What branch size is specified for a water closet that uses less than or equal to 1.6 GPF?

When selecting the branch size for a water closet that uses less than or equal to 1.6 gallons per flush (GPF), the appropriate size is important to ensure proper drainage and system performance. In plumbing systems, the branch size is determined based on the fixture's flow rate and the potential volume of wastewater it will produce.

For a water closet with a flow rate of 1.6 GPF or less, a branch pipe size of 3 or 4 inches is proper. This size is capable of accommodating the wastewater generated by modern, efficient toilets, ensuring they have adequate drainage without risking blockages or backflow. The 3 or 4 inches provides enough capacity to handle the volume of water being discharged, as well as the ability to maintain a proper flow velocity within the drain system.

Choosing a smaller size, such as 1 1/2 inches or 2 inches, could potentially result in insufficient drainage, as these sizes may not be able to handle the volume efficiently, leading to clogs and other issues. On the other hand, increasing the size to 5 inches is unnecessary for water closets that use only 1.6 GPF, and it can lead to increased costs and installation complexity without providing additional benefits in