A plumbing crew is installing a combined waste and vent system (wet vent) for two back-to-back lavatories on a common branch. The upstream fixture is vented conventionally, and the downstream fixture's trap arm is wet vented through the upstream fixture's branch drain. What is the maximum permitted length of the wet-vented section of the trap arm, and what is its minimum diameter, to ensure proper function and prevent trap seal loss for a 38 mm trap?

Question: A plumbing crew is installing a combined waste and vent system (wet vent) for two back-to-back lavatories on a common branch. The upstream fixture is vented conventionally, and the downstream fixture's trap arm is wet vented through the upstream fixture's branch drain. What is the maximum permitted length of the wet-vented section of the trap arm, and what is its minimum diameter, to ensure proper function and prevent trap seal loss for a 38 mm trap?

Answer options:

• 1.5 m, 38 mm ✅ 2.5 m, 50 mm
• 3.0 m, 38 mm
• 3.5 m, 50 mm

Correct answer: 2.5 m, 50 mm

Explanation: For a wet-vented trap arm from a 38 mm trap, the National Plumbing Code of Canada typically allows a maximum developed length of 2.5 m, and the wet-vented section itself must be at least 50 mm in diameter. This larger diameter and specific length limit are critical to prevent siphoning and ensure the trap seal is maintained, as the pipe is functioning as both a drain and a vent for the downstream fixture.