It would be difficult to calculate the load for this service. There must be some incredibly low demand factors (or under loaded) on the distibution circuit breakers. I'm glad you posted the drawing, since I thought from your original description that the 800A main board was being repalced, which it isn't.
Unfortunately this design is faulty, in that the circuit breakers can never be coordinated for a distribution breaker to trip before the main. The main breaker is smaller than two of the three distribution breakers. The other thing to be sure is, that there is some type of shunt trip activation on some of the dirstibution to accomodate the smaller generator breaker as compared to the main breaker.

[This message has been edited by Ron (edited 01-24-2003).]