egc/gec 50kw satcon subpanel interconnect

[natron chief]

i am connecting a rooftop mounted 50kw satcon inverter to a nearby 400A 480V subpanel with an 80A breaker. the grounding between the subpanel on the roof and the main panel two floors below is #2. the gec for the main panel is #4/0. the largest dc conductor is #3/0. do i really need to run #3/0 all the way downstairs two floors to the AC grounding electrode?

[jaggedben]

Yes, you do, unfortunately.