Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is installing a 3-inch horizontal drainage pipe. What is the maximum number of drainage fixture units (DFUs) that this pipe can serve if it has a slope of 1/8 inch per foot?
- A42 DFUs
- B20 DFUs
- C30 DFUs
- D60 DFUs
Show answer & explanationAnswer & explanation
Correct answer: C. 30 DFUs
According to plumbing code tables (e.g., IPC Table 709.1), a 3-inch horizontal drainage pipe with a slope of 1/8 inch per foot can serve a maximum of 30 DFUs. While a 3-inch pipe at 1/4 inch per foot slope can serve 42 DFUs, the question specifically states 1/8 inch per foot.
Why the other options are wrong
- A. This capacity is for a 3-inch horizontal drainage pipe with a 1/4 inch per foot slope, not 1/8 inch per foot.
- B. This is too low for a 3-inch pipe at 1/8" per foot slope.
- D. This capacity is for larger pipes or steeper slopes.
Horizontal Drainage Pipe Sizing (DFU)
Horizontal drainage pipes are sized based on the total drainage fixture unit (DFU) load they carry and their slope. Proper sizing ensures adequate flow capacity and self-scouring velocity to prevent blockages.
- DFU load determines pipe size.
- Slope impacts capacity.
- Steeper slope = higher capacity.
- Refer to code sizing tables.
Memory trick: Pipe Size and Slope Dictate the DFUs it Can Cope.