Tanks should be listed and labeled, and conform to standards for underground and above-ground tanks. The maximum amount of fuel oil stored above ground or inside a dwelling should be 660 gallons (2,498 L). The cross-connection of two tanks is permitted if the total aggregate capacity does not exceed 660 gallons. Gravity flow from one tank to another is permitted, provided that the two tanks are on the same horizontal plane. 14.1.1 Inside and Outside