MWBC trouble shooting help needed

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Old 10-18-17, 09:55 PM
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MWBC trouble shooting help needed

Trying to tidy up and clean up some messy wiring from past ownership in the panel, I found ten #12 conductors going into a 3/4" PVC conduit at the panel.
  • 6 hot conductors (6 different circuits - refrigerator, microwave, kitchen receptacle, kitchen receptacle, garbage disposer, dishwasher)
  • 3 neutral conductors
  • 1 green conductor

It looks like two circuits are sharing a neutral, hence six circuits and three neutral conductors.

However, when I followed each hot conductor to it's respective breaker, something is not making sense. This is a picture of the left side of the panel, GE breakers - the thin ones.



The six circuits going into this conduit are circuits 1A, 5A, 5B, 7A, 7B and 9A.

7B and 9A are double pole breakers, one is the dishwasher and the other is the disposer. They go to one split wired receptacle under the sink cabinet. These two circuits look OK.

1A, 5A, 5B, 7B are all single pole breakers. I have no idea which neutral conductor is shared between which circuits. What is more troubling is if I assume 5B & 7A are the other pairs that share a neutral, then 1A and 5A don't make sense. The breakers are not next to each other and worse yet, they are on the same leg!

I am tempted to swap breakers 1A and 3B and call it done...but I think I will sleep better at night if I can find out which neutral conductor is for which two circuits, and to make sure they are handle tied and on opposite legs. However, is there any easy way to find out what I need to know without opening up each junction box and pull out all the wires in each and study them and trace them?

I have a tone generator and probe. If I turn off all the breakers, and disconnect the three neutral conductors from the bus bar, if I then send tone into one of the neutral conductors at the panel, I should get a response at all the neutral conductors of circuits sharing the neutral, right?
 
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Old 10-18-17, 11:36 PM
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One common problem with GE panels if half size double pole breakers are used or half size SP breakers are used for multiwire circuits there is nothing to prevent both poles from being put on the same leg. Those look like half size breakers to me. Are they? if both are on the same leg they will read 0v between the breaker poles and both poles will read 120v to neutral. Move the breaker(s) up or down one half space to correct.
 
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Old 10-19-17, 12:48 AM
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Yes they are half size breakers, I called them "thin" breakers in my original post.

I have already did the voltage difference tests, and 7B and 9A are on opposite legs, so is 5B and 7A etc etc etc...

1A and 5A are on the the same leg, and yes I can swap 1A with 3B, and pairing 3B with 5A would seem like one quick way to put them on opposing legs and next to each other.

However, I am not sure I can assume 7B and 9A are sharing a neutral, or 5B and 7A are either. What if whoever did it wired it such that 9A and 5A are sharing a neutral, and 5B and 7B are? All I see is three neutral conductors going up in that conduit with six hot conductors, I have no idea how they are "paired". Don't I need to hunt it down to make sure which two hots are paired with which neutral, then I can take those two circuits and put them next to each other on opposite legs?
 
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Old 10-19-17, 01:12 AM
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Don't I need to hunt it down to make sure which two hots are paired with which neutral, then I can take those two circuits and put them next to each other on opposite legs?
No if these are individual wires in one conduit since they all come from the same panel it doesn't matter. If cable though the answer would be yes in same cable. Code says all conductors of a circuit must be in the same raceway (cable, conduit, trough) That would be met with all hots and neutrals in the same conduit. However it is critical the hots be paired correctly. They must be together in the panel on adjacent breakers. Based on your statement: I "found ten #12 conductors going into a 3/4" PVC conduit at the panel." and assumption it is individual conductors not cables.
 

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Old 10-19-17, 06:00 AM
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It is also important that, at the far end of the circuits, each receptacle or switch or wired-in device the matching neutral be used with each hot conductor.

Neutrals for different circuits may not be interconnected therefore you can see more than one bundle of white wires in a junction box where more than one circuit enters.
 
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Old 10-19-17, 07:13 AM
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Alan thanks for adding a crucial point I missed.
 
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Old 10-19-17, 08:05 AM
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Bottom line is a MWBC shares a neutral so the two hot conductors that share a neutral need to be on opposite legs so not to overload the neutral and the breaker for the MWBC needs to be a DP or two SP's with a handle tie. If those SP breakers you have are serving MWBC's then they need handle ties and be on the correct legs for the circuit that they serve..
 
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Old 10-19-17, 09:18 AM
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Originally Posted by ray2047 View Post
No if these are individual wires in one conduit since they all come from the same panel it doesn't matter. If cable though the answer would be yes in same cable. Code says all conductors of a circuit must be in the same raceway (cable, conduit, trough) That would be met with all hots and neutrals in the same conduit. However it is critical the hots be paired correctly. They must be together in the panel on adjacent breakers. Based on your statement: I "found ten #12 conductors going into a 3/4" PVC conduit at the panel." and assumption it is individual conductors not cables.
I agree completely that it is crucial that the hots be paired correctly, placed adjacent to each other and on opposite legs.

However, the only way I can tell if the hots are paired correctly is to find out which two hots are paired, and the only way to find out which two hots are paired is to traced them in downstream junction boxes, expecting the three neutral conductors to be paired with their respective hots, right?

Right now, I have circuits 1A, 5A, 5B, 7A, 7B and 9A along with three neutral conductors going into one conduit at the panel. If we ASSUME 7B/9A is a pair - based on the fact that they are on a pair of double pole 20A breakers seated on opposite legs, but 1A and 5A are obviously a problem. I wonder if it may have been wired properly, and at some point some DIYer played musical chairs with the breakers and moved one circuit without knowing what he did.

It seems my only safe option is to find out out of the six circuits, which should be three pairs of hots each sharing a neutral, to determine which hots are paired based on investigating how the neutrals are wired in downstream boxes, then each pair should be paired such that their respective breakers are adjacent to each other with handle ties, and on opposing legs.

If as AllanJ said if I find some of these neutrals are not clearly delineated downstream and spliced not logically, I will have a big mess to untangle. I hope not.
 
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Old 10-20-17, 08:33 AM
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... the two hot conductors that share a neutral need to be on opposite legs so not to overload the neutral...
Educate me. I understand that the hots will be out of phase but still isn't that shared neutral carrying current from TWO loads and will get hotter than a normal circuit? Does wire sizing & code assume this possibility when matching gauge to breaker size?
I just can't seem to wrap my mind around why this is permitted.
 
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Old 10-20-17, 09:06 AM
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On a shared neutral between opposite legs it only carries the current imbalance. So if you are pulling 15A on each leg the neutral basically gets 0A. If pulling 15A on one and 0A on the other the neutral gets 15A. Both hots on the same leg pulling 15A each the neutral gets 30A. To be honest I have no clue how the physics works. You can read up on it here... Characteristics of the Neutral Conductor | Electrical Construction & Maintenance (EC&M) Magazine
 
 

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