Journeyman Electrician Practice Exam

What is the full-load running current of a 208-volt, 3-phase, 50 hp, continuous-duty squirrel-cage AC motor?

1. 120 amperes

2. 143 amperes

3. 156 amperes

4. 170 amperes

The correct answer is: 143 amperes

To determine the full-load running current of a 208-volt, 3-phase, 50 hp continuous-duty squirrel-cage AC motor, you can use the formula for calculating the current in a three-phase motor: \[ I = \frac{P}{\sqrt{3} \times V \times \text{efficiency} \times \text{power factor}} \] Where: - \( I \) is the current in amperes, - \( P \) is the power in watts (1 horsepower is approximately 746 watts), - \( V \) is the voltage. For a 50 hp motor, the power in watts would be: \[ P = 50 \, hp \times 746 \, watts/hp = 37,300 \, watts. \] Using the formula, and assuming a typical efficiency of around 0.90 and a power factor of about 0.85 for a standard squirrel-cage motor, the calculation proceeds as follows: First, calculate the apparent power \( P \) in watts: \[ P = 50 \times 746 = 37,300 \, watts. \] Next, input the values into the formula: \[ I = \frac{37