Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a battery of four lavatories in a public restroom. Each lavatory has a 1 1/4-inch trap and discharges into a 2-inch horizontal branch drain. What is the maximum developed length allowed for the trap arm of each lavatory to its vent connection according to common plumbing codes?
- A3 feet 6 inches
- B5 feet
- C2 feet 6 inches
- D6 feet
Show answer & explanationAnswer & explanation
Correct answer: C. 2 feet 6 inches
For a 1 1/4-inch trap, the maximum developed length of the trap arm to its vent connection is typically 2 feet 6 inches (30 inches). This ensures proper venting and prevents trap siphonage.
Why the other options are wrong
- A. 3 feet 6 inches (42 inches) is too long for a 1 1/4-inch trap arm and could lead to siphonage.
- B. 5 feet (60 inches) is the maximum developed length for a 2-inch trap arm, not 1 1/4-inch.
- D. 6 feet is excessively long for a trap arm of this size and is not allowed by code.
Trap Arm Developed Length (1 1/4-inch)
The maximum permissible length of the horizontal pipe connecting a 1 1/4-inch fixture trap to its vent pipe, measured from the trap weir to the vent connection.
- Prevents trap seal loss due to siphonage or backpressure.
- Length limit is specific to the trap arm diameter.
- Measured along the centerline of the pipe.
- For 1 1/4-inch traps, typically 2 feet 6 inches (30 inches).
Memory trick: Small pipes have short arms, or they'll lose their charm!