A lot is to do with the distance between the trap and the vent. I have a 1.5 inch p-trap on a 1.5 inch pipe for my laundry tub. But it is just over 42 inches away from the vent. The trap gulps after the tub has drained. I am going to have to shorten the distance.a 2" trap arm/drain is sufficient to avoid siphoning on a 1-1/2" p-trap. Can anyone corroborate with a real-life anecdote?
If you have just shy of five feet of pipe between your trap and your vent then you will need 2 inch pipe. Any more than 5 feet and you will run into the same problems as me.
Although Table 12.8.1 shows something different. Where am I going wrong here?