Each doorway leading into a transformer vault from the building interior shall be provided with a tight-fitting door having a minimum fire rating of ___ hours.
See answer and applicable 2014 NEC reference
The correct answer is: b) 3
Article 450 outlines the installation requirements for transformers and transformer vaults. Part III of this Article covers transformer vaults. As noted in Sec. 45043(A), "Each door leading into a vault from the building interior shall be provided with a tight-fitting door that has a minimum fire rating of 3 hours. The authority having jurisdiction shall be permitted to require such a door for an exterior wall opening where conditions warrant."