Subscriber Discussion

How To Power A PTZ Camera 400 Meters Away?

one of my customer has a ptz camera at 400 meter which can only be powered by external 24vac. It is an oil n gas application and they are not allowed to pull 230vac to the camera. Can u suggest an option for me how I cam power the cam ?

Hello Samuel:

You can push 24 VAC over 400 meters, as long as your power supply accommodates for the voltage drop.

1. Find Camera Amperage

First, you need to establish how much current your camera pulls during operation. This can be determined from the listed wattage on the spec sheet:

I = P(W) / (PF × V )

Where I = Amps,P(W) = Watts, PF = Power Factor (We'll assume it's 1), and V = volts. Therefore:

.291 A = 7 /(1 x 24)

The example above assumes a camera is listed at 7 W, yours will differ. It pulls about 0.3A.

2. Determine Wire Resistance

From there, we can use this formula to determine 'Voltage Drop' due to the resistance of the wire and distance of run:

Ohms x (Wire length/1000) x Current = Voltage Drop

Luckily, 'Wire Resistance' is a standardized value for copper cable, and those factors are commonly provided by cable manufacturers. Here's a table of the most common gages used in Low Voltage work:

3. Calculate Voltage Drop

So for our example, we'll use 18 gage wire over 1350 feet (~400m):

1350 feet of 18 gage wire with a camera that draws 0.3A plugs into this formula:

Voltage Drop = (Wire Resistance) / ((Distance)/(1000)) x (camera amps)

2.6 Volts Dropped = 6.51 x (1350/1000) x .3 Amps

This means your power supply needs to nominally deliver ~+15% over 24VAC to power the camera

4. Specify the right Unit

Luckily, most supplies are rated to deliver a voltage within a range, and some are built with this loss in mind. Take for example this Altronix unit:

This particular unit will perform the task of our example, with voltage drop and amperage considered. It costs about $120 USD online. However, like I noted up top, your camera's amperage draw makes a big difference.

So making a long story short: No 'special' power supply is needed - just one that's properly specified!

Brilliant Brian,

Thanks a ton!!

Adding to Brian's comment, power supply capability is only one factor. The other is the wire gauge of the power cable. As he showed in his Wire Resistance Chart, the larger the conductor size (lower gauge), the lower the voltage drop. While I would not agree on his example of 0.3A power draw by a PTZ, the math is correct.

Pelco has a pretty good chart at Pelco Wire Gauge Table that can be used to determine the correct gauge of wire based on distance and power draw. It also takes into account power supply voltages of 24VAC and 28VAC but it only goes to 1,000 feet (400 meters equals 1,312 feet). Typical draw of a Pelco Spectra non-environmental (no heater) PTZ is around 20VA, so you would need 10-12 gauge wire. Environmental Spectras draw on the order of 60-80VA when the temperature drops and the heater kicks on. If that is what you are deploying, you would need very large diameter wire (probably around 8 gauge or larger).

On that basis, I would recommend locating the power supply closer to the PTZ.

Hello, Carl:

Thanks for the comment. My example of 7W draw wasn't intended to depict a PTZ, it was just a general example of what is typical for 'a camera'.

Indeed, the PTZ the OP is working with draws ~60W at full operation. Wow!

By the way, that is another issue to take into consideration. Environmental enclosures have fairly large swings in power requirements. An environmental Pelco Spectra HD PTZ, for example, only draws 23VA when the temperature exceeds the thermostat's setting, which is nominally 10 degrees celsius (50 degrees farenheit). When the temperature drops below that, the PTZ draws substantially more. That can cause wild voltage swings at the PTZ, especially at longer distances, where the voltage drop of the cable under differing loads varies more.

Pelco Spectra HD spec below:

24 VAC 23 VA nominal (without heater and blower);
73 VA nominal (with heater and blower)
I've run across this a few times where the wire gauge was chosen poorly. In those cases, the PTZ functioned properly on warm days but stopped working at night or when the ambient temperature dropped.

Run 12 gauge solid copper with a ground wire as if you we're going to run 230vac. Don't use stranded

If installing in an oil/gas area check if you must follow explosion proof regulations

Mark, 12 gauge will not provide sufficient power at 400meters (1,312 feet). At approximately 1.6 ohms per 1000 feet, the resistance of each conductor would be 2.1 ohms or 4.2 ohms for both conductors combined. 60 watts is approximately 2.5A so 2.5*4.2=10.5V drop for 12 gauge wire. 24-volt PTZ's won't operate at 13.5 volts. Even if the P/S supplied 28 volts, the PTZ would only see 17.5 volts. Most 24V PTZ's require a minimum of 18 volts (Pelco Spectras are rated at 18-32VAC) and I wouldn't recommend less than 20 volts to prevent intermittent operation.

10 gauge wire would have a total resistance (sum of both legs) of approximately 2.6 ohms (1.3 ohms for each conductor). 2.5*2.6=6.5 volt drop. While a 24VAC supply would only provide 17.5 volts to the PTZ, a 28VAC supply would provide 21.5 volts - sufficient to assure operation.

Now, the big question: how do you run 10 gauge power wire 400 meters?

You are correct Carl about the 10 gauge My fat thumb one finger typing error