MWBC and AFCI??

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Ponchik

Ponchik

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Electronologist
In a dwelling, I saw a MWBC with one leg on AFCI single pole and the 2nd leg on a NON afci breaker. The neutral conductor terminates on the AFCI breaker.

I can not understand how this would work?:? But it does.
 
It may be a GE or other AFCI breaker that has no GF component at all.
If so, there is still a chance that a ground fault on the non AFCI circuit could trip the AFCI breaker via an arc fault signature appearing on the neutral.
 
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It is a SQUARE-D and been that way for about 2 years. No problem yet.
 
Did you confirm (with clamp ammeter) that the return current from the other circuit is really coming back through that neutral into the breaker?
There might be another neutral wire running via a different route, or even a bootleg independent neutral over the EGC.
 
GoldDigger said:
Did you confirm (with clamp ammeter) that the return current from the other circuit is really coming back through that neutral into the breaker?
There might be another neutral wire running via a different route, or even a bootleg independent neutral over the EGC.
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No I have not confirmed any measurements. Just did a visual confirmation when on site.
The circuit seems to be working fine.
This set up should not work (at least in my book)
 
edward said:
No I have not confirmed any measurements. Just did a visual confirmation when on site.
The circuit seems to be working fine.
This set up should not work (at least in my book)
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I have a copy of the same book, which makes me suspect hidden factors or a defective breaker.
I would try generating 50ma of ground current and see if it trips!
 
curt swartz said:
Are you sure there are not actually 2 neutral conductors in the cable? 14/2/2 or 12/2/2?
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Positive. 12-3

I did not get a chance to test the circuit today. But I am going back for additional work this week and will test that MWBC.
 
After troubleshooting, the NON afci was feeding a guest bathroom receptacle that apparently never had anything plugged in it. Hence the AFCI that was feeding the room had never tripped.

After I loaded the bathroom receptacle with my load bank, the AFCI tripped like a charm.
 
FREEBALL said:
Isnt this a violation anyway seeing that a mwbc sharing a neutral is to be on a common trip two pole breaker? just asking
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Just answering:
The requirement is that manual actuation affects both at the same time, so installing a handle tie would be just as good as a common trip for that.
 
yea I thought of that after I posted but still I pondered the functionality of the circuit w/ the AFCI forgive me if Im wrong, but does'nt the AFCI work on the theory of neutral current to the phase its operating on if so wouldn't the second (non afci breaker) have to be on the same phase in order to work without nuisance tripping. If this is so then the circuit would still be in violation. Im still in wonderment on the AFCIs. thanx
 
FREEBALL said:
yea I thought of that after I posted but still I pondered the functionality of the circuit w/ the AFCI forgive me if Im wrong, but does'nt the AFCI work on the theory of neutral current to the phase its operating on if so wouldn't the second (non afci breaker) have to be on the same phase in order to work without nuisance tripping. If this is so then the circuit would still be in violation. Im still in wonderment on the AFCIs. thanx
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An AFCI which includes a 30ma ground fault detector would have to have its own dedicated neutral for the hot wire it protects.
But there are AFCI breakers with no GF trip.
Not having the neutral running through might decrease the sensitivity to AF though.
 
In my mind I can imagine a 14-3 to switch box. Red & 14-3 neutral protect the AFCI portion of the circuit. Black & neutral from a different circuit serve their own loads. Wouldn't this setup satisfy edward's description & work ? Granted there is the chance to overload the neutral with the black wire
 
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