I recently had my framing inspection and the inspector was sort of glancing at my plumbing and electrical work while we walked through the house.
in the basement he said that my 2” drain pipe was too small for the clothes washer.

he said that a clothes washing machine needed to have a 3” pipe after the p-trap.
it seemed excessive to me, so asked if it was something new.

For clarity, my clothes washer is the furthest away from my main stack, so water from the washer and utility sink is the first thing to go into the drain pipe. the drain is 2” pvc for the both the washer, which then captures from the utility sink. Then there is the p-trap. Then the drain pipe goes down to the basement, where it turns a couple times, and then picks up drains from the kitchen sink/dishwasher and then the shower drain.

Looking at it now, it probably should go into a 3” drain after the kitchen sink because it picks up two more DFU from the shower before it runs into main drain, so I’m going to have to fix those fittings anyway and running a few more feet of 3” pipe isn’t the end of the world.

My main question would be if there is really a requirement that the clothes washer needs to empty into a 3” pipe after the P-trap. To me it seems like it would depend on what else is draining into the pipe.

Oh well, back to Lowe’s to buy more 3” fittings.

 

You didn't mention anything about venting but I wonder if it has to do with your plumbing vent location. 3" is not "required" after the trap. But, the distance allowed to the vent varies with the pipe size. Going to a larger diameter pipe would allow the trap/fixture to be located further from the vent.