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

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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.

Copyright © 2011 IDG Communications, Inc.

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