I'm running 4/0, 4/0, 2/0 Al plus #6 Cu about 220' from main to sub-panel on a metal building. The cable will be buried. I was planning to run through Sch 40 gray carlon, but because of needing to work under/around tree roots and possibly having some (minor) bends along the way, I was thinking it would be much easier to snake some polyethylene pipe through the trench than having to slide long sections of PVC. Also save lots of gluing as I'd have one continuous conduit; no joints. I'd transition to PVC where the cable came above grade. Some additional information: This is out in the country. No code/inspection issues to deal with. Also, the conduit will not be buried in areas with any heavy traffic running over it, so no issues with crushing. If I need to go under a location where there may be heavy traffic, I'll run the poly tubing through a larger diameter sch 40 or sch 80 pipe to provide greater protection. Planning to bury about 18" - 20" deep. Again, this is on land in the country, and I'll know where it is, so I'm not concerned with it being cut or damaged at that depth by other activity. Just need to know if there is any reason you can't pull these cables through poly pipe. I'd be pulling all 4 cables/wires through 2" conduit. Based on prior research, I think this will allow for sufficient heat management. Thanks.