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

100%

Answer

150%

Answer

250%

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

Blue

Answer

Red

Answer

Yellow

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

4-wire Wye

Answer

3-wire Delta

Answer

Open Delta

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

50%

Answer

0.5%

Answer

95%

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

1 hour

Answer

2 hours

Answer

4 hours

Question 6

What is the primary purpose of tap connections on a transformer's primary winding?

Accepted Answer

To adjust the voltage ratio for variations in supply voltage

Answer

To connect multiple transformers in parallel

Answer

To provide multiple secondary voltage levels

Answer

To increase the transformer's kVA capacity

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?

Accepted Answer

57.7%

Answer

50%

Answer

66.7%

Answer

75%