Typical porch light cable will be a 2 or 3 wire big (compared to Ethernet) cable (plus ground). That is not the type of cable you would use for Ethernet - it just won't work.
Could you replace that cable with an Ethernet cable - i.e., instead of running a separate cable? Maybe:
The existing cable would have to be either in a conduit all by itself, or NM (Romex) cable. If it is conduit then you can't share that conduit with any other high voltage cable (e.g., you might find that you have a switched hot for the light but also additional power cable going elsewhere in the same conduit). If it is Romex and not in a conduit then using the old cable to pull a new cable won't necessarily be so easy if it is stapled or otherwise secured in place.
At the other end (e.g., switch box), you would need to either pull the Ethernet cable out at a different location or remove any electrical cables from the box as you can't mix low voltage (Ethernet) with high voltage (e.g., 120V).
You wouldn't have a porch light any more, which means your camera would be in the dark.
I really wouldn't recommend trying to replace the electrical cable with PoE - just not worth it unless the electrical is really "extra" - e.g., two separate switched lights and you combine the two together and then reuse the conduit/replace cable going to one of them.
The good news, as you already know, is that at least PoE saves you from having to run separate power to the camera.
There is one other possible solution, depending on distances and equipment. If the porch light is wired in a way that power is always available (e.g., panel -> porch light -> switch instead of panel -> switch -> porch light) then you could:
- Install a receptacle in a box next to the porch light, pulling power from the porch light always-on power cable.
- Use a WiFi camera with power from the new receptacle.
And one more possibility: If no unswitched power is available at the porch light, you could leave the switch always on (or rewire so that the porch light fixture is always on and remove the switch) and install a receptacle next to the porch light, sharing power, and replace the porch light itself with either a motion sensor or a dusk-to-dawn photocell fixture. That way you can have power all the time for a WiFi camera without having the light on all the time.
I generally prefer wired cameras, but this is one case where WiFi might save you a lot of trouble running a new Ethernet cable.
