MWBC breaker question

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  #1  
Old 12-30-13, 12:27 PM
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MWBC breaker question

I know breakers of a MWBC must be on opposite legs. However, can one be a 15A and the other a 20A, or do they have to be identical? Please explain.

Thanks.
 
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  #2  
Old 12-30-13, 04:54 PM
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You can have mixed sizes as long as the neutral is sized for the larger circuit.
 
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Old 12-31-13, 07:09 AM
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rather than starting a new topic I wanted to ask....

Neutral at outlets need a pigtail.

What if a light fixture was hooked up to the neutral where they split?

For example if you had under one wirenut 4 wires: (the shared neutral going back to panel, neutral for circuit A, neutral for circuit B and the neutral for the light fixture)..... are you supposed to add pigtail and then connect the fixtures neutral to that pigtail instead of bundling the fixture wire with the others?

I hope that makes sense....
 
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Old 12-31-13, 08:07 AM
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A light fixture doesn't need an additional pigtail, it basically already is one.

The reasoning for requiring a pigtail at MWBC receptacles is this:
Picture the MWBC neutral going to one screw of the first receptacle, then the second screw daisy-chaining the neutral to the next device. If the screw ever loosens or someone removes the receptacle live (which of course you should never do), the MWBC downstream loses its neutral. In the case of MWBC, that could mean any device downstream can now have somewhere between 120v and 240v going through it... which is a bad thing.

Thus the NEC requires neutrals to be pigtailed since they are more secure and less likely to be removed, causing the above issue.


Long answer to your short question
 
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Old 12-31-13, 08:36 AM
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For example if you had under one wirenut 4 wires: (the shared neutral going back to panel, neutral for circuit A, neutral for circuit B and the neutral for the light fixture)..... are you supposed to add pigtail and then connect the fixtures neutral to that pigtail instead of bundling the fixture wire with the others?
In your hypothetical example, yes, I'd use a pigtail and connect the fixture neutral to the neutral pigtail. The reason is I would want the neutral connection for the circuits to remain solid even if the fixture were being changed.
 
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Old 12-31-13, 08:50 AM
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So yes and no.

I was thinking as Joe said, the wirenut would have to be removed to replace the fixture. Does code consider the fixture as a device (as per the mwbc pigtail rule)

That is how my situation is currently wired, should I be changing this?
 
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Old 12-31-13, 08:52 AM
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I was thinking as Joe said, the wirenut would have to be removed to replace the fixture.

That is how my situation is currently wired, should I be changing this?
I would. .................................
 
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Old 12-31-13, 08:57 AM
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I would. .................................
Is your reasoning for changing it because an unsuspecting person changing the fixture may be working live and could open the neutral or is the fact I have the fixture wire bundled with the rest making the joint any less secure if I added the extra pigtail (and wirenut) to the box.

Does NEC consider this a code violation?
 
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Old 12-31-13, 08:57 AM
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Both responses are acceptable. Zorfdt's is correct as the light fixture has its own pigtail, the wires coming from the fixture. Joe's would also be acceptable but is redundant IMO, a pigtail to a pigtail.

Make a quality splice by twisting the wires of the neutrals together. Then add the wire of the light fixture and twist on a wirenut. This has always been the reason that I suggest twisting splices.

You should not be working on a circuit hot anyway. This is why multi-wire circuits require multi-pole breakers to disconnect all hots.
 
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Old 12-31-13, 06:44 PM
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Joe's would also be acceptable but is redundant IMO, a pigtail to a pigtail.
You should not be working on a circuit hot anyway. This is why multi-wire circuits require multi-pole breakers to disconnect all hots.
Of course, it's all up to the interpretation of the AHJ and there are probably many points in the NEC that we on this forum may not agree on entirely. The way I see it, the fixture doesn't have pigtails on it, but fixture wires. IMO, the intent of the code is to avoid disturbing the neutral connection where the neutrals split on a MWBC. Adding a neutral pigtail for connection to the neutral fixture wire would do that. With today's code requiring either handle ties or a 2 pole breaker, this point is not as critical as it once was.
 
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Old 01-01-14, 07:13 AM
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With today's code requiring either handle ties or a 2 pole breaker, this point is not as critical as it once was.
So the reasoning behind this is so the neutral connection is not inadvertanly removed while servicing... not that having a fixture wire attached makes the joint any less secure....

I kind of assumed that the requirement for pigtails at outlets was because the device was LESS SECURE than a wire nut connection. Zordft points out its because its being used to extend the continuity of the neutral connection. A fixture wire in this case does not do that. So that's is where my confusion is.
 
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Old 01-01-14, 07:48 AM
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So the reasoning behind this is so the neutral connection is not inadvertanly removed while servicing... not that having a fixture wire attached makes the joint any less secure....
IMO, that would be correct. If BOTH circuit breakers are turned off when servicing, the potential higher voltage situation does not exist and that is the reasoning behind the requirement for handle ties or a 2 pole breaker. That being said, the NEC requirement provides for both poles to be disconnected, but the NEC cannot make the person doing the servicing actually turn the circuits off.
 
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