Each transformer is considered an SDS. (Separately Derived Service)
That includes even the dinky 5 kVA center-tapped one-phase job.
So you'll need a #8 GEC for it -- I usually bond it inside the transformer. (Xo) [250.66]
The 75kVA unit will typically require #4 GEC, also best bonded on the Xo lug.
It's common practice to route these GECs back up the line -- bonding them with the larger Service GEC. This ensures an equipotental plane without having to jump out from each transformer to bond with the building's steel, gas and water and whatnot.
Look up SDS in the NEC Handbook for drawings, reasoning, and all the rest.
Additionally, these GEC runs require bonding bushings at every step of the way -- so as to avoid choking conditions during a transient fault.
Fault energies are addressed in Ugly's... in the transformer pages.
Bonding bushings are not required if the conductors are routed via PVC (underslab?) -- as it's non-ferrous.
Table 250.122 is used for all EGCs loadside of the OCPD. (the breaker/fuses) The GEC, sized per 250.66, has to reach all the way into the can that holds the OCPD. (bonding it)
One rarely sees this type of layout run under the slab. Hitting the 'windows' is such a chore/ risky. This goes double for most dry type transformers. Their bottoms/ points of entry are not that consistent. Routing the conductors from underslab is sure to be a real chore. Don't be surprised if the clearance is lousy or that the terminal bending radius in the secondaries is brutal. The tap rail is usually close above the bottom chassis support -- which must always be massive.
The usual drill is Greenfield and flex-ninety connectors with bonding bushings. This assembly is best performed before the transformer is shoved back and anchored down. The result is a clean look -- with the secondaries routed from behind and around one side, down low, inside the transformer body.
Nothing will go quickly, and many steps are two-man jobs. Aluminum secondaries will be much easier to bend -- if the contract permits them. If copper is required, bump up the Greenfield to the next size, you'll be glad you did.
Expect to feed the 208Y120 daughter panel from the bottom. Routing secondaries up to the top is a waste in every sense.
Stock up on multi-barrel Xo lugs.
If you can do so: check out previously installed dry type transformers. The first time out the door, most journey-men screw up... on just these issues.
I can't recommend the NEC Handbook too highly. You'll get your investment back on just this one job.
