What is the nameplate FLA for a 20 hp, 208V, three-phase motor with a 90 percent power factor and 80 percent efficiency?

• A. 48A
• B. 58A
• C. 68A
• D. 78A

Answer: (B)

To determine the nameplate full-load amperage (FLA) for a 20 horsepower (hp), 208V, three-phase motor, you can use the motor's horsepower, voltage, power factor, and efficiency.

First, you need to calculate the input power of the motor in watts. The formula for power in a three-phase motor is:

[ P = \frac{hp \times 746}{\text{Efficiency}} ]

For a 20 hp motor with 80 percent (0.80) efficiency:

[ P = \frac{20 \times 746}{0.80} = 18650 \text{ watts} ]

Next, you need to calculate the apparent power (in volt-amperes) using the formula that accounts for power factor:

[ S = \frac{P}{\text{Power Factor}} ]

In this case, the power factor is 0.90:

[ S = \frac{18650}{0.90} \approx 20722.22 \text{ volt-amperes} ]

Now, to find the full-load amperage (FLA), use the formula for current in a three-phase system:

[ I = \frac{S}{