What is the maximum vertical distance between the top of the fixture drain outlet and the horizontal center line of the trap outlet?

• A. 5 inches
• B. 10 inches
• C. 15 inches
• D. 20 inches

Answer: (C)

This asks about the maximum vertical separation between the top of the fixture drain outlet and the horizontal center line of the trap outlet. The code sets a limit of fifteen inches for this distance to keep the trap within a reasonable height so the trap arm can run to the wall drain with a proper slope and, just as importantly, to ensure the trap has adequate venting. If the trap outlet were much lower, you could end up with a configuration that is difficult to vent properly and prone to siphoning, which can siphon the trap seal and let sewer gases escape. Fifteen inches is the value chosen to balance practical cabinet/fixture heights with reliable venting. Choices that are smaller would unduly constrain install options, while a larger value would risk venting problems, so fifteen inches is the correct maximum.