New York State Master Electrician Practice Exam

What is the short circuit capacity of a 72 KVA transformer with a 240V secondary and an impedance of 3%?

1. 8,000 A

2. 10,000 A

3. 12,000 A

4. 15,000 A

The correct answer is: 10,000 A

To determine the short circuit capacity of the transformer, you can use the formula for calculating the short circuit current based on the transformer's kVA rating, the secondary voltage, and the impedance. The formula to find the short circuit current (Isc) is: \[ Isc = \frac{KVA \times 1000}{\sqrt{3} \times V \times \text{(Impedance)}} \] For a 72 kVA transformer with a 240V secondary and 3% impedance, we first convert the percentage impedance to a decimal for calculations: Impedance in decimal = 3% = 0.03. Next, substituting the values into the formula gives: \[ Isc = \frac{72,000}{\sqrt{3} \times 240 \times 0.03} \] Calculating the values step by step: 1. Calculate \(\sqrt{3} \approx 1.732\). 2. Then calculate: \[ Isc = \frac{72,000}{1.732 \times 240 \times 0.03} \] 3. Calculate the denominator: \[ 1.732 \