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Article 310.15(4) of the NEC

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rlwhite

Electrical
Mar 25, 2003
2
In article 310.15(4(a) regarding the Neutral Conductor, we have differences of opinions on what it says. The article says "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shal not be required to be counted when applying the provisions of 310.15(B)(2)(a)."
If you have (3) hots A, B,and C phase and (3) neutrals (one for each leg) are you required to count the neutrals when applying the provisions of 310.15(B)(2)(a)?
We have one guy that says that this is only used whe you have a multi pole circuit (i.e. 2 or 3 pole) that also requres a nuetral. Tell me what you think.
 
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Part (c) of the same section deal specifically with multi-wire circuits with non-linear loads. Part (a) deals with the "unbalanced current from other conductors of the same circuit", indicating that the circuit is multi-wire, but linear.
If you have three separate single-phase circuits from A, B, amd C phases, and each has a separate neutral, I would think that Part (a) does not apply. If the three neutrals are carrying the unbalanced current from the three phase conductors(same load), then I would think that it does apply.
 
DanDel is correct. This is clear if you read the next paragraph 310.15-4(b) which describes a slightly different but similar situation where you would have to count the neutral.

The issue in this entire section is the necessity to derate the specified conductor ampacity in the NEC Tables which is based on the assumption of three current carrying conductors in a conduit.

In a three-phase, 4-wire system (with no triplen harmonics), the total rms heating effect is always the same regardless of how the current divides. (Disregarding conductor resistance changes related to temperature).

In the situation where each hot wire has a neutral conductor that is carrying the same current as the hot wire, all conductors must be considered current-carrying, since all are generating heat and there is no cancellation of current.
 
Exactly. Don't get too hung up on the 1-phase vs. 3-phase issue.

The main point is that if the neutral IS carrying current, regardless of the circuit configuration, then it's putting off heat and MUST be counted.

If it's NOT carrying current, as in a well-balanced 3-phase system, then it's not putting off heat and you DON'T need to count it.
 
peebee, it may be more accurate to say, rather than 'no current', that any current that the neutral is carrying in a three-phase circuit is current that would normally be carried by one of the phase conductors.
Since the vector sum of the total currents in a three-phase, four-wire circuit will equal zero, whether the total currents are in three conductors or four.
 
Yup.

By the way, nobody's mentioned harmonics here yet, which is key here too. If there's harmonic loads on that balanced 3-phase circuit (PC's, fluorescent lighting, etc.) then not only must the neutral be counted, it most likely needs to be DOUBLED, and both neutrals counted.
 
peebee, that's why I mentioned Part (c) of Art. 310.15 about multi-wire circuits with non-linear loads in my first post.
 
This has been a matter on whic I have watched local inspectors flip-fllop, of course all the while denying ever having thought the opposite. The current thought among them for the last 10 years has been that if 3 phase conductors share the same neutral, regardless of single or multiple loads, then the neutral need not be counted, being that it is carrying the unbalanced load between them collectively. However, in the instance of 1 neutral per hot, then the neutral must be counted, since it is basically used as the current return.

Hope this helps.
 
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