Posted on April 9, 2013 by Henry Hsu
Despite the growing ubiquity of 3-phase power distribution (at 208V) in North American data centers, data center operators are still not sufficiently fluent with the real-world capacity implications of 3-phase power in their cabinets.
The principal reason is because—at 208V—the math required to understand 3-phase power distribution at the cabinet is completely counter-intuitive.
THE TWO MOST COMMON QUESTIONS I GET are some form of the following:
Whatever you think is reasonable in your head is, in fact, not actually the way it works. So the answer to both of the above questions is, “no”.
In a previous post, I attempted to explain at a lower-level the reasons why this is true—including a link to a very useful Excel spreadsheet to help you calculate 3-phase rack PDU loading and capacity planning.
But I realize that most people are like me (lazy). You don’t need to know the electrical rationale for the counter-intuitive math, you just need to know the answers! So in this post you will find just the main information you need to do simple capacity planning (without explaining the reasons why). I guess I really should have posted this first. Whoops!
THE SHORT VERSION: 2 Primary Tips
1. If you don’t totally, 100% understand 3-phase power distribution, it is best if you do NOT think or speak in terms of amps. In all likelihood, you will say something incorrect that confuses your electrician.
Instead, think about how many watts your equipment consumes, and how many watts your 3-phase power strip can provide. Watts are universally comparable, regardless of the supplied electrical configuration available at your rack: both your power strip manufacturer and your IT equipment vendor will tell you how many watts can be supplied. No matter what voltage you use; the rated current (amps); or whether you have 1-phase power or 3-phase power; etc… you can always compare watts.
If you absolutely must think using amps (i.e., “My Dell salesperson says this server will draw a maximum of 1.4A; and that’s all he will tell me.”), then I strongly suggest you: (a) download and use our 3-phase calculator here; and (b) consult the right-most column of the following cheat-sheet.
2. Print out this chart, pin it on your bulletin board ... and trust no other document on earth. (Click to view as high-resolution PDF).
THIS INFORMATION IS VENDOR NON-SPECIFIC
Please note that all the information in this post is not specific to Raritan power strips—but is vendor-agnostic… it’s just math. (Of course, if you do find this information beneficial, I sure would appreciate your considering Raritan as a potential provider for critical power distribution in your next build.)