Tracer wire is run right beside the pipe. I use a 14 gauge wire, but I would think anything would work. It is used so you can trace the pipe later on if you need to. Say you had to dig and area up, you can now locate the pipe and not put a shove though it.If you watched the video link that mitchgo posted, at the end the guy took a soil probe. What if he put the prob into the pipe? Tracer is just a safety net
This post goes to all the pros or I guess anyone that wants to play. Share a story of your hardest or most memorable trouble shooting problem. Was there a time when you couldn't figure out what was going on with a system. Something wouldn't work or something was acting weird and it took you a moment to realize what was going on. Maybe you worked on system and you tried everything you knew and it turned out the water was shut off. lol You learn from mistakes, so share yours so you can teach other...
If you are going to do this, I would spend the extra money and run a tracer wire with the pipe. It will save you in the long run.