My community follows the NEC codes for installing electricity between my house and my detached garage. There are no additional local codes. I live in Wisconsin and we have rough winters. I want to go underground and pull 3 - 12AWG thwn rated wires in eletrical PVC. My question is, would I need to use schedule 80 rather than schedule 40 anywhere in that run, and why?

Schedule 80 is considered as equal to steel rigid conduit. For residential purposes, Sch 40 is fine.

Actually no conduit is required at all.

You could use 12AWG UF cable directly buried to a depth of 12" provided the circuit is 20 amp GFCI protected.

You would then only need conduit as a riser to protect the cable as it emerges up from the ground from a depth of 12" up to and including all points above grade.

Under the 2008 NEC you will still be required to run at least 2 grounding electrodes at the garage and provide an intersystem bonding connector unless there will be only 1 branch circuit feeding this garage.

But agree that if you do decide to use conduit, sch 40 is all that would be required... and 6 inch burial depth is permitted.

If however you plan for future circuits, you will need to bury the conduit at least 18" below grade.

The 6 inch depth is only allowed for rigid metal conduit, since Sch 40 is nonmetallic, it must be 18" deep, unless it contains this one 120 volt 20 amp GFI protected circuit, then it can be 12 inch deep.

First and foremost, don't listen to the guy who says you don't need pipe... I am a licensed residential electrician and trust me, I see so many jobs where some one just laid some UF wire in the ground and two or three years later (especially in wisconson where I'm sure you get some pretty good frost) has been stretched or broken by frost heaves, it's definitely worth it to spend the extra money laying pipe... As far as the schedule 80/40 thing - if you are going to be driving over pipe (i.e. if it crosses the driveway) it should definaltly be schedule 80... if its just a simple run through the back yard to the garage, 40 is perfectly fine... two things to think about... if you are putting just those three #12's in you can get away with 1/2", however, again with the things i see everyday, if you ever plan on needing more power out there (i.e. a welder recept or likewise) you will need bigger pipe for those wires... I would recommend at least using 1 1/4" as that is big enough to run wires for a small sub panel... you don't want to be the guy who has to rip up an inferior size piece of conduit when you decide to upgrade, rather the guy who just pulls new wire through the existing... and the second thing to keep in mind, again with your winters and all is expansion joints... I cannot stress this enough as well, USE EXPANSION JOINTS in your run or you will run into the same problems as the guy who just dug a ditch and used UF... The pipe should by code should be 18" in the ground by code (however, i live and work in upstate NY where we probably have similar ground, our local inspectors make us go 12" because 18" without a machine is about impossible up here, and the pipe will give you that much more protection anyway...

Sch80 is NOT required anywhere underground, even under a driveway. This is not to say you can't use it if you like.

Typically Sch80 is required anywhere exposed that is subject to physical damage, such as most outside installations below 6' or so.

As TK stated, in the eyes of the code Sch80 is about equal to steel conduit. Sch40 is about equal to NM cable.

How much money is the diff?

We're talking DIY here, yeah? If the run only costs a few dollars more, would the sch 80 be overkill for only a few buck more? I always vote for DURABILITY - we've got one poster in frost heave land.

My 2 cents.

Schedule 80 has a smaller inside diameter would be one reason you may not want to use it. You may want room for future wiring.