What is the maximum horizontal distance between the vertical centerline of a fixture drain outlet and the vertical centerline of the trap inlet?

• A. 10 inches
• B. 15 inches
• C. 20 inches
• D. 25 inches

Answer: (B)

The main idea here is that there is a limited horizontal offset allowed between where the fixture drains and where the trap begins, to keep the trap arm properly vented and the drainage functioning reliably. The trap must be close enough to the fixture outlet so the trap arm isn’t too long; if the offset is too large, the venting distance becomes excessive and siphonage can occur, risking the trap seal.

For standard residential fixtures, the code sets this maximum horizontal offset at 15 inches. Keeping within 15 inches ensures the trap arm remains short enough to be effectively vented and to maintain proper drainage performance.

Distances smaller than this maximum would still be acceptable, but they aren’t the limit. Distances greater than 15 inches would exceed what the code allows for the fixture drain-to-trap connection, which is why those options aren’t correct.