AFCI required on MWBC?

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Sparks4All said:
Back in the day, the MWBC circuits did not need to be on the same breaker so they were technically two different circuits. You can still do this but a handle tie is required. Usually everyone just installs a 2-pole breaker.
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Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


JAP>
 
Sparks4All said:
Back in the day, the MWBC circuits did not need to be on the same breaker so they were technically two different circuits. You can still do this but a handle tie is required. Usually everyone just installs a 2-pole breaker.
Click to expand...

jap said:
Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


JAP>
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Sparks is saying that at one time you could run a 3 wire, shared neutral, MWBC and use any two circuits in the panel that weren't one the same leg or phase. That is no longer a viable option because of the handle-tie requirement.
 
jap said:
Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


JAP>
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So the NEC is 'incorrectly' speaking of a split phase as "2 different phases??" Or are we supposed to be wiring our homes on 3-phase power?
 
PaulMmn said:
So the NEC is 'incorrectly' speaking of a split phase as "2 different phases??" Or are we supposed to be wiring our homes on 3-phase power?
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The "different phases" requirement is for a multi-phase system, the "Ungrounded Conductors that have voltage between them" is from a single winding center tapped system.
 
infinity said:
A MWBC is two circuits when applying the requirements of 210.12 for AFCI protection. It can be considered a single circuit elsewhere in the NEC such as 225.30.
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210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." Which also means it is not required to be considered as multiple circuits and may generally be considered as one circuit.

So what text says that for purposes of 210.12, a multiwire circuit must be considered as multiple circuits?

Cheers, Wayne
 
We love to see how complicated we can make a simple question, don't we?

For the sake of the OP's question, a residential MWBC (where a single load is not supplied by both ungrounded conductors) is two 120v circuits.

The only reason "1 phase vs 2 phases" comes up here is because of the tendency of some to call every ungrounded conductor a phase.
 
wwhitney said:
210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." Which also means it is not required to be considered as multiple circuits and may generally be considered as one circuit.

So what text says that for purposes of 210.12, a multiwire circuit must be considered as multiple circuits?

Cheers, Wayne
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210.12(A) says all 120 volt circuits, if you run MWBC for two 120 volt circuits then it requires AFCI protection.

210.12(A) Dwelling Units. All 120-volt, single-phase, 15- and
20-ampere branch circuits supplying outlets or devices in-
stalled in dwelling unit kitchens, family rooms, dining
rooms, living rooms, parlors, libraries, dens, bedrooms,
sunrooms, recreation rooms, closets, hallways, laundry ar-
eas, or similar rooms or areas shall be protected by any of
the means described in 210.12(A)(1) through (6):
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Darn, maybe I should give up.

Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.

As a solar installer I've been able to almost entirely ignore arc-fault stuff until recently we got into doing battery backup and now I'm dealing with rewiring house loads and it's starting to come up at inspections. I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.
 
jaggedben said:
Darn, maybe I should give up.

Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.

As a solar installer I've been able to almost entirely ignore arc-fault stuff until recently we got into doing battery backup and now I'm dealing with rewiring house loads and it's starting to come up at inspections. I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.
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The bold part is specific to where you earn your living. If you're in a place that follows the NEC to the letter then you will need AFCI protection on a MWBC that is providing 2-120 volt circuits.
 
jaggedben said:
Darn, maybe I should give up.
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Please don't.

Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.
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There may not be a consensus to be had. Here in VA, we're fortunate enough to still on the '14 NEC, with the option to use the '11 until September. Only bedroom outlets require AFCI protection.

I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.
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You're answering your own question; or rather, you're saying that there is no single answer where you are. When and where AFCI protection is required for you is inconsistent.

The most direct answer we can give you is that having a MWBC does not have any effect on whether AFCI protection is required in any given installation, new work or otherwise.
 
infinity said:
210.12(A) says all 120 volt circuits, if you run MWBC for two 120 volt circuits then it requires AFCI protection.
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Sure. But the Article 100 definition of "Voltage, nominal" says:

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).
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So the argument would be that a MWBC may be considered to be one 120/240 circuit, rather than two 120 volt circuits. Let's say that every outlet on the circuit has both ungrounded conductors present, along with the neutral.

Cheers, Wayne
 
infinity said:
210.12(A) says all 120 volt circuits, if you run MWBC for two 120 volt circuits then it requires AFCI protection.
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I admit I have never thought of this, nor ever heard of the interpretation/philosophy of MWBC's being exempt from AFCI, but see the ambiguity now. Consider a NEMA 14-20 recep (as someone mentioned a few pages back). Does that need aFCI? I say no. But then I also say that it follows that a MWBC's doesnt require AFCI UNTIL it splits in to two 120V circuits.
 
electrofelon said:
I admit I have never thought of this, nor ever heard of the interpretation/philosophy of MWBC's being exempt from AFCI, but see the ambiguity now. Consider a NEMA 14-20 recep (as someone mentioned a few pages back). Does that need aFCI? I say no. But then I also say that it follows that a MWBC's doesnt require AFCI UNTIL it splits in to two 120V circuits.
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How often is a 14-20 used in the rooms mentioned in 210.12? Mostly only used at a dwelling as a standby generator inlet and that is about it, and not typically in any the rooms mentioned in 210.12 either.
 
electrofelon said:
How long have you been on this forum? Its what we do here is come up with the most bizarre and unlikely things to debate and prove how poorly worded the code is ;)
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I know, I suggested there is a loophole in using only MWBC's to avoid needing AFCI's - but I know that won't go over very well even if you have pretty convincing justification prepared.
 
If a 20 amp small appliance multi wire branch circuit is not two branch circuits than thousands of kitchens have been wired in violation and passed inspection by thousands of inspectors for 50 plus years

interesting no one questioned that application for years, now that AFCI protection is a requirement all of a sudden a multi wire circuit for the same application is no longer two 120 volt branch circuits

Not a logical argument and seems self serving
 
david said:
If a 20 amp small appliance multi wire branch circuit is not two branch circuits than thousands of kitchens have been wired in violation and passed inspection by thousands of inspectors for 50 plus years

interesting no one questioned that application for years, now that AFCI protection is a requirement all of a sudden a multi wire circuit for the same application is no longer two 120 volt branch circuits

Not a logical argument and seems self serving
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My argument exactly. :cool:
 
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