• Oil Filled Transformer Vault Requirements

    The requirements listed here are intended to provide guidance in applying certain regulations and are for informational use only. They cannot be used as a substitute for the construction codes or for other city codes. They are intended for use to prepare for an inspection. This is only a general list and is not intended to address all possible conditions.

    Applicable codes are called out in brackets. The group responsible for inspections and review is noted in parenthesis - Building (B), Electrical (E), Fire (F), Mechanical (M), Plumbing (P), Land Use (LU), Puget Sound Energy (PSE). Be aware this information comes from the City of Bellevue and the PSE requirements are only advisory. Please confirm those requirements with PSE.

    Location Requirements

    1. Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wherever such an arrangement is practicable. [NEC 450.41] (B) (E) (PSE)
      Exception: When the location of the vault is not practicable on an outside wall, the ventilation duct to the outside air shall have that same rating as the vault and shall not have a fire damper. [COB policy]
    2. Any pipe or duct system foreign to the electrical installation must not enter or pass through a transformer vault room. [NEC 450.47] (E)
    3. Ventilation openings/louvers located on the exterior of the building may require land use approval due to their impact upon the building appearance. [COB Ordinance] (LU)

    Vault Construction Requirements

    1. The walls, floors and ceiling of the vault must provide not less than a 3-hour fire-resistive rated separation from all other areas of the building [NEC 450.42] (F).
      Exception:  Where transformers are protected with automatic fire sprinklers (in accordance with NFPA 13) and installed no more than five stories above finished grade, the rated separation can be reduced to 1 hour. [WCEC 450.42]
    2. Stud and wallboard construction of walls and ceiling of the vault enclosure is not permitted. Floor construction of the vault shall not be less than 4-inch thick concrete. [NEC 450.42] (B) (E)
    3. Stud and wallboard construction of the rated ventilation shaft to the exterior is permitted. The use of fire rated duct-wrap to obtain the required rating for the ventilation shaft is not permitted. [COB policy] (B)
    4. An automatic closing device is required on rolling fire shutter doors when installed. Personnel doors must swing out and be equipped with panic bars (pressure plates that are normally latched and locked, but open under simple pressure for egress). [IBC 716.5.11]  [3hr/1hr NEC 450.43] (B). The minimum size of an equipment door is 10 feet wide and 10 feet high. (PSE)
    5. All in-building vault doorways serving non-occupiable transformer rooms or vaults shall be permitted to have a removable oil containment sill of 6 inches minimum and 24 inches maximum in height above the finished floor level. The sill shall be installed on the push side of each doorway and extend not less than 6 inches past the door frame on both sides. The sill shall be connected to the wall with flush-mount internal-threaded anchors and be sealed to the floor and wall with oil-resistant caulk. The sill shall be painted bright yellow.
    6. Fire sprinklers are to be installed in accordance with NFPA code. [NFPA 13] (F)

    Plumbing Requirements

    1. Vaults containing a transformer larger than 100 kVA capacity shall be provided with a drain or other means of carrying off oil and water. Plumbing drain and piping shall be non-combustible to the gate valve and the valve shall have a label that says, “This valve shall be normally closed and opened only when contained oil has been removed from the containment area.” The piping shall extend to an oil/water separator. [NEC 450.46] [COB Water Quality] (P)

    Ventilation Requirements

    1. Transformer cooling ventilation opening sizes, location and fire damper requirements will be reviewed by the mechanical reviewer and PSE. Fire dampers for ventilation openings inside the building shall be installed per manufacturer installation requirements. To achieve cross ventilation, one ventilation opening is typically located inside the building and the other opening is located on the outside wall of the building or is ducted in a shaft to the exterior of the building. [NEC 450.45 A through F] [IBC 714] (M) (B)
    2. Natural ventilation shall be a minimum of 3 square inches for each kVA. Mechanical ventilation is measured in CFMs and shall be 1.6 times the transformer capacity (measured in kVA). (M) (PSE) 
      Example: 500 kVA transformer requires 1500 square inches natural ventilation or 800 CFM or 800 CFM mechanical ventilation. To detect a defective air ventilation system or potential fire or if the temperature exceeds 140 degrees within the vault, an alarm will sound in the building management system. (PSE)

    Electrical Requirements

    1. Wiring methods and materials within the vault shall comply with NFPA code. [NFPA 70] (E)
    2. Spill control and secondary containment requirements shall be in the form of a sill meeting the requirements in item 5 under Vault Construction Requirements above. The sill height shall be determined based on the volume needed to confine the oil from the largest transformer and the volume of water discharged from fire sprinklers equivalent to a 30-minute duration within the vault. Provide the volume calculations for verification. [IBC 414.5.4 and IFC 5004.2] (E) (PSE)
    3. The electrical inspector is the gatekeeper ensuring all City of Bellevue inspections are completed for the vault. PSE inspections and approvals are not part of the City of Bellevue inspection process. [COB Policy] (E)