Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is installing a main building drain that will connect to the public sewer. The building drain will serve a total of 1200 drainage fixture units (DFU). If the drain is installed with a slope of 1/4 inch per foot, what is the minimum required diameter for this building drain?
- A8 inches
- B10 inches
- C12 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 8 inches
According to common plumbing codes (e.g., UPC or IPC tables for building drains), a building drain with a slope of 1/4 inch per foot can typically carry a maximum of 720 DFU for a 6-inch pipe, and 1600 DFU for an 8-inch pipe. Therefore, for 1200 DFU, an 8-inch diameter pipe is the minimum required size.
Why the other options are wrong
- B. 10 inches would be oversized for 1200 DFU at this slope.
- C. 12 inches would be significantly oversized for 1200 DFU at this slope.
- D. A 6-inch pipe at 1/4 inch/foot slope can typically handle up to 720 DFU, which is insufficient for 1200 DFU.
Building Drain Sizing (Large DFU)
Determining the minimum diameter of large-capacity building drains based on high total drainage fixture units (DFU) and specified slope.
- Critical for commercial/industrial buildings with many fixtures.
- Sizing charts are essential for accurate determination.
- Slope plays a significant role in DFU capacity.
- Undersizing leads to backups and system failure.
Memory trick: Big Building Drain: DFU, Slope, Chart, No Joke.