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.
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.
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.
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.
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.
There are several types of power distribution unit available ranging from the most basic format the most advanced feature-rich design.
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.
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.
As soon as you start to add multiple devices to your server rack, power management can become an issue. The use of Power Distribution Units (PDUs) with some form of metering or intelligent IP connection can help to overcome this and provide a pathway for the future scale-up of your IT deployment.