First is this the service panel to the building?
Is the transformer owned by the power company or is it customer owned?
400 amps of secondary protection is already to much for a 75 kva transformer @ 208 volts.
A 75 kva transformer has a rated output at 208 volts of 208 amps. (Yes thats 208 amps at 208 volts)
How where you planing on getting power to this new panel?
You can not 'tap' the existing secondaries, you have to go right back to the transformer terminals using double lugs.
Also one last thing.....please do not set a 200 amp single phase panel off a 3 phase transformer....use a three phase panel.
It is important to keep the load on the transformer balanced and that will be all but imposable with a 200 amp single phase panel connected to a 3 phase transformer with an 208 amp rating.
