Another factor in wiring sizing is electrical resistance in the wire itself. The actual factor limiting the number of amps in a circuit (and the breaker/fuse size) is the heat generated by the electricity flowing through the wire. Too much electricity, too much heat, with the insulation being the weak link.
Resistance is mainly determined by wire diameter and length. The larger the wire, or shorter the length, the less resistance. The more amps carried, the greater the resistance and the greater the resistance, the more the voltage drops. The more the voltage drops, the more amps the motor will draw, and on in a viscous cycle.
Running a 11.5 amp motor on a 15 amp circuit (14 gauge) that isn't too long (including both the house wiring and the cord length) will work fine, as long as there aren't more loads on that circuit. Running the same motor on a 20 amp circuit (12 gauge) will give less voltage drop and more cushion if other loads draw on that circuit.
I always try to use larger conductors. I've used 10 gauge house wire on a 20 amp circuit that has long runs, or multiple loads.
The only way most people experience voltage drop is when the motor heats up or lacks full power.
