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Same basic steps, calculate the load or proposed load, select the conductor , select the overcurrent device. All this with Art 450 in mind also.
There is one "trick" on transformer secondaries and that is 240.4(F) does not allow "next size up" for transformer secondaries.
Keep in mind Art 450 is for transformer protection, and Art 240 is for the conductors (primarily 240.21)
That help any ?