A plumber is asked to calculate the minimum size of a building drain in a commercial occupancy. The total drainage fixture unit (DFU) load from all fixtures connected to this drain is 450 DFU. Based on typical National Plumbing Code (NPC) sizing tables, what is the minimum required diameter for this building drain, assuming a horizontal slope of 1 in 100?

Question: A plumber is asked to calculate the minimum size of a building drain in a commercial occupancy. The total drainage fixture unit (DFU) load from all fixtures connected to this drain is 450 DFU. Based on typical National Plumbing Code (NPC) sizing tables, what is the minimum required diameter for this building drain, assuming a horizontal slope of 1 in 100?

Answer options:

• 75 mm
• 100 mm ✅ 150 mm
• 200 mm

Correct answer: 150 mm

Explanation: According to NPC sizing tables for building drains, a 150 mm (6-inch) drain pipe laid at a slope of 1 in 100 can typically handle a maximum of 480 DFU. Therefore, for a total DFU load of 450 DFU, a 150 mm diameter pipe would be the minimum required. A 75 mm (3-inch) or 100 mm (4-inch) pipe would be undersized for this DFU load, while a 200 mm (8-inch) pipe would be oversized and unnecessary.