The connection of the building sewer into the public sewer shall be made at the Y branch, if such branch is available at a suitable location. If the public sewer is twelve inches in diameter or less, and no properly located Y branch is available, the owner shall make connection in the manner specified by the engineer. Where the public sewer is greater than twelve inches in diameter, and no properly located Y branch is available, a neat hole may be cut into the public sewer to receive the building sewer, with entry in the downstream direction at an angle of about forty-five degrees. A forty-five degree ell may be used to make such connection, with the spigot end cut so as not to extend past the inner surface of the public sewer. The invert of the building sewer at the point of connection shall be at the same or at a higher elevation than the invert of the public sewer. A smooth, neat joint shall be made, and the connection made secure and watertight by encasement in concrete. Special fittings may be used for the connection only when approved by the engineer.