PoE Calculations and Switch/Injector Limitations – Part One

Kavita Boodram

On December 4th In IP Solutions, Power1 Comment

One of the great advantages of using IP-enabled physical security devices is the common availability of PoE or Power over Ethernet. This technology can provide all the power necessary to operate IP cameras, intercoms, access control panels, video encoders, and other device types that can be connected to a local network. The power for the remote device is delivered on the same four-pair UTP Cat5e or Cat6 cable that is used for Ethernet communications. Not only does PoE provide centralized power for remote devices, eliminating the need for separate power supplies or connections; it also makes the connection to each device a single cable. This can greatly reduce installation time and material costs.

PoE is a standardized device powering method that is available in many network switches. If a network switch doesn’t have PoE capabilities a mid-span PoE injector can be connected. Usually this mid-span injector will be co-located with the network switch, and connected using 2 to 6 foot Cat5e jumper cables.  Whether the power comes from the switch or a mid-span injector the connection to the remote device is a single cable. 

For remote devices to be able to use PoE power they must have been built to accept the PoE current; this information should be readily available on the device itself or within its instructions/specifications. Usually the simple connection of a UTP cable to the remote IP camera or device that is hooked up to a PoE switch or mid-span injector will automatically power the device. In a few cases I’ve seen where there is a slide-switch or some other “hardware” method of setting a remote device to accept PoE, but this isn’t common.  It is very important that when planning an installation technicians determine how they plan to power the remote devices. Many devices provide alternate methods of powering such as DC power supplies or AC transformers. It is critical that only one power source is selected and connected to each device, otherwise damage to the remote device may result.

PoE was originally standardized with the classification of 802.3af in 2003. This standard provided for a maximum current of 12.95 watts of usable power at 48 volts DC. The latest PoE standard is 802.3at (2009), which is sometimes referred to as “PoE+.” This PoE provides roughly double the current at 25.5 watts at the same voltage as the older standard. These two standards are backwards compatible so for example an IP camera that can be powered using the 802.3af PoE can be connected to a network switch or mid-span injector that is on the 802.3at standard and will receive the necessary current for functionality.

It’s also interesting to see that while the 802.3at and af standards are designed so that the DC current is provided over two of the four pairs in the typical Cat5e UTP cable, there are many vendors who are providing PoE over all four pairs simultaneously. This provides their devices the capability of pushing up to 45+ watts of PoE power out to a remote device, while retaining the ability to perform to the standards for devices with lower wattage power requirements.

In the next post we’ll examine the calculations needed to ensure that your PoE power will work for the IP devices you’re installing.