What IT should know about AC power

After all, it's just electricity, right? How hard could that be? Surprisingly hard -- if you don't know what you're doing. Here's the lowdown

If there's one thing people tend to screw up in medium-size data center buildouts, it's AC power specification. Even some of the brightest network and server administrators seem to think there's a limitless number of power outlets in any given rack or room, and if there isn't, well, you can just plug a $3.99 power strip into the UPS and be done with it, right?

Technically, they're right. You can plug that $3.99 power strip into a UPS and run a few servers or small switches off it, even if it pushes the UPS near overload. That doesn't mean it's a good idea or even a safe one. But it will probably work for a time. Oddly, I've found that a few of these same folks bristle mightily when they discover that a user has brought in a $19.99 five-port 10/100 switch to plug into the network jack in their cube -- pot, kettle, and so forth.

[ See "8 radical ways to reduce data center power costs" by InfoWorld's Mel Beckman. | Still power hungry? Read the classic but still controversial "10 power-saving myths debunked" by InfoWorld's Logan Harbaugh. ]

The power vacuum
The sad fact is that far too many good, solid network and system folks simply don't know much about power. They see an outlet and assume there's juice. Occasionally they get upset when they can't find an outlet that will accept a NEMA 5-20 plug (with one horizontal blade), because the outlet's only 15 amp and has a 5-15 receptacle. I've seen many situations where someone has gone to Home Depot and built a cable with a 5-20 receptacle on one end and a 5-15 plug on the other to "adapt" the power cable. This is known as a bad idea.

Sure, there are plenty of power-savvy IT folks out there, but the trend is toward letting the ephemeral "someone else" deal with the problem. The "someone else" usually turns out to be a UPS and/or generator vendor or a local electrician. That's where the fun begins. It can be like an IT version of "Who's on First?" We could call it "How Many Watts?" or "What's an Amp Again?"

When power guys who know a little about IT and IT guys who know a little about power get together, there's a reasonably good chance that things will turn out OK after some tweaking. When power guys who know nothing about IT get together with IT guys who know less than a little about power, that's generally a recipe for disaster. This is how you wind up with wildly over- or under-spec'd in-rack power situations.

An AC power primer
Let's take a brief look at power, centered around IT needs. (Note: The following is greatly simplified in places. Do not take this as canonical; your service and situation may differ greatly depending on location and a variety of other factors.)

We'll start with the mains. You probably have three-phase AC service to your location. You don't really care about what's coming in unless you own the building outright, but you do care about delivering that power to your IT gear. If you have a generator, then there's a relay panel in the middle of that service, ready to cut over to the generator should mains power fail. Thus, everything you place on the load side of that panel must reside below the operational limit of the generator. If you have a 50kW generator, don't expect to be able to run 65kW of equipment, and so on.

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