75kVA, 480 delta-208/120 wye
Primary Amps=90.2A
Based on 450.3(b) Primary OCPD=90.2*2.5=225A; set to 225A
Primary wire size=1 set of 3#4/0 CU
Secondary Amps=208A
Secondary OCPD=208*1.25=260A; set to 250A
Secondary wire size=1 sets of 4#250kcmil CU
The secondary conductors are governed by a series of rules in 240.21(C), which are each analogous to each of the tap rules in 240.21(B). The secondary conductor extends from the terminals of the transformer to the first OCPD of each secondary connected circuit. Once it lands on the first OCPD after the transformer, the circuit on the load side of that OCPD is now a feeder.
The primary can be based on the full load amperes, and where applicable, the next-size-up rule is allowed to be applied. It is treated as if it were any other load off of the source that feeds it. The secondary by contrast, specifically requires at least as much ampacity as the secondary OCPD. Using an 250A OCPD on the secondary, gotta use 250A of wire. Even if the load on the load side of that OCPD would only require 230A of wire, and be allowed to “round up” to 250A per 240.4(B). It’s a little non-intuitive that you’d have to have 250 kcmil’s on the secondary conductors, but you get to have only 4/0’s from your secondary OCPD onward, should the load require no more than 230A.
Some transformer topologies allow you to protect the secondary conductors by the primary OCPD (after scaling it by the voltage ratio to apply it to the secondary), due to the fact that fault currents are guaranteed to line up directly across the winding pairs. I like to call this “protect by proxy”. This only applies specifically to 2-wire:2-wire single phase, and 3-wire:3-wire delta:delta systems specifically. Any time you have a wye system on either side or a system with a center tapped winding, forget it. A secondary fault could go “unnoticed” by a primary OCPD if there is a wye system on either side, which is why 350A primary OCPD on the 480V side doesn’t automatically become 808A worth of secondary wire protection on the 208V side. It only does this on certain topologies of the transformer. Where it doesn’t apply, which is most of the time in my experience, secondary protection is needed the instant you terminate the transformer secondary conductors in the first device after the transformer.