Transformer Requirements
Article 100 of the National Electrical Code defines a transformer as equipment, either single-phase or polyphase, that uses electromagnetic induction to convert current and voltage in a primary circuit into current and voltage in a secondary circuit. Transformers are general-use equipment and are required to follow the general provisions in the NEC, and specifically the rules in Article 450. Transformers are typically provided with a nameplate that meets the requirements in 450.11(A). These data nameplates usually include current values, but this is not a required marking.
1. In the NEC, notes to tables are applicable rules and are not like informational notes, which are unenforceable.
Correct Answer: True
2. For autotransformers rated 1,000V, nominal or less, an overcurrent protective device shall be permitted to be installed in series with the shunt winding (the winding common to the autotransformer’s input and output).
Correct Answer: False
3. For a 480Y/277V transformer protected by a primary-only protective device, what is the general requirement for percentages of transformer rated primary current?
Correct Answer: 125%
4. If the primary current rating for a 45-kVA transformer (three-phase 480V delta primary, and a 208Y/120V, three-phase, 4-wire secondary) is not provided on the nameplate, what is the transformer primary current value?
Correct Answer: 54.21
5. What is the ampacity for a 10 AWG copper TW conductor (without any correction or adjustment factors applied)?
Correct Answer: 30
6. Overcurrent protection for transformer conductors is not covered in Section 450.3.
Correct Answer: True
7. Transformers over 1,000V nominal shall be provided with overcurrent protection in accordance with Table 450.3(A).
Correct Answer: True
8. Which of the following information on a transformer nameplate is not required by the NEC?
Correct Answer: Rated current for primary and secondary
9. Which of the following is not a standard overcurrent protective (fuses and inverse-time circuit breakers) device rating?
Correct Answer: 55
10. What is the maximum rating of overcurrent protective device (fuse and inverse-time circuit breaker) for primary-only protection for a 150-kVA transformer, three-phase, 480V (primary) to 208Y/120V (secondary)?
Correct Answer: 250
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