The latest Power over Ethernet Standards, IEEE 802.3bt-2018, enable up to 90W (71.3W delivered) to be carried over standard twisted-pair cabling. This development has exponentially expanded the potential for PoE beyond the typical realms of IT. 71.3W is enough to drive motors, such as those found in window shades or stand-sit desks, and is even enough to charge a laptop or power AV equipment. A PoE-powered smart building now has a huge scope of systems and equipment to make use of.
Power over Ethernet uses much of the same basic components as unpowered Ethernet. As we know, when implementing a structured cabling system, the choice of cable Category is essential. But delivering both data and power on the same cable inevitably has an impact on how we choose our components.
Cabling Category
Category 6A cabling is superior to Cat6 and Cat5e in many ways, but when it comes to PoE performance, the Category is not important.
What are the differences between Cat6A and Cat6? Cat6A can support data transfer rates of up to 10G/s at a maximum bandwidth of 500MHz – substantial improvements over Cat6’s 1G/s and 250Mhz. The copper conductors in Cat6A are twisted tighter, making the cable most robust and resistant to crosstalk. Cat6A also has additional shielding built in – even in U/UTP cables – eliminating alien crosstalk. While these improvements are significant to the transfer of data, none of them affect the rate or ease at which the cables carry power.
For PoE, the gauge is far more important than Category. Gauge is a measure of the physical thickness of the cable. The thicker the cable, the less resistance there is to the flow of electricity, and therefore the better performance. AWG22 cable will always perform better than AWG23 or AWG24 and certainly better than AWG28, and an AWG22 Cat5e cable will perform roughly the same as an AWG22 Cat6A cable – from a strictly PoE standpoint.
Many IoT devices, such as sensors and actuators, do not require large amounts of power or data. So when designing a cabling system that is exclusively intended for PoE – for example, a smart building system or IoT sensor network – many will choose to opt for a higher-gauge Cat5e.
When to use Cat 6A
However, it’s not the case that Cat5e is sufficient for every PoE application. The major example is when designing a structured cabling system architecture that will also be required to carry PoE (not a standalone PoE network). Here Cat6A is required, because you need to comply with both PoE Standards and structured cabling Standards.
For structured cabling, the relevant Standards are ANSI/TIA-568.0-E:2020; ANSI/TIA-568.1-E:2020; and ISO/IEC 11801.x:2017. There are equivalent EN and AS/NZ Standards for those working outside the Americas.
For PoE, the relevant Standards are ANSI/TIA-184-A:2017 and addendum A1:2019; ISO/IEC TS 29125 Ed 2.0:2014; and IEC 60512-99-001.
The Standards are clear that your design must be universal, transparent to any ratified protocols, flexible and future-ready. The critical clause from ANSI/TIA is A.2:
“ Minimum cabling Category - Category 6A or higher performance 4-pair balanced twisted-pair cabling as specified in ANSI/TIA-568-C.2 and ANSI/TIA-568-C.2-1 is recommended. Category 5e or higher cabling is acceptable for existing installations.”
This clause is very clear in precluding the installation of Cat 5e or Cat 6 cables for new structured cabling installations.
There are a few reasons for this Standard’s insistence of higher Category cabling:
AVoIP. Converging audio-visual solutions onto an IP network requires high performance cabling. New installed links should be able to support SDVoE, which requires up to 90W of power and 10G/s, and HDBase-T, which requires up to 100W of power and 10G/s. This automatically demands the use of Cat 6A.
Wi-Fi 6 / Wi-Fi 7. New installed links should be able to support Link Aggregation for WAPs. Again this requires up to 90W of power and 10G/s, demanding the use of Cat 6A.
New high power PoE devices. New installed links should be able to support 10G Base-T and power for devices which require higher power and data transmission such as pan-tilt-zoom IP Cameras, high-power WAPs, Point of Sales (PoS) readers, Video Conferencing, TVs, Nurse call, Info kiosks, Access control, Laptop computers, etc… – Again, all of these require up to 90W of power and 10G/s – Which demands by default the use of Cat6A.
Using Cat6A to manage heat buildup
When designing a PoE network, particularly for higher powered systems, it is essential to calculate and counteract the build up of heat. Excessive heat impacts the rate of flow of both power and data, and can cause permanent damage to components. Managing heat is primarily a matter of ensuring that there is sufficient airflow around cables or cable bundles, to allow the heat to escape. Smaller bundle sizes, open cable trays and air conditioning can help. The type of cable is also relevant, as Category 6A and / or shielded cable types offer superior heat dissipation qualities. For the purposes of managing temperature rise the use of these cable types may therefore be beneficial.
ISO/IEC TS 29125:2017 recommends that the maximum temperature between the center of a cable bundle and the outer edge should be no more than 10°C / 50°C.
When to build PoE into an SCS
If cheaper Cat5e can be used for a PoE-only network, for example for IoT and smart building applications, is it possible to create a PoE network separate to the structured cabling? From a cabling perspective this is entirely possible, and there are some potential benefits to this solution.
If the PoE switches are in the field and the cables go back to local zone enclosures, a Cat5e could be an acceptable solution. This would reduce the cost of materials and installation and result in separate networks for the PoE elements and structured cabling.
If all the equipment was to be housed in the comms room, it would make sense to look at a total Cat6A solution. This would provide increased flexibility and future-readiness to the network, but would increase the size of comms room and the heat load, both in the comms room and in powered cable runs.
Ultimately which is the best solution for any given installation will depend on the situation and the complexity of the PoE network.
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