3-phase, 208V Power Strips (Rack PDUs) Demystified, Part II : Understanding Capacity

Posted on April 9, 2013 by Gento

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:

• On a single-phase power strip, I was able to power 10 servers. So how many will I be able to plug into this three-phase power strip? 30?
• I have a three-phase, 30A power strip. Does that mean I can have up to 30amps on each of the three phases of the power strip? That is, 90A total?

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).

SOME OBSERVATIONS

• This chart (and this blog post) applies ONLY to 208V, 3-phase power. If your facility utilizes 400V, 3-phase; or 415V, 3-phase power [i.e. most of EU, many other countries, and many of Raritan’s largest customers in North America]—then this information does not apply to you!!!

• Compare A and C in the chart above. Note that going from a single-phase (30A) power strip to a three-phase (30a) power strip does NOT get you 3x more power. That is a common misconception. Instead, you get 1.732x more power [the square root of 3]. Again, you will have to consult my previous post for an explanation why.

• Most rack PDU Manufacturers offer two versions of “50A, 3-phase” power strips—Options D and E in the chart above. In both cases, your co-location facility (or data center facility) provides a 50A, 3-phase input. But one power strip costs much less than the other. That is to say, moving from D to E increases the power strip’s costs materially, while offering only ~14% more power. That is why Rack PDU vendors often recommend Option D.  EXCEPTION TO THE RULE: Most co-location customers [i.e., not owner/operator].

• Similarly, many rack PDU Manufacturers offer two versions of “60A, 3-phase” power strips—Options F and G in the chart above. This is less common, but does occur on occasion.

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.)

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