solved problem of power combined mode in 4500 switch.

Unanswered Question
Jan 23rd, 2009

Hi every body!

I made up a problem to check if i understand the concept correct.

In 4500 series switch,(combined mode) power supplies provide p+2/3p , where p is the power of a power supply.

Let say we have a 4500 switch with two power supplies p1=100 watts(max),p2=100 watts(max).

Let the total load be 150 watts.


Since total load (150 watts) exceeds the max wattage of power supply(100 watts), we must use combined mode.




p=90 watt

p(2/3)= 90(2/3)

= 60 watt

so one power supply is supplying 60 watt while the other power supply is supplying 90 watt.

Did i solve the above problem correctly?

Thanks a lot!

I have this problem too.
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Giuseppe Larosa Sat, 01/24/2009 - 01:41

Hello Sarah,

I think both PS will provide 75W each.

the combined mode allows to increase the threshold on maximum power required from chassis above the maximum wattage of a single PS.

but then when comes the time to use the two PS or only one is used (redundancy mode) or they share fairly the power load.

Hope to help


sarahr202 Sat, 01/24/2009 - 08:03

Thanks Giuseppe!


Let me quote from the above link.

"When your switch is configured to combined mode, the total available power in not the mathematical sum of the individual power supplies. The power supplies have a sharing ratio predetermined by the hardware. In combined mode, the total power available is P + (P * sharing-ratio), where P is the amount of power in the power supply. "

Giuseppe Larosa Sun, 01/25/2009 - 00:40

Hello Sarah,

my understanding was :

when the power management strategy is combined the threshold to decide if the chassis can be powered or not is:

P + P*(sharing-ratio)

However, the sentence

"The power supplies have a sharing ratio predetermined by the hardware"

makes me think you understood this better then me.

Best Regards



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