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Old 11-29-2016, 03:15 PM
joshdmorris joshdmorris is offline
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Join Date: Nov 2016
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Default Circuit Size

I apologize up front as I'm new to this whole game and got thrown into this position. Many of my assumptions may be wrong or inaccurate so any guidance is absolutely welcome.

We're trying to plan out some new racks in our DC and in 1 of them, we've got (20) Dell XD 730s going in and the PSUs are rated for 750W ea. We've got 3-phase 208 circuits in the suite. We've got 2 legs for redundancy but obviously I'm only planning on one leg.

So, if my math is right... 750*20 = 15,000W. 15,000W / 208V = ~72Amps. Spreading that across 3 phases spreads the load a little and you could arguably also reduce this number to 80-85%. This is obviously a power scenario you'd never actually hit, but in my mind, it doesn't make sense to not plan for what things are rated for (because why not?).

First question, is my thinking/planning method incorrect? Because I'm getting questioned when I'm requesting larger (60A 3phase) circuits for some of these dense racks. We have other dense racks that are operating with similar Amp requirements (on paper) but real world numbers don't hit close to that, and these are only on 30A 3phase circuits!

Second, what methods do you use as professionals to plan for your power needs? Do you plan for what is rated on the box or more real world numbers, or something different? Using my method above, we seem to have some racks that are already oversubscribed on paper, but real world numbers are nowhere close. I'm having trouble understanding this and properly assessing risk.

Thanks for any help!
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