What size trap must be present if an open receptacle connects to a floor drain line without being vented?

In plumbing systems, traps are essential for maintaining a seal that prevents sewer gases from entering buildings while allowing wastewater to flow through. When an open receptacle connects directly to a floor drain line without venting, it creates a specific condition that necessitates a larger trap size to accommodate the increased potential for flow and to maintain proper drainage.

A 4-inch trap is the appropriate size in this situation because it is capable of handling the volume of wastewater that may be discharged from an open receptacle. This size ensures adequate drainage without creating excessive pressure buildup that could potentially lead to backflow issues or siphoning of the trap seal.

Using a trap that is too small could lead to blockages or inadequate drainage, which might result in water pooling and not effectively clearing the wastewater. In essence, the 4-inch size provides a balance between functional flow and the ability to maintain a water seal, making it the correct choice for the specified application.