Master Electrician Practice Exam

What is the minimum kVA rating required for a transformer supplying a secondary load of 416 amperes per phase?

1. 100 kVA

2. 150 kVA

3. 200 kVA

4. 250 kVA

The correct answer is: 150 kVA

To determine the minimum kVA rating required for a transformer supplying a secondary load of 416 amperes per phase, we first need to calculate the kVA based on the load current. The formula to calculate kVA for a three-phase system is: \[ kVA = \sqrt{3} \times Voltage \times Current \] Assuming a common secondary voltage of 480 volts (which is often standard in industrial applications), the calculation would be as follows: 1. Convert the current to kVA: 2. Calculate \( kVA \): \[ kVA = \sqrt{3} \times 480 \times 416 / 1000 \] 3. This results in: \[ kVA ≈ 0.866 \times 480 \times 416 / 1000 ≈ 200 kVA \] Thus, the calculation yields approximately 200 kVA, indicating that a transformer rated at this capacity can handle the load safely without being overloaded. This kVA rating takes into consideration that transformers should ideally operate below their maximum capacity for efficiency and longevity. A transformer with a smaller kVA rating could become overloaded under such a load, leading to overheating and potential failure. Hence,