IEC Transformers and Distribution Systems 1 — Questions and Answers
Question 1: What is the turns ratio of a single-phase transformer with a 480V primary winding and a 120V secondary winding?
- 2:1
- 4:1 (Correct answer)
- 8:1
- 16:1
Correct answer: 4:1
Turns ratio equals primary voltage divided by secondary voltage: 480V ÷ 120V = 4:1.
Question 2: A 25 kVA, single-phase, 480V/240V transformer is operating at full load. What is the approximate rated secondary current?
- 52A
- 104A (Correct answer)
- 156A
- 208A
Correct answer: 104A
Secondary current = kVA × 1,000 ÷ secondary voltage = 25,000 ÷ 240 ≈ 104A.
Question 3: Which three-phase transformer connection provides a neutral conductor on the secondary side?
- Delta-Delta
- Delta-Wye (Correct answer)
- Wye-Delta
- Open Delta
Correct answer: Delta-Wye
A Delta-Wye transformer provides a neutral from the center point of the wye-connected secondary windings.
Question 4: According to NEC 450.3(B), a 480V transformer with a rated primary current of 9A requires primary overcurrent protection not exceeding what value?
- 9A
- 11.25A
- 15A (Correct answer)
- 25A
Correct answer: 15A
NEC allows 125% of 9A = 11.25A; since no standard device matches that value, the next standard size of 15A is permitted.
Question 5: What is the primary purpose of a K-rated transformer?
- To provide voltage regulation above 10%
- To handle harmonic currents from non-linear loads (Correct answer)
- To improve the system power factor
- To reduce audible transformer noise
Correct answer: To handle harmonic currents from non-linear loads
K-rated transformers are built to withstand the additional heat generated by harmonic currents produced by non-linear loads such as computers and variable frequency drives.
Question 6: A 75 kVA, three-phase, 480V/208Y/120V transformer is operating at full load. What is the approximate full-load secondary line current?
- 104A
- 156A
- 208A (Correct answer)
- 360A
Correct answer: 208A
Full-load secondary current = kVA × 1,000 ÷ (secondary line voltage × √3) = 75,000 ÷ (208 × 1.732) ≈ 208A.
Question 7: What type of transformer is designed to make small voltage adjustments, typically within ±5–20% of the supply voltage?
- Autotransformer
- Isolation transformer
- Buck-boost transformer (Correct answer)
- Potential transformer
Correct answer: Buck-boost transformer
Buck-boost transformers are designed to raise (boost) or lower (buck) supply voltages by a small percentage to compensate for voltage variations.
IEC Transformers and Distribution Systems 1
What is the turns ratio of a single-phase transformer with a 480V primary winding and a 120V secondary winding?