What is the maximum depth in inches for the trap seal of a 3" p-trap?

The maximum depth for the trap seal of a 3" p-trap is 4 inches. This depth is specified to ensure that there is a sufficient water barrier to prevent sewer gases from entering the living space through the drain. A trap seal that is too shallow risks allowing these gases to escape, resulting in potential health hazards and unpleasant odors in the environment.

Moreover, maintaining a proper trap seal depth helps to facilitate effective drainage and ensures that the trap functions correctly by retaining an appropriate amount of water. The trap must have enough water to seal off gases but not so much that it becomes sluggish or malfunctions under varied operating conditions.

While the other options present shallower or deeper measurements and do not comply with plumbing codes or practical drainage requirements, a 4-inch depth strikes the right balance between functionality and safety for a 3" p-trap.