HOLT Electricians Practice Exam

For a 10 horsepower 208-volt, 3-phase motor, what is the correct rating for overload protection?

1. 25-30 amperes

2. 30-33 amperes

3. 33-38 amperes

4. 38-42 amperes

The correct answer is: 33-38 amperes

To determine the correct rating for overload protection for a 10 horsepower 208-volt, 3-phase motor, it's essential to apply the appropriate formulas and standards. The full-load current for a 3-phase motor can be calculated using the formula: \[ \text{Full-load current (I)} = \frac{HP \times 746}{\sqrt{3} \times V} \] Where: - HP is the horsepower (10 in this case), - 746 is the number of watts per horsepower, - \( V \) is the voltage (208 volts). Plugging in the values: \[ I = \frac{10 \times 746}{\sqrt{3} \times 208} \] Calculating this gives: \[ I = \frac{7460}{360.8} \approx 20.7 \text{ amperes} \] According to the National Electric Code (NEC) guidelines, the overload protection device rating is typically set at 125% of the motor's full-load current for properly sized protection. Calculating the appropriate rating for overload protection: \[ \text{Overload protection} = 20.7 \times 1.25 \approx 25.875