I am running about 20 feet of 8 AWG 4 wires through 3/4 inch PVC piping for a electrical power generator (transfer switch to L14-30 outlet). Wanted to know, codes aside, if it is fine to use white instead of grey PVC. The are both schedule 40, but the white is half the cost locally. I'll be running along side the corner of the wall and roof of the inner side of my garage.
I understand that white is specifically for plumbing and grey is specifically for electrical, but want to put aside the codes for the sake of argument.
Best Answer
Don't mix your (PVC) streams
Electrical PVC conduit and PVC sanitary (supply, DWV) piping may look similar (save for the color), but are made to very different standards. Plumbing PVC needs to be rated to a higher working pressure for supply piping use, and also is designed to be used from fitting to fitting, with female-female couplers used to join straight lengths together. However, most of it isn't rated for outdoor use, or for its behavior when subject to ignition sources. (In addition -- the listing/rating for most plumbing PVC is through the NSF, which isn't competent to issue electrical listings.)
Electrical PVC conduit, on the other hand, while not rated for pressure, is rated for UV exposure for outdoor use and for its ability to resist ignition for a period of time; neither of these are true of garden-variety (white) PVC pipe. It also is designed so that lengths of PVC conduit can be fitted together without a coupling, thanks to a flare on one end of the PVC, and has specialist fittings available for it that don't exist in the DWV world (such as expansion joints, to keep longer runs from doing the worm due to temperature changes). Electrical PVC is also UL (NRTL) listed for electrical service, which is a NEC requirement (352.6).
As a result of these differences, they are not interchangeable or even compatible products, so just get the right stuff, even if it costs more.
As to sizing, though...
4 8AWG THHN wires take up just under 96mm2 of fill, while a 3/4" Schedule 40 PVC conduit has 131mm2 of fill available, so you are fine with your conduit sizing. I would recommend using conduit L bodies whenever possible and prefabricated elbows for inside bends (where L bodies cannot be used) -- this makes installation the simplest, and stands the best chance of not violating the 360 degree rule for bends between pull points.