The "100-amp" switch is rated to be able to switch and deliver 100 amperes (at 240VAC) to the loads. Is your generator rated to deliver 100 amperes? The answer is simple - no. It is rated for 37.5 amperes on Natural Gas and 41.6 amperes on LP. The internal breaker (in the generator, protecting the cables feeding the transfer switch) is a 45-amp breaker. These ratings are the maximum continuous current levels that can be delivered by the generator. The reason that the supplied wiring is smaller than that needed for a generator delivering 100 amps (minimum size is #2 AWG THHN wire - wire size based on NEC 445.13) is cost. Why use wire that is much larger (and far more expensive) than is really needed for a 10KW generator? Back to an older question - a 50-ampere breaker in your main panel (to feed utility power to the transfer switch) would probably be sufficient for matching the capability of the generator. I would suggest that you do not connect loads into the transfer switch that - when all turned on - exceed the capacity of the generator. If you need 100-amp service from your generator, you need another generator. It's that simple.

All you would have to do to use the full 100-amp capacity of the transfer switch (on the utility power side) is to install a two-pole 100-ampere breaker in the service panel (your main breaker panel) and connect that breaker to the transfer switch with #3 AWG or larger THHN wire. The problem, though, is that the generator is capable of supplying less than half that energy. You would not want to be drawing 80 or 90 amperes on the utility side when power failed. The generator would get running, the transfer switch would swap over to the generator feed, and then the breaker in the generator would trip out. You'd have no feed whatsoever then. The best game plan is to choose which branch circuits that you want to move over to the breakers in the transfer switch with a total load (assuming everything will be running at the time of the transfer) of about 40 amperes. Some electrical inspectors will go ballistic if, for example, you hooked up your system to run double the load that the generator can handle. A manual transfer switch could solve some of the overloading types of problems because you'd have instructions for whoever's doing the transfer to turn off the breakers to heavier loads before transferring to the generator power. By the way, I don't know if you're running your generator set on natural gas or propane. If it's propane, then you have a little more load capacity headroom.