What IT Should Know About AC Power

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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.

If you have a substantial amount of gear and want to use a substantial UPS to protect it, you'll probably wind up bringing three-phase power directly to a PDU and UPS in the room. These two pieces will handle the switching to/from the UPS during a power outage and will be used to distribute power to the rest of the room.

Using three-phase gear can also greatly simplify rack power distribution because three phase basically means that there are three 120v circuits delivered to each rack, generally within a single vertical PDU. By combining two of those circuits, you can run gear that requires 208v, such as beefy switch and blade chassis power supplies. However, within the same PDU, you can also spread out those three circuits into individual 120v outlets for 120v gear, such as standard 1U servers and the like. Without three-phase delivery, your PDUs are generally only 120v or 208v, and unless they have step-down transformers or you decide to make the switch and run all your gear at 208v, you'll need separate PDUs for each set of gear.

Regardless of whether you can use three-phase service, you'll need to be familiar with the range of NEMA plug types and what they mean. For instance, you're certainly familiar with the standard NEMA 5-15 plug that the vast majority of all electronic gear uses. This means it's suited to be plugged into a 120v 15-amp circuit. If that plug is a 5-20, it needs at least a 20-amp circuit. If the designation includes an "L" before the first number, it's a round, locking plug type, such as a NEMA L5-20. That type would again denote that the device requires 120v 20-amp service. If the first number is a 6, as in a NEMA L6-20, then we're talking a maximum of 250v and 20 amps. If it's L6-30, it's 250v and 30 amps. An L6-50? You guessed it, a 250v 50-amp circuit.

There are also IEC 60320 plugs and receptables. You're already intimately familiar with the C13/C14 coupler -- you probably have a half-dozen within reach right now with a NEMA 5-15 on the other end. Some larger gear such as blade chassis will use C19/C20 couplers on the back of the gear and offer a variety of cable options to connect to a number of service types. A common example is a C19-to-L6-20 cable that will deliver 208v 20-amp service to a particular power supply. These are usually found on blade chassis, big storage, and large switching chassis. There are many additional NEMA plug types, but these are some of the more common types used in IT. A handy reference for straight plugs and receptacles can be found here, and locking plugs and receptacles are right here.

There's an entire discussion to be had on how to gauge your current and future power consumption, wattage calculations and so forth, but that's a discussion all unto its own. Perhaps we'll look at that next week.

Before embarking on a new data center design or even before modifying or updating an existing data center, spending a little time researching the power situation at your facility and power delivery in general. It's neat stuff, and adding that knowledge to your arsenal can only help you now and in the long run.

This story, "What IT should know about AC power," was originally published at InfoWorld.com. Read more of Paul Venezia's The Deep End blog at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.

This story, "What IT Should Know About AC Power" was originally published by InfoWorld.

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