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Is MWBC 1 or 2 circuits for purposes of 310.15 (B) (5)

J

jkruse

Member
Location
Pennsylvania
Occupation
Mechanical Engineer
I’m getting conflicting information on an engineering plan that uses Multiwire Branch Circuits and the Neutral Conductor clause of 310.15 (B) (5) to keep the current carrying conductor count low for conduit derating. The conflict revolves around whether a MWBC is a “single circuit” as required in 310.15 (B)(5)a and how to apply the “shall be permitted” portion of 210.4 (A) allowing a MWBC to be considered multiple circuits.

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...

310.15 Ampacities for Conductors Rated 0–2000 Volts.
(B) (5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).

In 225.30, this is clearly defined for outside branch circuits, but other sections are silent.

225.30 Number of Supplies.
A building or other structure that is served by a branch circuit or feeder on the load side of a service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.

My hunch is the 210.4 (A) note is intended to clarify that the separate branches can be counted individually for locations requiring a particular circuit count (ie. kitchen receptacles). If a MWBC can always and arbitrarily be considered “multiple circuits”, how would 310.15 (B) (5) be of practical use?

Technical drawbacks of MWBC aside, is there further clarification of a “single circuit” applicable to 310.15 elsewhere in NEC?
Note code references are 2017 NFPA 70/ 2018 IRC
 
Before I allow anyone to answer your question, I need to know why you are asking it. Forum rules do not permit us to assist a person who is not an electrician to perform their own electrical installation work. I will therefore (at least temporarily) lock this thread. Please send me a PM with your explanation.
 
In conversations with the OP, I concluded that this is an allowable question. It is work-related, not a DIY situation.
 
In my opinion, the "single circuit" rule is only there to allow a MWBC to be used where toe Code restriction calls for a single circuit such as the separate building clause you noted.
As far as the neutral being counted as a current conductor, this guide by one of our members (maybe charlieb, I don't recall) should help.


Here's some examples of when to count the neutral as a CCC:

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
 
For the purpose of 310.15(B)(5), an MWBC is the 'same circuit'. Otherwise it would be impossible for a neutral to carry only the unbalanced current of the other conductors. For a two wire circuit there would only be one other conductor, and both conductors would carry all the current.
 
Thanks for the responses, I think LarryFine's response nails the conundrum. Everyone in our discussion agrees on the CCC count, the electrical details, pros and cons of MWBC and current splititng, etc. The heart of the question is who gets to declare per 210.4 (A) that a MWBC is 1 or multiple circuits? One party is citing the code that says "Shall be permitted" so I choose to treat it as multiple to exclude the use of 310.15 (B)(5), the other is saying " 'shall be permitted to...' doesn't mean the same as 'must' in code and I choose to not use this option".

At this point we will probably just upsize everything to make the issue go away and continue the job ASAP, but I'm still interested in hearing others thoughts. I'm sure many here have dealt with clients or inspectors that treat "may" as "must" and no amount of logic changes their mind, but I don't want to just write this off like that as I truely see where both are coming from. I'm certainly not questioning the wisdom of NEC, just curious if there is guidance elsewhere in the code that both parties have missed.
 
jkruse said:
Thanks for the responses, I think LarryFine's response nails the conundrum. Everyone in our discussion agrees on the CCC count, the electrical details, pros and cons of MWBC and current splititng, etc. The heart of the question is who gets to declare per 210.4 (A) that a MWBC is 1 or multiple circuits? One party is citing the code that says "Shall be permitted" so I choose to treat it as multiple to exclude the use of 310.15 (B)(5), the other is saying " 'shall be permitted to...' doesn't mean the same as 'must' in code and I choose to not use this option".

At this point we will probably just upsize everything to make the issue go away and continue the job ASAP, but I'm still interested in hearing others thoughts. I'm sure many here have dealt with clients or inspectors that treat "may" as "must" and no amount of logic changes their mind, but I don't want to just write this off like that as I truely see where both are coming from. I'm certainly not questioning the wisdom of NEC, just curious if there is guidance elsewhere in the code that both parties have missed.
Click to expand...
I'm confused as to why it even matters what you call it, be it one circuit or multiple circuits. In either case the number of CCC's does not change so why is the distinction even necessary?
 
infinity said:
I'm confused as to why it even matters what you call it, be it one circuit or multiple circuits. In either case the number of CCC's does not change so why is the distinction even necessary?
Click to expand...
Somebody's stupid argument is this:

(2017) 310.15(B)(5)(a) says "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted." Well, I choose to count the MWBCs as multiple circuits, so all neutrals must be counted, as while they are carrying only the unbalanced current overall, it's not the unbalanced current from conductors of the same circuit, it's from conductors from multiple circuits. Therefore you must use a higher number of CCCs and upsize conductors for ampacity adjustment accordingly.

This argument has zero merit and would render 310.15(B)(5)(a) a waste of space. 310.15(B)(5)(a) obviously is written with the idea that an MWBC is one circuit. End of discussion.

Cheers, Wayne
 
A 3-wire MWBC will have either 2 or 3 CCC's depending on the system it is connected to. For a single phase, 120/240 volt system it's 2 CCC's for a 3 phase, 208Y/120 volt system it's 3 CCC's.
 
Thanks everyone, I tend to agree this is a silly discussion with no technical merrit for all the reasons cited. I'm still in a bind with the stubborn party as none of the arguments worked previously. We will just write this off as the cost of doing buisness and hope that we don't encounter them again when using MWBC in a full conduit. I'm glad this isn't a common occurance as the code (and logic) seems pretty clear to me. That said, it's still interesting to see how 225.30 explicitly resolved this with 1 sentence. It should seem obvious from the entire premise of 310.15(B)(5) that a similar statement isn't even needed, but perhaps it will be added in the future to eliminate this argument.

Regards,
Jkruse
 
Would a 'neutral conductor that carries only the unbalanced current' ever not be derived from a split phase system?
Seems like a neutral would always be a CC in 480Y/277 or 240 corner grounded, 120/208 open wye MWBC etc...
EDIT I was thinking 3 wire branch circuit off two pole breaker.
 
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The words 'of the same circuit' in 310.15(B)(5) aren't even necessary and if they were deleted that might have resolved our OP's issue.
 
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