Pertanyaan ini muncul saat ingin melakukan design untuk trafo power 480V/400V, 50Hz, Dyn-11 dengan kapasitas 150KVA yang akan ditempatkan disebuah portable container. Dalam NEC (NFPA 70) artikel 450.21 dijelaskan demikian. NEC 450.21 Dry-Type Transformer Installed Indoors. (A) Not over 112½ KVA. Dry-type transformer installed indoors and rated 112½ 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 fire resistant, heat-insulated barrier. Exception: this rule shall not apply to transformer rated for 600 volts, nominal, or less that are completely enclosed, with or without ventilating openings. (B) Over 112½ KVA. Individual dry-type transformers of more than 112½ 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