My Next Data Center: Power Distribution

Now that we've resolved that I'm using AC power coming out of a modular UPS in my next data center, we can look at how I'll get power from the UPS to my server and storage racks.  Over the years I've seen all sorts of solutions, from L5-20 twist-lock outlets in the ceiling and orange extension cords under the raised floor, to APC's pre-built whips running in cable troughs on top of the racks. After all that, I definitely know what I like, and don't.

Bear in mind that I'm a New York guy who spent most of his consulting life working with midmarket companies. The New York part means that data centers are squeezed into re-purposed office spaces that don't have the ceiling height for 24" raised floors or multiple levels of cable trays above the racks, so size is a bigger issue than for those of you lucky enough to be working in purpose-built data centers with 12' ceilings.  

Being in NY means that my clients have to use not just union electricians, but the electrical contractor that the landlord approves of, so I'm always looking for solutions that need less of that expensive labor. It also means I'm looking for flexibility since the building or facilities department may pay for the initial build-out, but IT is going to pay for changes. Just a few years ago you could plan on a rack using 1-2KVA of power that could be supplied by a standard 120v 15 amp circuit, but today a single blade chassis can draw 7500VA.

To get that much juice to my racks, I'm going to run 208v three-phase power to the rack. Using 208 means I get almost twice the power for each 20 or 30 amp circuit as I get with simple 120. Most equipment in the data center has universal power supplies that can run from 100-240 volts. I can even get PDUs that have both 208V outlets connected between two of the three phases, and 120V outlets that are connected between a phase and neutral.  A single five-wire L21-20 plug can supply 5.7KVA to my racks.

While some leading edge data centers are using 412/240v three-phase power, the 2 percent efficiency change just doesn't seem worth it to me.For a small datacenter of a dozen racks or fewer, the output of the UPS will either use APC's output breaker panel and bypass switch in a second rack along with their overhead power distribution whips to feed the server racks or Eaton's equivalent three-phase rack PDUs. In a larger, or more dynamic data center, I'd want busway.  

Busways, like Universal Electric's Starline, or similar systems from Square-D or Liebert/Siemens, are the data center version of track lighting. A single busway carries 100-225 amps of three-phase power on copper bus bars. Above each rack, you plug a drop into the busway that includes the branch circuit breaker and a connector into which you plug the rack PDUs.

Busways eliminate under-floor conduits and power cables, and they eliminate the need to spec out the power needs of each rack and run cables from a central breaker panel or PDU. No conduits, and once the system's installed, no electricians.