Illinois Plumbing Code Practice

In plumbing systems, the standpipe is a vertical pipe that receives wastewater from a washing machine or utility sink. The standpipe's inlet must be positioned a specific height above the trap's weir to ensure proper drainage and prevent backflow. According to the Illinois Plumbing Code, the maximum allowable height for the standpipe inlet of an automatic clothes washer above the weir of a trap is 48 inches. This height requirement helps maintain adequate water pressure to effectively transport wastewater without causing blockages or improper drainage. Therefore, option C, 48", is the correct answer.