Basic PoE Power Facts and Design Recommendations

Important notice to readers: This post is only about PoE power with active endpoint detection. It intentionally does not address passive PoE power, which is often used by ISPs to power wireless links. The main disadvantages of passive solutions are:

  • Inability to detect the end device (the source does not know if there is a PoE compatible device at the end of the cable)
  • the inability to centrally monitor the power system using SNMP protocol

PoE power supply provides huge advantages due to its low demand on the cabling used. Unlike conventional power systems, it does not require separate cables for data and power. In addition, the communication between the power supply and the powered device provides short-circuit protection and, last but not least, easy monitoring of the entire power system via SNMP.

Since the ratification of the first PoE standard in 2003 (IEEE 802.3af), its use has expanded dramatically into a wide range of new applications. As time has progressed, however, its limiting factor has proven to be its maximum power rating of 15.4W/port. While the power was sufficient for conventional fixed cameras and IP phones, it was limiting for IP cameras with IR illumination, PTZ cameras, videophones, and other devices. Depending on the pairs used for power transmission, the standard distinguishes between PoE mode A (over data pairs) and mode B (over spare pairs). Each PoE-PD (Powered Device) should support both modes, A and B, according to the standard. In practice, it sometimes happens that camera manufacturers try to save money and only fit components for PoE A. For PoE-PSE (Power Supply Equipment) devices (PoE switches and injectors), it is sufficient to support one mode. However, all our LAN-RING BOX series switches support both A and B modes.

For this reason, the IEEE has issued a new standard, IEEE 802.3at 2009, with a power rating of up to 30 W per port. Each PoE-PD (Powered Device) should again support both modes A and B according to the standard.

But the development went on rapidly. Outdoor PTZ security cameras, POS terminals, LED lighting, 802.11ac and 802.11ax access points and other devices requiring more than 30W to operate began to proliferate on the market. High demand usually triggers a battle of different standards. The IEEE organization continued to work on a new standard, which was ratified in September 2018 under the designation IEEE 802.3bt with a maximum delivered power of 90 W/port. But back in 2011, the HDBaseT Alliance created the Power over HDBaseT (PoH) standard, which allows for a maximum power delivered of 95 W over four Cat5e pairs.

This process results in 2 mutually incompatible power standards, 90W PoE according to IEEE802.3bt and 95W PoE according to POH HDBaseT Alliance. All METEL LAN-RING PP switches with hardware design 2020 support both solutions, see table below.

 LAN-RING PP / 2020 hardware design - overview of supported PoE power modes
Typ Standard PoE-PSE maximum delivered power Maximum power consumption for PoE-PD
 Cable category Cable length Number of pairs for power transmission
 Minimum voltage at PoE PD input
1 802.3af 15.4 W 12,95 W Cat5e 0 - 100 m 2 37 V 
2 802.3at 30 W 25,5 W Cat5e 0 - 100 m 2 42.5 V
3 802.3bt 60 W 51 - 60 W Cat5e 0 - 100 m 4 42.5 V
4 802.3bt 90 W 71 - 90 W Cat5e 0 - 100 m 4 41.1 V
POH POH 95 W 95 W Cat5e 0 - 100 m 4 38.125 V

 

UPOE

These are basically 2 separately detected and classified Class 4 devices. Typical examples of UPOE are outdoor PTZ cameras with PoE heating or fixed cameras in enclosures with PoE heating.

PoE power supply according to IEEE802.3bt

The transmission of high power over UTP/FTP/STP cables places increased demands on their quality. Therefore, only cables with copper conductors of category Cat5 and higher should be used to ensure proper operation. For example, CCA cables with aluminium core conductors and copper coating are completely unsuitable. The table below shows the results of power and voltage loss calculations for different input voltages and cables for convenience.

The calculation further shows the advantage of higher input voltage. That is why we use power supplies with regulated output in the minimum range of 48-56 VDC in OH switchgear.

Approximate loss calculation for Cat5e CCA cables with a conductor resistance of 130 Ohm/km
Distance between PoE PSE and PD PoE-PSE output voltage Power delivered by PoE-PSE Total loop resistance Current through the loop Losses on the line Input voltage PoE-PD Minimum voltage at PoE-PD input according to the standard
10m 56 V 90 W 0.65 Ω 1.61 A 1.04 V / 1,68 W 54.96 V 41.1 V
10m 48 V 1.88 A 1.22 V / 2.29W 46.78 V
100m 56 V 6.5 Ω 1.61 A 10.45V / 16.79 W 45.55 V
100m 48 V 1.88 A 12.19V / 22.85 W 35.81 V

 

Approximate loss calculation for all-wire Cat5e cables with a conductor resistance of 93.8 Ohm/km (BELDEN 1594A)
Distance between PoE PSE and PD PoE-PSE output voltage Power delivered by PoE-PSE Total loop resistance Current through the loop Losses on the line Input voltage PoE-PD Minimum voltage at PoE-PD input according to the standard
10m 56 V 90 W 0.47 Ω 1.61 A 0.75 V / 1.21 W 55.25 V 41.1 V
10m 48 V 1.88 A 0.88 V / 1.65 W 47.12 W
100m 56 V 4.69 Ω 1.61 A 7.54 V / 12.11 W 48.46
100m 48 V 1.88 A 8.79 V / 16.49 W 39.21 V

 

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PoE power according to HDBaseT Alliance

The POH power standard was published by the HDBaseT Alliance back in 2011. It is based on the IEEE 802.3at standard and allows up to 100 W to be safely transmitted over an Ethernet cable. It was published many years before the competing IEEE802.3bt standard was published, so it is used by the vast majority of IP cameras with power consumption above 25.5 W, in addition to many audio-visual devices.

Approximate loss calculation for Cat5e CCA cables with a conductor resistance of 130 Ohm/km
Distance between PoE PSE and PD PoE-PSE output voltage Power delivered by PoE-PSE Total loop resistance Current through the loop Losses on the line Input voltage PoE-PD Minimum voltage at PoE-PD input according to the standard
10m 56 V 95 W* 0.65 Ω 1.7 A 1.10 V / 1.87 W 54.9 V 38.13 V
10m 48 V 1.98 A 1.29 V / 2.55 W 46.71 V
100m 56 V 6.5 Ω 1.7 A 11.03 V / 18.71 W 44.97 V
100m 48 V 1.98 A 12.86 V / 25.46 W 35.14 V

 

Approximate loss calculation for all-wire Cat5e cables with a conductor resistance of 93.8 Ohm/km (BELDEN 1594A)
Distance between PoE PSE and PD PoE-PSE output voltage Power delivered by PoE-PSE Total loop resistance Current through the loop Losses on the line Input voltage PoE-PD Minimum voltage at PoE-PD input according to the standard
10m 56 V 95 W* 0.47 Ω 1.7 A 0.8 V / 1.35 W 55.2 V 38.13 V
10m 48 V 1.98 A 0.93 V / 1.84 W 47.07 V
100m 56 V 4.69 Ω 1.7 A 7.96 V / 13.5 W 48.04 V
100m 48 V 1.98 A 9.28 V / 18.37 W 38.72 V

* 95W is the maximum power supported on PP switches with hardware designed in 2020

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