Mission Critical - Winter 2008 - (Page 68)

Data Center Racks continued Table 1. North American electrical circuits Table 2. International electrical circuits survey of 100 enterprise data center operators found 81 percent had load densities greater than the typical traditional power density of 3 kW/rack. Tables 1 and 2 show the maximum load capacity that can be handled by various common circuits used in and outside of North America. The tables show that data centers typically have threephase power available. Because lower power densities can easily be handled by single-phase circuits, data center operators often choose to separate out the phases at the output of floor-mount power distribution units and feed individual phases to separate racks. For load densities greater than 5 kW/rack, three-phase power all the way to the racks should be considered because of the benefits: • Lower cost of cabling. Loads greater than 5 kW within a rack can be handled by a single three-phase circuit rather than multiple single-phase circuits. • Higher reliability of electrical infrastructure. Phaselevel metering on three-phase rack PDUs allows better balancing of loads. Balanced loads minimize harmonics and overheated neutral wires. • Higher reliability of IT infrastructure. A lower number of circuits maximizes air flow under raised floors and within the racks, lessening the chance for equipment to overheat. • Scalability. Higher capacity allows room for additional equipment later. Three-phase power requires a decision about using a wye supply or a delta supply to the rack. Wye supplies pull a neutral wire into the rack; delta supplies do not. In North America, if all equipment is operating at 208 V, a neutral is not required and a delta supply can be used. For equipment operating at 120 V a wye supply is necessary. Outside North America all loads require a neutral wire to run 220 – 240 V, thus a wye supply is needed. structure within data centers can provide 120 V (phase to neutral) and 208 V (phase to phase), requiring voltage selection as part of the data center strategy. Data center operators should consider choosing the higher voltage, which produces a lower current draw translating into lower power losses and greater efficiency. Operating servers at 208 V has resulted in as much as 2 percent efficiency increase from the servers alone. Adding the benefits of lower required cooling, gains for the entire data center might be as high as 3 to 4 percent. Several enterprise servers and large networking switches are in fact designed to only accept 200 – 250 V. Outside North America the electrical infrastructure allows only high voltage options (220, 230 or 240 V) that fall within the acceptable range. Outlets on Rack-Mount PDUs Three major physical characteristic considerations for rackmount power distribution units are: • Form factor. Rack PDUs are available in vertical and horizontal form factors. Horizontal PDUs are typically limited to up to 10 outlets within 1U of rack space. Vertical versions can offer up to 42 outlets. • Type of outlets. Most IT equipment features IEC input power connectors with modular input cords allowing manufacturers to ship worldwide by localizing the power cord. Jumper cables can connect the IEC plugs on the equipment to the corresponding IEC outlet on the PDU. A PDU with IEC outlets will not work in North America, if the IT equipment has a hardwired input cord with a NEMA plug. Such cases require a PDU with NEMA outlets • Number of outlets. A PDU should have the number of Electrical Circuits Much IT equipment includes dual redundant power supplies. This feature benefits a data center design that uses a minimum of two sets of independent circuits within each rack. Ideally, both sets of circuits should come from different power sources capable of independently handling the entire anticipated load capacity of the rack. Data center operators need to pay close attention to determining the anticipated loads within the racks because of rising power densities. As shown in figure 1, the Aperture 68 | Mission Critical Winter 2008

Table of Contents Feed for the Digital Edition of Mission Critical - Winter 2008

Mission Critical - Winter 2008 Table of Contents Editorial: What Have We Been Doing? Cronin’s Workshop: Imagining the Perfect Data Center Power And Cooling Solution Digital Power: The Next Edge in Business Resiliency Zinc Whiskers: Keeping Up With The Green Movement Mission Critical Care: Shipping, Staging, and Installation Legal Perspectives: “A Reliable Energy Supply Is Key to Data Center Success” Continued Payback, Instant Protection Beyond the Nines The Green Information Technology Revolution Health Care Network Offers Glimpse of the Future of Backup Power High-Density Data Centers – Discuss, Design, Deploy High Performance Data Centers Call for Innovative Cooling Techniques More Data in the Data Center Five Considerations for Powering Data Center Racks New Products Industry News Events Classifieds Letters Ad Index Datacenter Wiki: wiki-Mission Critical

Mission Critical - Winter 2008

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