What is the maximum number of back-to-back clothes washers allowed on a four-inch horizontal drain?

The maximum number of back-to-back clothes washers allowed on a four-inch horizontal drain is 30. This limit is based on plumbing codes and standards that specify the maximum fixture units that can drain effectively through a given size of pipe.

For a four-inch drain line, the capacity is determined by the number of fixture units assigned to each type of waste. Clothes washers are assigned a specific value, and when multiple units are installed in close proximity, they collectively must not exceed the capacity of the drain. The number 30 aligns with the established guidelines to ensure proper drainage and prevent any issues related to backflow, clogs, or inadequate drainage that could arise from exceeding the recommended number of fixtures.

Thus, adhering to this maximum helps maintain a well-functioning plumbing system within residential or commercial buildings.