Allied Telesis AT-S63 - Manual (Page 178)

Chapter 15: Power Over Ethernet
178 Section II: Advanced Operations
PoE Device Classes
The IEEE 802.3af standard specifies four levels of classes for powered
devices that are defined by power usage. The classes are:
ˆ 0 - 0.44 W to 12.95 W
ˆ 1 - 0.44 W to 3.84 W
ˆ 2 - 3.84 W to 6.49 W
ˆ 3 - 6.49 W to 12.95 W
(The standard actually specifies five levels; the fifth is reserved for future
use.)
The class of a powered device is set by the manufacturer and it cannot be
changed. This is mentioned here because you can view the classes of the
powered devices through the switch’s management software.
According to the IEEE standard, the maximum amount of power a
powered device should consume is 12.95 W. However, the ports on the
switch can deliver up to 15.4 W. The reason for the difference is because
some power is lost on the twisted pair cable as it travels from the switch to
a powered device. For those devices needing 12.95 W, the extra watts act
as compensation for the possible loss.