2-6
Operating Rules
Provisioning Power for PoE
Operating Rules
PD Power Classification
A PD is classified based on the maximum power it draws across all input
voltages and operational modes. The most common class is 0, in which the
switch will allow a maximum draw of 15.4 watts per port. As an example, 15.4
watts - Power Loss (16%) = 12.95 watts. See table 2-2.
Table 2-3. Power Usage
As you can see in the table, 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.
PD Power Requirements
When a PD is initially connected to a PoE port, a minimum of 17 watts of
available power is required to begin the power-up sequence. This 17 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 ports have a Low priority. If the
switch has less than 17 W of PoE power available, 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.
Class Usage Minimum Power Levels
at Output of PSE
Range of Maximum
Power required by
the PD
0 Default 15.4 Watts 0.44 to 12.95 Watts
1 Optional 4.0 Watts 0.44 to 3.84 Watts
2 Optional 7.0 Watts 3.84 to 6.49 Watts
3 Optional 15.4 Watts 6.49 to 12.95 Watts
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