. Matthew Turmer Jan 04 '21 at 22:23
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    Correct wiring of only 120v devices on a MWBC some devices should be connected to only A leg and other devices to only B leg, with all devices sharing the neutral. When the A leg circuit breaker is turned off the devices on A leg should see only the 0v from the neutral. Someplace when you shut off A leg the devices on that leg are still seeing 120v. The only way for A leg devices could see voltage is if the wire feeding B leg is connected to A leg. Normally this would be a short from A leg to B leg, but since the breaker is mislocated on same leg they just create a parallel paths to devices. – NoSparksPlease Jan 05 '21 at 01:22
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    Look particularly for a duplex intended to be wired as a split receptacle, with leg A connected to one half, and leg B connected to the other half. Normally the little tab on the side would need to be broken to prevent a short, but if somebody replaced a damaged receptacle they may have not observed that the tab was broken. They may have thought they "fixed" the tripping problem by relocating the breaker, but that actually resulted with both wires in parallel on the same leg. – NoSparksPlease Jan 05 '21 at 01:32
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    I understand the lack of ability to understandably explain in detail how the voltage and current interact in just a few paragraphs in an online forum, and because of that I think this is a real good situation to hire a Certified Electrician to properly correct the problem. – NoSparksPlease Jan 05 '21 at 01:39
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    You can get a book on home electrical wiring from the library, or you can web-search the term "multi-wire branch circuit". – A. I. Breveleri Jan 05 '21 at 02:03
  • Thank you both so much! This all makes perfect sense now. I know exactly what you mean about the duplex receptacle tab, and since there are two receptacles well-positioned for air conditioners, I think I'll start there. And if the solution isn't clear to me, I will definitely hire an electrician. :) – J. Matthew Turmer Jan 05 '21 at 02:22
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    somewhere the red wire is linked to the black wire, if both red and black from the same cable go to the same duplex outlet, then the joining tab should be removed on the live side. but it could easily be some different mistake. and it's not unusual for the red to change to black at some junction which can make tracing this harder. – Jasen Jan 05 '21 at 13:08
  • Thank you @Jasen! At this point, I'm going to investigate all the receptacles and light switches on these circuits. If I can't find a clear short, there isn't much else I can do -- maybe I can find a junction box in my ceiling if I'm lucky, but I doubt it. More likely, at that point I'll have to hire someone to come take a look with more practiced eyes and more skilled hands. Thanks again for all your help! – J. Matthew Turmer Jan 05 '21 at 17:58