A plumber is connecting a battery of three lavatories, each with a DFU value of 1, to a common branch vent. The branch drain serving these fixtures is 51 mm (2 inch) in diameter. What is the minimum required diameter for the common branch vent, according to the National Plumbing Code of Canada?

Question: A plumber is connecting a battery of three lavatories, each with a DFU value of 1, to a common branch vent. The branch drain serving these fixtures is 51 mm (2 inch) in diameter. What is the minimum required diameter for the common branch vent, according to the National Plumbing Code of Canada?

Answer options:

• 32 mm (1 1/4 inch) ✅ 38 mm (1 1/2 inch)
• 51 mm (2 inch)
• 64 mm (2 1/2 inch)

Correct answer: 38 mm (1 1/2 inch)

Explanation: According to the National Plumbing Code of Canada, a branch vent serving a branch drain carrying up to 8 DFU (as three lavatories equal 3 DFU) must have a minimum diameter of 38 mm (1 1/2 inch) if not serving a water closet. The vent diameter generally cannot be less than one-half the diameter of the drain pipe served, or less than 38 mm.