PoE/PoE+ Planning and Implementation Guide 2011-09

12-6
Planning and Implementation for the E3800 Switches
Planning Your PoE or PoE+ Configuration
The lowest loaded bank of ports (1-24 or 25-48) has 22 watts reserved. That
power is available for use by the two highest priority ports in the bank, (in a
default configuration ports 1 and 2, or 25 and 26).
For example, the switch starts with 700 or 1000 watts. Depending on which
voltage is applied to the switch, 110 or 220. The switch will use 185 W for
switch operation. This drops the usable PoE/PoE+ wattage down to either 515
W for 110 voltage or 815 W for 220 voltage.
Then it reserves 22 watts per bank leaving 471 W at 110 voltage or 771 W at
220 voltage for PoE/PoE+ allocation. If ports 1-24 are chosen to be used then
the 22 watts that was held in reserve for that bank of ports will be added back
in for a total of 493 W at 110 voltage or 793 W at 220 voltage.
It takes 720 watts to fully provision 24 ports (plus 5 watts to account for load
fluctuations, 725 watts), therefore, at 110 voltage and 493 W, only 16 ports can
be provisioned. If a second power supply is added at 110 voltage, that would
add 700 watts because the 127 watts for system power would be coming from
the initial power supply. This would allow for a maximum of 1080 W for PoE/
PoE+ provisioning. This would allow for 36 ports to receive up to 30 W of PoE/
PoE+ power.
However, if the switch is supplied with 220 volts then there would be 815 W
for PoE/PoE+ provisioning. This would provision 27 ports with 30W of PoE+
power. If a second power supply is added at 220 volts, that would add 1000
watts because the 185 watts for system power would be coming from the initial
power supply. This would allow for a maximum of 1080 W for PoE/PoE+
provisioning. This would allow for 36 ports to receive up to 30 W of power.
These examples hold true unless the second power supply is configured as a
redunant power supply. Then all wattages must be figured on a single power
supply.
Another example would be to load balance or split the number of devices and
wattage between the two banks of ports. In this example the total wattage
would be divided in half where half of the available watts would be allocated
to ports 1-24, and the other half would be allocated to ports 25-48.
One 1000 watt
Power Supply
185 W for
system use,
and 815 W for
PoE+ @ 220 V
27 @ average 30 W each for
a total of 720 W
48 @ average 15.4 W each
48 @ average 7.5 W each
None
Source of
Power
Watts
Available
# of Ports Powered and
Average watts/Port
Redundant # of Ports
Powered and Average watts/
Port