RCW 35.92.130

At the option of the legislative authority of the city or town various series and issues of outstanding warrants or bonds, or parts thereof, issued for the purpose of acquiring or constructing any public utility, or for making any additions or betterments thereto or extensions thereof, may be funded or refunded by a single issue of funding or refunding bonds. No proportion or part of the revenue of any one such public utility shall be pledged for the payment of funding or refunding bonds issued to fund or refund warrants or bonds issued for the acquisition or construction, or the making of additions or betterments to or extensions of, any other public utility.