PoE/PoE+ Planning and Implementation Guide 2011-09

2-7
Operating Rules
PoE Power Characteristics
As you can see in the table for classifications 0-3, any 802.3af compliant PD
will never require more than 12.95 watts. The switch provides a minimum of
15.4 watts at the port in order to guarantee enough power to run a device, after
accounting for line loss. For classification 4, the switch provides 30 watts at
the port in order to guarantee enough power to run a device, after accounting
for line loss.
PD Power Requirements
When a PD is initially connected to a PoE port, a minimum of 17 watts for PoE
and 33 watts for PoE+ of available power is required to begin the power-up
sequence. This 17 or 33 watts is needed to determine the type of PD requesting
power (see “PD Power Classification” on page 2-6). Once the power
classification is determined and power is supplied, any power beyond the
maximum power requirements for that class of PD is available for use.
In the default switch configuration all PoE/PoE+ ports have a Low priority. If
the switch has less than 17 W of PoE power available or 33 W for PoE+, the
switch transfers power from lower-priority ports to higher-priority ports.
See “Switch Priority Class” on page 2-5 for information on the use of PoE port
priority classifications. Within each priority class, a lower numbered port is
supplied power before a higher numbered port.
Disconnecting a PD from a port causes the switch to stop providing power to
that port and makes that power available to other ports configured for PoE
operation.