What is the approximate current in the primary of a 100 kVA, 480:208/120 volt, 3-phase wye transformer?

• A. 100 amps
• B. 110 amps
• C. 120 amps
• D. 130 amps

Answer: (C)

To determine the approximate current in the primary of a 100 kVA, 480:208/120 volt, 3-phase wye transformer, you can use the formula for calculating the primary current in a three-phase system:

[ I = \frac{S}{\sqrt{3} \times V} ]

where ( I ) is the current, ( S ) is the apparent power in volt-amperes (VA), and ( V ) is the line-to-line voltage for the primary side.

In this case, the apparent power (S) is 100 kVA, or 100,000 VA. The primary voltage (V) is 480 volts. Plugging these values into the formula gives:

[ I = \frac{100,000}{\sqrt{3} \times 480} ]

Calculating ( \sqrt{3} ) gives approximately 1.732. Therefore:

[ I = \frac{100,000}{1.732 \times 480} ]

Calculating the denominator yields approximately 829.44. Dividing 100,000 by this value results in an approximate primary current of:

[ I \approx 120.1 \text{ amps