When is it necessary to use 2" PVC SCH 40 drain lines instead of 1 1/2"? I'm assuming it has to with the volume of water/number of fixtures draining into it. Any info would be appreciated!

We haven't used 1 1/2" drainage for some years now. And in some codes the kitchen drainage is increased from 2 to 3". You are correct, 2" will carry more volume and have less clogs. Why do you ask? Regards, Tom

Oh wow, OK. I thought 1 1/2" was pretty standard on kitchen/bath waste lines but I'm far from being a professional. I'm plumbing a kitchen/utility area that has a double bowl sink, DW, another smaller sink, and a utility tub. The main soil pipe is 4" cast iron and there's a 2" galv. arm that Ys off of it that reduces down to 1 1/2". The previous owners threaded a PVC female adapter onto it. I tried to unthread the 1 1/2" galv. reducer off to get to the 2" and run 2" PVC from there but half a can of liquid wrench later and it wouldn't budge with the pipe wrench so I pretty much had to go 1.5" from there--especially since I didn't want to chisel out the lead and bushing from the cast iron and try to do a rubber bushing or something like that. Nothing is glued yet, just dry fitting everything so I can go to 2" but it would just be increasing it at the 1 1/2" galv. So it would still technically be draining into a smaller line. Also, you eventually have to reduce to 1 1/2" before the sink traps right? or do they make 2" slip joint connectors? Sorry if this seems muddled, again thanks for any expertise you can lend.
Ollie

you eventually have to reduce to 1 1/2" before the sink traps right? Yeah! For a few inches until the 2
Trap. This is not the same as running 1 1/2" all the way to the 4"
Main. I hope you're not planing on adding a disposal. This wouldn't be a good idea. As for using 1 1/2" for your kitchen drain, for years we used 1/2" drainage, however with dishwashers and disposals the volume was too much for 1 1/2". If you don't add appliances on the drain line you should be OK.
However, If you wanted to increase to 2" PVC you could cut out the bushing and convert to PVC using a 2" shielded clamp.(see image) Good luck. Tom