IEC Transformers and Distribution Systems 2

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?

Accepted Answer

125%

Answer

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?

Accepted Answer

Orange

Answer

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?

Accepted Answer

4-wire Delta with center-tapped neutral

Answer

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?

Accepted Answer

5%

Answer

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?

Accepted Answer

3 hours

Answer

NEC 450.41 requires transformer vault construction to have a minimum 3-hour fire resistance rating to contain fire and protect the surrounding structure.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC Transformers and Distribution Systems 2