This part may be a bit tough to grasp. But there is no such thing as a “total current” in a three phase system. Yes, you can serve 60 amps of single phase line-to-neutral load from a 60 amp breaker, and yes that means you have a total single phase load of 180 amps. That, however, does not mean that the breaker is seeing a “total current” of 180 amps. The three phases are separated from each other, in time, by one third of a cycle. So when phase A is at its positive peak, both phases B and C have negative currents, and they are at one half of their negative peak.
Here is what that means. 60 amps of current is leaving the phase A leg of the 60 amp breaker. It runs through the 60 amp single phase load. It returns to the panel on the neutral wire. At that same moment, 30 amps of current is leaving the panel via the neutral wire associated with the B phase load. That current goes “backwards” through the 60 amp load on the B phase, and returns to the panel via the phase B leg of the breaker. Phase C does the same thing at that moment as phase B.
Now look at it from the perspective of the upstream transformer’s secondary windings. The three neutral currents described above cancel each other out. The wire between the panel’s neutral bar and the transformer’s neutral point does not see any current. The current leaving the transformer’s phase A goes through the phase A load. Half of it returns to the transformer via the phase B load, and the other half through the phase C load. Simply put, the transformer will see 60 amps on each phase, but it is the same current. You don’t have 60 + 60 + 60 = 180. You have 60, 60, and 60, for a total of 60.
