(1) The city engineer of a city of the first class shall, when requested by the mayor or city council, make estimates of the cost of labor and material which may be done or furnished by contract with the city and make all surveys, estimates, and calculations necessary to be made for the establishment of grades, the building of culverts, sewers, electric light systems, waterworks, power plants, public heating systems, bridges, curbing, and gutters, the improvement of streets, and the erection and repair of buildings and shall perform such other duties as the city council may require. When the city has appointed a board of public works, and the mayor and city council have by ordinance so authorized, such board may utilize its own engineering staff and may hire consulting engineers for the design and installation of extensions and improvements of the works under the jurisdiction of the board of public works. Whenever the mayor and city council have authorized the same, the board of public works may purchase material and employ labor for the enlargement or improvement of the systems and works under the jurisdiction of the board.

(2) Except as provided in section 18-412.01, no contract for enlargement or general improvements, such as water extensions, sewers, public heating systems, bridges, work on streets, or any other work or improvement when the cost of such improvement is assessed to the property, costing over thirty thousand dollars shall be made unless it is first approved by the city council.

(3) Except as provided in section 18-412.01, before the city council makes any contract in excess of thirty thousand dollars for enlargement or general improvements, such as water extensions, sewers, public heating systems, bridges, work on streets, or any other work or improvement when the cost of such enlargement or improvement is assessed to the property, an estimate of the cost shall be made by the city engineer and submitted to the city council. In advertising for bids as provided in subsections (4) and (6) of this section, the city council may publish the amount of the estimate.

(4) Advertisements for bids shall be required for any contract costing over thirty thousand dollars entered into (a) for enlargement or general improvements, such as water extensions, sewers, public heating systems, bridges, work on streets, or any other work or improvement when the cost of such enlargement or improvement is assessed to the property, or (b) for the purchase of equipment used in the construction of such enlargement or general improvements.

(5) A municipal electric utility may enter into a contract for the enlargement or improvement of the electric system or for the purchase of equipment used for such enlargement or improvement without advertising for bids if the price is: (a) Thirty thousand dollars or less; (b) sixty thousand dollars or less and the municipal electric utility has gross annual revenue from retail sales in excess of one million dollars; (c) ninety thousand dollars or less and the municipal electric utility has gross annual revenue from retail sales in excess of five million dollars; or (d) one hundred twenty thousand dollars or less and the municipal electric utility has gross annual revenue from retail sales in excess of ten million dollars.

(6) The advertisement provided for in subsections (3) and (4) of this section shall be published at least seven days prior to the bid closing in a legal newspaper in or of general circulation in the city. In case of a public emergency resulting from infectious or contagious diseases, destructive windstorms, floods, snow, war, or an exigency or pressing necessity or unforeseen need calling for immediate action or remedy to prevent a serious loss of, or serious injury or damage to, life, health, or property, estimates of costs and advertising for bids may be waived in the emergency ordinance authorized by section 16-405 when adopted by a three-fourths vote of the city council and entered of record.

(7) If, after advertising for bids as provided in subsections (3), (4), and (6) of this section, the city council receives fewer than two bids on a contract or if the bids received by the city council contain a price which exceeds the estimated cost, the mayor and the city council may negotiate a contract in an attempt to complete the proposed enlargement or general improvements at a cost commensurate with the estimate given.

(8) If the materials are of such a nature that, in the opinion of the manufacturer and with the concurrence of the city council or board of public works, no cost can be estimated until the materials have been manufactured or assembled to the specific qualifications of the city, the city council or board of public works may authorize the manufacture and assemblage of such materials and may thereafter approve the estimated cost expenditure when it is provided by the manufacturer.