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Practice Questions: Transformer

Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.

NEC NEC 450.3(B)Question 1

Per NEC 450.3(B), what is the maximum overcurrent protection rating for a transformer rated 600 volts or less with currents of 9 amperes or more on the primary side?

  • A100% of rated primary current
  • B125% of rated primary current
  • C150% of rated primary current
  • D200% of rated primary current
Show explanation

NEC 450.3(B) and Table 450.3(B) require primary-only protection not to exceed 125% of rated primary current for transformers 9 amps or more.

NEC NEC 450.9Question 2

Per NEC 450.9, transformers shall be installed in a manner that provides adequate ventilation. What is the minimum required clearance from walls for dry-type transformers rated over 112.5 kVA?

  • A6 inches
  • B12 inches
  • C18 inches
  • D24 inches
Show explanation

NEC 450.9 requires transformers rated over 112.5 kVA to have at least 12 inches of clearance from walls and other obstructions for ventilation.

NEC NEC 450.13(A)Question 3

Per NEC 450.13(A), each transformer rated 600 volts nominal or less shall be protected by an individual overcurrent device on the primary side. What is the exception regarding location of the primary OCPD?

  • ANo exception exists; OCPD is always required
  • BThe OCPD may be omitted if the transformer is rated less than 50 VA
  • CThe primary OCPD is not required where the primary circuit overcurrent device provides the protection specified in 450.3(B)
  • DThe OCPD may be located on the secondary side only
Show explanation

NEC 450.13(A) permits omission of a separate primary OCPD where the feeder overcurrent device provides protection per 450.3(B).

NEC NEC 450.21(A)Question 4

Per NEC 450.21(A), dry-type transformers installed indoors and rated 112.5 kVA or less shall have a separation of at least how far from combustible material?

  • A6 inches
  • B12 inches
  • C18 inches
  • D24 inches
Show explanation

NEC 450.21(A) requires dry-type transformers rated 112.5 kVA or less to be installed with at least 12 inches of separation from combustible materials.

NEC NEC Table 450.3(B)Question 5

A transformer rated 1000 volts or less is to be protected with primary overcurrent protection only. If the rated primary current is 5 amps, what is the maximum percentage of primary current permitted for the overcurrent device per NEC Table 450.3(B)?

  • A125%
  • B150%
  • C167%
  • D250%
Show explanation

Per NEC Table 450.3(B), for transformers rated 1000 V or less with primary-only protection, the overcurrent device may be rated at a maximum of 167% of the rated primary current when that current is less than 9 amperes. For 9 A or more, the limit is 125%, and the next higher standard size is permitted where 125% does not land on a standard rating.

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