TRANSFORMERS AND TRANSFORMER VAULTS 70-455

rating of 3 hours. The authority having jurisdiction shall be permitted to require such a door for an exterior wall opening where conditions warrant. Exception: Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted. (FPN): For additional information, see Fire Doors and Fire Windows, NFPA 80-1992 (ANSI). (b) Sills. A door sill or curb of sufficient height to confine within the vault the oil from the largest transformer shall be provided, and in no case shall the height be less than 4 inches (102 mm). (c) Locks. Doors shall be equipped with locks, and doors shall be kept locked, access being allowed only to qualified persons. Personnel doors shall swing out and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure. 450-45. Ventilation Openings. Where required by Section 450-9, openings for ventilation shall be provided in accordance with (a) through (f) below. (a) Location. Ventilation openings shall be located as far away as possible from doors, windows, fire escapes, and combustible material. (b) Arrangement. A vault ventilated by natural circulation of air shall be permitted to have roughly half of the total area of openings required for ventilation in one or more openings near the floor and the remainder in one or more openings in the roof or in the sidewalls near the roof, or all of the area required for ventilation shall be permitted in one or more openings in or near the roof. (c) Size. For a vault ventilated by natural circulation of air to an outdoor area, the combined net area of all ventilating openings, after deducting the area occupied by screens, gratings, or louvers, shall not be less than 3 square inches (1936 sq mm) per kVA of transformer capacity in service, and in no case shall the net area be less than 1 square foot (0.093 sq m) for any capacity under 50 kVA. (d) Covering. Ventilation openings shall be covered with durable gratings, screens, or louvers, according to the treatment required in order to avoid unsafe conditions. (e) Dampers. All ventilation openings to the indoors shall be provided with automatic closing fire dampers that operate in response to a vault fire. Such dampers shall possess a standard fire rating of not less than 1½ hours. (FPN): See Standard for Fire Dampers, ANSI/UL 555-1972. (f) Ducts. Ventilating ducts shall be constructed of fire-resistant material. 450-46. Drainage. Where practicable, vaults containing more than 100 kVA transformer capacity shall be provided with a drain or other means that will carry off any accumulation of oil or water in the vault unless local conditions make this impracticable. The floor shall be pitched to the drain where provided. 70-456 NATIONAL ELECTRICAL CODE 450-47. Water Pipes and Accessories. Any pipe or duct system foreign to the electrical installation shall not enter or pass through a transformer vault. Piping or other facilities provided for vault fire protection, or for transformer cooling, shall not be considered foreign to the electrical installation. 450-48. Storage in Vaults. Materials shall not be stored in transformer vaults.