My boss want me to read up on this. I'm studying for my Journeymans test
Not an electrician but::
I would ask your boss for slightly more detail. Is the question about the electrical theory of how the neutral (grounded) conductor functions or since you are asking about the NEC, is the question about when to count the neutral (grounded) conductor should be counted when considering the number of current carrying conductors and when neutrals (grounded) conductors are to be counted as such.
If we are talking the National Electrical Code (as you requested) the best resource to begin with is reading the following references within the NEC and asking direct questions related to them so that we can elaborate and possible help educate.
Let's start at Article 100 because some of the most important aspects of learning the NEC is to learn definitions. If you don't truly know the meanings then you can most certainly never answer the question.
Article 100 - Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. (CMP-5)
So let's take a simple circuit you are probably very familiar with. A traditional Type NM-B Cable used in residential applications for typical branch circuits (and sometimes feeders) applications. A simple Black and White conductor in a complete closed loop setup from the source of the power (panel) to the load (lamp). Again for simplicity sake we will use the flow analogy in order to avoid getting into too much theory. The current flows from the power source to the load, in this case a lamp, and the flows back to the power source to complete the closed loop. The current traveling on the black conductor will also be returning back to the source on the white conductor. This white conductor is referred to as a neutral conductor ( which is a grounded conductor as well) and will carry current under normal conditions.
Article 100 - Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system. (CMP-5)
So again keeping it simple, on a typical transformer that will provide 120/240V or 277/480V (for brevity) be it a wye connected multiphase (referred to as polyphase or 3-phase) or as stated above the mid-point o a single-phase portion of a 3-phase delta system which is traditionally what you will see in residential applications when you see those 3 conductors (one being bare) coming down from the pole mounted transformer outside your house.
Sticking with the three conductors coming down to your house concept, the bare supporting conductor is the grounded (neutral) conductor that is providing the return path back to the source (transformer) for these two other conductors that are also with that bare supporting conductor...made sense?...it will in a minute.
Now lets look at the Article 220 and more specifically section 220.61. If you are doing calculations and determining the "neutral" loads then review the following because it is important to understand the concept.
220.61 Feeder or Service Neutral Load. (A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor. This will be important information when it comes to when a reduction in size of the neutral can take place and when it is prohibited. So just tuck that back into your mind until which time to start to perform calculations on service conductors and feeders.
Lastly, lets look at section 310.15(B)(5) :
(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).
(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a).
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.
So as you can see, a simple 2 conductor (with ground) Type NM-B has to be counted as a current carrying conductor while it's counterpart a 3 conductor (with ground) type NM-B where the neutral is being used as a multiwire branch circuit (eg. the neutral is being shared among two different phases/legs within the circuit resulting in the neutral carrying only the imbalance of the load) results in the neutral not being counted as depicted in (a) above.
It is important to note that in our earlier example of the 3 phase delta system where the midpoint between two phases/legs are tapped results in a typical 120/240V (for our example) from a 240V - 3 phase delta transformer out on your pole. However, we are getting very deep at this point and in order to keep it very basic we will stop there. Some folks may chime in and say more detail is needed....resist the temptation at this point because we are just talking the introductory basics and it is being explained in very basic (not all inclusive) terms.
So those are some areas of the NEC to review and if you have any additional questions by all means post them. I am sure others will chime in as well. The above is only to serve as a basic understanding as clearly it can get more complex where applicable.
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