Sharing a neutral

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roger said:
Charlie, where is this stated in the NEC?
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210.2. In most of the code, it is presumed that the absence of a "no you can't" statement is the same as directly saying "yes you can." But here, you have a specific statement that "branch circuits shall comply with this article." That leaves no room for a "but you didn't prohibit XXX" interpretation; if it isn't in 210, you can't do it, because 210.2 says you have to comply with this article.

What was your basis for saying the A,B,C,A,C,N was not code compliant?
 
mxslick said:
Oh boy, here we go again. :roll:

Ok guys, let me throw this real-world installation I ran into out for this discussion, and you all can tell me if it is Code-Compliant or not:

3 phase, 208y/120v panel
Total of five circuits, phased A,B,C,A,C All circuits 20 amp, 120 volt
A single (one) #10 neutral

What say the experts?
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WOW I've never seen that done. Even if it were code compliant I feel it's a accident waiting to happen. How does one see that a # 10 can handle the unbalanced load of 3 properly phased circuits and then add 2 additional circuits. Who designed such a "rube Golberg"?

Generally it is common to use a oversized neutral because of Inherent imbalance in non-linear loads. To do what your company has done it is lucky it did not get sued. It is far more common for an experienced EC to NOT use any multi-wire circuits in such installations. In Our experience it has only lead to issues. Interesting when it comes to HUMM and noise in a video signal we have observed the twisted wires in MC or AC cable to aide in a cleaner signal. Of course you can order from the wire supplier
pre-twisted groups of wire from a wire wholesaler.
 
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Sierrasparky said:
WOW I've never seen that done. Even if it were code compliant I feel it's a accident waiting to happen. How does one see that a # 10 can handle the unbalanced load of 3 properly phased circuits and then add 2 additional circuits. Who designed such a "rube Golberg"?

Generally it is common to use a oversized neutral because of Inherent imbalance in non-linear loads. To do what your company has done it is lucky it did not get sued. It is far more common for an experienced EC to NOT use any multi-wire circuits in such installations. In Our experience it has only lead to issues. Interesting when it comes to HUMM and noise in a video signal we have observed the twisted wires in MC or AC cable to aide in a cleaner signal. Of course you can order from the wire supplier
pre-twisted groups of wire from a wire wholesaler.
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Bold/italic added by me.

Let me make it clear that we DID NOT do that, it was done by another "A/V" company that has been known for cutting corners, especially when it comes to the hiring of their EC's.

Our company cleaned up the mess by running the dedicated neutrals..in fact if you search my posts you'll see that I have a "hatred" of MWBC's for ANY audio/video applications.
 
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charlie b said:
210.2. In most of the code, it is presumed that the absence of a "no you can't" statement is the same as directly saying "yes you can." But here, you have a specific statement that "branch circuits shall comply with this article." That leaves no room for a "but you didn't prohibit XXX" interpretation; if it isn't in 210, you can't do it, because 210.2 says you have to comply with this article.
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I don't see anything that would say we are not complying with the article.

charlie b said:
What was your basis for saying the A,B,C,A,C,N was not code compliant?
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 240.4(D), the #10 could not be protected for 40 amps and with two 20 amp breakers sharing this grounded conductor it could see 40 amps.

Roger
 
roger said:
I don't see anything that would say we are not complying with the article.
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That article says that we can do lots of things, and installing MWBCs is one of them. As you also pointed out, this would not be a MWBC. So we are left without something that says we can do this (meaning the A,B,C,A,C,N thing). And back at the beginning, 210.2 says we have to install BCs in accordance with the article.
 
roger said:
240.4(D), the #10 could not be protected for 40 amps and with two 20 amp breakers sharing this grounded conductor it could see 40 amps.
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So in theory, if that neutral was a #8, it would have been compliant IAW 240.4(D)?

Charlie, wouldn't the provisions of 210.2 make it non-compliant? Or am I misreading what you said here: (Bold added by me)

charlie b said:
210.2. In most of the code, it is presumed that the absence of a "no you can't" statement is the same as directly saying "yes you can." But here, you have a specific statement that "branch circuits shall comply with this article." That leaves no room for a "but you didn't prohibit XXX" interpretation; if it isn't in 210, you can't do it, because 210.2 says you have to comply with this article.
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This discussion is why I love this site, I am learning something new here..

EDIT: Whoops, I see what you guys typed while I was typing, it makes sense now.. :)
 
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charlie b said:
That article says that we can do lots of things, and installing MWBCs is one of them. As you also pointed out, this would not be a MWBC. So we are left without something that says we can do this (meaning the A,B,C,A,C,N thing). And back at the beginning, 210.2 says we have to install BCs in accordance with the article.
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And what are we not installing per the article? Where does it specifically say we can install two wire circuits?

Roger
 
Ok, so in summary:

What I see is even with a #8 neutral (for ampacity's sake) that setup is NOT compliant because it does not meet the definition of a MWBC IAW 210.2

So it would appear that the AHJ on the original install dropped the ball big-time.
 
LOL ok, so where does that leave us in regards to 210.2?

I think it would have forced at least one other neutral wire as the doubling up of the A and C phases negates this being a single MWBC, right?

Edit: In the 1999 Code (only edition I have handy) I noticed there is no clear definition of what constitutes a single MWBC..just says a MWBC can be "multiple circuits".
 
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charlie b said:
See post #11 above. Article 210.4 allows us to use MWBCs, and article 100 tells us what those are. Two hots from the same phase plus a shared neutral do not make a MWBC, and thus are not allowed.
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If I cannot find a circuit definition in Article 100 that means I cannot install that circuit?

That is going to be pretty limiting. :)
 
roger said:
Where does it specifically say we can install two wire circuits?
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Article 210.1, in which it says that the article applies to branch circuits. A two wire (for example, A + N) meets the article 100 definition of "branch circuit."
 
mxslick said:
I think it would have forced at least one other neutral wire as the doubling up of the A and C phases negates this being a single MWBC, right?
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Right. In fact, it turns it into a pair of MWBCs, if you select the wire associations correctly (A,B,C,N and then A,C,N).
 
mxslick said:
Edit: In the 1999 Code (only edition I have handy) I noticed there is no clear definition of what constitutes a single MWBC..just says a MWBC can be "multiple circuits".
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All MWBC's are single circuits by their very nature, the NEC allows them to be considered "multiple circuits" which they are not.

Roger
 
That's what I thought..but here's an edit I made while you guys were posting:

me said:
Edit: In the 1999 Code (only edition I have handy) I noticed there is no clear definition of what constitutes a single MWBC..just says a MWBC can be "multiple circuits".
Click to expand...

So once again the Code is clear as mud....at least in the '99 cycle.
 
5-7 Log #3640 NEC-P05 Final Action: Reject
(100.Common Neutral (New) )
_______________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Add new text to read as follows:
Common Neutral. A neutral conductor used in a circuit with two or more
ungrounded conductors having no potential between them.
Substantiation: The term common neutral is used in Articles 215 and 225. It is
not defined in either article and the term needs to be defined for proper
understanding of the code rules.
Panel Meeting Action: Reject
Panel Statement: The proposed definition does not add any clarity to the use
in those sections cited in the substantiation. The proper usage of a common
neutral is adequately covered in 215.4 and 225.7(B). Section 215.4 deals with
multiple feeder circuits using one neutral and is not one circuit as implied in
the proposed definition.
Number Eligible to Vote: 16
Ballot Results: Affirmative: 16
 
_______________________________________________________________
5-49 Log #3644 NEC-P05 Final Action: Accept in Principle
(200.8 (New) )
_______________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Add new text to read as follows:
200.XX Common Neutral Conductors. Common neutral conductors shall not
be used unless specifically permitted elsewhere in this code.
Substantiation: It appears that CMP 2 is attempting to prohibit the use of
common neutral conductors by specifically permitting them in 215.4(A) and
225.7(B). There is no reasonable reading of the words ?shall be permitted? that
can lead the code user to the conclusion that these words actually prohibit the
use of common neutral conductors in other cases. The act of specifically
permitting something in no way prohibits something else. Section 3.1.2 in the
NEC Style Manual says that the words ?shall be permitted? are to be used to
permit an alternate installation method. The words ?shall not? are required to
be used to prohibit an installation method per 3.1.1 of the Style Manual. This
change will make the wording in 215.4 and 225.7 comply with the style
manual rules. Also the prohibition of the use of common neutrals should rest
with CMP 5 as they have control of Article 200, Use and Identification of
Grounded Conductors and not with CMPs 2 and 4.
Panel Meeting Action: Accept in Principle
Revise the recommendation to read:
200.4 Neutral Conductors. Neutral conductors shall not be permitted to be
used for more than one multiwire branch circuit or for more than one set of
ungrounded feeder conductors unless specifically permitted elsewhere in this
Code.
Panel Statement: CMP-5 revised the proposal to more specifically apply to
multiwire branch circuits and feeders.
Number Eligible to Vote: 16
Ballot Results: Affirmative: 16
Comment on Affirmative:
JOHNSTON, M.: Continue to accept this proposed revision as amended by
the action of CMP-5. This additional section provides clarity to users about
how neutral conductors should be used and coordinates with the newly defined
terms neutral conductor and neutral point. Additional uses of the term common
conductor are not necessary and the NEC should migrate to removal of the
term as it is undefined and can cause inconsistency in enforcement.
 
From the draft copy of the 2011 NEC.


200.4 Neutral Conductors. Neutral conductors shall not be permitted to be used for more than one multiwire branch circuit or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this Code. [ROP 5-49]
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So, after CMP 5 got done with it, the end result does not stop us from using a common grounded conductor for circuits that are not MWBC's.

Roger
 
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