Is the secondary feeder from a 75 kva xfrm protected from the primary breaker? I have a 100 amp 480 v breaker on the primary with a 200 amp 208 v secodary to a panel. Do I need a 200 amp main or can I use main lug and have the 100 amp breaker protect the 200 amp feeder?
Proably not, and in this case I believe "NO" The OCPD installed per 450.3B protects the transformer proper, The secondary conductors are Tap conductors and must be protected in accordance with 240.21 (C)
The truth of the matter is if it's a single voltage secondary and a 3 phase Delta you can protect the secondary conductors with overcurrent protection on the primary. You could even use a MLO panel. Chances are tho, you have a secondary that would be more than one voltage and you need to protect the secondary conductors at their ampacity. You will be using the transformer tap rules in Article 240 for protecting these conductors. As for protecting the secondary of the transformer, you can protect it via the overcurrent protection on the primary sized @ 125% or limit the load on the secondary to the ampacity of the secondary with no more than 6 taps to overcurrent protection that does not exceed the ampacity of the secondary of the transformer.
It is if my opinion that there is a lot of info missing such as if it's a 1ph 480-208v transformer with a 2w secondary(probably not though), a 480v 3ph delta-208vdelta (probably not either, but a educated "guess" would assume that it is a common 74kva 480v-208y/120 with a 3ph4w secondary which means that it would have both pri. and sec. OCPDs. As a former application engineer to provide an answer based upon incomplete information as illustrated with this question is not wise.
I'm not sure that you can go MLO if the panel qualifies as a lighting and appliance BC panel board.
If you are working on one of those somewhat rare cases where the transformer primay OCPD is permitted to protect the secondary conductors, then you would have overcurren protection on the line side of the lighting and appliance branch cicuit panel board as required by 408.36(A). Don