The minimum sizes of vertical and horizontal drainage shall be determined from the _____.

The minimum sizes of vertical and horizontal drainage are determined by the total fixture units. This is because fixture units represent a standardized measure of the probable demand that a plumbing system will experience, taking into account the number of fixtures and their likely usage. By calculating the total fixture units, plumbers can effectively determine the appropriate diameter and capacity of the drainage pipes necessary to handle the expected flow, prevent clogs, and ensure efficient wastewater removal.

Understanding the relationship between total fixture units and pipe sizing is critical for designing plumbing systems that meet the Uniform Plumbing Code's requirements. Larger fixture units indicate a higher potential demand on the drainage system, necessitating larger pipe sizes to adequately accommodate the flow without experiencing backups or failures. This approach ensures that the plumbing system can handle drainage effectively under various conditions.