Design Guidelines for Dry-Type Transformers

Section 450.21 of the 2011 NEC notes the specific provisions applicable to dry-type transformers installed indoors.

July 17, 2014

Indoor transformers of greater than _____ rating shall be installed in a transformer room of fire-resistant construction.

a) 75kVA
b) 87.5kVA
c) 112.5kVA
d) 250kVA

See answer and applicable Code reference

The correct answer is: c) 112.5kVA

Article 450 covers the installation of all types of transformers. Section 450.21 notes the specific provisions applicable to dry-type transformers installed indoors. As per 450.21(B), "Individual dry-type transformers of more than 112.5kVA 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."