18awg is small for power wires. With 5VAC 2A:
- At 20ft you're going to get ~4.5V
- At 40ft, about ~4.0V.
For comparison, if it was 16awg, at 20ft you'd get ~4.7V. With 22awg, at 20ft you'd get ~3.7V.
You should check the specs on the camera: they'll often give a voltage tolerance. On top of that, some transformers (especially cheap ones) don't exactly supply the voltage they say they will, especially as the current draw is higher. A crappy transformer coupled with under-sized wiring may mean not enough power for the camera, which will result in it either not working or being "glitchy".
Because you also have a wifi camera, the lower the wifi signal it gets, the more power it will take. I'm not sure how significant this actually is, but wifi is likely to be one of the main demands for power in the camera.
You can compensate for the voltage loss with a slightly higher input voltage (a 5.5V transformer would work perfectly for you). Power over Ethernet compensates for this by supplying a higher voltage (48VDC) and requiring devices can accept as low as 44V DC.
You could also potentially use the existing wire as a fish to pull a larger wire though, but that of course depends on if it's exposed in an unfinished basement or how willing you are to open and patch drywall. If you're going to pull wires, it may just be easier to pull ethernet and use a PoE hard-wired camera (more reliable, plus the power problem is taken care of).
All the ones I've seen seem to be sized for installation on a light box. You want to use as shallow a box as you can get away with so you don't decrease the wall insulation. If you use the really shallow ones, you might be able to surface mount them on the sheathing over the tyvek.
Do you even need a box? If the camera is PoE, you're running low voltage and tiny currents.
I think your best bet is to look up some of the cameras, then go to the Mfgs website, and read the installation guide.
Don't put them all at 8 feet. That's really easy to hit with a spray can of paint. Mind you: Any camera is subject to a paint ball gun
You can often find dummy cameras. These can give a bit of false security while you are waiting to afford the others, and they can mark the places where your cable comes through the wall.
Place one camera where it can read the license plate, both coming and going.
Place at least one camera where it can get a good face shot of someone leaving the house. These cameras are much more difficult to scout, and most thieves aren't actively looking for surveillance on their way out.
Best Answer
There's a couple of ways to play this, depending on how easy it is to get above/behind the mount point above the camera
Use a one-way screw. Not impossible to remove, but not doable with hand tools
Use a non-standard screw head. Star bits are a bit harder to come by than Phillips or straight. There's even some rather exotic heads, with the goal being security through obscurity
Mind you, there's nothing to stop a determined thief from drilling the fastener out and taking the camera (which would involve noise), but these ideas should make it more difficult than using a regular pocket screwdriver.