Licensed Electrical Contractor Transformers and Transformer Calculations 2

Question 1

What is the full-load primary current of a 45 kVA, single-phase transformer with a 480V primary?

Accepted Answer

93.75A

Answer

46.9A

Answer

187.5A

Answer

375A

Question 2

A single-phase transformer has a 3:1 step-down turns ratio. If the primary current is 10A, what is the secondary current?

Accepted Answer

30A

Answer

3.33A

Answer

10A

Answer

20A

Question 3

What is the full-load secondary current of a 25 kVA, 240V single-phase transformer?

Accepted Answer

104.2A

Answer

52.1A

Answer

208.3A

Answer

416.7A

Question 4

What is the full-load primary current of a 75 kVA, three-phase transformer with a 480V primary?

Accepted Answer

90.2A

Answer

52.1A

Answer

156.3A

Answer

180.4A

Question 5

A transformer has a nameplate impedance of 5%. If the available short-circuit current at the primary is 2,000A, what is the transformer's full-load primary current?

Accepted Answer

100A

Answer

40,000A

Answer

400A

Answer

10A

Question 6

A transformer nameplate reads 480V delta primary and 208Y/120V secondary. What is the secondary line-to-line voltage?

Accepted Answer

208V

Answer

120V

Answer

240V

Answer

480V

Question 7

What is the typical efficiency range for a well-designed distribution transformer?

Accepted Answer

97–99%

Answer

75–80%

Answer

85–90%

Answer

90–95%