A sanitary building drain is to be sized for a hydraulic load of 300 fixture units. If the pipe is laid at a grade of 1:50 (2%), what is the minimum required pipe size?

Question: A sanitary building drain is to be sized for a hydraulic load of 300 fixture units. If the pipe is laid at a grade of 1:50 (2%), what is the minimum required pipe size?

Answer options:

• 3 inch
• 4 inch
• 5 inch ✅ 6 inch

Correct answer: 6 inch

Explanation: Using the NPC sizing tables for sanitary building drains, a 4-inch pipe at 2% grade can handle 216 FUs, and a 5-inch pipe can handle 485 FUs. However, the next common size up for larger FUs is 6 inch. A 6-inch pipe can handle 840 FUs at 2% grade, making it the correct choice exceeding the 300 FU requirement.