[quote]I will feed a 30 kva with a 90 amp breaker and feed the L 123 lugs
I then will take H 123 for my load. [quote]
I assume you mean X1, X2. X3 are 208 (no neutral/grounded conductor)and H1, H2, H3 480 VAC?
That is up to you the NEC does not require it.
BUT IMO, this is a bad installation. I speak from a multitude of experience, where contractors call my shop to check out WIERD voltages at a facility we get there, and we find this connection. In addition some transformer manufactures do not list their transformer for reverse feeding. Lastly there is a higher inrush on the 208 VAC side then there would be on a 208 delta to 480/277 Wye due to winding design They make the right transformer for the application, why not use it? 208 Delta to 480/277 Wye.