Bus track and busway power distribution: Power anywhere?

Bus track and busway power distribution in the data center is a growing trend that allows for greater flexibility and accommodation of rising power densities. This in-depth exploration of bus-type power systems will help you decide whether this is a good option for your data center power infrastructure.

Massive power distribution in today's data center is a given; nonetheless, more power seems to be required almost every month. New, constantly changing IT gear, each with different power, voltage and receptacle requirements, seem to be the norm and has become a significant challenge to the electrical infrastructure.

The traditional method of a fixed floor-mounted power distribution unit (PDU) or a wall-mounted distribution panel permanently hardwired to each rack has been besieged by the constant need for additional circuits. And the under-floor labyrinth of hard pipe, Greenfield or flexible whips has impeded the raised-floor airflow. As a result, the trend is to try to move power cabling up and out of the floor and run it overhead, primarily using the same conduit or flexible whip systems. However, even this alternative is challenged by the ever-growing and changing high-density loads.

A more flexible alternative has begun to make inroads in the data center to meet this power distribution challenge: bus track and busway power distribution systems.

Bus track vs. busway systems
Bus track units resemble track lighting on steroids, with a continuous opening in the bottom of the track, while busway systems have fixed insertion points for power module taps located on the side at regular intervals (typically every 1 or 2 feet). Both types represent a flexible method to rapidly distribute power where and as needed.

While relatively new to the data center, the bus power concept has been used for many decades on industrial factory floors, where it provided an easy method to reconfigure and add power for new machines as needed. Bus power distribution has begun to appear in the data center only recently. The idea was originally introduced to the data center marketplace (at the rack level) in the late 1990s with Starline Track Busway by the Universal Electric Corporation, which also previously made industrial-type bus track. For most of the last decade, it has been alone in the market, slowly making inroads and gaining market acceptance for this type of data center power distribution system.

According to Joel Ross, president of Universal Electric Corporation, "Busway systems are gaining wider acceptance and popularity in the data center market, replacing traditional cable whips under raised floors."

Busway systems are gaining wider acceptance and popularity in the data center market, replacing traditional cable whips under raised floors.
Joel Ross,
presidentUniversal Electric Corporation

Ross sees the colocation market as a perfect candidate for bus track products, especially since they service many different customers with constantly changing power requirements. "The data center industry is realizing that busway's reliability, the added value of its unique features and its large volume of installations makes a compelling case as a best-practice technology in data center design."

Of course, where there is market acceptance for a product there comes competition. This year, several of the major data center equipment vendors (Eaton, Emerson, PDI and Siemens) have also introduced their own bus-type products specifically aimed at data center applications. These new units are somewhat different than the tradition large industrial and commercial busway or "bus duct" systems, which are normally used for main power feeds, that are also manufactured by most of these same vendors.

According to Steven E. Kuehn of Siemens, the company's XJ-L Busway has been around for a while but is only now being adopted for the data center at the rack level.

"We've been producing Siemens busway solutions since 1960," Kuehn said. "Over the years we have refined this offering for different applications. Currently we have more than a million feet in service."

He said that Siemens has learned a few things about offering bus-type power to data centers, including the demand for power takeoffs with simple installation and multi-circuit feeders. "We've found this is a must for data center system designers -- they insist on having enough flexibility to be able to meter and monitor loads at varying intervals. All of this is necessary so IT engineers can draw operating data from more points."

Now let's look at what these products offer. They are all designed to provide three-phase power along the length of the track/bus (single-phase power is also available via the circuit configuration of the tap-off box). The typical track section length is 10 feet (some vendors offer lengths of 3-12 feet) and can be extended up to 100 feet by simply adding sections using an end-splice box, right angle or "T" connectors. Power takeoff to each rack is derived by the use of a plug-in or slide-in power tap-off box. The power tap-off boxes are available in different configurations and capacities, ranging from 120 V/20 A single-phase up to 208 V/100 A three-phase (or 415/240 V). The tap-off box typically contains circuit breakers and outlets (or a short drop cord or cords with receptacle).

The bus: How much power can be delivered?
The bus systems come in different amp ratings, ranging from 60-600 A, with most vendors offering them in three standard ratings of 100, 225 and 400 A.

With rising power densities, such as cabinets housing up to four blade servers (or up to 40 1U servers), these systems need to supply a significant amount to power to each rack. For example, with four blade servers (each requiring 4-8 kVA), each cabinet could require 16-32 kVA. So how do these systems address these challenges?

Bus power delivery capacity
Maximum available three-phase power* at different voltages/currents

Amps Voltage KVA
100 208/120 36
100 415/240 72
225 208/120 80
225 415/240 160
400 208/120 144
400 415/240 288
600 208/120 216
600 415/240 432
800 208/120 288
800 415/240 576

*Example for three-phase power assuming balanced phases and 200% neutral. (Not de-rated -- example assumes the busway is being fed by 100%-rated breaker.) Note: Single-phase loads will reduce this since the phases will not be fully balanced.

Supported number of cabinets
Power per cabinet: Approximate number of cabinets supported at various kVA

5 KVA 8 KVA 10 KVA 15 KVA 20 KVA
7 4 3 2 1
14 8 7 4 3
16 10 8 5 4
32 20 16 10 8
28 18 14 9 7
56 36 28 19 14
43 27 21 14 10
86 54 43 28 21
57 36 28 19 14
115 72 57 38 28

As can be seen from the above chart, the higher-density cabinets require the larger bus ratings in order to support more than a few cabinets. If you are using 208/120 V power, consider planning for high-density 400 A systems (or larger), which offer the greatest power capacity and flexibility for future growth.

All the three-phase products are rated to at least 480 V (some at 600 V), but in the data center the typical distribution voltage to the rack in the U.S./North American market is 208/120 V. In Europe, the common distribution voltage is 415/240 V. In addition, all the vendor's products can be specified with 200%-rated neutral bars, a virtual necessity in a data center.

Most manufacturers offer busways up to 400 A. Some offer (or plan to offer) units rated at 600 A or possibly 800 A in the future. Check with each vendor to make sure that the system you are considering will use the same power-tap modules throughout the product line, otherwise you may be stuck with power taps that will not fit the larger-rated bus systems in the future.

While this article is about busway-type power distribution, the above chart also clearly shows the advantage of considering the use of 415/240 V power in a U.S. data center, especially if high-density equipment such as blade servers are contemplated, since it can deliver twice the power over the same size busway. Interestingly, nearly all the vendors interviewed indicated that they had some customer queries or actual installations of 415 V systems in the U.S. (For more on this topic, check out this tip on choosing a power distribution voltage for your data center.

The power tap-off module
Each manufacturer has its own proprietary power tap-off modules. These vary in size and shape, as well as total ampacity and the number of circuits or receptacles they can support. In some cases this can severely limit the practical amount of power that can be delivered to the rack-level power strips, a potential problem for high-density cabinets. For example, each blade server has two to three power supplies, each requiring a 208-240 V/20 A circuit (non-redundant). Of course, you must double that for full A-B power redundancy. This adds up to a high number of circuits per tap, per cabinet. Some manufactures offer power taps that can accommodate up to 12 circuit breakers to be able to deliver up to four 208 V three-phase circuits, six 208 V single-phase circuits or twelve 120 V circuits, while other are limited to a single three-phase breaker or three single-phase circuits.

Example: Power delivery to support a three-phase rack-level power strip

Circuit rating in Amps w/ 80%-rated breaker Max. rated kVA (fully balanced phases): 208/120 V Max. rated kVA(fully balanced phases): 415/240 V
20/16 5.7 11.5
30/24 8.6 17.2
40/32 11.5 23.0

As can be seen from the above chart, you will need to carefully consider how many circuits as well as the type of connector each power tap will need to properly support enough rack power strips for the higher IT power loads. Moreover, it is nearly impossible to have fully balanced phases when connecting single-phase loads such as most IT equipment so that the actual usable power will be less. While some manufacturers' power taps offer ratings up to 100 A per tap, they may only offer a single large output breaker and receptacle. You may want to consider power taps that can support multiple breaker configurations such as (2) x three-phase or 6 (or more) single-phase breakers. Check that the vendor you are considering offers the number and type of receptacles or drop cords for your particular requirements.

Potential benefits of bus power
Improved cooling efficiency. In addition to power distribution flexibility, one reason to consider moving to an overhead bus-type distribution system is improved cooling. As power densities increase, more power cables are needed. If the additional power is run under a raised floor, it further impedes airflow, which makes it even more difficult to deliver adequate cooling, especially to high-density cabinets. Moving to overhead power distribution helps alleviate this issue. Moreover, since the busway would normally be located above the cabinets, it partially blocks the front-back overhead airflow, helping to limit overhead cross-mixing of hot-aisle/cold-aisle airflow by creating a continuous barrier.

Improved electrical efficiency. Some vendors claim that their bus systems offer better energy efficiency than individual power whips, since a large value bus conductor offers a lower voltage loss over a long run. In theory this may prove somewhat true, but the only fair way to compare this claim is with an actual length-based layout. It is important to evaluate each vendor's voltage drop specification per a given length (which is based on the size of its bus bars) in order to compare energy-efficiency gains or losses.

Potential drawbacks to bus power distribution
Reliability and availability. One concern is that a short or overload would cause the loss of an entire track feeding a row of cabinets. While not likely to happen in a properly managed system, this factor should not be overlooked in a non-redundant system. In a fully redundant A-B system, it should not be as major of a concern.

Physical space and overhead clearance issues. If you are planning for A-B redundancy, the ability to physically offset the tap-off points of the bus and/or the tap-off modules themselves to allow enough space for redundant power tap-off modules is important. Some systems are well designed and allow you to stack or stagger the bus to allow for redundant A-B power paths, while others have some restrictions or physical limitations. You also need to be aware of the total overhead space requirements of the system you are considering.

Some open bus track systems are designed to only hang from the ceiling while other busway systems could also be supported by brackets mounted to the top of the cabinets. Other systems can be used either overhead or under floor. So if you are planning to design or upgrade your power distribution system using a bus-type system, make sure that you review the physical installation considerations, not just the power ratings.

None of the products are interchangeable, which means that you need to commit to the vendor's proprietary system. Since some of these products are just now in their first generation (others are just adaptations of the vendor's existing industrial busway), check that the system you're considering allows you to use the same power tap-off on different size/ampacity busways to avoid early obsolescence.

Safety consideration: Applicable standards
All systems are designed to have power taps inserted into a live busway or track. UL 857 seems to be one of the primary U.S. standards for bus-type products that allow for insertion or removal of a power tap-off in a live bus system without special protective safety gear. While all the systems claim to meet the IEC IP2X "finger safe" rating, each has a unique design. In most cases, electricians will probably still be the ones inserting in the tap-off boxes; however, it is possible for non-electricians to just plug in the power taps.

Some systems have a continuous open track that may make some users uncomfortable, especially while trying to insert or remove a power tap into a live track. Other busway systems have openings at regular intervals, which have safety covers that must first be removed in order to insert the power-tap box. Eaton's Bus Pow-R-Flex system features a unique "safety shutter," an automatically interlocking sliding access cover that only opens as the power tap is inserted, which keeps the live conductors fully covered.

"Pow-R-Flex busway is one of the most significant innovations in data center power distribution," said Mark Mull, global product line manager of busway products at Eaton Corporation. He said that Eaton's Pow-R-Flex is UL marked to UL 857 and IEC 60439-2, and offers ampacity to 600 A, 600 V short circuit ratings as well as bus plugs fitted with receptacles or drop cords and receptacles through 600 V.

Emerson Liebert's Cameron Nowak, Product Manager for Power Conditioning & Distribution products, said the Liebert MB busway can be used under floor as well as overhead for customers that still prefer to keep the power out of sight. "Liebert views bus as one piece of a complete data center and offers easy integration with other data center components" such as floor-mounted PDUs, racks and power strips. Furthermore, he said that the MB Modular Busway is Liebert's "newest product designed to optimize power distribution at the rack level and complete the power distribution from the UPS to server."

Energy monitoring and management
Each vendor offers its own optional system to provide energy monitoring and management. Some are fully integrated within the power tap-off while others rely on external rack-level PDUs.

One system by Power Distribution Inc. (PDI) offers a special feature: A segment within its PowerWave Bus System can contain an integrated communication path that eliminates the need for separate signal cabling to each power tap, according to David Mulholland, VP of marketing at PDI.

Mulholland describes the Powerwave Bus System as "the first overhead bus system specially designed for data centers." He adds that "PDI's product includes built-in power-monitoring communication through its patented Branch Circuit Monitoring System."

The bottom line
If there is one constant in the data center, it is change. IT equipment evolves and is replaced constantly. Moreover, all data center operators and IT departments are being asked to do more with less. If you are evaluating your data center's power infrastructure, bus-type power distribution systems should be given serious consideration. While traditional power distribution will not suddenly disappear, flexible distribution systems will start to make a stronger appearance, especially in new installations and upgrades. In any event, all the major vendors seem to be jumping in with products, and that means that the data center designer has a greater variety of options from which to choose.

ABOUT THE AUTHOR: Julius Neudorfer has been CTO and a founding principal of NAAT since its inception in 1987. He has designed and managed communications and data systems projects for both commercial clients and government customers.

What did you think of this feature? Write to SearchDataCenter.com's Matt Stansberry about your data center concerns at  [email protected].

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