Licensed Electrical Contractor Transformers and Transformer Calculations 3

Question 1

Per NEC 450.3(A), what is the maximum primary overcurrent protection for a transformer with a primary voltage over 600V when no secondary protection is provided?

Accepted Answer

300% of primary FLA

Answer

125% of primary FLA

Answer

200% of primary FLA

Answer

250% of primary FLA

Question 2

According to NEC 450.21(A), what minimum separation is required between a dry-type transformer rated 112.5 kVA or less and combustible materials?

Accepted Answer

12 inches

Answer

6 inches

Answer

18 inches

Answer

24 inches

Question 3

Per NEC 450.21(B), dry-type transformers rated over 112.5 kVA installed indoors must be located in:

Accepted Answer

A fire-resistant transformer room or enclosure

Answer

A liquid-filled vault

Answer

An area separated by 12 inches from combustible materials

Answer

A weatherproof NEMA 3R enclosure

Question 4

When is a transformer vault required for an indoor installation under the NEC?

Accepted Answer

For liquid-filled transformers containing flammable liquid installed indoors

Answer

For any transformer over 15 kVA

Answer

For all transformers operating over 600V

Answer

For dry-type transformers over 112.5 kVA

Question 5

Per NEC 450.14, where must the disconnecting means for a transformer be located?

Accepted Answer

Within sight of the transformer or lockable in the open position

Answer

Within 10 feet of the transformer in all cases

Answer

On the secondary side only

Answer

At the main service panel

Question 6

According to NEC 450.3(B), what is the maximum primary overcurrent protection for a transformer with primary voltage 600V or below when the transformer has no secondary protection and the primary FLA is 9A?

Accepted Answer

125% of primary FLA

Answer

167% of primary FLA

Answer

250% of primary FLA

Answer

300% of primary FLA

Question 7

Which of the following is NOT required on a transformer nameplate per NEC 450.11?

Accepted Answer

Maximum available fault current

Answer

Rated kVA

Answer

Primary and secondary voltages

Answer

Frequency