If you have a circuit breaker box containing 650 amps of circuit breakers it is 3 phase 480 volts what size kva generator do you need?

The question isn't the number of amps total on your branch circuits, but rather, what your MAIN breaker(s) are rated at. This will determine what size of generator you will need. And be certain that the generator is 3-phase.

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The formula you are looking for is Amps = kva x 1000/1.73 x voltage.