Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a new 4-inch building drain that will serve a total of 150 drainage fixture units (DFUs) and has a slope of 1/8 inch per foot. According to plumbing codes, what is the maximum permissible DFU load for this pipe?
- A250 DFUs
- B384 DFUs
- C180 DFUs
- D216 DFUs
Show answer & explanationAnswer & explanation
Correct answer: D. 216 DFUs
For a 4-inch building drain with a slope of 1/8 inch per foot, the maximum DFU load allowed by most plumbing codes is 216 DFUs. The proposed 150 DFUs falls within this limit.
Why the other options are wrong
- A. 250 DFUs is too high for a 4-inch drain at this slope; it might be for a larger pipe or steeper slope.
- B. 384 DFUs is the maximum for a 4-inch drain at 1/4 inch per foot slope, not 1/8 inch per foot.
- C. 180 DFUs is typically the maximum for a 4-inch drain at 1/4 inch per foot slope, not 1/8 inch per foot.
Building Drain Sizing (DFU & Slope)
The process of determining the minimum required diameter for a building drain based on the total drainage fixture units (DFUs) it serves and its installed slope.
- DFU load increases with more fixtures.
- Steeper slopes (e.g., 1/4 inch/foot) generally allow for higher DFU capacity than shallower slopes (e.g., 1/8 inch/foot) for the same pipe size.
- Referencing plumbing code tables is essential for accurate sizing.
Memory trick: Slope and Size are the twin keys to DFU capacity's maze.