A vault room is a reinforced concrete structure used for the purpose of housing liquid cooled transformers, switchgear and other electrical distribution equipment. Requirements for vault rooms for liquid filled transformers are defined by many specifications including NEC 450.26. Vault requirements are outlined in NEC 450, Part III, beginning with 450.4. Typical requirements might include:
Ventilation with outside air via a dedicated ductwork
Three hour fire-resistant construction including a minimum 4″ concrete floor
Oil containment capable of containing the entire liquid contents of the largest transformer
Some exceptions will allow lower construction standards such as the use of sprinklers, carbon dioxide or halon systems and/or the use of “less flammable” liquids.