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 fixtures totaling 300 drainage fixture units (DFU). The drain will have a slope of 1/8 inch per foot. What is the minimum required diameter for this building drain?
- A5 inches
- B4 inches
- C8 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: D. 6 inches
According to common plumbing codes (e.g., UPC or IPC tables for building drains), a building drain with a slope of 1/8 inch per foot can carry a maximum of 180 DFU for a 4-inch pipe, and 390 DFU for a 6-inch pipe. Therefore, for 300 DFU, a 6-inch diameter pipe is the minimum required size.
Why the other options are wrong
- A. 5-inch pipes are not a standard size commonly used for building drains in this context.
- B. A 4-inch pipe at 1/8 inch/foot slope can typically handle up to 180 DFU, which is insufficient for 300 DFU.
- C. An 8-inch pipe would be oversized for 300 DFU at this slope, though it would function adequately.
Building Drain Sizing (DFU & Slope)
Determining the minimum diameter of a building drain based on the total drainage fixture units (DFU) it serves and its installed slope.
- Larger DFU requires larger pipe diameter.
- Steeper slope allows for smaller pipe diameter for the same DFU.
- Plumbing codes provide tables for specific DFU capacities based on pipe size and slope.
- Building drains must be sized to prevent overloading and ensure proper flow.
Memory trick: DFU, Slope, and Size: The Plumbing Code's Wise Advice.