The purpose in standardization of PoE is to ensure that any complaint power sourcing equipment (PSE) and powered device (PD) will work with one another, as well as that superimposing power on a network cable is done in a safe way and does not reduce data rates. That said, technically speaking, PoE standards just like most electronic standards, are voluntary rather than legally required.

PoE standardization began around 1999 when PowerDsine along with 3Com started IEEE PoE study group. The first PoE standard IEEE 802.3af was ratified in 2003. It specified 15W PSE that can deliver of up 12.95W to a PD device over two twisted pairs to a distance of up to 100 meter. Looking ahead I can say that so far all PoE standards limit the length of the Ethernet cable to 100 meter. Note that the difference between PSE output power and PD input power represents the losses in the cable. The next specification IEEE802.3at (2009) extended the PSE output to 30W yielding at least 25.5W at PD input. Some companies also implemented a modification of IEEE802.3at to deliver up to 51W over all four pairs (so-called UPoE or PoE++). These quasi-standard solutions were not always inter-operable.


About the same time, in 2011 HDBaseT Alliance ratified so-called Power Over HDBaseT standard. Initially it specified 95W PSE that could deliver up to 72W to an end device over four pairs. Later on they extended the PSE power level to 100W. In 2012 Linear Technology published a proprietary specification called LTPoE++™ that allowed up to 90W over 4 pairs. LTPoE++™ PSEs and PDs are said to be backward compatible and inter-operable with with IEEE 802.3at Type 1 and Type 2 devices. The latest IEEE802.3bt specification (2018) likewise enabled power transfer over all four pairs with 90W PSE. Among other things, IEEE802.3bt introduced 8 classes of PoE devices. Although IEEE were a bit late to the 4-pair party, as far as I can see, the systems compliant to IEEE standards usually are the most common ones on the market.

IEEE802.3bt document categorizes PSE and PD as a Type 1 through Type 4 depending on their supported parameters and supported classes. For reference, the following table provides maximum capabilities of PoE systems per various standards.

PoE standards chart

STANDARDS	PSE OUTPUT watt (min)	OUTPUT VOLTAGE (volt)	MAXIMUM CURRENT (A)	PD INPUT watt (min)	TYPE
IEEE 802.3af	15.4	44-57	0.35	12.95	1
IEEE 802.3at	30	50-57	0.6	25.4	2
IEEE 802.3at 4-pair (UPoE)	60	50-57	0.6 per pairset	51	2
HDBaseT (PoH)	100	50-57	1.0 per pairset	75	3
IEEE 802.3bt	98.6 (99.9max)	52-57	0.948 per pairset	74.9	3,4

Note. The numbers in the PD wattage column represent minimum power that can reach PD input after 100 meter cable. It is calculated as PSE wattage minus power dissipation in the cable at rated current in the pairset(s). Of course, the shorter cables the higher power that theoretically can be delivered to PD input. It does not mean however that the PD will necessarily get this amount of power. PSE with advanced power management will decide how much watt to actually allocate to a given port. Of course, there are also losses in PD front-end section and DC-DC converter (see PoE system block diagram here). So, the end application will always get less power than listed in the above chart. To estimate available power to the network device you need to take into account these losses. Given typical efficiency of PD is 90%, power delivered to the end application is about 10% less than that at PD input (see our PoE calculator).