Article 310.15(4) of the NEC
Article 310.15(4) of the NEC
(OP)
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.
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.






RE: Article 310.15(4) of the NEC
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.
RE: Article 310.15(4) of the NEC
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.
RE: Article 310.15(4) of the NEC
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.
RE: Article 310.15(4) of the NEC
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.
RE: Article 310.15(4) of the NEC
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.
RE: Article 310.15(4) of the NEC
RE: Article 310.15(4) of the NEC
What DanDel said.
RE: Article 310.15(4) of the NEC
Hope this helps.