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

NEC 450.3(A) allows a maximum primary overcurrent device of 300% of rated primary current when no secondary overcurrent protection is provided for transformers over 600V.

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

NEC 450.21(A) requires a minimum separation of 300 mm (12 inches) between dry-type transformers rated 112.5 kVA or less and combustible materials.

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

NEC 450.21(B) requires dry-type transformers rated over 112.5 kVA to be installed in a fire-resistant room or enclosure to prevent fire spread.

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

NEC 450.26 requires a vault for liquid-filled transformers containing flammable liquid when installed indoors, to contain potential leaks and prevent fire spread.

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

NEC 450.14 requires that a transformer's disconnecting means be within sight of the transformer or be capable of being locked in the open position.

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Licensed Electrical Contractor Transformers and Transformer Calculations 3