Master Electrician Practice Exam

What is the maximum standard size overcurrent protection device permitted on the primary side of a 50 kVA transformer with a 480-volt, 3-phase primary?

1. 80 amperes

2. 100 amperes

3. 125 amperes

4. 150 amperes

The correct answer is: 150 amperes

To determine the maximum standard size overcurrent protection device permitted on the primary side of a 50 kVA transformer with a 480-volt, 3-phase primary, one must first calculate the full-load current of the transformer. The formula used is: Full-load current (I) = kVA × 1000 / (Voltage × √3) For a 50 kVA transformer at 480 volts, the calculation would be: I = 50 × 1000 / (480 × √3) Calculating √3 gives approximately 1.732, thus: I = 50000 / (480 × 1.732) I ≈ 50000 / 831.36 I ≈ 60.1 amperes According to the National Electrical Code (NEC), for transformers, overcurrent protection on the primary side can be sized larger than the full-load current by using the 125% rule for overcurrent protection for transformers rated above 9 kVA. Thus, the size of the overcurrent protection device can be calculated as follows: 125% of full-load current = 1.25 × 60.1 amperes ≈ 75.125 amperes The next step is