Is this MWBC and is it done wrong?

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Old 02-13-20, 01:10 PM
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Is this MWBC and is it done wrong?

Moved into a new house and tracing and documenting the panel and breakers I see something that looks wrong.

Inside the panel I see a 12-3 NMb wire with the black conductor connected to one breaker and the red conductor connected to another breaker directly below it. The wire runs somewhere to a bedroom, one breaker controls the lights the other breaker controls the receptacles. They share the same neutral conductor. This is a MWBC correct?

This is a GE panel and some of the breakers are the half size skinny breakers. So the 12-3 is attached to circuits 8A and 8B, my understanding is that with MWBC you need to be on opposite legs, and on the panel circuits 2, 6, 10 are on opposite legs of circuits 4, 8, 12 right? So 8A and 8B would be on the same leg? I need to rearrange them so they are on something like 6B 8A or 8B 10A?

There is no handle tie either.
 
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Old 02-13-20, 01:19 PM
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Correct. You need to be on opposite legs of the service.
8A and 8B would be on the same leg.
 
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Old 02-13-20, 01:21 PM
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You are correct that the circuits need to be on opposite legs and the breakers need a handle tie.
 
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Old 02-13-20, 01:24 PM
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And probably an AFCI DP breaker.
 
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Old 02-13-20, 02:52 PM
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This is a MWBC correct?
Correct
my understanding is that with MWBC you need to be on opposite legs,
That is correct.
So 8A and 8B would be on the same leg?
Correct
6B 8A or 8B 10A?
That won't work. The breakers need to be beside each other so the handles can be tied together so they go off together. You would need something like 7 and 8 or 8 and 9. If you want to use half size beakers then you need a quad breaker with the two inside and the two outside tied.
It will look something like this one.

 
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Old 02-13-20, 05:55 PM
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Handle ties have been required for quite some time. Since it is a GE then something like this is easier to find (THQP220):
 
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Old 02-14-20, 05:34 AM
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The THQP220 single wide breaker above illustrates the idea of a handle tag but is fed only by one leg of the 120/240 volt service. But for a multiwire branch circuit you would need two of them side by side to span two fins behind them and both legs. Illustrated further above is a needed handle tie configuration to feed the MWBC from breakers covering both legs.

Because the fins behind the breakers might not be arranged, polarity-wise, as expected, it is necessary to use a voltmeter to be sure that 240 volts is measured across the two breaker terminals to which the MWBC conductors will be connected.
 

Last edited by AllanJ; 02-14-20 at 06:10 AM.
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Old 02-14-20, 07:29 AM
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AllanJ
The THQP220 single wide breaker above illustrates the idea of a handle tag but is fed only by one leg of the 120/240 volt service.
Are you sure about that? The THQP220 is listed as a 2 pole breaker. It should be able to be installed across opposite legs for 240V.
 
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Old 02-14-20, 07:33 AM
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That won't work. The breakers need to be beside each other so the handles can be tied together so they go off together. You would need something like 7 and 8 or 8 and 9. If you want to use half size beakers then you need a quad breaker with the two inside and the two outside tied.
It will look something like this one.
joed, thank you. However I am trying to understand why it won't work. This is what I have right now.



Just to make sure, I showed them in blue and white to indicate different legs of the service, and I also showed the breakers in different thicknesses as I have a mix of regular and skinny ones.

The current MWBC breakers are in 8A and 8B, clearly on the same leg as you have confirmed.

Why can't I swap the 8A and 10A skinny breakers, to have them result in opposite legs, and add handle ties?
 
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Old 02-14-20, 07:37 AM
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Because the fins behind the breakers might not be arranged, polarity-wise, as expected, it is necessary to use a voltmeter to be sure that 240 volts is measured across the two breaker terminals to which the MWBC conductors will be connected.
Thanks, I will confirm with a voltmeter to make sure they are on the same leg first.
 
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Old 02-14-20, 08:54 AM
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Do the 8A and 8B (and the 10A and 10B) skinny breakers come apart and snap out individually?

You could leave the breakers in place and swap the wires of 8A and 10A if needed provided that the breaker ratings match and you are not messing up the polarity for another multiwire branch circuit.
 
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Old 02-14-20, 10:54 AM
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Those skinny GE DP breakers are designed to fit only when they hit both phases and only work in GE panels. The rejection tabs are set up to work that way.
 
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Old 02-14-20, 12:12 PM
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My mistake on the breaker number system. You are correct. But a single tandem won't install across 6b-8a. You would need the quad version.
 
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Old 02-14-20, 09:48 PM
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Those skinny GE DP breakers are designed to fit only when they hit both phases and only work in GE panels. The rejection tabs are set up to work that way.
Those are not DP breakers, those are a pair of single skinny breakers, not a problem relocating them. I just need to tie the handles together.
 
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Old 02-15-20, 12:16 AM
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What I was trying to say is it is probably easier to replace the two breakers you have with a GE DP breaker since the handle ties may not be in stock. Skinny DP breakers normally are in stock.
Handle tie: https://www.homedepot.com/p/GE-THQP-...THT1/100152228
 
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Old 02-15-20, 09:59 AM
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I just need to tie the handles together.
By today's code, yes. How old is the house? Back in the day we never tied the breaker handles on a MWBC because it wasn't required or even suggested. I don't recall when the requirement first appeared in the code, but suspect your house was built before the requirement.
 
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Old 02-22-20, 09:37 AM
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I pointed this thread to an electrical friend I have known for decades as to his take on the question. Below is his reply in a quote from a e-mail (he's not a member here, he ask me to post this);
The info, after it was corrected by others, is all correct.
The reasons, I've now learned, NOT to do this:
1. By using one 12-3 wire instead of two 12-2 wires, saves a person almost nothing in money because 12-3 wire is double the cost of 12-2.
2. If you want to change ONE of the two circuits to an AFCI or GFCI breaker, you CAN'T.
3. If the neutral ground opens up (say at the breaker panel) the two load circuits on either side of the phase will be in series across 240 volts. Since the two loads will obviously be dissimilar, They will no longer be 120 volts each. (Example: On one side, the load will have 60 volts while the other side has 180 volts. This will cause your equipment to BLOW UP. This happened to ME personally and blew out a $1000 copier.)
4. The first electrical box on the 12-3 circuit needs to be oversized to accommodate a 12-3 coming in and two 12-2 wires leaving this box.
5. As mentioned in this article, if someone moves a wire to a different breaker and has now placed the red and the black on the same side of the phase, the neutral wire will now carry up to 40 amps instead of a maximum of 20 amps. This will cause a fire.
>>> Henceforth, don't try to save a couple pennies. RUN TWO 12-2 WIRES FOR TWO CIRCUITS instead of one 12-3 wire.

Daniel from DDK Electrical
Lancaster NY
 
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Old 02-22-20, 10:38 AM
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Your friend makes some valid points, but I also disagree with a few. Multiwire branch circuits do save money because they can be a great labor saver. They save even more money in commercial work where up to 6 circuits can be installed in a single 3/4" conduit. With electrician labor near the $40 mark plus benefits each hour is valuable. I believe there are two major issues when dealing with multiwire branch circuits. One is low skilled handymen and the other is even lower skilled homeowners, neither being qualified to even open a panel much less start moving wires around.
 
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