I have to connect my new 2 inch shower pan to existing 1.5 inch drain.
Should I change the p trap to to inch or leave it 1.5 and increase the pipe to 2 inches at the shower pan opening?

Hi Hambeek...

2" is required by plumbing codes in every state that I know of so if there is any way to increase the drain to 2" I strongly recommend that you do that.

Otherwise, if you simply can't (or won't) then install the 2" ptrap if the floor joist depth will allow it and then reduce to 1.5" after that.

Mark

What material are the 2" trap and the 1 1/2" line you're connecting to? Are they the same or will you have to convert? Let me know, Tom

Thanks Mark.
Tom I have abs drain line and plan to use abs p trap and raise it to fiberglass shower pan

OK... then advice stands as presented above... should just need a reducing coupling, or even better, if the main drain is readily accessible you could cut in a wye fitting with a 2" branch and run that over to the shower... best job here and up to code.

Now, does the manufacturer recommend setting the fiberglass shower pan in a bedding substrate like mortar or structolite? If so, I highly recommend that you don't skip this step.. makes a big difference when stepping into the shower and helps to assure that the drain seal won't move over time!

Mark