___________________ is the minimum size plastic soil or waste pipe that may be installed underground?

The minimum size for plastic soil or waste pipe that may be installed underground is 2 inches. This size is established to ensure adequate flow and to accommodate the volume of waste and water that the system may handle. Smaller pipes, such as 1 1/2 inch, do not provide the necessary capacity to prevent clogs and backflow, especially in systems that carry waste or have the potential for high volumes of drainage. Additionally, 3 inch pipes, while larger and certainly acceptable in many cases, exceed the minimum requirement and are typically used in specialized applications or larger systems. The 2 inch minimum aligns with common plumbing practices and regulations, ensuring proper performance and reliability in underground installations.