What size is required for a vent pipe serving a 3 inch fixture drain?

For a vent pipe serving a 3-inch fixture drain, a 2-inch vent pipe is the appropriate size. This is based on plumbing code requirements, which dictate that vent pipes should generally be at least half the diameter of the drain they serve. The primary purpose of a vent pipe is to provide air to the drainage system, helping to maintain proper pressure and prevent the siphoning of trap seals while allowing for the escape of sewer gases.

In this case, the design ensures that the 2-inch vent can effectively serve a 3-inch fixture drain, facilitating the necessary airflow and preventing potential blockages or pressure imbalances. Therefore, a vent pipe sized at 2 inches strikes a balance between functionality and compliance with established plumbing standards for drains of this size.