PoE Confusion

S

Stratman

n3wb
May 27, 2016
3
0
Louisiana
I've Googled for hours and I'm still not clear on PoE voltages. I understand the current draw and VD over distance but can't figure out how what voltage is safe to use.

I'm installing 4 bullet cams using CAT5e copper and maybe a PTZ at a later date. The longest run may be 75Ft. The cam specs say 12v but I see lots of 48v PoE switches. The VD over 75Ft of CAT5e of course drops the voltage at the camera to 9.6v.

If I use a higher voltage PS will the camera step it down to 12v or should I look for a PS of 15-16 volts to compensate for the VD?

Maybe I'm overthinking this but I haven't found a black & whit answer.
 
Stratman said:
I've Googled for hours and I'm still not clear on PoE voltages. I understand the current draw and VD over distance but can't figure out how what voltage is safe to use.

I'm installing 4 bullet cams using CAT5e copper and maybe a PTZ at a later date. The longest run may be 75Ft. The cam specs say 12v but I see lots of 48v PoE switches. The VD over 75Ft of CAT5e of course drops the voltage at the camera to 9.6v.

If I use a higher voltage PS will the camera step it down to 12v or should I look for a PS of 15-16 volts to compensate for the VD?

Maybe I'm overthinking this but I haven't found a black & whit answer.
Click to expand...

If your camera is compliant with PoE standards 802.3af, or 802.3at, you just need the PoE switch to also be standards compliant. 802.3at is capable of delivering more power. The distance is usually limited to 100 meters for the standards. Also, make sure the CAT5e cable you use is solid copper, not CCA.
 
The device will only draw the power needed. The switch will not push all 48v to the camera. As long as both devices are standards compliant, there will be a handshake at connection. This need determination also allows non-poe devices to be connected to the switch without voltage overload. Switch wattage will need to be sufficient for the cameras plus the potential ptz added later.
 
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The 802.3xx standard POE voltage is around 50, and that is what is supplied by standard POE switches. When a POE-compliant camera says it uses a 12 volt supply, that refers to when it's powered using its separate power connector instead of via the network cable. The camera "probably" converts the ~50 volt POE power to 12 volts to power itself.
 
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