§ 120. Board of contract and supply. There shall be a board of contract and supply, composed of the mayor, comptroller, commissioner of public works, corporation counsel and city engineer. Except as otherwise provided by law, it shall be the duty of such board, after public notice and in accordance with regulations to be prescribed by general ordinance of the common council, to let to the lowest bidder, who will give adequate security therefor, all contracts for the performance of any work or for the supply of any material required by or for the use of any officer, board, body or department of the city, in all cases where the expense of such work or materials, or both, shall exceed the sum of five hundred dollars, unless by ordinance of the common council adopted by a vote of not less than four-fifths of all the members thereof and unanimously approved by the board of estimate and apportionment, it is determined that a public emergency exists in which case said ordinance shall state the circumstances and conditions that created the public emergency, which shall be limited to those arising out of an accident or other unforeseen occurrence or condition affecting public buildings, public property or the life, health, safety or property of the inhabitants of such city require immediate action which cannot await public bidding, and shall designate the officer, board or department to procure such work or purchase such materials. In case of public emergency involving accident or other injury by which the heating or plumbing of any of the public buildings or any of the fire or water-works apparatus or any of the machinery used in sewage-treatment plants, or for garbage disposal, or any equipment used by the water-works or department of public works shall become disabled, the commissioner having jurisdiction thereof shall cause repairs thereto to be made without a letting by contract, upon filing with the board of contract and supply a certificate, approved by the mayor, showing such emergency and the necessity for such repairs. The board shall have power to reject all bids or proposals if in its opinion the lowest bid or proposal is excessive. The said notice shall describe the work and materials for which contracts will be let and the day and hour and place of the meeting of the board at which proposals therefor will be opened. Specifications for the performance of any work and for the supply of any materials shall be prepared and set forth with sufficient details to inform all persons proposing to bid therefor of the nature of the work to be done and of the materials to be supplied, and written or printed copies thereof shall be delivered to all applicants therefor. Every contract for a public improvement shall be based upon an estimate of the whole cost thereof, including all expenses incidental thereto and connected therewith, to be furnished by the proper officer, board or department having charge of such improvements. No bid or proposal shall be received or contract awarded, other than for a local improvement or work to be performed by the city, which involves the construction or maintenance of any structure, erection, obstruction or excavation within, under, over, along or upon any street or public place within the city, unless the person to whom such contract shall be awarded shall have a franchise permitting the same.