Sorry for the confusion I’ve been reading up on this for two days and all I get is more confused. The one job is already done but was done wrong.
Main concern is the my job of providing 480V power to two machines both 3 phase one 50amp the other 30amp the 50 draws about 22amps the 30 draws 12.5 amps at 480V.
My transformer supply voltage is 208 some 130’ away. Ideally I’d like to install a 3 pole 100amp breaker in the 208V panel run that the 130ft to a disconnect feed transformer come out of transformer secondary twice with 480V to two fusible disconnects installed within 10ft of the transformer to feed each machine. Confusion has risen when trying to use there own transformer sitting on the shelf which is 480V pri and 208 sec I get that it would need to be corner grounded and unfused grounded phase initial influx etc but other factors with the EGC and the overall stress on a unknown used transformer etc. So option two is to supply a proper step up transformer and basically install as stated above. All the references and different chapters concerning this install all the advice given sometimes contradictory has left me confused as to which is the best way to proceed. If I can avoid a panel board with two breakers for the loads and simply use the disconnects that would offset the cost of a new transformer and would be my recommendation to the client. Now I bet anyone who reads this is confused as well sorry. Bottom line is to let them know of the code violations on previous install and provide the client two options for hooking up there two 480V machines. One with there transformer and one with a step up transformer. Pros and cons of each.
Some considerations.
As Don mentioned inrush when energizing a backfed transformer is likely higher than would be for similar capacity unit that is designed for 208 primary. This may mean the 100 amp breaker you wish to use may hold for the conventional type unit but may not on the backfed unit, but then the 130 foot supply conductors might have enough resistance to help out here as well.
VD will be bigger issue for 130 feet of conductor @ 208 volts then it is for same VA load @ 480 volts, plus smaller conductor can be used even before factoring in VD so there is reason to consider placing transformer near the 208 volt supply. Even if you run two separate circuits 130 feet you are looking at a set of 12 AWG and a set of 10 AWG most likely vs a set of 3AWG or maybe even 2AWG for VD, and larger raceway as well. Probably will need a disconnect near the loads supplied, but can likely just be a non fused safety switch or for no more current than involved here even motor rated switch (some are called manual motor switches). Would only need to be 30 amp switches, your 30 and 50 amp "circuit" is possibly recommended circuit breaker to allow motor starting? Time delay fuses can be less, and are still fine on 12 AWG and 10 AWG (depends on details) if supplying motor loads of the ratings you mentioned even though the overcurrent devices are over 20 and 30 amps. (your loads sound just about right for a 10 HP and a 20 HP motor or maybe smaller motors and other combination load types included.
Another option is a single 480 volt feeder and feeder taps near the load end to two disconnects. need to run pricing on all these options because each one has higher cost in some areas and lower cost in others, but nothing is necessarily equally proportional in price in each of these options. Single 480 volt feeder will need your primary breaker, a secondary disconnect, then your two disconnects near the load plus possibly a splice box or something to make the taps in, but only single raceway run or no need to have to adjust ampacity for multiple current carrying conductors. This off top of my head may be able to be as little as 8 AWG feeder conductor and if supplying motor loads only still could possibly be on 60 amp time delay fuses. 8 AWG might be borderline too small, all depends on more details.
Using less copper and smaller raceway may offset enough cost to justify spending some more an additional 600 volt disconnect.
Add: you also need a GEC on the secondary of the separately derived system. If you have building steel as part of your GES and it is everywhere in the facility this may not make much difference, if your SDS is close to the service equipment and you don't have other electrodes near the other end of this run it may mean spending more on a GEC, though I believe if you are running a EGC with the 208 volt supply it would likley be able to also be used for GEC in this instance, but would be a larger EGC than for a 480 volt feeder also.