Master Electrician Exam 2025 – 400 Free Practice Questions to Pass the Exam

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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?

80 amperes

100 amperes

125 amperes

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

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