When the legislative authority of a city or town determines to issue such funding or refunding bonds, it shall provide therefor by ordinance, which shall create a special fund for the sole purpose of paying the bonds and the interest thereon, into which fund the ordinance shall bind and obligate the city or town to set aside and pay a fixed amount without regard to any fixed proportion out of the gross revenue of the public utility as provided therein. In creating such special fund, the legislative authority shall have due regard to the cost of operation and maintenance of the utility as constructed or added to, and to any proportion or part of the revenue thereof previously pledged as a fund for the payment of bonds, warrants, or other indebtedness, and shall not bind and obligate the city or town to set aside into the fund a greater amount of the revenue of the utility than in its judgment will be available above the cost of maintenance and operation and the amount or proportion of the revenue thereof so previously pledged.