No grant of franchise and no extension, renewal or amendment thereof shall be valid for a greater period than twenty years from the day of such grant, renewal, extension or amendment, provided and except, however, a grant of franchise, extension, renewal or amendment thereof shall be valid for a period of twenty-five years for operating street railways from the day of such grant, renewal, extension or amendment and providing such grant, renewal, extension or amendment be passed by an affirmative vote of at least all of the members elected to Council less two of the Council.