The first step is converting horsepower to wattage. According to the University of Virginia, one horsepower is equal to 745.7 watts. That means 150 horsepower is equal to 111,855 watts of energy. This is the energy input needed to drive the motor, presuming 100 percent efficiency.
Ohm's Law states that wattage is equal to volts multiplied by amps. You need 111,855 watts of power to drive the motor. Common voltages are 120, 240 and 480. Working the equation for 120 volts yields a need of 932 amps. This is unrealistic, since the wires will have to be as thick as telephone poles to carry that much amperage. If you work the equation for 480 volts, the need is 233 amps. This is more realistic, since many power cables are available to carry that amount of current.
The aforementioned equations presume 100 percent efficiency, but no electric motor has that. Most electric motors operate between 70 and 80 percent efficiency. Engineers use a complex series of equations to calculate the efficiency of the power input to account for the extra 20 percent loss.
A 150 horsepower electric motor is more than likely three phase. In single-phase power, only one wave of electricity is present at one time. In three-phase power, three waves of electricity are present, but one wave lags a little bit behind the other. The end result is that three times more energy is transferred to the motor. Engineers design most high-power systems to be three phase because this is a more efficient method of power transference. Again, complex equations and even computer programs are used to calculate the exact power needs of a 150 horsepower, three-phase motor.