Illinois Plumbing Code Practice

The minimum air gap required for overflow pipes from gravity tanks connected to the drainage system is established to prevent any potential contamination of the water supply. The correct answer identifies a specific air gap that ensures there is a sufficient physical distance to prevent backflow, which could occur if there were any blockage or backpressure in the drainage system.

An air gap serves as a barrier that protects potable water from contaminants. In Illinois Plumbing Code, the required air gap is typically measured from the bottom of the overflow pipe to the flood level rim of the receiving receptacle or drain. The specified minimum air gap for overflow pipes is intended to provide safety against syphoning and other backflow issues, ensuring a reliable drainage without compromising the quality of the water.

In the other provided choices, while they may specify various dimensions, they do not meet the criteria set forth in the Illinois Plumbing Code for adequate protection against backflow contamination. Proper adherence to these standards is essential for maintaining safe plumbing practices and protecting public health.