“Permanent distribution mains” means mains six inches or larger in diameter of cast or ductile iron, asbestos-cement or steel, with a minimum wall thickness of three-sixteenths inch or seven gauge. Four-inch mains of the above material may be classified as permanent mains in residential districts if not over 300 feet in length and standard fire protection can be furnished without connecting a hydrant to the four-inch main. (Ord. 780 § 3, 1975; Ord. 416 § 1.07, 1968)