Final answer:
A horizontal wet vent must have a minimum diameter of two inches for 24 dfu or less, which is set by plumbing codes to ensure adequate drainage and venting in plumbing systems.
Step-by-step explanation:
A horizontal wet vent shall be not less than two inches in diameter for 24 dfu or less. This guideline comes from plumbing codes that specify the minimum requirements for building and fitting drainage systems, such as the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC). DFU stands for Drainage Fixture Unit and measures the probable discharge into the drainage system by various types of plumbing fixtures. The dfu value is used to size piping and venting systems in plumbing to ensure proper drainage and venting without causing backups or slow drainage.
According to these codes, a wet vent serving one bathroom group containing fixtures such as a bathtub, shower, toilet, and sink typically allows for up to 24 dfu. This ensures that a minimum of a 2-inch diameter pipe is used, which is adequate for the average flow from these fixtures while maintaining the necessary venting requirements.