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Why Do Manufacturers Produce PoE Switches If Used At Full Max Specified Will Be Over Drawn?

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Wolsey Colley
Oct 12, 2017
IPVMU Certified

I don't understand why manufacturers would produce PoE switches that will clearly be overdrawn if all the ports are used to power devices at the specified 802.3at and/802.3af. I just don't get it.

 

(1)
U
Undisclosed #1
Oct 12, 2017
IPVMU Certified

Most cameras don't use even half of the power of just 802.3af.

Power per port is arguably the most expensive item in a POE switch.  More power generally makes a heavier and noiser unit as well.

People don't want to pay for what they may never use.

Related: IP Camera PoE Power Consumption Measurements

JH
John Honovich
Oct 12, 2017
IPVM

Wolsey, good question. I agree with #1's answer. Moreover, switches are typically produced for general market usage where they expect a mixture of devices including some that require no PoE.

You did not explicitly mention NVRs with embedded PoE switches but the same pattern can be find there often. They typically expect that most cameras will be fixed with no heaters and blowers. In those cases, PoE consumption is typically 5w or less, only a 1/3 of max 802.3af standard.

This does leave out the small number of cameras that require PoE+ 802.3at but switch / NVR manufacturers assume those are uncommon and given the relatively high cost for those cameras, the user will purchase niche powering devices for them.

(1)
JH
Jay Hobdy
Oct 12, 2017
IPVMU Certified

Look in your house and look at the electrical panel. Probably a 200A panel. Now add up all the breakers. I am willing to bet way more than 200A. That is because it is never expected to use every circuit at full capacity at the same time.

 

Same concept here.

Cameras are only using their max wattage when everything is on such as IR, PTZ etc. Otherwise they are using minimal power.

 

Then clients would complain about expensive POE switches. Ubiquiti sells some of their switches in 2 models with different wattages being the only difference.

 

If you are using the entire power budget, you are probably the exception, not the rule.

 

(1)
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Mike Dotson
Oct 17, 2017
Formerly of Seneca • IPVMU Certified

When we were looking at systems with integrated POE capability to make part of our lineup, we had the same sort of question.
In the lab, when we were testing these, we would see an underpowered supply not handle turning on a set of POE cameras that had their IR LEDS active.  The dynamic current at turn on was the issue and some vendor cams did not like getting less that the expected 'min PD spec voltage' when they turned on.

We decided to design a build our own integrated POE system and we put a 500W supply in it JUST for the POE so that we did not have the dynamic current issue mentioned above and the steady state power is quite sufficient for 16 ports.

I even designed and build a POE Power test rig to pull 25+W from the ports to validate the PSU selected was up to the task.   I attached a pic of this for your amusement.POE-Power-test-rig

(1)
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Sean Nelson
Oct 17, 2017
Nelly's Security

They do make POE switches that have all ports rated at 30 watts (even when all ports are being used) if thats what you need though. Its not "normal" but its far from rare.

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