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How to Select Server Rack PDUs

The number of options available when specifying server rack power distribution units is immense. One of our server rack PDU manufacturing partners has over 5,000 drawings covering permutations that have either been manufactured and delivered or at least quoted over the last decade. Such a number is possible due to the number of PDU options available which can range from outlet socket types, to cable connections, form factors, voltage, communications and even paint colours. Our rack PDU guide details a step-by-step process to help demistify their specification and help you to select the righ server rack PDUs for your comms room, server room or data centre projects.

Server Rack Capacity List

The first stage in designing a rack-level power management system is to make a list of all the devices that are installed or going to be installed within the server cabinet. Details to include on a spreadsheet include make, model, power requirements (VA or Watts, single or three phase) and plug or socket type. This should be a relatively easy step for a new rack but may require an equipment audit for existing cabinets.

A PDU’s power requirements can be difficult sometimes to assess. It is not uncommon to refer to manufacturer sources for load ratings such as rating labels, manuals, or data sheets. Most computer and server manufacturers provide specification information on their websites and may also include load calculation tools. Best practice is to size for the maximum load, even though most power supplies do not reach this. It is better to over than under-estimate. Alternatively consider a server rack power audit to assess individual load ratings (both at start-up and running Amperages).

Once you have the list of equipment and loading(s), the next step can include drawing a single line diagram showing. This can highlight other factors that will affect power distribution within the rack including the use of A and B supplies requiring separate PDUs, power cord lengths and where the PDUs are to be connected into for their own power.

Whilst most devices will be standard AC powered loads (230Vac 50Hz single phase or 400Vac 50Hz three phase in the UK), there may be some that require a DC (direct current) supply and for these separate DC PDUs should be considered, powered from a suitable source i.e. 48Vdc.

PDU Socket Outlet Types

The two most common types of socket and outlets for servers and IT devices are IEC320 C13/C14 and C19/20. C13 and C19 are the socket types and C14 and C20 the plug types. Both are single phase but are rated at 10A and 16A maximum outlet load, respectively. You may also find some loads require a BS1363 UK socket. This is not unusual for some peripheral devices that are DC powered from an AC/DC adaptor plug.

On the load equipment list completed in the first stage of the design process, note against each load the socket outlet type required. When totalled this will provide the quantity of outlets required. You may only require C13 or C19 or the UK square-pin outlet or a combination of these. Remember to add more outlets for future expansion as you may not have the room available for additional rack PDUs later.

Further considerations revolve around power cords or power cables as they are also known. These are the cables that connect the load to the power distribution unit within the server cabinet.

  • Locking or Unlocking: locking mechanisms can prevent accidental disconnection of a server or other IT load from the rack PDU. A typical locking solution will consist of a plug and outlet that lock into place when connected and can only be disconnected if a button is pressed to release them.
  • Outlet Orientation: : this is an important consideration of you have A and B supplies and mount two PDUs on either side of the server cabinet. Custom made to order PDUs can be supplied with the sockets at 45 or 90⁰ angles to make it easier to connect the loads into them.
  • Power Cord Lengths: the length of power cord or power cable is also important for cabinets with multiple loads within a server cabinet. All power cords and cables should be identified with a tag or label which relates directly back to the original load table. This makes it easier to safely manage disconnections for swap outs and upgrades. It is also important of course to have suitable cable lengths to ensure that the equipment can be placed where you need it to be within a rack and have the cable length required to connect to the PDU. Too much power cord cable can also be an issue in terms of safe and aesthetic cable management.
  • PDU’s Power Cord Position: another factor that can be accommodated or ‘made-to-order’ bespoke PDUs is the position of the input power cable. Positions available include the front or rear, and top or bottom of the PDU.

Rack PDU Input Power and Plug Types

Where your rack PDU is powered from and how it is connected to its power supply is as important a consideration. Almost all critical server installations will be powered from an uninterruptible power supply (UPS). The UPS system may be installed as a centralised system, powering a complete computer room, server room or data centre. Alternatively, individual UPS systems may be installed at the server rack level in a decentralised power protection plan.

The UPS arrangement will dictate the connection requirements for the PDU. For server rack UPS, the PDU may be installed with C14, C20 or even hardwired connections. Alternatively, the PDU may require a IEC309 Commando type plug to connect underfloor or overhead power feeds from a UPS powered sub-distribution panel. For single phase applications it is worth noting that the IEC309 plug is blue. For three phase PDUs the plug is red.

At this stage, it also worth double checking that that the load capacity (from the equipment list) can be supported by the UPS system and still allow sufficient headroom for future expansion and overload start-ups. Best practice in UPS sizing is to run at 80% of the UPS rating (kVA/kW) and leave 20% headroom.

Power splitter boxes can help to play a role in the electrical distribution to a server rack. Splitter boxes generally have a single feed e.g. 32A and split this into two separate feeds e.g. 16A and 16A. In this scenario, only a single 32A single phase feed is available, but the requirement is for two 16A rated supplies.

PDU Form Factor : Horizontal or Vertically Mounted PDUs

In terms of server racks, the term ‘form factor’ is used to define certain physical characteristics of the equipment to be installed. Typically, a form factor may be horizontal or vertical and the term can also be used in relation to depth (into the server rack) i.e. a short or long form factor.

In terms of server rack PDUs, the choice is between horizontal and vertical orientations. A horizontal PDU will have a height quoted in ‘U’ such as 1U. This means that the PDU Is designed to be installed across the width of the 19inch rack mount cabinet and has a height of 1U / 44.45mm. If you see a PDU described with form factor of ‘0U’ is a vertical type design.

In terms of the number of outlets, horizontal PDUs are limited by the width of the rack. 2U or even larger horizontal PDUs are available, providing more than a single row of outlets. Vertical PDUs are limited by the height of the server rack or cabinet and are generally the preferred choice for data centre server racks and cabinets. Horizontal PDUs may be more suitable for smaller cabinet and IT system, audio visual and telecoms installations.

Types of PDU

There are several types of power distribution unit available ranging from the most basic format the most advanced feature-rich design.

  • Basic PDUs: a basic PDU provides outlets to which loads can be connected. Protection will generally be in the form of a fuse (or circuit breaker) and the basic PDU may also be available with surge/spike and EMC/EMI protection. A single basic PDU can provide one or more branch circuits dependent upon the design arrangement i.e. A, B and C circuits.
  • Metered PDUs: a metered PDU provide measurement of the connected loads. This can be at the overall total PDU or individual outlet levels. Metering information can include load voltage (V), amps drawn (A), energy usage (kWh) and power factor (pF). Metering information may be available from an LED/LCD built-into the PDU or over an HTTP/HTTPS IP connection.
  • Switched PDUs: this type of switched power distribution unit allows connected loads to be rebooted for power cycling or isolated for maintenance or upgrades. The switch may be a circuit breaker to provide an ON/OFF function at the PDU level or more sophisticated switching at the individual outlet socket level.
  • Metered and Switched PDUs: combine the above features into a more sophisticated type of power distribution unit. Metered-by-outlet and switched-by-outlet providing more functionality than PDU level metering and switching.
  • Intelligent PDUs: are the most sophisticated type of smart PDU used within a data centre environment. This type of PDU offers all the above functions and generally includes SNMP communications for connection to a data centre infrastructure management package. Additional options can include environment monitoring sensors for temperature and humidity, and access control to the server cabinet they are installed within. Intelligent or ‘Smart’ PDUs provide a way to remotely manage an estate of server racks and identify load issues such as under-utilised servers, thermal ‘hot-spots’ within cabinets and assist load capacity planning and equipment layouts.

PDU manufacturer specific software may also provide additional features in terms of PDU monitoring, email and SMS text alerts for power or environment data drifting outside pre-set thresholds and/or equipment failures.

Custom Designed PDUs

It is probably fair to say that the more sophisticated the server rack PDU, the less like it is to be available off-the-shelf. This is not surprising when you consider the number of options available. The greater the number of options, the longer the lead time required.

In addition to the general options already described, custom colours can also be specified for power outlets, power cords and even the metal bodies of the PDUs themselves. In a data centre environment, the use of colour coding for power distribution circuits can make identification easy at the rack level.


The more feature-rich a PDU, the less likely it is to be available off-the-self and will have to be ‘made- to-order’. This may not be an issue for new build sites but can be for comms room, server room or data centre upgrade and refresh programs with tight ‘go live’ deadlines to meet. Our server rack PDU planning and installation service can help here coupled with the fact that we work with several PDU manufacturers and suppliers. Where necessary, a site survey by one of our project managers can assist at the planning stage, as can our PDU installation service.

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