AC vs DC power in data center

There has been a debate going on for some years if the traditional AC or new DC power distribution is best approach to power a data center. The DC power side has been pushing their technology with claims of quite considerable power savings. In many published articles, expected improvements of 10% to 30% in efficiency have been claimed for DC over AC. I have had my doubts of the numbers on their promises. Now there is some new data on AC vs DC issue available.

White paper compares AC vs. DC power distribution for data centers article tells that a new white paper from APC by Schneider Electric provides a quantitative comparison of high efficiency AC vs. DC power distribution platforms for data centers. The latest high efficiency AC and DC power distribution architectures are shown by the analysis to have virtually the same efficiency, suggesting that a move to a DC-based architecture is unwarranted on the basis of efficiency

A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers paper demonstrates that the best AC power distribution systems today already achieve essentially the same efficiency as hypothetical future DC systems. It also tells that most of the quoted efficiency gains in the popular press are misleading, inaccurate, or false (like I have suspected to be for some time). And unlike virtually all other articles and papers on this subject, this paper includes citations and references for all of the quantitative data (which is very good).

The paper first describes that there are five methods of power distribution that can be realistically used in data centers: two basic types of alternating current (AC) power distribution and three basic types of direct current (DC) power distribution. These five types are explained and analyzed.

One AC and one DC, offer superior electrical efficiency. The paper focuses on comparing only those two highest efficiency distribution methods, which are very likely to become the preferred method for distributing power in future data centers. The data in this paper demonstrates that the best AC power distribution systems today already achieve essentially the same efficiency as hypothetical future DC systems.

The best AC system is based on the existing predominant 400/230 V AC distribution system currently used in virtually all data centers outside of North America and Japan. Increasing Data Center Efficiency by Using Improved High Density Power Distribution white paper gives details how it could be used in USA. It says that the use of the international 230/400 V distribution system instead of the USA standard 120/208 system can save 56% in the lifetime cost of the distribution system, and save floor space and weight loading.

The preferred DC system is based on a conceptual 380 V DC distribution system (consensus in the literature as a preferred standard) supplying IT equipment that has been modified to accept DC power. In the proposed international ETSI standard for DC distribution for data centers, the 380V DC system is actually created with the midpoint at ground potential to keep the maximum system voltage to ground to within +/- 190 V.

Based on the data I think the 400/230 V AC distribution system is the best way to go in data centers around the world.


  1. Tomi Engdahl says:

    Bureau of
    Indian Standards
    Redefining Electricity
    First International Conference on Low Voltage Direct Current
    New Delhi, India,
    & 27 October

  2. Tomi Engdahl says:

    ETSI EN 301 605 (Grounding and Bonding)
    - Summary

    ETSI EN 301 605 (Grounding and Bonding)

    Both system
    arrangement comply with relevant safety requirements

    IF the continuity of operation is placed in the forefront THEN the symmetrical IT
    with earthed high
    point is the first choice
    In cases where an
    IT system is used for reasons of continuity of supply, automatic disconnection is not usually required on the occurrence
    of a first fault (single fault) to an exposed
    part or to earth. This is valid on condition that an
    Monitoring Device (IMD)
    indicates the first fault by an audible and/or visual signal which shall continue as long as the
    fault persists.

    IF similar system
    arrangement as for today’s
    system is requested
    THEN the TN
    S system +400
    may be chosen

  3. Tomi Engdahl says:

    In this article, we look at low-voltage dc at 380 Vdc,
    the new industry specification and the single worldwide
    standard in data centers.

    The Advantages of 380 Vdc
    subsequent r
    &D was able to fully articulate the
    benefits and advantages of 380 Vdc for the data
    center. t
    hey show that 380 Vdc:

    is 28% more efficient than 208 Vac systems at that

    is 7% more efficient than 415 Vac

    results in 15% less up-front capital cost in production

    requires 33% less floor space

    has a 36% lower lifetime cost

    is 1,000% more reliable

    uses 20–100 times less copper than –48 Vdc

    introduces no harmonics and is safe

    in fact, lower capital and operating cost is the reason that
    380 Vdc is the ideal voltage today.
    the third secret of mak
    ing a cost-effective, efficient voltage distribution standard is
    to stay below 420 V so as to use parts that share the volume
    economics with desktop personal computer P
    s. t
    hat is
    why the industry selected 380 Vdc (and a specification that
    requires operation up to 400 Vdc and must survive exposure
    of up to 410 Vdc).

    table 2. Analysis of IEC protection against electric shock safety requirements
    shows that distributing ±190 Vdc with a midpoint ground can provide 380 Vdc that has a
    comparable safety margin to ac at 208–250 Vac.

    380 Vdc is a hazardous energy, no more or less than ac at
    these voltages. Whether it is more or less hazardous than ac
    should not be the issue; it is enough to note that both ac and
    dc are hazardous at these voltages.
    national and interna-
    tional standards exist for the proper safety procedures for
    both ac and dc, and they need to be followed.
    at the same time, the data do exist, and they tend to
    show dc is actually safer than ac at the same voltages

    iec 23
    e W
    g 2
    analyzed; according to these data, 380 Vdc distributed
    at ±190 Vdc is actually as safe as ac voltages from 208
    Vac up to 250 Vac

    Debunking Common Myths

    DC is only 1% or 2% more efficient:
    False. t
    foregoing data clearly support that 380 Vdc is 7–8%
    more efficient than low-voltage ac will ever be.

    DC requires big conductors and can only go a cou
    ple of meters for a reasonable cost:
    While that may
    be true of dc at 12 or 24 or 48 V, it is certainly not true
    at 380 V

    Running off the battery bus eats up all your effi
    False. While it is true that the 380-Vdc P
    is 1–2% less efficient when it has to tolerate the wide
    260–400-Vdc input, this was judged more than worth
    while to get a 1,000% reliability gain

    Arc flash is an unacceptable hazard with 380 Vdc:
    False. t
    he connectors have been specifically designed
    with features to fully enclose the arc for 5–10-
    currents. c
    onnectors with magnetic arc breakers
    and switched interlocks exist in the market today
    for greater currents. a
    nd the iec
    -309 connector is
    already fully rated for up to 450 Vdc.

    AC is safer because the voltage crosses zero 50–60
    times a second:
    this is only true if the current is not
    leading or lagging. a
    nd if that were true for servers,
    they wouldn’t have PF
    c circuits.

    An ac server PSU is more efficient than a dc PSU:
    this can never be true for a given design. s
    imply clip
    the bridge and PF
    c circuits from said ac supply, and
    it is now a 1–2% more efficient 380-Vdc supply.

    What to Expect Next
    a worldwide industry consensus is building around
    380 Vdc, and it is being led by the 95-member
    alliance. t
    he group’s vision of l
    Vdc in the four areas
    of commercial buildings—occupied space, data center,
    building services, and outdoors—has led to initial indus
    specifications for 24 Vdc and 380 Vdc (distributed as

  4. Tomi Engdahl says:


    The ETSI standard states that the voltage drop in the entire 48 V DC loop has to be limited
    to 1V [12]. This leads to oversized cables, especially on long distances in order to meet

    At its origin, the telecom industry relied on 48 V DC technology, for safety reasons in
    wireline communication networks. As the years went by, this voltage was kept because of the large
    installed base, which led to lower equipment cost.

    AC and DC topology comparison

    The choice that was made to mitigate the losses in this ve
    ry high power density
    environment was to increase the voltage, and reduce the amount of conversion steps (only two in
    the 380 V DC topology) in order to mitigate the conversion and conduction losses associated with
    the AC setup.

    The voltage was
    determined to be 380 V DC because it is a voltage that was already present in
    the power supply at the time, thus facilitating the deployment of the technology. 380 V DC is
    also considered a low voltage from a regulatory standpoint.

    380 V DC topology g
    ot rid of all phase balancing issues, as well as harmonics present in the
    legacy AC system. Also, DC offers much better integration of renewable resources
    , such as PV
    , wind power
    and fuel cells
    , that are natively DC.

    of the improvements resulting from the use of 380 V DC
    in the data center industry can
    be applied
    to a cell site facility

    Cabling modeling
    Cabling was selected following the principles of the ETSI EN 300 132-2
    for the 48 V DC
    system (less than 1 V of voltage drop in the entire loop).
    The regulations for a 380 V system were unclear up to two years ago. The Japanese Electric
    code states that the voltage drop rate has to be 1.5% or less for a branch circuit, or 2.5% or less
    including the main line. These specifications are defined for facilities carrying sensitive ICT


    The voltage originates from the DC source at 169 V DC, before being boosted to a voltage
    of 380 V DC. This voltage is subsequently
    to a voltage of 48 V DC to feed the loads at the
    top of the tower and in the shelter.

    As we would have expected, the 380 V DC system is more energy efficient than the 48 V DC system. The hybrid topology is the least efficient of the three systems

    The ETSI EN 300 132-
    2 makes a 48 V DC loop energy efficient at the expense of great
    amounts of copper.
    The 380 V DC system is energy efficient and costs less

  5. Tomi Engdahl says:

    Telecommunications cable

    Telecommunications power cable products, as described in Telcordia GR-347, consist of a stranded copper conductor used in AC/DC circuits up to 600 V that are insulated with non-halogen, limited smoke, polyolefin materials that are heat-resistant, moisture-resistant, and flame-retardant. These cables are provided as either Class B (standard) or Class I (flexible) products.

    Telecommunications power cable is intended for use in AC/DC distribution circuits, wireways, racks, and conduits installed in telecommunications Central Offices (COs), transmission stations, cell-tower sites, and other remote sites. These environments are normally dry, but cables may be placed in partially covered or protected porches, crawl spaces, or in underground vaults where water and high moisture levels can occur.

  6. Tomi Engdahl says:

    Rack Power (PDU) terms and technology

    This video is a primer on the terms and technology behind the rack Power Distribution Units (PDU) that are used in data centers.

    What Functionality Do I Need in a Power Distribution Unit?

    Power Distribution Units can offer a variety of features such as local displays, remote monitoring capabilities and even outlet control. There are six main PDU functionalities available in the market today, and it’s important to understand the benefits of each. Today we will be discussing PDU functionalities, focusing on their main features, and deciding what features are important to your application.

  7. Tomi Engdahl says:

    Key grounding and voltage
    considerations in the data center

    Designing a data center’s power system consists of numerous decisions about the
    components in the power path. In most of the world, there are two primary voltage
    schemes (three-phase) available, which are based on either the North American
    480/208/120 V (600/208/120 in Canada) or the 400/230 V system in used Europe and
    some parts of Asia.

    we are generically referring to the 400/230 V system (this is the midpoint voltage that represents 380/220 V through 415/240 V).

    some data centers are exploring the use of direct current (DC) to improve overall efficiency

    Rack-level power density and distribution

    Here in the North America, the common use of 120 V worked fine when a rack used 1-2 kW
    per rack and a single 20 A circuit was all that was needed (two for A-B redundancy). With
    the advent of blade servers, which typically require 208 V or 230 V circuits and use five or
    more kilowatts as well as racks full of 1U servers, the new baseline is now 5 kW per rack.

    almost all IT power supplies are now autosensing and universal voltage-capable (100-250 V)
    more efficient at 208 V or 230 V than at 120 V (or even lower at 100 V in Japan)

    by making three-phase power available in the rack, you will increase the available power bya factor of 300%, yet increase your cable conductor count and its cost by only 66%

    by deploying three-phase 208/120 V power to the racks, you can supply either 208 V single
    -phase or 208 V three-phase power and also provide 120 V for older or specialized IT gear

    consider using three-phase connectors such as NEMA “Twist-Lock”

    In the 400/230 V system, all output circuits are 230 V single-phase (from any phase to neutral and ground).

    In North America, we commonly use 208/120 V to end-user equipment using standard plugs and receptacles.
    At 480 V, the danger of Arc Flash is substantially greater

    Will European voltages work in U.S. data centers?
    In Europe, only single-phase 230 V is distributed to plug-in devices via standard IEC C13-
    and C19-type receptacles and plugs, at up to 16 A. However, three-phase 400 V power is also commonly available via the larger IEC type 309 receptacles at up to 60 A. Also
    in Europe, 400 V work in the panel is commonly done (with appropriate safety gear), since
    that is the basis of all their power distribution systems.

    It is clear that European data centers will continue to use the 400/230 V system since it is already native to their overall existing power system

    In North America, several vendors now offer 400/230 V products as a higher-efficiency alternative to traditional 208/120 V distribution systems.
    the North American high-utility voltage would be transformed only once to down 400 V (instead of 480 V).
    hope of gaining a 2-5% potential increase in energy efficiency

  8. Tomi Engdahl says:

    INTELEC 2012
    400V DC Microgrid Small Scale Demo System for Telecom and Datacom Applications

    EN 300 132-3-1 normal service voltage range of 260V – 400V DC

    Safety concerns for 400V DC distribution have been systematically addressed with a high resistance mid-point ground (HRMG)
    a. On the bulk power source by midpoint resistive grounding with fault detection
    b. On the distribution system by using ±190V instead of 0V-380V
    c. On the equipment front end conversion, with fully isolated, safety extra low voltage (SELV)-output high voltage bus
    Midpoint resistive grounding with fault monitor provides for a safe and reliable distribution line; fully isolated, SELV output high voltage bus converters used as equipment front-end provide for an extra layer of safety for the operators.

    Existing 48V DC loads can be transitioned to 400V DC distribution by implementing simple, minimally invasive adapters.

  9. Tomi Engdahl says:

    Why Use AC Instead of DC at Home??

    Isn’t AC more dangerous than DC?? So why do we use AC instead of DC to power our homes? Did we go wrong somewhere?

  10. Tomi Engdahl says:

    Rack Power (PDU) terms and technology

    This video is a primer on the terms and technology behind the rack Power Distribution Units (PDU) that are used in data centers.

  11. Tomi Engdahl says:

    The Top 3 Reasons for Transformer Failures

    The culprits are all sins of omission: a lack of craftsmanship, high-quality materials, and good design.

    When it comes to big-ticket items, power transformers are near the top of the list. So, when they fail prematurely, it’s all the more painful: Damages can far exceed the cost of a replacement. The added expenses may include the loss of production time, damaged credibility, and regulatory fines and civil lawsuits.

    “After a transformer failure, the first thing out of the customer’s mouth is inevitably, ‘Hey, it’s just a year old! What happened?’”

    But it’s no mystery, according to Jones. Ultimately, you get what you pay for.

    Plant engineers, facilities managers, general contractors, and specifying electrical engineers can learn a lot from the “post-mortem” experiences of a CSI tech such as Jones. In most cases, the premature failures of transformers could have been avoided, and the culprit is often an inadequately designed or constructed unit.

    “You want equipment that is cost-effective, not cheap,” says Jones. “Cheaper transformers often cost you more in the long run, especially if [they are] critical to your business process or data center. Then the extra $10,000 or $20,000 for a better unit represents inexpensive insurance.”

    “Probably the most influential factor in transformer longevity is the level of craftsmanship, the attention to detail in the manufacturing process, and the quality control,” says Jones. “This is often overlooked in today’s rush to automate every manufacturing process.”

    “Look where they put transformers these days: in tight spaces when the buildings are first being erected or on the roof where you’ll need a crane to replace it,” s

    At that point, this manufacturer put a single wrap of Kevlar, whereas a high-quality transformer would have double or triple layering of insulating material because it’s a high-stress area.”

    The manufacturer tried to avoid the rap by explaining that their transformers needed a snubber, a capacitor-filter-resistor network that connects in parallel with the primary winding to absorb high-voltage transients.

    “But a snubber is a $25,000 add-on,” explains Jones. “If you have a cheaply manufactured transformer and need to buy this extra equipment, where are the savings in that? The university had 10 of these transformers, so they were facing a quarter-million-dollar jump in costs. Now who is going to pay for that? It becomes a big fight.”

    Jones also discussed the importance of the iron core material. Pure original—as opposed to recycled—magnetic silicon steel is best. Also, the thinner the core steel pieces, the better.

    Commonly used M6 steel has a thickness of 0.014 in. per piece, whereas the M3 steel is only 0.009 in. thick. To cover the same volume or area, you have more pieces with M3. The more pieces, the lower the no-load losses and the higher the efficiency.

    “High heat contributes to transformer failures,”

    Whether wet- or dry-type, the way coils are wound around the transformer’s core greatly affects its durability. Because of increased axial forces acting at the corners of rectangular-wound transformers, energy gets wasted and noise is created. With round-wound designs, however, voltage stresses are lower, so they stay cooler, run quieter, and present less risk of short-circuit with the sheet wound secondary.

    Beyond the improved reliability factor, the round-wound designs further increase efficiencies and save costs in real time as the plant consumes less electricity.

    “In the past, they made transformers that were somewhat overdesigned in terms of capacity and durability,”

  12. Tomi Engdahl says:

    AC vs DC Switching Demonstration with Arcs

    The difference between switching AC and DC.
    With DC, the arc formed as the switch contacts open is far more difficult to extinguish, and can cause significant damage.

    Explains why relays have different voltage rating for AC and DC.

  13. Tomi Engdahl says:

    Rack Power (PDU) terms and technology

    This video is a primer on the terms and technology behind the rack Power Distribution Units (PDU) that are used in data centers.

    Selecting a rack PDU

  14. Tomi Engdahl says:

    AC/DC Electrical Panel/Wiring Set Up For a Tiny House or Camp Cabin


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