High-power PoE considerations: What you need to know

By ROY KUSUMA and MATT GENTILE, General Cable --Power over Ethernet (PoE) technology has been quietly gaining traction in the market ever since its launch in 2003, especially in Voice over Internet Protocol (VoIP) telephony and IP surveillance cameras. Today’s PoE landscape has effectively split into two broad categories: The first being non-standard applications with higher power delivery capabilities, such as Cisco’s UPoE 60W and HDBaseT’s 100W; and the second, IEEE standards-compliant 24.5 W 802.3at. These non-standard applications create a significant paradigm shift in PoE possibilities. Power delivery through 1000 mA per pair and beyond is looming on the horizon. Steps will be necessary to ensure continued compatibility with this rapidly changing environment. To address this, the Institute of Electrical and Electronics Engineers (IEEE) is actively working on 802.3bt with expectations of powering schemes up to a maximum of 100W. This standard is expected to be ratified in 2017.High-Power PoE Considerations When contemplating a major increase in power, there are many key considerations. In order to go from a four-pair 60-watt output to a 100-watt output the current load changes from 600mA/pair to 1000mA/pair when the voltage of the power sourcing equipment (PSE) is kept at 50 volts. At such a drastic current capacity change, issues such as heat generation, power losses and safety protocols at the equipment end need to be studied. Heat generation, in particular, can be a serious issue for cables as highlighted in the figure below.

Figure 1: Comparison chart of 600mA/pair (60 W at 50V) versus 1000mA/pair (100 W at 50V) on typical category 5e and 6 cables. Note that this is the heat rise over ambient temperature. Due to concerns about increased temperature rise, IEEE commissioned the Telecommunications Industry Association (TIA) to not only define expectations and details of the new PoE system, but also peak operating parameters and equipment requirements. The excess heat generated from cabling systems (not designed for the increased power consumption) can cause insulation and cabling heat-aging degradation as well as data transmission attenuation issues.
Testing heat rise In order to provide guidelines for 802.3at standards, TIA had previously developed a method of testing heat rise comparison performance in current-carrying category cables for the initial release of TIA TSB 184:2009.

The figure above shows the configuration for a 91-cable bundle. The trial consists of powering each conductor (within this bundle) with a test current and measuring the temperature rise of the center cable at a steady state temperature. By assuming a 45°C maximum ambient temperature and a generalized cable operating rating of 60°C, they allowed for 15 degrees of heat rise for durability and effective data cabling use.Category cabling option and heat rise The conventional recommendation of PoE systems is that increased power will relate to increased category cabling. For example in 802.3at PoE+, category 5e cabling is the most basic construction, which provides sufficient conductivity without heat generation issues, therefore it became the minimum requirement. The same recommendation by TIA and the IEEE task force can then be expected with 802.3bt with the specified 600mA/pair maximum. The reduced heat generation of higher category cables is inferred mostly from the more stringent attenuation requirements of higher category cabling, causing cable manufacturers to increase conductor size. For example, a typical category 5e cable is constructed with 24 AWG conductors, while typical category 6A has 23 AWG conductors. However, now that 1000mA/pair is a real possibility, it will redefine what is acceptable for PoE between 60 and 100 watts. To better demonstrate the differences between these cabling types with the high-current load, we have performed an extensive analysis on the difference in heat generation.

Figure 2: Observed temperature rise over ambient temperature of different category cables in a 91-cable bundle with 1000mA/pair power. The data presented above is open air. However, in many cases a large cable bundle may be present under the floors, behind walls or enclosed in an insulated space. In the last circumstance, the heat rise figures are significantly worse with numbers as high as 50°C above ambient temperature for the typical worst-case category 5e construction. It is quite clear that heat generation becomes a real issue in many of the most common cable constructions installed (or available) today. However, it is not an ideal assumption to use category designation as a PoE classification rating, because the majority of the heat generated from running amperage is due to conductor size or the presence of a shield. Currently, in applications where higher power usage is expected in excess of 50 watts, yet higher data transmission rates are not required, there are few, if any, options. To appropriately address the increased temperature rise one could be confined to using category 6A or category 6A F/UTP. The limiting factor of many devices, especially IP cameras, nurse call systems, building management controls and point-of-sale is power consumption, not data. This places a difficulty on premise designers to justify the use of higher category cabling or the need to pull a power source onto a location.Cables specifically designed for PoE General Cable has created a new product line of EfficienC Max cables, designed for high-powered PoE applications with superior performance on insertion loss, heat rise and temperature capabilities.Specifically designed with larger conductors, these EfficienC Max cables offer reduced resistances, which directly reduces the amount of heat generated within a current-carrying cable. The large conductors also provide improved attenuation performance (insertion loss) to further protect against higher temperature data transmission losses. General Cable’s EfficienC Max cable is also rated to 90°C and constructed of 100% fluoropolymer insulation, offering higher protection against increased operating temperatures and preventing material degradation from elevated temperatures over extended periods.