Some explanation about POE classes

I want to clarify meaning of  POE classes.
For example class 4 is defined in range min 12.95W- max 25.5W.

What really means min 12.95 W. ?

What is sense of declaring minimal power.?

For example our system should power 8-10W in normal case, but also 18 W

in some phases (battery loading).

Of course, we must select class4, but what for us stand 12.95W ?

In my opinion nothing.

My doubt is coming of definition class 0 (0-12.95W) and

also class 3 ( 6.xx W-12.95W). What is sense of definition class 3

when is covered by class 0.

Thank in advance

  • View the concept of classes in the light of multiport PoE sources.   The ability to alocate power based on declared class offers the benefit of more ports served for a fixed power source (i.e. cost per port).  The assumption is that an oversubscription results in an overload that results in all PDs loosing power (at least temporarily).

    Class 0 was implemented for those PDs that did not want to waste operating power, but chose to not implement a switched detection signature (cheap, maybe used as one per installation).  The range of classes is a roughly 2:1 set, and class 3 completes the set.  It was seen as a competitive advantage for PDs to use lower power (lower class) and to declare it, as the end implementer could extend the number of devices per multichannel PSE.  So for a 250W capacity, strictly speaking, a PSE could support 250/15.4 Class 0 PDs  or 250/4 Class 1 devices. 

    I believe that the spec states that the declared hardware class is the maximum the power the PD will draw from the PSE.

    With the advent of DLL class in .at, a PD may reserve and release differing power over time.  This will not be true for all PSEs.

    The presence of class 3 means that there is a class 2 and then class 1 ... This is a "binning" algorithm.

  • In reply to Martin Patoka:

    Thank you very match Mr. Patoka

    In you mail you probably thought 25 W (not 250W) ?

    As I understand you well, you agreed that  POE class4  (25 W) should

    suit for my case (nominal power will be 8-10 W and maximal 16-18W, very seldom) .

    Expecting your final confirmation, best regards.

  • In reply to Semir Fazlagic:

    See 802.3at or 802.3 base standard (if/after update to include .at).  Sections 33.3.2, 33.3.5, 33.3.6, 33.3.7 (esp table 33-18 and 33.3.7.2), and 33.6.

    It is always best to understand the base specification.  

    In case you don't have a copy:  https://standards.ieee.org/about/get/802/802.3.html

  • In reply to Martin Patoka:

    OK, thank you.

    From document seems very clear.

    Thank one more and best regards