If you have a friendly plumber, check with him, they are always replacing black plastic well pump lines and have to get rid of the old lines. I picked up a roll of used line free and use it to protect any buried electric lines. It's easy to work with, cuts with a hacksaw. I always seal the ends of the pipe after installing the wire, using expanding foam, to keep knawing critters out. Be sure to install marker posts at intervals so 10 years from now you know where the line actually is. Someone posted to bury the line below the freeze level, I can't imagine why that is necessary, as long as the line is deep enough to protect against accidental contact with mowers etc., 15", you should be OK.