If your home wiring has a metal cable around the wire, pbx cable I believe, and it's connected to the box, the switchbox is the ground or the cable. Otherwise you have to run a pbx cable or a third wire. A porch light doesn't take too much current. If your connections are good and you have a fuse or circuit breaker, with the low watt bulbs today, you shouldn't have too much trouble. That's all people had for years. I guess your thinking about someone getting a shock from the light, so in that case you're right, you need another wire or pbx (a heavy spiral metal cable with wires inside). You might look into what is legislated in your area. I seem to recall pbx being not so good, although I don't see why. Maybe something to do with whether it's connected right or something. You can call your county government. They'll want a license, of course, if you rewire to the fuse box or elsewhere. If you have pbx, that's not necessary.