For a 7.5-horsepower, 3-phase, 208-volt induction motor, what is the full-load current used for calculations?

For a 7.5-horsepower, 3-phase, 208-volt induction motor, what is the full-load current used for calculations?

• A. 18.0 A
• B. 24.2 A
• C. 28.4 A
• D. 36.0 A

Answer: (B)

Full-load current is the current the motor draws when it is delivering its rated output. For practical calculations, you don’t rely on horsepower and voltage alone; you use the motor’s full-load current rating from the nameplate because that rating already accounts for the motor’s efficiency and power factor at full load. For this 7.5-hp, 208-volt, 3-phase induction motor, the nameplate lists a full-load current of about 24.2 A, which is the value you should use in calculations such as sizing conductors or protective devices. If you tried to back-calculate it solely from horsepower and voltage, you’d need the specific full-load power factor and efficiency, which can vary between motors and operating conditions. Since those aren’t provided, the standard and most accurate reference is the motor’s full-load current on the nameplate, which in this case is 24.2 A.

Full-load current is the current the motor draws when it is delivering its rated output. For practical calculations, you don’t rely on horsepower and voltage alone; you use the motor’s full-load current rating from the nameplate because that rating already accounts for the motor’s efficiency and power factor at full load. For this 7.5-hp, 208-volt, 3-phase induction motor, the nameplate lists a full-load current of about 24.2 A, which is the value you should use in calculations such as sizing conductors or protective devices.

If you tried to back-calculate it solely from horsepower and voltage, you’d need the specific full-load power factor and efficiency, which can vary between motors and operating conditions. Since those aren’t provided, the standard and most accurate reference is the motor’s full-load current on the nameplate, which in this case is 24.2 A.