AFCI required on MWBC?

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infinity said:
A 3-wire MWBC is two 120 volt circuits. Why is that so hard to understand?
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It's not that it's hard to understand your point of view. And if that is the industry consensus (more or less) then I'm not going to fight it. It's just that I don't think it's at all obvious from reading the code.

Article 100 (Note the definition of MWBC as a singular circuit.)

Branch Circuit, Multiwire. A branch circuit that consists
of two or more ungrounded conductors that have a voltage
between them, and a grounded conductor that has equal
voltage between it and each ungrounded conductor of the
circuit and that is connected to the neutral or grounded
conductor of the system.
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Voltage (of a circuit). The greatest root-mean-square
(rms) (effective) difference of potential between any two
conductors of the circuit concerned.
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That seems to pretty clearly define an MWBC as one circuit of 240V. We can find more...

210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article
shall be permitted as multiwire circuits. A multiwire circuit
shall be permitted to be considered as multiple circuits.
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Note permitted, not required. (Emphasis added.)

(C) Line-to-Neutral Loads. Multiwire branch circuits
shall supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies
only one utilization equipment.
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So per the exception wording, two hots and a neutral that go to a single outlet are still an MWBC, and also still a single circuit. Per the voltage definition, this is a 240-volt circuit.

225.30 Number of Supplies. [...] For the purpose of this section, a multiwire
branch circuit shall be considered a single circuit.
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Granted, this is 'for the purpose of this section', but we now have multiple sections treating an MWBC as one circuit by the basic definition, vs. one section permitting it to be considered otherwise.


It just seems to me that 210.12 ought to say 'all 120V circuits, including MWBCs serving 120V outlets', if that's what it intends to mean. There's just too many other places in the code which imply that an MWBC is a single, 240V circuit. 210.12 needs it's own 'for the purposes of this section'. Or at any rate, people should not claim that the meaning should be bleedingly obvious to newbies.
 
jaggedben said:
So per the exception wording, two hots and a neutral that go to a single outlet are still an MWBC, and also still a single circuit. Per the voltage definition, this is a 240-volt circuit.
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The Code has requirements for 125V, 15A and 20A receptacles for dwelling units. The Code does not permit 125V, 15A and 20A receptacles to be installed on 240V circuits.

So your question shouldn't be is AfCI required on mwbc's, your question should be are mwbc's allowed to supply the required 125V, 15A and 20A receptacles in dwelling units.

You have already answered this by noting that the Code permits a mwbc to be considered as multiple circuits...multiple 120V circuits in the case of supplying the required dwelling unit receptacles. And 120V, 15A or 20A circuits in dwelling units require AfCI protection in most locations.
 
jap said:
If You go to the supply house and buy just enough black, white,and green the length you need for a 120v circuit, you will not have enough wire for a multi wire branch circuit because you will be missing the 2nd ungrounded conductor to make it a MWBC.

Why is this so difficult for you to understand?

Jap>
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:? Were you drinking (or smoking) and posting? I have no idea of what you are trying to have me "understand". A MWBC consists of either 2 or 3 120V circuits (depending on your service, single or three phase).
 
jaggedben said:
So per the exception wording, two hots and a neutral that go to a single outlet are still an MWBC, and also still a single circuit. Per the voltage definition, this is a 240-volt circuit.
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No, it is not. Because you have 240v loads and 120v loads intended to be served by a single outlet. If you are talking about a duplex receptacle split wired, by DEFINITION you have two outlets. Hint, that's why it's called a duplex receptacle.
 
jaggedben said:
It's not that it's hard to understand your point of view. And if that is the industry consensus (more or less) then I'm not going to fight it. It's just that I don't think it's at all obvious from reading the code.
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You can’t pick and choice if you are installing two 20 amp breakers to supply 120 volt loads normally two branch circuits and you instead run one multi three wire circuit with ground , you can’t then define that circuit as a 240 volt branch circuit

Lets pull away from residential for a second and supply 26 duplex receptacles with one 20 amp multi wire circuit in an office occupancy

How many branch circuits do you have ?

Its not that you can arbitrarily define the circuit to your choosing, the application of the branch circuit defines it as 120 volt or 240 volt circuits

When you use a multi wire circuit instead of two 120 volt branch circuits, the application of the circuit defines it as a 120 volt multi wire branch circuit for code purposes
 
OK, here's my best attempt to parse everything and support the latest argument presented by jaggedben, by distinguishing between the specific term branch circuit and the more general term circuit:

Say you have a 20A 2 pole breaker in a 120/240V panelboard feeding the ungrounded conductors of a 12/3 cable that hits several junction boxes in series. Each junction box has a single duplex receptacle which is fed from just one ungrounded conductor and the neutral, alternating ungrounded conductors along the run.

- Per the article 100 definition of branch circuit, and the optionality of 210.4(A), this may be considered just one branch circuit.

- Per the article 100 definition of voltage (of a circuit), the voltage of this branch circuit is 240V.

- Within each junction box, a small 120V circuit is created when the 125V receptacle is connected to just one ungrounded conductor and the neutral. This complies with the 406.4(A) restriction on the circuit feeding the receptacle.

- Since 210.12 references the voltage of the branch circuit, rather than just the (local) circuit, AFCI is not required.

Cheers, Wayne
 
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oldsparky52 said:
:? Were you drinking (or smoking) and posting? I have no idea of what you are trying to have me "understand". A MWBC consists of either 2 or 3 120V circuits (depending on your service, single or three phase).
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I don't drink, and I don't smoke, so that comment doesn't apply to me.

I stand by my statement.

A MWBC is not A 120v circuit,and, A 120v circuit is not MWBC, because It takes more than a single 120v circuit to make a MWBC.

As far as the AFCI being require,I would say more times than not, it's going to be required just as it would be if it were required for a 120v circuit.

Jap
 
wwhitney said:
- Since 210.12 references the voltage of the branch circuit, rather than just the local circuit, AFCI is not required.
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This is not correct. "Local circuit" isn't a thing in the NEC. "Branch circuit" is.

The receptacles connect to a branch circuit. AFCI protection is required for most of those branch circuits in dwelling units
 
david luchini said:
This is not correct. "Local circuit" isn't a thing in the NEC. "Branch circuit" is.
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Fine, ignore the word local in my last post, I subsequently parenthesized it.

My point is that some language in the NEC uses the phrase "branch circuit" and some language uses the single word "circuit". The former term has a specific definition in Article 100; the latter term does not and is more general. So a circuit may refer to just a portion of a branch circuit, or a portion of a feeder, etc. It is not correct to substitute the phrase "branch circuit" every time the NEC uses the bare word "circuit".

The upshot is that an MWBC (from a 120/240V panelboard) is a 240V branch circuit, but it may supply 120V circuits.

Cheers, Wayne
 
wwhitney said:
The upshot is that an MWBC (from a 120/240V panelboard) is a 240V branch circuit, but it may supply 120V circuits.

Cheers, Wayne
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Wayne, IMO you have that backwards. A MWBC is by 210.4(C) multiple 120v circuits. There are two exceptions that allow you to use it in a 240v or 120/240v application. It is not a 240v circuit 1st, a 240v MWBC is an exception to the rule.
 
wwhitney said:
Say you have a 20A 2 pole breaker in a 120/240V panelboard feeding the ungrounded conductors of a 12/3 cable that hits several junction boxes in series. Each junction box has a single duplex receptacle which is fed from just one ungrounded conductor and the neutral, alternating ungrounded conductors along the run.

- Per the article 100 definition of branch circuit, and the optionality of 210.4(A), this may be considered just one branch circuit.

- Per the article 100 definition of voltage (of a circuit), the voltage of this branch circuit is 240V.

- Within each junction box, a small 120V circuit is created when the 125V receptacle is connected to just one ungrounded conductor and the neutral. This complies with the 406.4(A) restriction on the circuit feeding the receptacle.

- Since 210.12 references the voltage of the branch circuit, rather than just the (local) circuit, AFCI is not required.
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I disagree. You defined the MWBC as being two 120v circuits, and not a single 240v circuit, when you connected each receptacle between either line and the neutral.

Had you connected each receptacle to both lines, then I'd agree that you have a 240v circuit. I say that the load connections, as well as the source, defines the circuit.
 
oldsparky52 said:
I think the loading alone defines the circuit.
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Well, a 2p breaker is required for a 240v circuit, while two handle-tied 1p breakers can supply a MWBC, and the two breakers need not even match.

Added: I guess this is still saying that the loading defines the circuit because the loading determines the breaker requirements that I just stated.
 
oldsparky52 said:
:thumbsup:

I think the loading alone defines the circuit. If you have 2 hots and a neutral feeding all 120v loads you have 2 120v circuits not a 120/240v circuit even if the hots are connected to a 2-pole breaker.
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I agree, it's really that simple. :)
 
wwhitney said:
The upshot is that an MWBC (from a 120/240V panelboard) is a 240V branch circuit, but it may supply 120V circuits.
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The upshot is that the 120V circuits are also branch circuits, and therefore require AFCI protection. There is no way around that.

The MWBC/AFCI "loophole" that was discussed earlier does not exist.
 
david luchini said:
The upshot is that the 120V circuits are also branch circuits, and therefore require AFCI protection.
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Let's look at the definition of branch circuit:

Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
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Your interpretation would be correct if the definition started out "Any portion of the circuit conductors . . ."

However, the definition does not say that. So the "branch circuit" is all of the 12/3 cable circuit conductors in my example, starting at the 2p 20 amp breaker. There is one branch circuit, comprising the entirety of those circuit conductors. The "branch circuit" is reducible into a sum of smaller circuits, but those smaller circuits are not branch circuits.

Cheers, Wayne
 
oldsparky52 said:
Wayne, IMO you have that backwards. A MWBC is by 210.4(C) multiple 120v circuits. There are two exceptions that allow you to use it in a 240v or 120/240v application. It is not a 240v circuit 1st, a 240v MWBC is an exception to the rule.
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210.4(C) is moot in my example, as I specified a 2p circuit breaker, so the 2nd exception applies.

210.4(A) clearly tells us that an MWBC may be considered a single circuit.

Cheers, Wayne
 
wwhitney said:
However, the definition does not say that. So the "branch circuit" is all of the 12/3 cable circuit conductors in my example, starting at the 2p 20 amp breaker. There is one branch circuit, comprising the entirety of those circuit conductors. The "branch circuit" is reducible into a sum of smaller circuits, but those smaller circuits are not branch circuits.

Cheers, Wayne
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In what jurisdictions have they agreed with you? Is it okay for me to call them? :)
 
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