In wet venting between two fixtures, the drain between the two fixtures must be at least one pipe size larger than the upper fixture drain and not smaller than what diameter?

• A. 1 inch
• B. 1-1/4 inch
• C. 2 inches
• D. 2-1/2 inches

Answer: (C)

In wet venting between two fixtures, the connection between them must be sized to carry the wastewater from the upper fixture and still function as a vent, which means the drain between them must be at least one pipe size larger than the upper fixture’s drain. At the same time, there’s a minimum diameter you must meet, regardless of the upper fixture size, to ensure proper venting and flow. That minimum is 2 inches. So the drain between the two fixtures cannot be smaller than 2 inches in diameter. If the upper fixture drain is 1-1/4" or 1-1/2", the next standard size would be at least 2", which satisfies both requirements. Options smaller than 2 inches don’t provide adequate venting, while a larger size like 2-1/2" is allowed but not required by the stated minimum.