To answer this question, wire size is rated in the amount of amperage that it can legally carry. Fuse sizing is based on the amperage that the wire carries.
The formula to find amperage when the HP is known is I = HP x 746/1.73 x E x %eff x pf.
A standard motor's efficiency between 5 to 100 HP is .84 to .91.
A standard motor's power factor between 10 to 100 HP is .86 to .92.
Amps = 30 x 746 = 55950 = 55950/ 1.73 x 415 x .87 x .87 = 55950/550 = 101 amps.
The electrical code states that a motor conductor has to be rated at 125% of the motors full load amperage. 101 x 125% = 126 amps
A #2 copper conductor with an insulation factor of 90 degrees C is rated at 130 amps.
Fusing for this motor will be, non time delay fuse 300 amps, time delay fuse 175 amps or a circuit breaker rated at 250 amps.
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