Any city, whether organized under the general laws of this state or by special charter or by constitutional charter, by vote of the constitutionally required percentage of the voters thereof voting thereon, may incur an indebtedness in an amount not to exceed an additional ten percent of the value of the taxable tangible property therein as shown by the last completed assessment for state and county purposes for the purpose of paying all or any part of the cost of purchasing or constructing waterworks, electric or other light plants to be owned exclusively by the city. The total general obligation indebtedness of such city shall not exceed twenty percent of the assessed valuation of such city as shown by the last completed assessment for state and county purposes.