Any city, by vote of the qualified electors 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 shown as provided in section 26(b), 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, provided the total general obligation indebtedness of the city shall not exceed twenty percent of the assessed valuation. For elections referred to in this section the vote required shall be four-sevenths at the general municipal election day, primary or general elections and two-thirds at all other elections.