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I would like to install 16 surveillance cameras All the cameras are connected to a DVR, 25 meters away (at most) All cameras are powered by 5V-DC, around 3W-6WS for each camera.

I have two strategies for the powering the cameras.

  1. Use standard 220V-AC already spread around and have a small AC-DC converter for each camera.
  2. Install a Main AC-DC (around 150W) near the DVR and connect all the camera with 12AWG wires.

There are many pros and cons for each strategy. I would like to get your opinion on the subject. Feel free to assume that 16 small power ACDC costs like one high power ACDC.

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  • \$\begingroup\$ Did you do the math for 25m (actually 50m, since there's also a return) of wire yet? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 28 '15 at 15:38
  • \$\begingroup\$ Since you mention that 220VAC is already "spread around", why not use some compact and very inexpensive mobile phone USB chargers, one for each camera? The halfway decent mobile chargers incorporate a DC-DC circuit, and provide an output within reasonable tolerances (between 4.7 and 5.3 Volts on the half dozen USB chargers I have around the house). The USB chargers typically work on universal AC supply voltage, such as between 95 and 265 Volts AC. Hence they won't be affected much by supply voltage variations. \$\endgroup\$ – Anindo Ghosh Jun 29 '15 at 8:53
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The closest table of conductor resistances I have to hand is for 50Hz AC resistances of metric wire. For the purpose of this exercise, assume that the DC resistance is approximately the same as the AC resistance at 50Hz.

Suppose each camera requires 6 watts at 5 volts. This is 1.2 amperes.

  • The a.c. resistance of 2.5mm² conductor (common household mains wiring size) is 9.01 Ω/km at 75° C.

  • You need 50 metres of it (25 metres each way), so the circuit resistance is 9.01 Ω/km × 0.050 km = 0.45 Ω.

  • By Ohm's Law, 1.2 amps × 0.45 Ω = 0.54 V of voltage drop.

  • Typically, a voltage supply is expected to be within 10% of the nominal voltage. In this case, 10% of 5V is 0.50V, so the voltage drop of 0.54 V is probably too much.

If you go up to 4mm² conductor, with 5.61 Ω/km resistance, the voltage drop is only 0.33 V which is probably tolerable.

Personally, I would be inclined to run 4mm² conductors at 5 VDC. Low voltage is less of an electrocution risk, and you can install it yourself. You should (must?) mitigate the risk of fire (from faulted circuit) by installing a 4 amp fuse on each circuit.

Running 240 VAC to each camera means that you would need an electrician to do the work, you need to buy 16 individual power supplies, and those power supplies will be in very inconvenient places when you need to replace them.

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