Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is installing a main building drain in a commercial facility. The drain will receive discharge from a total of 150 drainage fixture units (DFUs) and has a slope of 1/8 inch per foot. What is the minimum required diameter for this building drain?
- A6 inches
- B5 inches
- C3 inches
- D4 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 6 inches
According to plumbing codes (e.g., IPC Table 709.1), a building drain with a slope of 1/8 inch per foot can handle 180 DFUs for a 6-inch pipe. A 4-inch pipe with this slope can only handle 84 DFUs, and a 5-inch pipe can handle 128 DFUs. Therefore, a 6-inch pipe is the minimum required for 150 DFUs.
Why the other options are wrong
- B. A 5-inch pipe at 1/8 inch/foot slope can handle 128 DFUs, which is insufficient for 150 DFUs.
- C. A 3-inch pipe at 1/8 inch/foot slope can only handle 31 DFUs, which is insufficient.
- D. A 4-inch pipe at 1/8 inch/foot slope can handle 84 DFUs, which is insufficient for 150 DFUs.
Building Drain Sizing (DFU & Slope)
The determination of minimum pipe diameter for a building drain based on the total drainage fixture units (DFUs) it serves and its installed slope.
- DFU capacity increases with pipe size.
- DFU capacity increases with steeper slope.
- Critical for efficient waste removal and preventing blockages.
Memory trick: Slope and size, DFU's guide.