7

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

10
  • 1
    Are there receptacles on either or both of these circuits? – ThreePhaseEel Jan 4 at 0:48
  • 1
    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 4 at 5:16
  • 2
    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 4 at 15:24
  • 1
    @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 4 at 22:31
  • 1
    @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 4 at 23:46
19

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.

11
  • 3
    I suspect they're likely picking up phantom voltage on the turned-off leg... – ThreePhaseEel Jan 4 at 0:48
  • 2
    yeah some forgotten 240V load could be interesting... – Jasen Jan 4 at 5:46
  • 3
    Or disconnect the wires and use an ohmmeter to check for continuity. – Hot Licks Jan 4 at 16:17
  • 2
    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 4 at 22:53
  • 2
    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 6 at 1:53
11

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.

11
  • 2
    they may not be joined it may just be phantom voltage. – Jasen Jan 4 at 5:44
  • 3
    @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 4 at 6:10
  • 1
    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 5 at 1:22
  • 3
    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 5 at 1:32
  • 1
    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 5 at 1:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.