802.3bt 100w PoE - its benefits and implications for cabling

As Power over Ethernet (PoE) continues to grow in popularity, so does the demand for applications with higher power. The introduction of the new PoE standard IEEE 802.3bt, meets this demand by doubling or perhaps as much as tripling the amount of power delivered to the end device. The previous standard enabled power transmission over just two pairs of a four-pair cable, whereas the new standard takes advantage of all four pairs, spreading the flow and allowing up to 100W of power transmitted.

What are the benefits of the new standard and what has changed?


PoE's application base will increase enormously with the increase of power to the end device. 

The main advantage for design engineers is based mostly on the saving in installation costs through the delivery of power and data on the same link compared to running on separate lines. This makes the installation and relocation of devices as simple as moving a standard CAT5/6 cable, instead of requiring an electrician to move ac power sources.

While the need to provide more power to the end device was the aim of the new standard, IEEE 802.3bt will also enable more efficient operation.

One of the downsides of the transition from 13 W per port to 25.5 W per port is that cable losses increased exponentially. The most effective means of improving efficiency is to inject power into all four pairs because this solution reaps the benefits of the basic squared relationship between current and power.

Using a two-pair PoE solution, cable loss power (PCL) can be calculated as: PCL = I2 ✕ R. Where R is channel resistance and I is the current carried by the two pairs. For a four-pair solution, the equation becomes:

PCL = (I/2)2 ✕ R + (I/2)2 ✕ R 
PCL = 2 ✕ (I/2)2 ✕ R 
PCL = (1/2) ✕ I2 ✕ R

If the current remains the same, the four-pair solution experiences one-half the cable loss. If the load remains the same, upgrading to a four-pair power is slightly more than 50% more efficient than a two-pair powering system since the current delivery impedance halves. This lowers the total delivered current for a four-pair system to less than half of a two-pair system due to the squared relation of current to power. Therefore, the total power into the cable and the powered device is less because the overall current is less. The actual power saved depends on cable characteristics, but one example would be a 24-port IEEE 802.3at-2009 Type 2 system that delivers 25.5 W per port over a cable run of 100 metres. Upgrading the two-wire solution to a four-wire solution can save more than 60 W in this case.
The table to the right compares an average case with a worst-case scenario in terms of power saved. The average PoE installation does not require the maximum power available and has a cable run of about 40 metres. The worst-case scenario uses the maximum 25.5 W and the maximum recommended 100 metre cable run. 

Using a conservative estimate of 100 million deployed nodes and the data from the above table, the average cable loss with a two-wire PoE solution is 121.7 million kWh per year worldwide. With the four-wire PoE solution, the cable loss is 60.8 million kWh. In addition to reducing the environmental impact of PoE, a four-wire solution offers operators of large Ethernet-based installations with PoE systems considerable utility cost savings. 

Losses due to power conversion will always make PoE less efficient than plugging the powered device into a power main, however as power conversion technology becomes more efficient, this difference will decrease and also has to be weighed against PoE's considerable installation cost savings.

What are the cabling implications?

As the amount of wattage increases, so does the heat within the PoE cable bundles. Cable bundles with higher temperatures are more likely to experience power dissipation, which in turn will cause decreased cable ranges. There are three ways to mitigate this problem:

Minimise the number of cables per bundle

Larger cable bundles experience the highest amount
of temperature at their core, so splitting a large cable
bundle into smaller cable bundles is an easy way to decrease heat. Also consider no bundling and use cable trunking/baskets instead.

Increase the cabling category size

Higher cable categories consistently maintain lower temperatures than lower categories. Higher category cables often comprise of thicker jackets increasing distance between cables | larger dividers giving greater distance between wire pairs | larger wire size reducing heat creation

Install shielded cabling

Although more costly, a work around is to install higher shielding cabling for larger cable bundles. Shielded cable will help increase the insulation of your PoE cable bundles.

Higher amperage will also mean a larger breaking arc on the hardware. The PoE is not live until the powered device (PD) and powered sourcing equipment (PSE) handshake, therefore there is no arc on the initial mating process. When unplugging a live PoE, an arc (or spark) occurs between plug and jack contacts. The higher the current, the larger the arc will be. Substandard products or products not designed to allow for 802.3bt can lead to eventual failure of the products. DINTEK products are made to handle 802.3bt type 4. 

PoE is already a compelling option for supplying modest amounts of power to devices that are integrated into an Ethernet network because it significantly reduces installation costs for powering the end device. In that respect, PoE has already achieved significant market success. However, some applications have been locked out of using PoE due to the lower power delivery of the two previous standards, so by extending the power limit to accommodate new types of powered devices, the new PoE standard can certainly lead to growth.

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