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New Member

UPS Size for Cisco 3750

Hi There,

We have many comms cabinets on campus; I need to order in some new UPS for each to replace old devices. All new switches are 3750?s; all old switches are 3500's.

How can I correctly calculate the UPS size for each cabinet? On average we will have two switches per location. I would need a maximum of 1 hour uptime for two switches should I have a power failure.

It would be good if Cisco have a UPS sizing tool where you could drag and drop each cisco device, type in expected load etc and out pops the answer! :)



  • LAN Switching and Routing

Re: UPS Size for Cisco 3750

Hey Stuart,

Load cals for North American voltage works something like this:

Obtain load details from the rating plate on the back of the switch. The trick is getting a measurement called VoltAmps. (that would be a cool name for a Rock band...) The 3750 is given in terms of voltage and current (amps), so now just multiply the two to obtain VA. Just for the fun of it, sometimes the load is given in watts, if so, as a rule of thumb, divide given wattage figure by 0.7 to obtain For example, the 3750-48TS is close to 75 watts, therefore: 75/.7=107 volt-amps.

VA are commutative so now just add up each of your VAs. Toss in around 30% fudge factor so you can add more Cisco switches (I bet a ISR would look good in those racks...) You need to support 60 minutes for 214 (total VA for both 3750's) + 64 (30% fudge factor) and your grand total is 278 VAs.

Add there ya go! I would make sure the whatever UPS I used would do power conditioning and monitoring so I know when the battery is getting old.

New Member

Re: UPS Size for Cisco 3750

That's great, thanks heaps....