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HP ProCurve 2520 Planning And Implementation Manual: Poe Power Characteristics; Line Loss; Pd Power Classification

Procurve power over ethernet (poe/poe+).
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PoE Power Characteristics

Line Loss
A certain amount of power is consumed by the resistance of the wire in the
LAN cable connected from the switch to the powered device (typically less
than 16% loss), which can be influenced by cable length, quality, and other
factors. The IEEE 802.3af specification has addressed loss of power by
providing more power than a powered device requires. As well, depending
upon the classification (Class 0-4) of the device, the switch will provide more
or less power to address the specific power needs of that end device.
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 2-7.
Table 2-1.
Power Usage
Class
Usage
0
Default
1
Optional
2
Optional
3
Optional
4
Optional
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.
Minimum Power Levels
Range of Maximum
at Output of PSE
Power required by
the PD
15.4 watts
0.44 to 12.95 watts
4.0 watts
0.44 to 3.84 watts
7.0 watts
3.84 to 6.49 watts
15.4 watts
6.49 to 12.95 watts
30 watts
0.05 to 24.00 watts
Operating Rules
PoE Power Characteristics
2-7

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