California Code of Regulations
Title 8 - Industrial Relations
Division 1 - Department of Industrial Relations
Chapter 4 - Division of Industrial Safety
Subchapter 7 - General Industry Safety Orders
Group 20 - Flammable Liquids, Gases and Vapors
Article 145 - Tank Storage
Section 5590 - Spacing (Shell-to-Shell) Between Aboveground Tanks

(a) The distance between any two flammable or combustible liquid storage tanks shall not be less than three feet.

(b) Except as provided in (c), the distance between any two adjacent tanks any one of which stores Category 1, 2, 3 or 4 flammable liquids shall not be less than one-sixth the sum of their diameters, except when the diameter of one tank is less than one-half the diameter of the adjacent tank, the distance between the two tanks shall not be less than one-half the diameter of the smaller tank. Tanks used only for storing liquids with a flashpoint greater than 199.4 °F (93 °C) (formerly designated Class IIIB Combustible liquids) may be spaced as provided in (a) unless within a diked area or drainage path for a tank storing a Category 1, 2 or 3 flammable liquid.

(c) Crude petroleum tanks at production facilities in isolated locations having capacities not exceeding 126,000 gallons (3,000 barrels), need not be separated by more than three feet.

(d) For unstable flammable or combustible liquids, the distance between such tanks shall not be less than one-half the sum of their diameters.

(e) When tanks are in a diked area containing Category 1, 2 or 3 flammable liquids, or in the drainage path of Category 1, 2 or 3 flammable liquids, and are compacted in three or more rows or in an irregular pattern, greater spacing or other means shall be provided to make inside tanks accessible for fire fighting purposes.

(f) The minimum separation between a liquefied petroleum gas container and a flammable or combustible liquid storage tank shall be 20 feet, except in the case of flammable or combustible liquid tanks operating at pressures exceeding 2.5 psig or equipped with emergency venting which will permit pressures to exceed 2.5 psig in which case the provisions of (a) and (b) shall apply. Suitable means shall be taken to prevent the accumulation of flammable or combustible liquids under adjacent liquefied petroleum gas containers such as by diversion curbs or grading. When flammable or combustible liquid storage tanks are within a diked area, the liquefied petroleum gas containers shall be outside the diked area and at least 10 feet away from the center line of the wall of the diked area. The foregoing provisions shall not apply when liquefied petroleum gas containers of 125 gallons or less capacity are installed adjacent to fuel oil supply tanks of 550 gallons or less capacity.