Electric Wire Size for Grain Bin Motors
Your worst case scenario on the SERVICE is the 2 x 7-1/2HP 230V single phase motors on one bin while being unloaded, with the 10HP on the other one running the blower. 7-1/2HP 230V single phase motors are 40A each, the 10HP is 50A. You have 130A of load here. You cannot run that on a 100A service, continuous or not. So the wire size is moot.

But IF you get your service upgraded to let's say, 200A when you get the 2nd bin installed next year, then ASSuming you have a separate run from the service panel to each bin, the max worst case load would be 80A on each run, conductors must be 125%, so you size for 100A. You would need #3 Copper cable (if you can even find it), or #1 Aluminum, before de-rating for voltage drop (which I'm not going to do for you) so I would go one size larger. 

If you try to do one cable run out to the first bin, then sub-feed off to the 2nd one later, you will need 2/0 (Cu or Al because of a sizing anomaly), 3/0 if there is voltage drop at that distance. That will be a LOT more difficult to run, I'd do it with separate smaller runs.