Individual dry-type transformers rated over 112½ kVA shall be enclosed in a fire-resistant room with materials having a ___-hour minimum fire rating.

• A. 3
• B. 2
• C. 6
• D. 1

Answer: (D)

When a room houses a relatively large dry-type transformer, the safety goal is to confine potential fire and limit its spread to protect the surrounding area and occupants. For individual dry-type transformers rated over 112.5 kVA, the enclosing room or enclosure must use materials with a minimum fire-resistance rating of one hour. This one-hour standard provides time for detection, shutdown, and firefighting efforts to act before the fire can compromise adjacent spaces. Ratings beyond one hour aren’t required for this size, so the correct requirement is a one-hour fire rating.