As the power capabilities of power over ethernet (PoE) systems have increased in recent years, and the arrival of LED lighting that can be powered over Cat 6 cabling, many facility managers are understandably asking if the age of AC power is coming to end for commercial buildings. While the proclaimed demise of Nikolai Tesla’s creation of AC power has come very prematurely, there are real benefits that PoE can provide to building infrastructure, as long as facility managers are aware of basic limitations of these systems. The following is a brief primer on those benefits and limitations.

How does Power over Ethernet (PoE) Work?

In the PoE scheme, there are two types of devices involved: Power Sourcing Equipment (PSE) and Powered Devices (PD). PDs are PoE enabled network end devices equipped to accept low voltage power transmitted over structured Ethernet Cat 5/6 cabling. PSEs provide the DC power to PDs. PSEs provide this DC power at 48-volts to each PD. A PSE may be an endspan device, typically located back in the server room, or a midspan device connected in series with each end device, adding power to the line. Since each Cat 5/6 cable consists of four pairs of 24 AWG copper wiring that can be routed for up to approximately 270 feet from the server room to the furthest PD, PoE works by using some or all of the pairs of wire for delivering 48-volt DC power.

While earlier IEEE 802.3af standard PSEs were limited to just under 13 watts, later standards (IEEE 802.3at, 802.3bt) allowed for higher wattage PSEs, under standards nicknamed PoE+ and PoE++ respectively, to deliver more power to the PD: from 25.5 watts (802.3af standard) to 71 watts (802.3bt Type 4 standard). Even in the most high powered of PSEs, this still limits power delivery to 100 watts from the source, and amperage flow over the small gauge 24 AWG conductors to 1 amp per pair. In practical terms, this means that while a 277-volt, 20-amp lighting circuit can power up to 100 light fixtures on a single circuit, a PSE would need a dedicated cable to each 32-watt light fixture, consuming two 48-port servers for a large open office area.

What kind of devices can be powered from PoE?

 

 

A short list of the most common PoE devices include:

Common traits among the most successful devices that use PoE for power include:

For this reason, phones, WIFI, security, and public address systems have seen the biggest benefit to going with PoE.

What are the pros and cons of PoE for delivering power? Can I consider this “green” or “energy saving” technology?

Electrical or Technology engineers who work in construction are very aware of voltage drop losses, also called “I-squared R” losses, where power loss is directly proportional to wire resistance, and goes up exponentially with the amount of current on each wire. Thus, delivering higher power levels (over 1000 watts) over long distances (over 100-feet) at relatively low voltages (120-volts) require them use of larger gauge power wiring, going from 12 AWG (0.0808 inch diameter) copper to 10 AWG (0.0907 inch diameter) copper, for instance. Since PoE uses data cabling with smaller 24 AWG copper wiring (0.0201 inch diameter), the resistance losses are sixteen times higher in the smaller 24 AWG wire than in 12 AWG building power wiring.

So why use PoE at all? Newer PoE switches have integral power management features, that can reduce losses inherent to older PoE switches, boosting efficiencies to 94%. More importantly, they reduce material consumption and cost by using the same data cable for communications with the device for the power needs, and can eliminate the need for raceway along the entire cable path.

Taken together, we can summarize the pros and cons of PoE in the following table: