The Science Behind Voltage Drop 68

Voltage drop is a term that we hear about all the time in the surveillance industry. While many people talk about it and use the charts and calculators provided by different sources, I think a smaller amount of people fully understand the physics behind it. Today I want to give a quick review of the ideas behind voltage drop and then some practical tips for installing security cameras or any other equipment.

Voltage Drop for CCTV
Voltage Drop for CCTV

Equation

Here is the equation for Voltage Drop:

Voltage drop=( 2 * (length of run in ft.) * (resistance factor) *(current load in Amps))/1000

The length of the run is pretty simple; it is how long the cable is between the camera and the power supply.
The resistance factor is determined by the low voltage wire used and there is a chart in the NEC Chapter 9, Table 8 (rule of thumb, thinner wire will have higher resistance).
The current will be defined by the camera and will be found on the spec sheet.

Voltage Drop Example (300mA Security Camera)

Here is a quick example:  200 ft. run, 18 Gauge wire, 300mA camera
VD=2 *(200)*7.7 Ω *.3A/1000
VD=.924 V

Voltage Range

Video surveillance Cameras need a certain amount of voltage.  There is normally a range of ±10% of the rated voltage, so for this example we will say that a 12v camera can work in a range of 10.8v up to 13.2v (±10% of 12v is 1.2v). If a CCTV camera gets too much voltage, it will fry, and too little will not power it on.  So if the power supply is exactly 12V at the source, then it will drop .924V in 200ft and supply the camera with 11.076V, which is in the range of operation.

Voltage Drop

One thing to remember is that the number for the voltage drop is not dependent on the supplied voltage.  So if a 12V camera loses 2 volts, it wouldn’t power on because that is a 17% loss, but if a 24V camera loses 2 volts, it will work because that is only an 8% loss.  So 24V cameras can run farther.

So the final test in for your setup is determining the Voltage percentage drop:

VD%=( VD/Source Voltage)*100

From our example above:
VD%=(.924/12)*100
VD%=7.7% (which is in the range)

Wiring Tips

All of this stuff is neat and I hope that is explains the science behind it all little more. But in real life, you can just download a voltage drop chart or a calculator and use it without having to put a lab coat on.

Just remember:

Longer Run=more Voltage Drop
Thinner wire(higher gauge)=more Voltage Drop
Larger Amps(more powerful camera)=more Voltage Drop
Higher source voltage=Less Voltage Percentage Drop

For Installers

Final point, there are times in which installers have called in because a camera is not working and they claim that the wire is supplying enough volts. The problem with that is that they unplug the camera and connect a Voltmeter to the two wires. The Voltmeter has a very low amp draw, and will show less voltage drop because of that. Once the camera is reintroduced in the circuit, the voltage drop will change, making the measurement ineffective. So we need to run the numbers on paper to determine the drop.

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