PoE - Understanding Maximum Power

Power over Ethernet (PoE) is a technology that provides electrical power through Ethernet cables. This allows you to both power network devices and connect them to the network over a single cable.

When PoE capabilities are described in device documentation, you may see two values indicated for the Maximum Power that a PSE such as a PoE switch, or a PoE injector can provide. These are known as:

  • Maximum Power per PoE Port
  • Maximum Power to PD

The Maximum Power per PoE port is the maximum power that each switch or injector port can supply to a device. This is the maximum power that each port is rated. This is also the value that is subtracted from the total power budget of the PSE device itself, when this wattage is drawn from this port.

The Maximum Power to PD refers to the maximum power that can be delivered to the Powered Device (PD) after considering power loss due to cable length, heat dissipation, and other factors. This assumes that cabling follows all the correct standards (i.e. 100 meter maximum cable runs, use of correct Cat5/Cat6 cabling, connectors, plugs, and terminations etc). The power loss over the cable is due to resistance, which leads to a decrease in power as the length of the cable increases. So, if you were to measure the wattage received at the PD, the maximum you would receive is that value.

For this reason, Maximum Power per PoE Port > Maximum Power to PD.

For example, some PSE documentation may state something like a Maximum Power per PoE port of 30W, and a Maximum Power to PD of 25.5W.