Loading and unloading space. In addition to the off-street parking space required above, any building erected, converted or enlarged in any district for commercial, office building, manufacturing, wholesale, hospital or similar uses shall provide adequate off-street area for loading and unloading of vehicles. The minimum size loading space shall be 65 feet in depth, 12 feet in width, with an overhead clearance of 14 feet. In no case where a building is erected, converted or enlarged for commercial, manufacturing, or business purposes shall the public rights-of-way be used for loading and unloading of materials.