Transformers and Transformer Calculations Flashcards
7 cards from real Licensed Electrical Contractor practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Transformers and Transformer Calculations flashcards as text
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?
Answer: 300% of primary FLA
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.
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?
Answer: 12 inches
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.
Per NEC 450.21(B), dry-type transformers rated over 112.5 kVA installed indoors must be located in:
Answer: A fire-resistant transformer room or enclosure
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.
When is a transformer vault required for an indoor installation under the NEC?
Answer: For liquid-filled transformers containing flammable liquid installed indoors
NEC 450.26 requires a vault for liquid-filled transformers containing flammable liquid when installed indoors, to contain potential leaks and prevent fire spread.
Per NEC 450.14, where must the disconnecting means for a transformer be located?
Answer: Within sight of the transformer or lockable in the open position
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.
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?
Answer: 125% of primary FLA
Per NEC Table 450.3(B), without secondary protection for transformers 600V or below, the primary overcurrent protection must not exceed 125% of the rated primary current.
Which of the following is NOT required on a transformer nameplate per NEC 450.11?
Answer: Maximum available fault current
NEC 450.11 requires kVA rating, frequency, primary/secondary voltages, insulation class, and temperature rise on transformer nameplates, but maximum available fault current is not a required nameplate item.