IEC Transformers and Distribution Systems 2 — Questions and Answers
Question 1: According to NEC 450.3(B), for transformers rated 600V or less with a primary current of 9A or more, primary overcurrent protection must not exceed what percentage of rated primary current?
- 100%
- 125% (Correct answer)
- 150%
- 250%
Correct answer: 125%
NEC 450.3(B) limits primary overcurrent protection to 125% of the transformer's rated primary current for this category of transformer.
Question 2: Per NEC 110.15, what color must be used to permanently identify the high-leg conductor in a 4-wire delta system?
- Blue
- Red
- Orange (Correct answer)
- Yellow
Correct answer: Orange
NEC 110.15 requires the higher-voltage-to-ground (high leg) conductor in a 4-wire delta system to be identified with the color orange.
Question 3: Which secondary transformer connection creates a 'high leg' (wild leg) condition where one phase has a higher voltage to neutral than the other two phases?
- 4-wire Wye
- 3-wire Delta
- 4-wire Delta with center-tapped neutral (Correct answer)
- Open Delta
Correct answer: 4-wire Delta with center-tapped neutral
A 4-wire delta system with one winding center-tapped for neutral creates a high leg with approximately 208V to neutral on a 240V delta system, while the other two phases measure only 120V to neutral.
Question 4: A transformer nameplate lists '5% impedance.' At full load, this means the internal voltage drop is what percentage of rated voltage?
- 5% (Correct answer)
- 50%
- 0.5%
- 95%
Correct answer: 5%
Transformer impedance percentage directly represents the percentage of rated voltage that causes full-load current to flow through the transformer's internal impedance.
Question 5: According to NEC 450.41, transformer vault walls, roofs, and floors must have a minimum fire resistance rating of how many hours?
- 1 hour
- 2 hours
- 3 hours (Correct answer)
- 4 hours
Correct answer: 3 hours
NEC 450.41 requires transformer vault construction to have a minimum 3-hour fire resistance rating to contain fire and protect the surrounding structure.
Question 6: What is the primary purpose of tap connections on a transformer's primary winding?
- To connect multiple transformers in parallel
- To provide multiple secondary voltage levels
- To adjust the voltage ratio for variations in supply voltage (Correct answer)
- To increase the transformer's kVA capacity
Correct answer: To adjust the voltage ratio for variations in supply voltage
Primary taps allow the turns ratio to be adjusted in small increments (typically 2.5% or 5%) to compensate for supply voltage that is higher or lower than the transformer's nominal rating.
Question 7: An open-delta (V-V) transformer bank can supply approximately what percentage of the three-phase capacity of a complete closed-delta bank using the same size transformers?
- 50%
- 57.7% (Correct answer)
- 66.7%
- 75%
Correct answer: 57.7%
An open-delta bank delivers approximately 57.7% (= 1/√3) of the capacity of a full closed-delta bank because only two transformers carry the three-phase load.
IEC Transformers and Distribution Systems 2
According to NEC 450.3(B), for transformers rated 600V or less with a primary current of 9A or more, primary overcurrent protection must not exceed what percentage of rated primary current?