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My home has an old security system which has long been disabled.

I'd like to be able to use the wiring for the sensors to possibly provide DC power to some wireless IP security cameras in various areas. For example, at the end of one hallway is a motion detector formerly used by the system which I'd like to rewire for camera power.

The control box for the security system has wiring from the mains incoming for power, and a bundle of wires for sensors branching throughout the house. Would I be able to, say, replace the control box with a standard power outlet, attach a UPS, then attach the AC adapters for the cameras to the UPS, and (taking the wire lengths into account) extend the power cables accordingly?

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What's the gauge of wire? What's the current draw of the camera, or the current rating of its transformer? What's the voltage? Do you know (approximate) length of the wire? –  gregmac Jan 10 '13 at 17:27
    
The sensor wire looks like maybe 18 gauge stranded wire, and the camera's AC adapter shows 5V AC and 2 amp output. The length of the run for the motion sensor I'm guessing is around 20 feet. –  Darth Continent Jan 10 '13 at 19:22

2 Answers 2

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).

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The transformers primary voltage will affect the secondary voltage, which means you could be starting off below what the transformer specifies. If the voltage is too low at the primary, there are a few things that might help. Decreasing the length of run to the transformer, or increasing the wire size to the transformer. Both should reduce the voltage drop across the run, and maximize the primary voltage. –  Tester101 Jan 11 '13 at 12:56
    
Showing your math, and/or explaining how you calculated your values, might be useful. –  Tester101 Jan 11 '13 at 12:58

You might be able to, depending on the power requirements of the camera and the size (gauge) of the wire. If the wire is quite small and the camera demands a lot of current, it may not work. Length of wire is also a factor. The buzz-word for this is "voltage drop". Some electronic devices are less immune to voltage drop than other devices.

Some camera models can use "power over ethernet", so your ethernet cable can supply all your needs. Oops, you said wireless, so this paragraph doesn't apply to you.

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