California Code of Regulations
Title 8 - Industrial Relations
Division 1 - Department of Industrial Relations
Chapter 4 - Division of Industrial Safety
Subchapter 5 - Electrical Safety Orders
Group 2 - High-Voltage Electrical Safety Orders
Article 17 - Enclosures for Electrical Installations
Section 2806 - Vaults Containing Oil-Filled Equipment

(a) Construction. Walls, roofs, and floors (other than when laid on earth) shall be of assemblies of materials approved for three-hour non-combustible fire-resistive construction.

(b) Doors. Door openings to vaults shall be protected by approved three-hour rated fire doors and frame assemblies.

(c) Sills or Curbs. A door sill or curb of sufficient height to confine within the vault the oil from the largest oil-filled equipment shall be provided, and in no case shall the height be less than 4 inches.

(d) Ventilating Openings. Ventilating openings shall be located as far away as practicable from building doors, windows, fire escapes, and combustible material. All openings from vaults into buildings, except approved fire door openings and viewing ports, shall be connected to a non-combustible duct or flue leading directly to the exterior, or shall be equipped with approved three-hour rated fire doors or fire dampers.

(e) Vents. Ducts and flues shall not be connected with any other ventilating or air distribution system, except that ventilation may be supplied from conditioned air systems into the vault, provided approved three-hour rated fire doors or fire dampers are installed in each opening.

(f) Sprinkler Systems. Water-type fire sprinkler systems shall be prohibited.

(Title 24, Part 3, Section 3-110 - 36.)