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Meeting high-power PoE at longer distances


December 2, 2019  


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By Carol Oliver

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Photo: Remee.

It’s been a year since high-power PoE was defined and ratified by IEEE 802.3bt. So, what has changed in the low-voltage industry and what are the challenges for system designers and contractors when selecting and installing cables for high data and high power?

High-power PoE, more commonly known as Type 3 and Type 4, utilize all four pairs of a twisted-pair cable, hence IEEE 802.3bt refers to this as four-pair PoE. Type 3 can provide up to 60W of DC power from the source to a typical maximum of 51W to the device and Type 4 provides up to 100W from the source to up to 71W at each port.

Chart: Carol Oliver.

Turning up the heat

The good news is that more devices, such as digital displays, laptops, televisions, access points and advanced IP cameras, can be powered through the network cable versus having to connect with other cables and to an AC outlet. Think of the freedom. Think of the cost savings. So, what’s the downside?

Category cables can power the PoE-enabled device as long as there is sufficient wattage at the source (i.e. a powered switch) to power the unit all the way to the end of the cable run and assure that the voltage has not exceeded the specified voltage drop. The voltage drop is affected by the end device requirement (typically 48V-57V), cable construction based on individual specifications and distance of the run. To calculate the voltage drop (V), multiply the current or amps (A) by the cable ohm resistance (W) which can vary between cable types.

With higher power going through the cable on all four pairs, the identified inhibitors are heat build-up within the cable and the distance limitations. It’s still classified as “low-voltage” so be assured your cable won’t melt or burn, but be concerned that the internal temperature rise can result in increased insertion loss which decreases cable efficiency and affects the entire cabling system.

There are standards and codes to help with the design and selection of the proper cable for safety and performance — NEC NFPA-70 (2017), TIA-TSB-184-A and TIA-569-D-2. Note that TIA and NEC differ in their maximum bundle sizes. NEC’s focus is safety and the 2017 code provides an ampacity chart (Table 725.144) for up to 192 cables, based on the bundle size, maximum current (A) per conductor, AWG size and cable temperature rating. TIA is concerned with assuring the data and power arrive safely to the powered device and identifies the contributing factors as the current (A) per pair, cable category and number of cables in the bundle (not to exceed 100). TIA provides recommended mitigation techniques and best installation practices to include: reducing the bundle size (manufacturers’ recommendation is not to exceed 24) ; spreading the cable out within the pathway (such as open cable tray) to provide air circulation; selecting a cable with a larger conductor size (i.e. 22 AWG versus 23 or 24 AWG); and, adhering to the manufacturers’ specifications for ambient and installed temperature ratings.

Extending the distances

Since the dawn of category cable history, dating back to 1983, IEEE 802.3 defined the distance limitation of a four-pair copper cable at 100 meters (which includes the patch cords on both ends). Today this is an age-old dilemma for which many can’t figure out how this rule came into existence or why it still exists. One hundred meters was a convenient length due to legacy specifications for data transmission (10Base-T and 100Base-TX) and could be backwards compatible. The longer the run, the more the signal degraded and the problem was presumed to get worse at higher bandwidths.

For power, the voltage drop and the resistance of the copper affects the length of the copper distance. Basically, voltage drop depends on the power transmission strength sent at the powered source and the gauge of the copper cable.

The restricted reach of 100 meters can severely limit the viable locations where end users can operate a remote IP-enabled device. Most devices requiring the high power PoE extend beyond the 100-meter limitations – think wayfinding signage in an airport, or scoreboards at a stadium, or security cameras in a parking lot.

Existing alternatives to extending the reach include running a hybrid coper/fiber cable with media converters and transceivers (which will require AC power), or installing extenders or repeaters for the copper cable. These options will add cost, as well as more points of failure.

For more information, check out Paige Data Com Solutions’ previous blog: http://www.paigedatacom.com/blog/article/living-on-the-edge-the-good-the-bad-and-the-ugly