A plumber is designing the DWV system for a new commercial laundry facility. The largest washing machines have a discharge rate that results in a total hydraulic load of 30 fixture units (FU) for a branch drain line. What is the minimum nominal pipe size required for this branch drain, assuming a maximum slope of 2% and according to standard plumbing code tables?

Question: A plumber is designing the DWV system for a new commercial laundry facility. The largest washing machines have a discharge rate that results in a total hydraulic load of 30 fixture units (FU) for a branch drain line. What is the minimum nominal pipe size required for this branch drain, assuming a maximum slope of 2% and according to standard plumbing code tables?

Answer options:

• 75 mm ✅ 100 mm
• 125 mm
• 150 mm

Correct answer: 100 mm

Explanation: According to the National Plumbing Code of Canada (NPC), a 75 mm branch drain has a maximum capacity for approximately 20 to 24 fixture units, depending on slope. For 30 fixture units, a 100 mm pipe is required as it can accommodate up to 160 to 180 fixture units, making it suitable for this load. 125 mm and 150 mm would be excessively large for 30 FU.