I agree that the size of the tank is a factor. When I had a customer install a large tank near his switchgear, I figured 2o ft was a reasonable separation between something designed to spear (breakers and their arc vents) and something inclined to go 'boom.'
This was derived from table 513.4B1, under "motor fuel dispensing locations." This table discussed propane (LPG), and in this case, was being used as a motor fuel.
