Skip to content

More POwEr: How Much Can Power Over Ethernet Deliver?

SecuritySpecifiers
This content was provided by SIA member company Security Specifiers.

Finally, it happened. On Sept. 27, 2018, the Institute of Electrical and Electronics Engineers (IEEE) formally approved IEEE 802.3bt-2018 and published it as an amendment to the 802.3 standard in January. Titled “Physical Layer and Management Parameters for Power over Ethernet over 4 pairs,” this amendment “represents a substantial change to the capabilities of Ethernet with standardized power. The power classification information exchanged during negotiation is extended to allow meaningful power management capability. These enhancements solve the problem of higher power and more efficient standardized Power over Ethernet (PoE) delivery systems,” according to the published abstract.

For me, the surprise was that the IEEE committee did not stop at the 60W High PoE (PoE++) level but went for the gold by taking it 50 percent higher. This may help to explain the length of time to approve the amendment. The enabler for this was bringing all four pairs of a data cable into play.

So how much power can PoE deliver? Depending on what you read, you may see 90W or 100W. Not finding a definitive answer through ordinary searches (Wikipedia was about the best), I purchased the standard and found that there is a real risk of oversimplifying.

First, some terminology and definitions. PSE is power sourcing equipment, which, as the name implies, is the device (switch or mid-span) sourcing the power over Ethernet cabling (or to a conversion device) to the equipment using the power, known as a power devices, or PD. What is available at the PD is less than the level of power sourced due to losses in the cabling. Both PSEs and PDs are characterized by four types and eight classes.

Type 1 corresponds to the original PoE, IEEE 802.3af, providing for power up to 12.95W at the PD. It encompasses four power classes (0-3). PSE voltage is 44.0 – 57.0V. Classes will be discussed a little later.

Type 2, known as PoE+, was first implemented in 802.3at, adding a power Class 4 up to 25.5W at the PD. PSE voltage is 50.0 – 57.0V.

Type 3, referred to as PoE++ or High PoE, providing up to 51W at the PD, is now defined in this new amendment. It covers power classes 5 and 6. PSE voltage is 50.0 – 57.0 V.

Type 4 provides up to 71W at the PD and encompasses power classes 7 and 8. Note that the PSE has to provide a minimum of 90 W at its physical interface and no more than 99.9W. So, this is where the 90W/100W applies, but the average power at the PD over a sliding one second interval cannot exceed the PD levels mentioned above. PSE voltage is 52.0 – 57.0V, which calculates to up to 1.7 amp total current supplied from the PSA over four pairs.

Several other significant changes were implemented, including

  • 4 pair operation – Type 3 operation allows voltage supply over data pairs, spare pairs, or all 4 pairs. Type 4 operation is 4 pair.
  • Multiple Signatures – A single-signature PD (Figure 3) is a PD that shares the same detection signature and classification signature between both pairsets. A detection signature is based on the PSE sending initial voltage to the PD’s 25 K ohm resistor to determine a valid PoE device and begin a classification process. Classification signatures are based on 2 subsequent “probing events” to establish a class for the device’s power needs. A dual-signature PD is a PD with an independent signature on each pairset; allowing each pair set to have fully independent classification and power allocations.
  • Power disconnect – A PSE may disconnect power from a PD if no usage has been detected for a period of 320 – 400 ms.

The main reason for a class system to allow power requirements to be negotiated for better management of the PSE’s power budget. A summary of power classes is presented here.

Single-Signature PDs

ClassPD Power Available (W)
013
13.84
26.49
313
425.5
540
651
762
871

Dual-Signature PDs

ClassPairset PD Power Available (W)
13.84
26.49
313
425.5
540

One of the potential installation concerns is the potential heat build-up in a cable bundle as higher currents are pushed through the conductors. NFPA 70 (aka National Electrical Code) in 2017 added section 725.144, Transmission of Power and Data, that, for Class 2 and Class 3 circuits, details the maximum allowable ampacity of cables based on the size of the individual conductors and the number of cables in a bundle. Further, Section 840.160 requires compliance with section 725.144 for PoE installations. A new Section 840.170 provides listing requirements for PoE power sources.


Also, higher current PoE systems may affect the conducting surfaces on RJ-45 connectors via electrical arcing during mating/demating. Arcing may lead to corrosion and pitting damage on the plated contact surface. Leviton has a good white paper available here.  Recommendations include using connectors designed to minimize arcing effects, connectors and patch cords with 50 µm gold-plated tines, as specified by ANSI/TIA-1096-A and ANSI/TIA-568-C.2 standards, and metal connector bodies for better heat dissipation.

And don’t forget back-up power for the PSE equipment to keep everything running or think about local device power in the event of PoE failure. UPS systems will need to be rethought with these increasing demands.

At the recent PSA Tec, I shared a panel with Chuck Wilson, executive director at the National Systems Contractors Association. Chuck pointed out the battle in some states around who installs higher power PoE circuits, with some thinking that it’s the province of electricians. As the power needs of some devices have fallen (LED bulbs) while PoE has gone higher, devices and equipment that were once electrical installs are now in the sights of low voltage contractors.

That said, the promise of PoE is undeniable, and it will be most interesting to see new PSE and PD equipment introduced to take advantage of it.

Copies of this amendment may be purchased through IEEE/Techstreet.

Ray Coulombe is founder and managing director of SecuritySpecifiers and the CONSULT Technical Security Symposium. Ray can be reached at ray@SecuritySpecifiers.com, through LinkedIn or followed on Twitter @RayCoulombe.

Back to Top