7900 Power Calculator

Answered Question
Jul 29th, 2008
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Hi there !


I'm using the Cisco Power Calculator tool at: http://tools.cisco.com/cpc/launch.jsp


I'm in doubt since I tryied to use a 3750G (48 Ports) with 48 phones 7941G-GE .. and I returned me an error.. I had to reduce the number of IP Phones to 28, so I had 97% of power usage....


Is this right ? I have a 3750G Poe 48 ports and I cannot use 48 phones ?


Thanks.

Andrei.



Correct Answer by Rob Huffman about 8 years 11 months ago

Hi Andrei,


Great question :)


The problem here is that the 48 Port model WS-C3750G-48PS cannot provide 15.4W per port Class 3 power on all 48 ports (which is what the 7941G-GE requires). The 24 Port model can.


It can however provide 7.7W per port Class 2 power on all 48 ports so this would power the Class 2 devices like the 7941G/7961G.


Have a look under the "key features" in this doc;


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_brochure0900aecd803ca7c6.pdf


Q. How does Power Management work in the Cisco Catalyst 3750 PoE switches?


A. The Cisco Catalyst 3750-24PS switch supports a maximum of 15.4W per port of -48V DC power on all 24 ports over standard Category 3 and 5 unshielded twisted-pair (UTP) cable up to 100 meters, however all 48 10/100 ports in Cisco Catalyst 3750-48PS switch will be able to deliver 7.7W simultaneously and any given port can be configurable to support up to the maximum of 15.4 watts as long as the switch's - 48V power capacity is not exceeded. The Cisco Catalyst 3750 PoE switches monitor and track the requests for power, and only grants power when it is available. If the connected PD draws more power, which results in exceeding the system power budget that is available on the switch, then power will be denied to that port and the PD will not power up. If power is denied, the switch re-checks the power budget periodically (every 15 seconds), and continues to attempt to grant the request for power. If power is granted, the switch updates the power budget.


From this good doc;


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_qas09186a00801b0971.html


Hope this helps!

Rob


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Correct Answer
Rob Huffman Tue, 07/29/2008 - 06:11
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Hi Andrei,


Great question :)


The problem here is that the 48 Port model WS-C3750G-48PS cannot provide 15.4W per port Class 3 power on all 48 ports (which is what the 7941G-GE requires). The 24 Port model can.


It can however provide 7.7W per port Class 2 power on all 48 ports so this would power the Class 2 devices like the 7941G/7961G.


Have a look under the "key features" in this doc;


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_brochure0900aecd803ca7c6.pdf


Q. How does Power Management work in the Cisco Catalyst 3750 PoE switches?


A. The Cisco Catalyst 3750-24PS switch supports a maximum of 15.4W per port of -48V DC power on all 24 ports over standard Category 3 and 5 unshielded twisted-pair (UTP) cable up to 100 meters, however all 48 10/100 ports in Cisco Catalyst 3750-48PS switch will be able to deliver 7.7W simultaneously and any given port can be configurable to support up to the maximum of 15.4 watts as long as the switch's - 48V power capacity is not exceeded. The Cisco Catalyst 3750 PoE switches monitor and track the requests for power, and only grants power when it is available. If the connected PD draws more power, which results in exceeding the system power budget that is available on the switch, then power will be denied to that port and the PD will not power up. If power is denied, the switch re-checks the power budget periodically (every 15 seconds), and continues to attempt to grant the request for power. If power is granted, the switch updates the power budget.


From this good doc;


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_qas09186a00801b0971.html


Hope this helps!

Rob


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