A five commissioner public utility district is a district that (1) either: (a) Has or had a license from the federal power commission to construct a hydroelectric project of an estimated cost of more than two hundred and fifty million dollars, including interest during construction; or (b) has a population of five hundred thousand or more; and (2) voters of the district approved a ballot proposition authorizing the district to become a five commissioner district as provided under RCW 54.40.040. All other public utility districts shall be known as three commissioner districts.