HOLT Electricians Practice Exam

Question: 1 / 400

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

25-30 amperes

30-33 amperes

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

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38-42 amperes

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