So back to the original discussion.
You've decided that it is a 75 KVA transformer 480D-208Y/120
On the primary side we have a full load current of 90 amps
On the secondary side we have a full load current of 208 amps.

If we are to provide primary and secondary protection, the primary protection can't
exceed 250% [450.3(B)]. I usually choose about 125%. So I pick a 125A trip breaker for primary protection.

Primary conductors are chosen 125% of full load, so if terminations are 75 degree,
3-#2awg copper + 1-#6awg copper Gnd. I like a separate EGC even though you would be installing it in EMT or RGS,

Secondary protection (within 10' of secondary windings) is (calculated at 260A) I choose 300A trip.

I start by choosing secondary conductors as 4-350kCMIL copper + 1-4awg copper Gnd.
Since there is 4 current carrying conductors in the secondary conduit, I check that 80% of 350A (ampacity of 350kCMIL @ 90 deg) is 280A, so since it is not 300A to match the secondary protection, I will change the secondary wire size to 4-400kCMIL copper + 1-4awg copper Gnd. Again 80% of 380A is ok with the OCPD, so it's ok.

From 250.66, I choose a 1/0 awg copper for the n-g bonding jumper and grounding electrode.

Hopefully this gets the conversation going again. [Linked Image]

Ron