If you are installing a large transformer it may have to be installed in a Fire Rated Room.
450.21 Dry-Type Transformers Installed Indoors.
(A) Not Over 112
1⁄2 kVA.
Dry-type transformers installed
indoors and rated 1121⁄2 kVA or less shall have a separation
of at least 305 mm (12 in.) from combustible material unless
separated from the combustible material by a fireresistant,
heat-insulated barrier.
Exception: This rule shall not apply to transformers rated
for 600 volts, nominal, or less that are completely enclosed,
with or without ventilating openings.
(B) Over 112
1⁄2 kVA.
Individual dry-type transformers of
more than 1121⁄2 kVA rating shall be installed in a transformer
room of fire-resistant construction. Unless specified
otherwise in this article, the term fire resistant means a
construction having a minimum fire rating of 1 hour.
Exception No. 1: Transformers with Class 155 or higher
insulation systems and separated from combustible material
by a
fire-resistant, heat-insulating barrier or by not less
than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically.
Exception No. 2: Transformers with Class 155 or higher
insulation systems and completely enclosed except for ventilating
openings.
FPN: See ANSI/ASTM E119-1995,
Method for Fire Tests
of Building Construction and Materials, and NFPA 251-
1999, Standard Methods of Tests of Fire Endurance of
Building Construction and Materials.
-Ed