Calculating A.C. Current Draw and Circuit Requirements for Nexus 7710

[Michael Mertens]

I have a new data center deployment consisting of Nexus 7710s and I’m trying to solidify my power requirements to Facilities. We will deploy power supply redundancy (N+1) along with power grid redundancy.  I am trying to derive solid current/amperage requirement for Facilities.

The Cisco Power Calculator tool, under the “Power Consumption/Heat Dissipation Summary” indicates a “Total Output Power” of 4830 Watts. The term “Output Power” honestly confuses me, and I’m assuming this is the power consumed. In order to cover 4830 Watts, I’ll need qty (2) 3000 Watt Power Supplies to cover the 4830 Watts, which will give me 6000 Watts, plus 1 P.S. for N+1, which brings me to 3 power supplies. For Grid Redundancy, I’ll add 3 more.

These will connect to 240v A.C. 3-phase circuits. To derive circuit draw: Power/Voltage=Current;

4830 watts/240 volts = 19.5 amps drawn per Nexus. I   am ordering qty (2) 60 Amp circuits on diverse substations/circuits for every two Nexus. Each Nexus will have 3 power supplies connected to A.C. circuit A and 3 power supplies connected to A.C. circuit B.  

Under normal operating conditions, I’d assume each Nexus will draw ½ of its current from Ckt A and ½ of its current from Ckt B- which is approximately 10 Amps drawn from each Nexus on each circuit. A 30 Amp ckt would suffice; however, if Ckt/Source A is lost, then both nodes will draw 19.5 Amps EACH from the 30 Amp circuit (or try to) and the break will blow. THAT is why I’m requesting the qty (2) 60 Amp circuits per 2 nodes. (Please see attached diagram)

Does this reasoning sound correct?

Is “Total Output Power” the power consumed by the node? Really, I consider output power coming from the line source.

Does the Power Calculator assume 3-phase or does it even matter, as long as I never convert 3-phase to single phase with RMS calculations?

What does the “Total Output Current” mean on the calculator?

What percentage should I be at for typical run-rate vs. Ckt breaker for network gear? For a 60 Amp circuit, should I plan on never exceeding 50%, 70%, 80% or what % of that circuit rating?

Thanks for any help you can provide in answering any of these questions. Heck, thanks for even reading this long-winded question.

Mike

[pwwiddicombe · Accepted solution]

In the notes, Cisco indicates you take the output power expected and divide by .85 to get the power supply INPUT requirements (There's a 15% efficiency loss in the power conversion), so 4830/.85 = 5683 Watts, so use that in your calculations to facilities. 

I don't know if you're that close in your calculations, but I don't know what the draw is (and whether it's included in the calculations for the line cards), but SFP+ requirements might also have to be considered.  However, make sure it's enough; as sooner or later you may insert an additional line card for expansion...

Now, if you do all that you should be OK, as in reality I have found the Cisco specs are for startup transients; and as indicated, the operational draw is typically much less (once fans and initialization are up to speed).