I currently use a 7.5hp rpc, both motor and control box are 7.5. Idling, no load, I have 8.7 amps per leg on my two single phase legs that power the rpc.
I found a post from 2006 where a fella with a 20hp rpc said he figures 12 amps idling, no load, on the single phase inputs. If in that vicinity, that's not as terrible as I guessed:
How many amps to start a 20hp rpc
I'd really like 15hp max, and down grade the motor size in two machines, 15hp & 10hp, to 7.5hp. Simply to save on the electric bill
. But I won't run the larger rpc all day anyway. . . My 7.5hp rpc motor is 1200 rpm and nice and quiet. I just won a local auction on a 20hp motor, also 1200 rpm, and need to upgrade for two larger machines I have. The cost of the 20hp motor and no shipping was attractive, plus I won't have to change motors in those two machines.
My thinking was, if I get the 20hp motor spinning to 1000 to 1200 rpm before hitting start on a 15hp control box, that my amp draw will be minimal. And once started, I should have no issue starting machines with 15hp and 10 hp motors. Looking at 15 and 20hp control boxes, seems like not too much difference in capacitors, and I won't start motor from a dead stop.
15hp:
20hp: