Master Electrician Practice Exam

What is the approximate full-load ampere rating of the primary side of a 150 kVA service transformer with a 480Y/277 volt 3-phase primary?

1. 150 amperes

2. 180 amperes

3. 200 amperes

4. 220 amperes

The correct answer is: 180 amperes

To determine the approximate full-load ampere rating of the primary side of a 150 kVA service transformer with a 480Y/277 volt 3-phase primary, the correct calculation involves using the formula for three-phase systems: \[ I = \frac{P_{kVA} \times 1000}{\sqrt{3} \times V} \] Where: - \( I \) is the current in amperes, - \( P_{kVA} \) is the power rating of the transformer in kilovolt-amperes, - \( V \) is the line-to-line voltage. For this transformer: - \( P = 150 \, kVA \), - The line-to-line voltage \( V = 480 \, volts \). Substituting the values into the formula: \[ I = \frac{150 \times 1000}{\sqrt{3} \times 480} = \frac{150000}{830.84} \approx 180 \, amperes \] This calculation confirms that the full-load ampere rating of the primary side of this transformer is approximately 180 amperes, which aligns with the selected answer. This number is crucial for assessing the electrical load capacity