Also need to consider grounding/lightning protection for anything on the rooftop.
Best bet would be to use any existing AC power that is already up there, and save this drop for low voltage, data, RF, etc. Ground everything to the AC power system ground that is hopefully already up there.
I assume the old (steam?) pipe is metal? It would be good to figure out if there is electrical continuity from end to end of the pipe. If it is, then grounding one end to the building power ground should be sufficient. If the conduit is not electrically continuous from end to end, it might be best to make sure both ends are grounded, or find out where the gap is and bridge across it with some kind of jumper.
-steve