MWBCs are one of my favorite topics!!!!
As Bob showed in his diagram, you could have multiple(many) circuits, regardless of phasing, on one single neutral, so long as the neutral is sized large enough. I believe (code reference unavailable right now) that the neutral size limit for this type of installation is a #2.
Now, the fact that you could do this is seperate from the practicality of actually wiring this way. In general construction, this would not be used. Maybe in a control cabinet, MCC room, Cellular room, etc.... It would be very specific in its use.
Now when working with NM cable, you are obviously not going to find this, and generally when wiring a building for normal building power and light loads, you wouldn't even touch that.
Keeping it in the 15 to 20 Amp general loads(but not limited to), we use MultiWire Branch Circuits (MWBC). Some used to and still may call this a network. A MWBC consists of hot conductors that share a common grounded conductor (neutral). Note: hot conductors are also referred to as 'phase conductors' even though the 'phase' refers to the difference between the phases, not the conductors themselves.
In a single phase system, both hot conductors are 180 deg out of phase. Meaning their cycles and their sine wave are exactly opposite in their wave patterns. If the 2 conductors share a common neutral, their return currents actually cancel each other out. (example: 10 amps on one and 7 amps on another will actually have only 3 amps on the return conductor) This is referred to as balancing the neutral. If the circuits are equal, than they are balanced, if they are not equal (as in the example stated), then they are unbalanced. To be completely unbalanced, meaning one circuit is at maximum and the other is minimum, then the neutral would see the most current it will ever see. (example: 15 amps on one and 0 amps on another will have 15 amps returning on the neutral) There fore in a standard MWBC, the neutral conductor would need only be sized as large as the largest circuit conductor. In the above example, the neutrals would be #14's.
Now that same principle is transferred to 3 phase systems as well. The only difference is that the phasing is 120 deg apart (3 x 120 = 360). This difference in phasing does not change the fact that opposing phase return currents still cancel each other. Therefore we can have the 3 circuit conductors share one neutral, and all be the same sized conductor. The neutral just needs to be no smaller than the largest of the circuit conductors.
Note: this is why keeping your phasing straight when in the panels is very important. Running the circuits means jack squat if you don't land the conductors on appropriate phases.
Talk to your Master and have him show you the relationship in the panels of A,B and C phases, and how to keep things straight during installation, and the importance of such.
