A dialysis clinic has an existing 120/208 volt, 3 phase, 800 amp main service disconnecting means. How is the capacity determined for an optional power supply, in this case a generator? If the answer is found in NEC 220, Branch-Circuit, Feeder, and Service Calculations; would the example found in Annex D, Example D3 Store Building be the format for the calculation needed to size the generator ampacity? Thanks.