Conductor sizing 15kva transformer

[Smart $]

Kind of. You must check that the primary protection and the turns ratio are not oversized based on your choice of secondary conductors.
While the end result is probably the same, the basic steps I always try to use are: 1) secondary load, 2) secondary conductors, 3) secondary conductor protection (which might be located on primary), 4) transformer, 5) primary protection, 7) primary conductor.
Work from the load towards the source, not the source.

I'm with you on that, but more specifically I was referring to 240.21(C)(1)...

(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer
having a 2-wire (single-voltage) secondary, or a
three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary conductors
are not considered to be protected by the primary overcurrent
protective device.