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Old 02-15-2011, 06:09 PM   #16
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AFCI problem has me stumped


don't think they are even on the market yet!!!!

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Old 02-15-2011, 06:10 PM   #17
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AFCI problem has me stumped


If it was wired more than 3 years ago, I ran as many MWBCs as I could, as it saved time. And I have seen this time and time again, that people want to sell a house or upgrade and they think they can just install a new breaker to meet the new code. I have heard rumblings that certain manufacurers are working on making an AFCI breaker that will work with MW circuits. I hope this happens soon as it would definatly make this kind of things work out.
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Old 02-15-2011, 06:17 PM   #18
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AFCI problem has me stumped


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Originally Posted by SparkandArc View Post
If it was wired more than 3 years ago, I ran as many MWBCs as I could, as it saved time. And I have seen this time and time again, that people want to sell a house or upgrade and they think they can just install a new breaker to meet the new code. I have heard rumblings that certain manufacurers are working on making an AFCI breaker that will work with MW circuits. I hope this happens soon as it would definatly make this kind of things work out.
But if there were no arcing in the hallway circuit would it be a problem to be 'protecting' two circuits with one AFCI? In this case the homeowner is being warned of a problem that exists in his hall light circuit without having the AFCI on that circuit. A nuisance but perhaps it will prevent a fire in the process.

Last edited by trosenda; 02-15-2011 at 06:25 PM.
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Old 02-15-2011, 06:24 PM   #19
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AFCI problem has me stumped


Well, I guess I don't quite understand what you mean. I believe the only reason that you cannot run an AFCI on a MWBC is that you get nuisance tripping. I very well could be wrong but I believe that I remember that there isn't a code saying that you cant,(exept that you need to follow manufacturers specs), just that it doesn't work. You wouldn't be protecting both circuits with an AFCI, but instead only the bedroom. I have used this trick a couple times with only one success. In my opinion I would rather use this when it works because many home owners would rather take the AFCI out completely then pay to run a new homerun in a finished house.
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Old 02-15-2011, 06:27 PM   #20
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AFCI problem has me stumped


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Originally Posted by oleguy74 View Post
don't think they are even on the market yet!!!!
Quote:
Originally Posted by SparkandArc View Post
If it was wired more than 3 years ago, I ran as many MWBCs as I could, as it saved time. And I have seen this time and time again, that people want to sell a house or upgrade and they think they can just install a new breaker to meet the new code. I have heard rumblings that certain manufacurers are working on making an AFCI breaker that will work with MW circuits. I hope this happens soon as it would definatly make this kind of things work out.
I provided a link to a siemans one. Apparently at least one company does market them currently.
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Old 02-15-2011, 06:29 PM   #21
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AFCI problem has me stumped


Quote:
Originally Posted by SparkandArc View Post
Well, I guess I don't quite understand what you mean. I believe the only reason that you cannot run an AFCI on a MWBC is that you get nuisance tripping. I very well could be wrong but I believe that I remember that there isn't a code saying that you cant,(exept that you need to follow manufacturers specs), just that it doesn't work. You wouldn't be protecting both circuits with an AFCI, but instead only the bedroom. I have used this trick a couple times with only one success. In my opinion I would rather use this when it works because many home owners would rather take the AFCI out completely then pay to run a new homerun in a finished house.
Your trick with the afci recep is not code compliant (at least prior to 2008 but I believe it was continued the same) due to the fact you muct provide the afci protection within 5 feet of the supply (breaker panel). Unless the panel was in the wall immediately behind the bedroom, it would be impossible to build your system and remain compliant.
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Old 02-15-2011, 08:21 PM   #22
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AFCI problem has me stumped


my post was in reguards to sparkandarc about afci recp's see post 13.

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Old 02-15-2011, 08:29 PM   #23
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AFCI problem has me stumped


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my post was in reguards to sparkandarc about afci recp's see post 13.

Ah, my apologies. It was getting kind of confusing there. I was wondering about the afci recep myself. I didn't recall seeing any myself.
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Old 02-15-2011, 08:33 PM   #24
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AFCI problem has me stumped


like to see them get on the market...
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Old 02-15-2011, 08:41 PM   #25
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AFCI problem has me stumped


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like to see them get on the market...
doesn't seem like it would be too advantageous in most areas. Unless the circuit is fed with RMC, IMC, EMT, or AC cable, the afci protection has to be in the panel.
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Old 02-15-2011, 11:08 PM   #26
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AFCI problem has me stumped


most respects no different than gfci.its wirtten that way so they can soak us for more money.
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Old 02-16-2011, 07:47 AM   #27
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AFCI problem has me stumped


1) What is MWBC?

2) Why can't they share neutrals? Is neutral also multi-phase? This may be where my academic knowledge and the real-world differ, but (atleast physically speaking) neutral should be common to all 3 phases. The only thing that differentiates neutral from ground is that neutral allows for a return path for current in the event that loads aren't perfectly balanced.

3) How exactly do I go about locating this arc? Should I take a tone generator and trace to look for potentially bad places the wires may be going?

4) How exactly do I tell if they are on the same phase/same neutral?

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If that doesn't work, then I would ask the OP if there are any multi-gang boxes that share both circuits? Such as a 2-gang switch box that has both the hallway switch and one for the bedroom? There may be an interconnection in there ... somewhere ...
Already replaced the AFCI, still happening, not sure where the interconnection would be because this is the receptacles and the lights (all on switches). There is no logical reason to connect them.

-----------------

Also, they aren't side-by-side, one is directly above the other.

Last edited by WR_ECE; 02-16-2011 at 07:59 AM.
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Old 02-16-2011, 08:28 AM   #28
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AFCI problem has me stumped


Easiest way to check for a MWBC (Multi Wire Branch Circuit) is to look in the panel. It will be wired with three conductor cable, red to one breaker and black to the other, and the breakers will be on different legs of the service.

Using a single pole AFCI on either of these circuits will not work as the AFCI will trip when it senses current flow on the neutral without a corresponding current flow on the hot... and there will be current flow on the neutral of a MWBC unless both hots are carrying the exact same load.
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Old 02-16-2011, 09:15 AM   #29
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Quote:
WR_ECE;592061]1) What is MWBC?
multiwire branch circuit. It is where two circuits on different legs (or in the case of 3 phase power, different phases. phases is not truly a proper term for a split phase power supply such as with a 120/240 volt system used in residential supplies) share a neutral common to each of the circuits

Quote:
2) Why can't they share neutrals? Is neutral also multi-phase? This may be where my academic knowledge and the real-world differ, but (atleast physically speaking) neutral should be common to all 3 phases. The only thing that differentiates neutral from ground is that neutral allows for a return path for current in the event that loads aren't perfectly balanced.
they can but there are two potential problems:

1. if the two circuits are not on different legs or phases, rather than the neutral current being the difference between the current on the two circuits, it is additive. As such, if you have 2 20 amp circuits, you could end up with 40 amps of current on the neutral

2. when using AFCI, sharing a neutral can be a problem. Unless using the correct breaker (one designed for use specifically with a MWBC) you will generally have false trips

Quote:
3) How exactly do I go about locating this arc? Should I take a tone generator and trace to look for potentially bad places the wires may be going?
since the circuits tend to work, it won't be that easy. A tone generator will simply tell you where there is a break in the line. If there is an acr, it would mean there is some level of continuity which would mean the tone generator probably will not provide any benefit.

Quote:
4) How exactly do I tell if they are on the same phase/same neutral?
looking is the easiest way for the phase question. If they are directly next to each other, in all modern panels, they would not be on the same phase/leg. Other than that, using a voltmeter and reading from the output of one of the breakers to the output of the other breaker (with them switched on). If you have a voltage reading of 0, they are on the same phase/leg.

to the neutral; generally from a visual inspection. Follow the conductors as the feed from the breakers. If they go into a common cable with only 1 neutral, they are sharing a neutral.







Quote:
Also, they aren't side-by-side, one is directly above the other.
semantics. That would typically be considered to be side by side.
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Old 02-23-2011, 07:48 AM   #30
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AFCI problem has me stumped


Well... finally figured out the problem yesterday.

They were on different phases (the way my box is built side-by-side is same phase, above-below is different phase)...

Problem was they shared a neutral at the other end, so effectively the hallway light's neutral reference was going through my master bedroom's AFCI (they were both on a single 2-phase wire). Quite a stupid move on the part of whatever electrician wired my house since it was a standard breaker for my lights. It was causing the false-trips (so you got it right nap).

Only real choice I had was to put both circuits through the single AFCI without rewiring that entire circuit. Hopefully it will be ok, I am adding less than an amp to the overall power-draw from the receptacles in my MB. I'll have to get a different breaker for that circuit if they even make them.

Thanks for all the suggestions everybody! Much appreciated, I definitely learned alot about how my house is wired.

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