Master Electrician Practice Exam

For a transformer supplying a building with a secondary balanced computed demand load of 416 amperes per phase, what is the minimum required kVA rating?

1. 75 kVA

2. 100 kVA

3. 150 kVA

4. 200 kVA

The correct answer is: 150 kVA

To determine the minimum required kVA rating for a transformer supplying a secondary balanced load of 416 amperes per phase, it is essential to use the formula for calculating kVA, which is: \[ kVA = \frac{Volts \times Amps}{1000} \] Assuming a typical secondary voltage of 480 volts for three-phase systems, the calculation for each phase would be: 1. First, we find the total current for a three-phase load, which is computed as \( \text{Total Amps} = \sqrt{3} \times \text{Phase Amps} \) for balanced loads. However, since we already have the demand load per phase, we can directly use the given load. 2. Plug in the values: \[ kVA = \frac{480 \, \text{Volts} \times 416 \, \text{Amps}}{1000} \] \[ kVA = \frac{199680}{1000} \] \[ kVA = 199.68 \] This value rounds up to approximately 200 kVA. In practice, transformers are selected based on standard size increments, which means the minimum required rating