Power & Power Supplies

Unanswered Question
Oct 25th, 2007

When figuring overall UPS (Wattage) requirements to support a switch, are the module wattage requirements and power supplies wattages added together to determine electrical needs?

OR are the module wattage requirements added together to determine the size of the switch's power supply needed and then UPS (electrical) requirements are based on the power supply(s) used?

I have this problem too.
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lamav Thu, 10/25/2007 - 06:50

Steven:

From my understanding, you have two choices in power supply -- either AC or DC. Which you use depends on the wattage requirements and the power handoff at the data center.

To figure out th epower requirements, you need to add the power consumed by each module as well as the power you will need from the PSE when using PoE. Most Cisco switches offer 6.3W low power and then 15.4W high power for phones that require it.

Then, you can run the power supplies in 1+1 mode or redundant mode. 1+1 means that 1 supply is enough to power the switch on its own, and if it fails, the other takes the full load. Combined means they have to work together to support the power requirements, and if one fails, some modules will lose power.

I dont know if this answers your question or not.

HTH

Victor

steven.e Thu, 10/25/2007 - 06:54

Can you say for certain whether or not the modules AND the power supplies draw power from the UPS or if the power supplies draw power from the UPS and then the modules only draw power from the power supplies?

To answer your question, Yes. Both the Modules and the Power Supply draw power from the UPS (Mains).

The Power Supplies draw in (typically in an AC supplied system) AC power, then switch/transform the voltage to the necessary voltage, rectify and regulate it to meet the requirments of the chassis. This power is available to the modules. It's a chain. The modules draw what they need. A single chassis, with a single module drawing - say 250 watts, would only draw from the mains about 300 or so watts. Just because you have a 1000 watt power supply does not mean that power supply will always draw 1000 watts from the mains.

A power supplies rating is the MAXIMUM amount of power it is rated to deliver. Exceed this rating and if there aren't protection circuits in the power supply, you will burn it up. Because of these circuits, and the inherent inefficency of a power supply (depending upon it's design, this inefficency could be upwards of 25% of the TOTAL rated power), the power supplies consumes power at well. So if you had a power supply that was 90% efficent (10% inefficent) and was rated at 1000 Watts, then theoretically, you could consume up to a maximum of 900 watts for your modules - including any PoE those modules are supplying.

If I were sizing a UPS for a switch/router, if it was a smaller switch or router (35/36/37/38 hundreds) I would size it to the specs on the rear. If it were a larger switch/router (45/65/75/76 hundreds) I would do as previously described (adding all the wattage drawn as shown on the cli command) and adding about 25% of the total wattage rating for the power supply. This, however, would limit your growth potential. So plan accordingly.

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