The term 'totally enclosed' does not specifically appear in Art. 100 or 450 (2011 NEC)
Mention is made within 450.21
"450.21 Dry-Type Transformers Installed Indoors.
(A) Not over 1121⁄2 kVA. Dry-type transformers installed
indoors and rated 1121⁄2 kVA or less shall have a separation
of at least 300 mm (12 in.) from combustible material unless
separated from the combustible material by a fireresistant,
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 1121⁄2 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."
IMHO, a basic 480/120-208 transformer is 'enclosed' by def.; and 'completely' enclosed as the wording within exception 2 above.
A 'totally enclosed' transformer IMHO, is one that is encapsulated and only the pri & sec conductors are visable.