A plumber is sizing a horizontal drainage pipe that serves a water closet, a lavatory, and a shower stall. The water closet has a drainage fixture unit (DFU) value of 3, the lavatory is 1, and the shower stall is 2. What is the minimum diameter of the horizontal drainage pipe required if the total DFU load is 6?

Question: A plumber is sizing a horizontal drainage pipe that serves a water closet, a lavatory, and a shower stall. The water closet has a drainage fixture unit (DFU) value of 3, the lavatory is 1, and the shower stall is 2. What is the minimum diameter of the horizontal drainage pipe required if the total DFU load is 6?

Answer options:

• 38 mm (1 1/2 inch)
• 51 mm (2 inch)
• 64 mm (2 1/2 inch) ✅ 76 mm (3 inch)

Correct answer: 76 mm (3 inch)

Explanation: According to the National Plumbing Code of Canada, a horizontal drainage pipe serving a total DFU load of 6, including a water closet, requires a minimum diameter of 76 mm (3 inch). A 51 mm (2 inch) pipe is typically limited to a maximum of 3 DFU for horizontal drainage, or 6 DFU for vertical drainage, and cannot serve a water closet.