With a slope of 1:50, what is the maximum length for a 3-inch trap arm?

With a slope of 1:50, what is the maximum length for a 3-inch trap arm?

• A. 10 feet
• B. 12 feet
• C. 14 feet
• D. 16 feet

Answer: (B)

The key idea is that how far a trap arm can run horizontally depends on its size and the slope of the drain, because the trap must be vented before siphon can occur. For a 3-inch trap arm, a slope of about 1/4 inch per foot is the standard reference that keeps the trap vented within the allowed distance. A slope of 1:50 is essentially the same as that 1/4 inch per foot, since 1:50 equals 0.02 feet of drop per foot (about 0.24 inches per foot). Using that slope, the maximum horizontal run for a 3-inch trap arm is about 12 feet. Across 12 feet, the vertical drop would be roughly 2.88 inches, which aligns with the venting requirements at this size and slope. Running longer than that would increase the risk of siphoning and violate typical plumbing tables or code limits. So the 12-foot length is the correct maximum.

The key idea is that how far a trap arm can run horizontally depends on its size and the slope of the drain, because the trap must be vented before siphon can occur. For a 3-inch trap arm, a slope of about 1/4 inch per foot is the standard reference that keeps the trap vented within the allowed distance. A slope of 1:50 is essentially the same as that 1/4 inch per foot, since 1:50 equals 0.02 feet of drop per foot (about 0.24 inches per foot).

Using that slope, the maximum horizontal run for a 3-inch trap arm is about 12 feet. Across 12 feet, the vertical drop would be roughly 2.88 inches, which aligns with the venting requirements at this size and slope. Running longer than that would increase the risk of siphoning and violate typical plumbing tables or code limits. So the 12-foot length is the correct maximum.