What minimum separation from combustible material does 450.21 generally require for an indoor dry-type transformer rated 112.5 kVA or less, absent a fire-resistant, heat-insulating barrier?
Not quite. The answer is B: 12 inches
Indoor dry-type transformers rated 112.5 kVA or less must be separated from combustible material by at least 12 inches unless a fire-resistant, heat-insulating barrier is used (an exception exists for completely enclosed units rated 600 V or less). Units over 112.5 kVA are dealt with differently: they generally must go in a fire-resistant transformer room, with limited exceptions. The kVA threshold is what the exam tests.
Reference: NEC 2023 450.21(A) (Dry-Type Transformers Indoors — 112.5 kVA or Less)
Checked against multiple published sources
Two trade-press articles agree on the 12 in. rule for 112.5 kVA and under and the transformer-room rule above that; neither is the code text. Subsection lettering is as reported there.
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