The doing of all work and the furnishing of all supplies for the waterworks, electric power and light plant, or any other plant or work which may be under its supervision or control, shall be let out by the board of public works in the same manner as other public works are let out, except in cases where it is not practicable to do such work or furnish material by contract; and all contracts shall be submitted to the common council for approval. Said board may have charge of the purchase of all supplies needed by the city in its several departments, under such restrictions and regulations as may be provided by ordinance.