Selecting the Cable for Power over Ethernet (PoE): Which Factors Should Be Considered?

Power over Ethernet (PoE) is a data transmission technology that has gained immense popularity since the 1990s. It enables data transmission in the form of electronic signals via twisted pair cables, i.e. the copper cables. One of the key advantages of this technology is that a single cable can transmit communication data and electric power at the same time. Out of which, the data packets are transmitted between a transmitter and receiver, whereas the electrical signal powers devices such as Wireless Access Points (WAPs), Voice over Internet Protocol phones (VoIP), Internet Protocol (IP) cameras, etc.

Although PoE appears to be a simple technology, it is actually quite complex. When it comes to the application of PoE, there are several factors that make it complex. Out of such factors, the selection of appropriate cable for PoE transmission is a crucial process. Any inappropriate cable selection can lead to performance errors, heat generation, and a drop in efficiency of the PoE network. Therefore, when it comes to selecting cables for power over Ethernet (PoE) transmission, one must consider several factors. This white paper discusses how PoE technology operates, the significance of cables, and how to choose PoE cables along with several other factors of consideration.

Learn About Data Transmission via PoE Cable

The PoE technology enables simultaneous transmission of data and electric power via a single cable. To achieve dual transmission, two pairs of copper cables are utilized. One is used for data transmission and the other is used for electric transmission. When the simultaneous transmission is performed, it is essential to ensure that electric signals and data packets do not interfere with each other. This no-interference dual transmission is called phantom power. This phantom power is established when a stream of data passes via conducting copper on the application of current. Differential signaling takes place under phantom power, which establishes transmission via Power over Ethernet (PoE) protocols.

Due to phantom power transmission, heat generation takes place in Ethernet cables. To perform effective heat dissipation, an appropriate Ethernet cable selection must be done. The next section offers you insights on selecting cables for power over Ethernet.

Selection Criteria of Cables for Power over Ethernet (PoE) Transmission

A safe PoE operation is an outcome of different combining factors. Of these, the five important ones are - temperature ratings, conductors, cable material, cable design, and installation mechanism. A slight deflection in any of these fundamental factors can lead to an efficiency drop in the PoE transmission network. In extreme cases, it is possible to result in abrupt damage to the PoE transmission network. Therefore, consideration of the following five factors is essential while selecting cables for Power over Ethernet (PoE).

• Temperature Rating: The temperature rating means the highest amount of temperature that can be sustained by the cable’s jacket or shielding. The cables for PoE transmission should have more than 60˚C of temperature rating. A higher temperature rating is essential as the higher the temperature rating, the higher is the power dissipation via the copper cable. Therefore, while selecting cables for Power over Ethernet (PoE), the temperature rating of the cable must be checked. Commonly PoE cables with 60˚C, 75˚C, and 90˚C are the temperature ratings available in the market. However, temperatures over 200˚C of temperature rating can be achieved in PoE cables.
  Keeping the temperature range of the cable between 50 and 90 degrees Celsius is good for the network performance and long operation life of cables. If the temperature gradient of the cables is crossed, that means if the generated temperature in the cable exceeds the temperature gradient, then the electrical signal transmission degrades. Also, if the operating or environmental temperature exceeds the allowed temperature rating, it can affect the longevity and performance of the cable. If your application demands more energy, high gauge cables such as Cat 6e are useful in reducing energy wastage and resistance. This is especially useful in applications involving high working temperatures and closed environments.
• Size of a Conductor Gauge:  The conductor resistance is one of the factors responsible for heat generation in PoE cables. The conductor resistance is higher in small gauge diameter cables, which generate a high amount of heat. Thus, the purpose behind checking for a conductor gauge size is to reduce heat generation or increase heat dissipation through the cable body. To achieve this, a larger connector gauge diameter must be selected. A connector with a larger diameter increases the heat dissipation across the length of the PoE cable. The heat is generated due to connector resistance or insertion losses, and the generated heat is dissipated throughout the cable.
  It is always ideal to choose Cat6 or Cat7 PoE cables instead of Cat5e cables. Cat6 cables are known to have about 80% lesser connector resistance (DCR) as compared to the Cat5e cables. Therefore, cables with a larger diameter, lesser conductor resistance, lesser heat generation, and higher heat dissipation must be considered for PoE applications.
• Cable Construction:  The construction of copper cables contributes to heat generation and dissipation.
  There are two common types of copper cable construction, namely, unshielded twisted pair cable (UTP) and shielded twisted pair cable (STP). Generally, STP cables with foil or metallic shields are known to offer higher heat dissipation than UTP cables.  The higher the heat dissipation, the cooler is the cable. Therefore, selecting a category 6 or category 7 STP cable with foil or metallic shields are advised.
  Network cables such as Cat 5e and Cat 6 are commonly used. These cables comprise eight wires which are four twisted pairs. Two of these are called data pairs as they are used for data transfer. The other two twisted pair cables transfer electrical signals through them. Out of the four twisted pairs, POE can use the data pairs for either of these two functions.
• Installation Mechanism: The type of installation or connectivity mechanism of the copper cable determines thermal resistance and insertion losses. The installation configuration is expected to have low insertion loss, easy connectivity, and a good heat dissipation factor. Inappropriate use of installation mechanisms can result in conduit effect, and trapped heat which can further damage the physical and operational performance of the cable.
  For instance, cables connected to POE switches in the middle of a bundle produce more heat than those on the outer sides of the bundle. This happened more in closed environments as heat radiation reduces. So, two aspects are important in this case— a proper ventilation system in the environment and loosening and segregating large cable bundles.
• Cable Material: When it comes to selecting a PoE cable, one can get two common choices, namely, copper clad aluminum (CCA) and pure copper cable. The pure copper cable has been traditionally used in PoE applications and is expensive. The copper-clad aluminum cables are economic, however, these cables may lead to interruptions during PoE transmission due to inferior materials used. The copper-clad aluminum cables generate high resistance, which leads to radiant heat generation in the long run. On the other hand, pure copper cables offer less resistance and less heat generation, therefore, it can be used for a longer run without any impact on data transmission speed. Selecting the correct cable material for the PoE applications can help in enhancing PoE transmission efficiency.
  Ethernet cables such as SPF Cat 6 and PoE switches are crucial to build this network. Appropriate type of cable, their structure, and installation directly affect the quality, performance, and speed of the network.

After knowing these useful tips for selecting PoE cables, it is also essential to consider the application-centric factor too. Let us discuss some factors that must be considered for cable and application compatibility.

Application-centric Considerations for Selecting the Cable for PoE

The following application-based factors must be considered while selecting a cable for PoE applications.

• IEEE 802.3af-2003: This is the first PoE standard defined by the Institute of Electrical and Electronics Engineers (IEEE). Under this standard, the PoE standards the PoE devices are powered with up to 15.4-watt power at the voltage range between 44 to 57 DCV and 350mA. Under this standard 12.95 watts of power can be transmitted via PoE copper cable.
• IEEE 802.3at-2009: This is the first updated version of IEEE standards. This standard is also known as PoE+ or PoE plus IEEE standard. As an upgrade, under this standard, the PoE devices can be powered with 25.5 to 30 watts of power at 50-57 DCV. This standard prohibits the use of all 4 pairs of power cable. Also, the PoE devices that are compatible with this standard feature backward transmission as well. It is commonly used for type 2 PoE devices.
• IEEE 802.3bu-2016: This standard was established for single-pair Power over Data Line (PoDL) signal transmission. It can be used for Ethernet cables like 100Base-T1 and 1000Base-T1 cables for automotive and industrial applications.
• IEEE 802.3bt-2018: This is the latest IEEE standard for PoE devices. It is suitable for type 3 and types 4 PoE devices. It offers two versions individually for type 3 and types 4 PoE devices. For type 3 devices, it enables 60watts power transmission whereas, for type 4 devices, it enables 100 watts of power transmission. This standard is suitable for 2.5GBase-T, 5GBase-T, and 10GBase-T cables for high-performance PoE applications.

• Data Transmission Requirements: Various data transmission requirements like transmission rate, supported transmission length, etc must be considered while selecting a copper cable for PoE application. For short-distance transmission over PoE protocol, Cat5 cable might suffice. However, digital devices like high-megapixel IP cameras may require higher transmission grade cable like 1000Base-T. Generally, for moderate efficiency applications, Cat6 and Cat6a are used. For the high wattage PoE supplication support, 2.5 to 5 GB Ethernet data rates are required, therefore HDBase-T cables are preferred. In the case of supreme efficiency, premium applications, Cat7 and Cat8 cables are preferred.  Category 8 is the latest and the most upgraded type of copper cables, which offers robust performance with a maximum transmission rate of 25GB at stable channels.
• Power Consumption: The power consumed by the PoE devices throughout the operation decides the PoE standard of cable to be adopted. Each standard of PoE cables has its minimum set of requirements out of which power consumption is the one. Therefore, to know what standard of PoE cables to be used, it is essential to know estimated power consumption as each cable is designed for a specific power consumption capacity.
• Power over Ethernet (PoE) Standards: The power over Ethernet (PoE) is defined and updated by the standards stated by the Institute of Electrical and Electronics Engineers (IEEE). These standards define significant features of PoE transmission protocol and PoE devices. The following are the PoE standards to be considered while selecting PoE cables.

Conclusion:

PoE technology has revolutionized the networking industry. Although it may seem simple, choosing the right power sourcing equipment and cables is crucial. This white paper explains the complexities of PoE technology and the factors that impact cable selection. At Versitron, we are dedicated to helping you with your networking needs. If you need assistance with selecting the right PoE switches or have any questions regarding our products, please request a quote from us. Visit our website at https://www.versitron.com/pages/contact/ to get started.