Data centers must be highly available as well as efficient, requiring paying special attention to the power distribution strategy. As power densities and power consumption costs are both on the rise, traditional ways of distributing power to the racks are being challenged.

In an April 2007 survey by Aperture Research Institute nearly 27 percent of responding data center operators said they experienced one or more outages because of issues with power distribution. On the other hand, an EPA study showed data center power consumption can account for as much as 30 percent of an IT budget.

To combat the challenges of being “always on” there are five key considerations when choosing rack-mount power distribution products:
  • Voltage levels
  • Electrical circuits
  • Number and type of outlets on rack-mount PDUs
  • Required features on rack-mount PDUs
  • Remote management capabilities
Several of these considerations also help control power consumption costs of a data center.

Voltage Levels

Most IT equipment requires single-phase power to operate, and most can accept voltages ranging from 100 to 250 volts (V). In North America the electrical infrastructure 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 cur-rent 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.

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 survey of 100 enterprise data cen-ter 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.

Table 1. North American electrical circuits

The tables show that data centers typically have three-phase power available. Because lower power den-sities 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. Phase-level metering on three-phase rack PDUs al-lows 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 sup-plies pull a neutral wire into the rack; delta supplies do not. In North America, if all equipment is oper-ating 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.

Table 2. International electrical circuits

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 outlets required to power the entire IT load within the rack. Equipment can have multiple redundant power supplies with each requiring one or more power connections. A rack with several blade servers will require low number of high-power out-lets, while a rack with several 1U servers will require high number of low-power outlets.

Features to Require on Rack-Mount PDUs

Power distribution units that have metering as well as switching capabilities can help ensure the highest availability levels for mission critical equipment.

When new equipment is connected to a PDU, the plugged-in loads are susceptible to tripped breakers or fuses. To minimize the chance of overloads, metering capability plus the capability to set current thresh-olds within the PDU is important. If the PDU has several branch circuits or is three-phase, metering ca-pability should be available for all branch circuits or phases separately. Nameplate current ratings for most IT equipment is overrated. Metering capability can provide a gauge of the actual power draws of IT equipment. This information could allow operators to right-size upstream electrical infrastructure im-proving the efficiency of the entire power chain

Non-responsive servers or network equipment is a common issue. Switching capability, defined as the ability to remotely recycle, turn on or turn off power, can help minimize downtime caused by hung equipment. Reducing the number of trips to the physical location of equipment also helps lower opera-tional costs.

Several switched PDU solutions have additional features that can improve the availability of the loads:

Sequential startup & shutdown. IT equipment draws a current higher than normal operation at startup called the “inrush current”. Unless the circuit is capable of handling the inrush of all equipment, starting all loads within a rack at once can lead to tripped upstream breakers. Switched PDUs allow out-let turn-on sequencing so the circuit’s current limits are not exceeded. Sequencing also allows applica-tions running on several servers that are dependent on each other to be started or shut-down in a specific order to avoid issues.

Electronic overcurrent protection. When current thresholds are exceeded on a branch circuit, many switched PDUs can lock unused outlets on that branch. Therefore, no additional equipment can accidentally be plugged in and continuous availability to the powered loads is ensured.

Remote Management

To maximize monitoring and control features and ensure a “lights-out” data center, switched and metered PDUs should be remotely manageable. When evaluating the equipment, consider the following:
  • Does the remote management strategy of the PDUs align with tools used to manage other IT devices?
  • Do the PDUs provide an out-of-band management option? In-band management tools are great, but their limitation is the vulnerability to problems from the very devices they manage.
  • Does the remote management tool provide a global integrated view of all PDUs?
  • Does outlet access follow secure authentication schemes?
  • Does the remote management tool automatically discover all attached PDUs?
  • Does the tool provide grouping capabilities for outlets? Grouping allows power control of multi-ple outlets together, useful for when a server with multiple power supplies is hung. By recycling power to all power supplies together, grouping ensures the power to the entire server gets recycled.


A well-planned rack-level power distribution is important to ensure continuous power to all IT equip-ment as well as to monitor and control the power consumption. Proper consideration should be given to the electrical, physical and feature characteristics of the PDUs. Electrically, IT equipment should operate within the 200 – 250-V range when possible. For loads within a rack above 5 kW, a three-phase PDU is best; a single-phase PDU for loads below 5 kW. Physically, PDUs should be easily installable and have the appropriate number of outlets to power all equipment. Remotely manageable PDUs with switching and metering capabilities provide the ideal feature set for highest levels of availability as well as effi-ciency.