Bill,
All conductors of circuits and transformer configurations that are intentionally connected to earth ground are designated as "grounded conductors", but not all grounded conductors are neutrals. In order to be a neutral conductor it must have the same potential difference to all of the ungrounded conductors of the circuit. A grounded center tap of a single phase transformer is a neutral as is the grounded common point of a three-phase wye configuration. The grounded center tap of one transformer in a three- phase delta configuration is not a neutral because it has the same potential difference to only two of the other three ungrounded conductors. Whether the amperage is balanced or not has no bearing on a conductor being a neutral conductor. Only the voltage being equal to all other circuits is the factor on which a conductor is called a neutral. (see Article 100--Definitions Branch Circuit, Multiwire)
For linear loads the neutral current of a single-phase 3-wire circuit or a three-phase 4-wire wye circuit will be zero if currents on the ungrounded conductors are equal.
The neutral current in a three-phase 4 wire wye system is computed by finding the square root of the sum of the squares of the current in each line minus the sum of the products of the currents in all three lines. In a wye system line current and phase current are equal.
If the current on phase a is 3 amperes the current on phase b is 5 amperes and the current on phase c is 1 ampere the neutral current would be 3.46 amperes.
If current on phase a and b is 20 amperes and phase c is zero the neutral current is also 20 amperes. Thus the reason behind Section 310-15 (b)(4)(b) of the NEC making you count the neutral conductor as a current carrying conductor of a single phase 3-wire system when derived from a three-phase 4-wire wye system.
Yes all loads characteristics are not the same and neutral currents may be high on some systems. Section 310-15 (b)(4)(c) and also the FPN after Section 210-4 (a) of the NEC cautions about the fact that in the real world, you will probably have non-linear loads connected to a system and that harmonic neutral currents are to be expected and planned for. But alas, this still has no bearing on a conductor being called a neutral.
R Dimery
