Except as provided in this section, it shall be unlawful for any person to store more than 15 gallons of gasoline, kerosene or diesel fuel, except such greater amounts as are normally stored in the fuel tanks of motor vehicles in any residential area of the city. The provisions of this section shall not apply to above-ground liquid petroleum, central gas distribution systems for mobile home parks, as defined in section 110-1, in existence on the effective date of the ordinance from which this section derives. In no event shall above-ground storage of liquid petroleum, gasoline, kerosene or diesel fuel exceed 250 gallons in any residential area of the city.