The walls and roofs of vaults shall be constructed of materials that have approved
structural strength for the conditions with a minimum fire resistance of 3 hours. The floors of vaults in contact with the earth shall be of concrete that is not less than 100 mm (4 in.) thick, but, where the vault is constructed with a vacant space or other stories below it, the floor shall have approved
structural strength for the load imposed thereon and a minimum fire resistance of 3 hours. For the purposes of this section, studs and wallboard construction shall not be permitted.
Exception: Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.
Informational Note No. 1: For additional information, see ASTM E119—18a, Methods for Fire Tests of Building Construction and Materials.
Informational Note No. 2: A typical 3-hour construction is 150 mm (6 in.) thick reinforced concrete.