For the purpose of defraying the cost of purchasing, constructing, improving, enlarging, extending or repairing any system or project of any character mentioned in § 6-21-50 and, when the borrower is a city having a population of more than five thousand people and less than five thousand five hundred people according to the United States census of 1930, for the purpose of refunding temporary loans made for the purpose of financing the cost of purchasing, constructing, improving, enlarging, extending or repairing any such system or project, any borrower may borrow money and issue its negotiable serial bonds.