Licensed Electrical Contractor Transformers and Transformer Calculations 2 — Questions and Answers
Question 1: What is the full-load primary current of a 45 kVA, single-phase transformer with a 480V primary?
- 46.9A
- 93.75A (Correct answer)
- 187.5A
- 375A
Correct answer: 93.75A
Full-load current = kVA × 1000 / primary voltage = 45,000 / 480 = 93.75A.
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?
- 3.33A
- 10A
- 20A
- 30A (Correct answer)
Correct answer: 30A
Secondary current = primary current × turns ratio = 10A × 3 = 30A; as voltage steps down, current steps up by the same ratio.
Question 3: What is the full-load secondary current of a 25 kVA, 240V single-phase transformer?
- 52.1A
- 104.2A (Correct answer)
- 208.3A
- 416.7A
Correct answer: 104.2A
Full-load secondary current = kVA × 1000 / secondary voltage = 25,000 / 240 = 104.2A.
Question 4: What is the full-load primary current of a 75 kVA, three-phase transformer with a 480V primary?
- 52.1A
- 90.2A (Correct answer)
- 156.3A
- 180.4A
Correct answer: 90.2A
For three-phase: I = kVA × 1000 / (V × 1.732) = 75,000 / (480 × 1.732) = 75,000 / 831.4 ≈ 90.2A.
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?
- 40,000A
- 400A
- 100A (Correct answer)
- 10A
Correct answer: 100A
Full-load current = Short-circuit current × impedance (decimal) = 2,000A × 0.05 = 100A.
Question 6: A transformer nameplate reads 480V delta primary and 208Y/120V secondary. What is the secondary line-to-line voltage?
- 120V
- 240V
- 208V (Correct answer)
- 480V
Correct answer: 208V
In a wye (Y) secondary, the line-to-line voltage is 208V while the line-to-neutral voltage is 120V (120 × √3 ≈ 208V).
Question 7: What is the typical efficiency range for a well-designed distribution transformer?
- 75–80%
- 85–90%
- 90–95%
- 97–99% (Correct answer)
Correct answer: 97–99%
Distribution transformers are among the most efficient electrical devices, typically operating at 97–99% efficiency due to minimal losses in core and windings.
Licensed Electrical Contractor Transformers and Transformer Calculations 2
What is the full-load primary current of a 45 kVA, single-phase transformer with a 480V primary?