Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a 3-inch horizontal drainage pipe. What is the maximum number of drainage fixture units (DFUs) this pipe can serve if it has a slope of 1/4 inch per foot?
- A60 DFUs
- B42 DFUs
- C84 DFUs
- D20 DFUs
Show answer & explanationAnswer & explanation
Correct answer: B. 42 DFUs
According to plumbing codes (e.g., IPC Table 709.1), a 3-inch horizontal drainage pipe with a slope of 1/4 inch per foot can serve a maximum of 42 drainage fixture units.
Why the other options are wrong
- A. This DFU count is typically associated with a 3-inch pipe at a 1/2 inch per foot slope, or a larger pipe at 1/4 inch per foot.
- C. This DFU count is for a 4-inch pipe at a 1/8 inch per foot slope, or significantly larger for a 3-inch pipe.
- D. This DFU count is too low for a 3-inch pipe at this slope.
Horizontal Drainage Pipe Sizing
The process of determining the minimum diameter of a horizontal drainage pipe based on the total drainage fixture units (DFUs) it serves and its installed pitch or slope.
- DFU capacity varies by pipe size and slope.
- Adequate sizing prevents overloading and blockages.
- Sizing tables are provided in plumbing codes.
Memory trick: Slope and size, DFU's prize.