Traps, Interceptors and Special Wastes
Traps, Interceptors and Special Wastes tests trap seals, trap arm length, prohibited trap arrangements, grease and oil interception, and special waste handling. The exam focuses on protecting the trap seal and preventing fats, oils, grease, sand, or chemicals from entering the drainage system.
Trap seals and arrangements
Each fixture is separately trapped, except where code allows a common trap. The trap seal depth is generally 2 to 4 inches. S-traps, double traps, and traps that depend on a moving part are prohibited. A trap must be accessible for cleaning.
Trap arm length
Under IPC 1002.2, the trap arm length is limited by size: 1 1/2-inch pipe has a 6-foot limit, 2-inch pipe has 8 feet, 3-inch has 12 feet, and 4-inch has 16 feet. The trap arm must be vented within that distance, and the vent must rise vertically before offsetting.
Interceptors and special wastes
Grease interceptors are required for food-service establishments and must be sized for the fixtures served and vented properly. Sand and oil interceptors are required where those wastes are discharged. Special wastes such as acids may need neutralization before discharge. Interceptors must be accessible for maintenance.
Worked example
A 2-inch trap arm runs 10 feet to its vent. IPC 1002.2 limits a 2-inch trap arm to 8 feet, so the installation fails. The fix is to move the vent closer, increase the trap arm size if allowed, or reconfigure the fixture location.
Traps that cost marks
Using a 2-inch trap arm at 10 feet is a common wrong answer. Other errors include omitting venting on a grease interceptor, installing an S-trap, or forgetting accessibility for cleaning.