Drain line diameter reduction

We all know drain lines are not supposed to decrease in diameter from the drain connection. But what about 1-1/2" out of drain pan, 2" p-trap, then reduced back to 1-1/2"? Saw it today.


I’d call it out just to keep everything 1-1/2”. I don’t know why they went to 2”, weird.

Yep. Weird. I called it out.

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I wonder if it is prone to clogging due to decreased velocity.

Interesting thought. I always thought it was because of the step formed at the diameter change.

P-traps are sizes in a way to clean themselves as waste water goes through the trap. Oversizing a trap may lead to the trap not being able to clean itself properly.

This is correct.