Computer Room Power: Up or Under?

Electrical experts discuss ways to avoid busway and UPS configuration faux pas, share a new technology update and check in on the latest codes. CONSULTING-SPECIFYING ENGINEER: What are the main challenges involved in designing busway configurations for the ever-increasing power needs of computer rooms? ROSS: The first is underfloor conditions.


Electrical experts discuss ways to avoid busway and UPS configuration faux pas, share a new technology update and check in on the latest codes.

CONSULTING-SPECIFYING ENGINEER: What are the main challenges involved in designing busway configurations for the ever-increasing power needs of computer rooms?

ROSS : The first is underfloor conditions. It is not uncommon to remove a floor tile and find hundreds of wires running in various directions, which can make servicing, rerouting or adding cables difficult, as mistakes can be made in identifying and disconnecting the proper cable. Air circulation can also become restricted due to too many cables.

As for cabling itself, another potential problem is finding that pre-existing underfloor cabling is undersized to handle increased power loads. To proactively address this, specialty busway systems allow for larger power capacities so that additional or larger plug-in units can be added to existing busway runs as power requirements increase over time.

BOHN : Cable distribution and management hardware, overhead trays, ladders and supports being used in telecommunications rooms are finding good uses in computer rooms. But more and more, computer rooms are going without access flooring and beginning to look more like telecom rooms.

PICKETT : I agree with Peter. Underfloor systems, in general, create power distribution challenges. Owners want to distribute power in the floor plenum because of its clean look. However, when using a busway system, it cannot be mounted below the raised floor because the NEC requires that overcurrent devices attached to the busway be readily accessible. Instead, utilizing overhead busway distribution is a great way to solve the increasing power needs and flexibility requirements that are needed in a computer room.

CSE: What are the advantages and disadvantages of busway overhead drops vs. cord whips under raised floors? What about cabling in plenum areas?

PICKETT : Busways have significant advantages when compared to the traditional cord whips under the raised floor. The busway system uses a plug-in cord drop, which makes addition or relocation of cord drops very easy and fast. Using a busway system will also reduce the number of panelboards needed to serve the computer room, because instead of having the overcurrent protection at the panelboard for the receptacles that serve the racks, it is located at the overhead drop. The material cost for the busway system is higher than whips under the floor, but the labor to install the system is much less.

ROSS : Another advantage of busway is that servicing an individual cabinet can be done at the point of use without walking to a remote location power distribution unit to turn off the circuit to the cabinet, risking the human error of shutting off the wrong circuit.

SENESAC : By utilizing whips overhead, the floor can be cleared of "rats' nests" that prevent the proper flow of air under the floor. Also, there are many cooling solutions available today that do not require raised floor.

CSE: What should be taken into consideration as far as power density is concerned?

SENESAC : As power density grows, it is important to reduce the amount of one-time engineering that would be associated with the power system. Pre-engineered solutions that eliminate or simplify most planning and engineering would be preferred.

Power needs in the computer room are also not consistent across all IT enclosures. Therefore, a power system, in my opinion, must be designed from the "rack up" vs. the traditional "room down" approach. For all intents and purposes, the rack becomes the data center. Adaptability in the power system is essential, especially in retrofit applications, as you must plan for a power density that is increasing and unpredictable.

BOHN : To me, the key is agreeing to a prospective power density figure that adequately allows for future needs vs. move-in load. The "Tier" classification levels developed by The Uptime Institute can assist with this task by providing uniform sets of criteria to direct the discussions.

CSE: What are the most common mistakes made when designing electrical and cabling systems for computer rooms. What should be done to avoid these errors?

PICKETT : I think the most common mistake made is over-sizing the UPS. The loads normally given to the electrical engineer are inflated because they are often based on the amperage rating of the plug strips in the racks, the maximum capacity of a rack—if it is filled with servers—or the power supply capacity of the computer equipment.

I have normally witnessed UPS running loads at 15% to 30% once the computer room is populated and operating at full capacity. Of course, this still works, but the problem is that the owner has purchased capacity that will never be utilized, and maintenance costs will be much higher because of the added batteries in the system.

To avoid this, the electrical engineer needs to carefully analyze the loads and not just accept them without questioning the owner's representative as to how the numbers were generated. The engineer should also visit existing similar installations that the owner may have and measure the actual loads of the equipment.

SENESAC : In traditional design, there are typically five or more potential failure points between the UPS and the critical load. These points consist of the output breaker on the UPS, input breaker on the PDU, step-down transformer, output breaker on the PDU and branch circuit breaker. Because these failure points are downstream of the UPS, any failure will result in a dropped load in a non-redundant configuration. Better designs would eliminate failure points between the UPS and the load.

ROSS : A widespread mistake made back in the days of the boom was building out the entire collocation or corporate data center based on very high anticipated growth projections, which turned out to be wildly overestimated. The result was massive overcapacity. Instead, build as you grow, also known as facility scalability, emerged as a solution which preserved cash flow and kept the facility right-sized over time.

The recent trend has been to design facilities which utilize equipment that is easily scalable by breaking the systems down into smaller units, which can be quickly added at any time, including UPS units, backup generators, HVAC systems and power distribution methods, such as smaller busway systems with quick-connect plug-in units overhead.

CSE: What kinds of new technology, equipment and approaches are beginning to find their way into these applications?

SENESAC : A fully integrated approach is important. This includes a modular system that implements all aspects of power distribution from the air conditioning to the UPS through the power panels, all the way down to the mechanics of connection of the plugs in the rack enclosures.

For example, one integrated system on the market includes prefabricated multi-branch power distribution whips, quick-change multi-voltage metered outlet strips with various receptacle configurations, pre-engineered circuit breaker coordination, single- and dual-path power feed support configurable at the rack enclosure or row level, point-of-use DC capability and rapid conduit-free installation.

In addition to the capability of the adaptable rack enclosure power system to respond quickly and economically to change, there are cycle time and cost advantages associated with the initial installation of such a system, including a dramatic simplification to the up-front engineering and installation work associated with data center design.

Furthermore, the ability to adapt the rack enclosure power system can allow the system to be right-sized to the actual load requirement and grow with expanding needs.

BOHN : Highly reliable UPS module arrangements are satisfying the need for better long-term power reliability. At them same time, the use of dual-cord user equipment has also become quite common.

CSE: What are the latest methods for optimal power monitoring and data cabling management in computer rooms?

ROSS : Most major cabinet manufacturers offer plug-in strips in their products. Some of those power strips have a basic power monitor feature for the total load of the power strip.

The specialty busway systems offer power monitor capabilities, either in each plug-in unit to each cabinet, or for each busway run overall. Data acquisition units gather information from many power monitor points and report it to a locally or remotely located station. Various power limit threshold warning capabilities are also available in most systems.

BOHN : We have critical server installations that converse directly with UPS systems by SNMP protocols. This way the servers can prepare for power problems as they develop. This is especially effective for critical, but modest-sized systems where multiple redundancy may not be possible due to cost.

CSE: Are there any new codes or standards which are currently influencing how power and cabling systems are designed in computer rooms?

ROSS : The applicable sections of the National Electrical Code are Section 368 Busways and Section 645 Information Technology Equipment. Section 645 defines some restrictions regarding the use of busway under a raised floor. Even the use of cables and whips underfloor have some recently added requirements, such as mandatory removal of all abandoned cables in a raised floor. Comprehensive and consistent labeling of underfloor cables is another ongoing issue which is still not standardized.

Plenum rating is an issue sometimes brought up by facility designers and building inspectors. The local inspector is the final entity to decide what is permissible at a given site, and there are varying standards and interpretations from one local area to the next. However, by going overhead, the plenum rating question becomes a non-issue.

PICKETT : There has been a lot of confusion in application of this section over the years. Many inspectors and engineers believe if the room is called a "computer room," then this section applies, but that is not necessarily so.

For example, if the design team determines with the building officials that the walls separating the room do not have to be fire-rated, then article 645 is no longer available for use.

SENESAC : Although NFPA received over 3,500 proposals for changes to the NEC, no proposals are pending that would significantly change Article 645 rules for distribution.

However, the committee accepted several proposals that would provide consistency favoring the use of the term "exposed" over "open wiring" and "open runs." If approved, the code will clarify that input power supply cord on listed IT equipment that is installed under a raised floor does not have to be DP cable.

BOHN : Power conditioning and distribution for computer rooms is a maturing field. Telecommunications standards are under continual change and evolution, and performance criteria increase in bounds, so this area must be watched carefully.

For further discussion of power infrastructure in computer rooms, read Russell Senesac's white paper, located in the Deep Links section of .


Peter H. Bohn, P.E., Chief Electrical Engineer, The Austin Company, Schaumburg, Ill.

David Pickett, P.E., Senior Vice President, Interface Engineering, Inc., Portland, Ore.

Joel C. Ross , President, Universal Electric Corporation, Bridgeville, Pa.

Russell Senesac , InfraStruXure Product Mgr, American Power Conversion, W. Kingston, R.I.

Keeping UPS in Check

When starting a project, it is common to find that the owner's representatives already have their minds made up on the configuration of the uninterruptible power supply system.

For example, I was involved in an essential facility project where the owner's representative had already decided that the application should have three independent UPS. I was rather baffled by this requirement, and after discussing this with them, found that the basis of this decision was because historically there had been a blame game between vendors of the electronic equipment, claiming electrical interference between equipment. Even though the situation had been resolved years earlier, there was still the fear that it could repeat itself.

I explained to the owner's representative that for the same cost of three UPS, we could configure the system in a parallel redundant configuration that would give a much greater reliability. They were very pleased with this approach and accepted my recommendation.

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