Journeyman Electrician’s Exam Transformers and Electrical Theory 1 — Questions and Answers
Question 1: A single-phase transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio?
- 2:1
- 4:1 (Correct answer)
- 8:1
- 1:4
Correct answer: 4:1
Turns ratio = primary voltage ÷ secondary voltage = 480 ÷ 120 = 4:1, confirming this is a step-down transformer.
Question 2: What is the full-load secondary current of a 25 kVA, 480/240V single-phase transformer?
- 52.1 amperes
- 104.2 amperes (Correct answer)
- 26 amperes
- 208 amperes
Correct answer: 104.2 amperes
I = VA ÷ V = 25,000 ÷ 240 = 104.2 amperes full-load secondary current.
Question 3: Per NEC Table 450.3(B), what is the maximum primary overcurrent protection for a transformer whose primary current is 9 amperes or more (with no secondary protection)?
- 100% of primary current rating
- 125% of primary current rating (Correct answer)
- 150% of primary current rating
- 250% of primary current rating
Correct answer: 125% of primary current rating
NEC Table 450.3(B) limits primary-only protection to 125% of the transformer's rated primary current when primary current is ≥9A.
Question 4: Transformer efficiency is calculated by comparing:
- Input voltage to output voltage
- Output power to input power (Correct answer)
- Primary current to secondary current
- Primary turns to secondary turns
Correct answer: Output power to input power
Efficiency (%) = (Output power ÷ Input power) × 100; losses reduce efficiency below 100%.
Question 5: Which 3-phase transformer connection provides a neutral conductor on the secondary side for single-phase loads?
- Delta-delta
- Delta-wye (Correct answer)
- Wye-delta
- Open delta
Correct answer: Delta-wye
A delta-wye transformer provides a neutral on the wye secondary, enabling 3-phase 4-wire systems with single-phase branch circuits.
Question 6: In a 3-phase, 4-wire wye system where the line-to-line voltage is 208V, what is the line-to-neutral (phase) voltage?
- 208 volts
- 360 volts
- 120 volts (Correct answer)
- 240 volts
Correct answer: 120 volts
Phase voltage = line voltage ÷ √3 = 208 ÷ 1.732 ≈ 120V, the standard single-phase voltage in a 208Y/120V system.
Journeyman Electrician’s Exam Transformers and Electrical Theory 1
A single-phase transformer has a primary voltage of 480V and a secondary voltage of 120V.
What is the turns ratio?