Practice Questions: Transformer
Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.
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.
Per NEC 450.21(B), individual dry-type transformers rated over 112.5 kVA installed indoors shall be installed in a transformer room of fire-resistant construction having a minimum fire rating of how long?
- A30 minutes
- B1 hour
- C2 hours
- DNo fire rating required if separated 12 inches from combustible material
Show explanation
NEC 450.21(B) requires individual dry-type transformers rated over 112.5 kVA to be installed in a transformer room of fire-resistant construction having a minimum fire rating of 1 hour. (Exception No. 1 and No. 2 apply where Class 155 or higher insulation systems are used with appropriate separation or complete enclosure.) NEC 450.9 governs ventilation requirements but does not specify a numerical wall clearance.
Per NEC 450.3(B) and Table 450.3(B), transformers rated 1000 volts or less shall have overcurrent protection per that table. Under what condition may a separate primary-side OCPD be omitted?
- 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 450.3(B) and Table 450.3(B) govern overcurrent protection for transformers 1000 V and less. A separate primary OCPD is not required when the feeder or branch-circuit overcurrent device on the supply side provides protection that does not exceed the values in Table 450.3(B). NEC 450.13(A) is a separate provision addressing accessibility of open-mounted dry-type transformers and does not govern OCPD requirements.
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.
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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