AFCI required on MWBC?

Status
Not open for further replies.
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I searched through past threads but I didn't find anything that addressed the question directly to my satisfaction.

The code requires arc fault protection on "120-volt, single-phase, 15- and 20-ampere branch circuits" in parts of dwellings. (210.12)

Is a multi-wire branch circuit a '120-volt' circuit?

It seems that at least some people think so, and that some people are installing AFCIs on MWBCs now that two-pole breakers are available. But to me the language seems vague. Is there a code section that addresses this more directly?
 
Yes it is.
No specific code section as to AFCI and MWBC. It's a 120v load of to hots sharing a neutral .
Why would you think it is not?
 
In fact, if you put your meter across any 2 conductors of a 120/240v 1ph or 120/208v 3ph Y branch circuit, and, the highest reading your getting is only 120 volts, then your not dealing with a MWBC at all. :)

Therefore, a multi-wire branch circuit is not a 120v circuit.


JAP>
 
Sierrasparky said:
Yes it is.
No specific code section as to AFCI and MWBC. It's a 120v load of to hots sharing a neutral .
Why would you think it is not?
Click to expand...

I don't see a substantive physical difference between an MWBC and, say, a 240V outlet with neutral.

I admit that this is a 'letter' vs. 'spirit' kind of a thing. It certainly seems like the spirit of the requirement should apply to MWBCs but I don't think that the code clearly says that.

I'm curious what people's experience with AHJs is on this. I see some panels where everything but the MWBCs have AFCIs and some of them seem too new to be attributed to breaker availability.
 
Yes it is. It is two separate 120V circuits.. These two circuits need to have their breakers tied together because the are sharing the neutral. They do need to have an AIF depending on their location and usage.
 
I'd think that the clause of a MWBC being considered a "Single" circuit is where the catch is, and the 240 volt rating of the other circuits is what may exclude those.


JAP>
 
Sparks4All said:
Yes it is. It is two separate 120V circuits.. These two circuits need to have their breakers tied together because the are sharing the neutral. They do need to have an AIF depending on their location and usage.
Click to expand...

The Key to the wording is 2 separate 120v circuits derived from 2 different phases.


JAP>
 
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.
 
jap said:
The Key to the wording is 2 separate 120v circuits derived from 2 different phases.


JAP>
Click to expand...

No, no, no, I will not have this thread become another debate about whether 120/240 is one or two phases. All other posters are directed to not respond to this argument. :happyno::lol::slaphead:
 
The real question is, does a MWBC need to be on an AIF breaker? In my opinion, it does depending on its location and usage. That is the opinion of every inspector in my area.
 
jaggedben said:
No, no, no, I will not have this thread become another debate about whether 120/240 is one or two phases. All other posters are directed to not respond to this argument. :happyno::lol::slaphead:
Click to expand...



"Phasers-- on stun!"
 
jaggedben said:
No, no, no, I will not have this thread become another debate about whether 120/240 is one or two phases. All other posters are directed to not respond to this argument. :happyno::lol::slaphead:
Click to expand...

Then you'll just be left with your pre conceived notions I guess.



JAP>
 
jaggedben said:
No, no, no, I will not have this thread become another debate about whether 120/240 is one or two phases. All other posters are directed to not respond to this argument. :happyno::lol::slaphead:
Click to expand...

jap said:
Then you'll just be left with your pre conceived notions I guess.



JAP>
Click to expand...

120/240 is a single phase, split in half. You may speak of a split phase, or the other half of the phase, but it's still a single phase (ie 1/3 of a delta or wye). :slaphead: I just responded.
 
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.
 
Status
Not open for further replies.
Top