Illinois Plumbing Code Practice

The correct answer is that the air gap must be at least two times the diameter of the fixture drain or drainage pipe served. This requirement is crucial in preventing back-siphonage and ensuring that contaminants do not enter the potable water supply. In plumbing codes, the air gap acts as a physical separation between the outlet of the drainage system and the flood level of the receiving fixture. By maintaining an air gap of at least twice the diameter of the drain, it creates sufficient space to prevent any potential backflow, thus protecting public health. This safety measure is essential in indirect waste systems, where proper drainage and venting are necessary to avoid any risks associated with wastewater contaminating drinking water systems.