Transformer rooms are not automatically required to be enclosed in fire-rated construction. However, there are situations in which a transformer room may require a fire-rated enclosure.

Transformer rooms containing individual dry-type transformers of more than 112.5 kVA rating are required to be enclosed with 1-hour fire-rated construction in accordance with NFPA 70. The transformer ratings are not cumulative, so a transformer room is permitted to contain multiple transformers with a rating of 112.5 kVa or less without requiring enclosure.   There are two exceptions which permit the 1-hour fire-rating to be omitted:

1.  Transformers with Class 155 or higher insulation systems which are separated from combustible material by a fire-resistant, heat-insulating barrier or by not less than 6 feet horizontally and 12 feet vertically.

2.  Transformers with Class 155 or higher insulation systems and completely enclosed except for ventilating openings.