Colocation Series #4 – Rack Power

Colocation Series #4 – Rack Power

A typical datacentre rack is supplied with one or two power feeds. When two supplies are presented to a colocation solution, these would normally be an ‘A’ and ‘B’ supply. Two power feeds increases the level of resilience due to the fact they would normally be fed from different master PDUs, UPS systems and even generator systems. This end-to-end separation eliminates any single points of failure which is an important consideration where ensuring the highest uptime levels.

In our Heywood House datacentre, we typically provide one or two 16A or 32A supplies to the rack. The connector utilised is an IEC60309, also known as a commando socket.

At this point you are actually unable to connect any equipment to the rack supply – to do so, you will need an rPDU (rack power distribution unit). A PDU is, in effect, akin to an extension lead. The PDU plugs into the rack supply (IEC309) and presents a number of outlets (typically between 8 and 24) to be used by your in-rack equipment.

Type of PDUs

There are three main types of PDUs (explained below):

  • Basic / Dump PDUs
  • Metered PDUs
  • Smart/Switchable PDUs

Basic PDU

A selection of Eaton Rack PDUs

These PDUs consist of several outlets.  These could be C13, UK or US socket, with each outlet typically rated between 8A and 16A. A basic PDU is considered dumb as there is no form monitoring or control of the outlets. Some basic PDUs do feature a local LED current meter so the operator can view the current when observing the PDU in person.

Metered PDU

Metered PDUs are very similar to basic PDUs with one key difference. A metered PDU would typically feature a controller board, an Ethernet port and a built-in web server. The controller circuitry is able to monitor the current/load of the main input and each individual outlet. This functionality provides real-time data on the power usage and power characteristics. This data can then be accessed through a web interface or more commonly using the management protocol SNMP. Metered PDUs allow you to keep track of when and where your power is being consumed, an important consideration given power is now one of the key costs components in a datacentre environment.

Smart PDU

Building on the features of the Metered PDU, the Smart PDUs take it one step further. While the metered PDU allows you to monitor the status of individual outlets, a Smart PDU enables you to control individual outlets, powering them on and off and enabling you to power down your equipment remotely. These features are typically accessed through the same web interface, SSH or using SNMP.

Smart PDUs are popular with colocation customers who wish to retain full control of their equipment remotely without having to engage datacentre staff.

Wi-Manx uses metered, fully-managed Smart PDUs with all of our shared colocation packages. Using our secure API, customers can also control their allocated PDU ports through our customer portal.

PDU selection

The rack PDU will be rated to a maximum of 16A or 32A, so it is important that the total load never exceeds this rating. The rating ultimately determines the size and load-requirements of the connected equipment. You would normally size the PDU and the rack supply to the envisaged load within the rack.

For example, a typical 1U rack server may use 1A of power.  Connecting 24 x 1A servers to a 24-port 16A PDU is not possible nor would it be recommended. It is important to size your PDU accurately for today’s load and to also plan for any envisaged growth. The metered and smart PDUs make it easy to monitor load and they can be configured to alert you when you are approaching your limit or experience an overload condition.

This overload could be you approaching the limit of your PDU’s capabilities or it could be a logical limit such as the power commit you have negotiated with your colocation provider.

Determining the PDU size and power requirements

This is one of the most common questions we come across when dealing with our datacentre colocation customers – how can they forecast their power requirements?

In most cases, the number of devices and therefore the quantity and the type of power ports is known (e.g. 8/16/24, IEC or UK plug). Although this information has been considered, often the actual power load has not been calculated.

Fortunately, there are a number of ways to estimate the power draw of your equipment.


Dell Energy Smart Solutions Advisor (ESSA)

The ESSA tool enables you to simulate your hosting solution and therefore determine the power demands, both today and with increased load. The tool is available here.

HP Power Advisor

HP makes a tool that, like the ESSA, enables you to forecast power requirements. You can access the tool here.

Other vendors

Many vendors will quote the power and current requirements on their data-sheets. These figures are often the highest, worst-case figures yet do not reflect real-world deployments. In these cases, the recommended route would be to use an inline meter (plug-through or clamp-on) and take real-world measurements to determine the exact power requirements.

Connector Type

In the UK, datacentre PDUs usually have two types of outlets for equipment; UK Plug and C13 (IEC 60320). C13 is the most common power connector when it comes to rack-mountable servers, network and security equipment. Most equipment will feature a C13 socket and come supplied with an IEC lead.  In the majority of cases, certainly in the case of colocation, the customer would use an IEC power lead to connector their equipment to the rack PDU.

Occasionally there are devices that require an AC/DC adapter and therefore a UK plug would be required.

Some PDU manufacturers do build multi-port PDUs that feature both IEC and UK plug sockets but this flexibility can add to the cost. The compromise is a small (e.g. 4 or 6) port UK plug PDU attached to your existing C13 PDU.

IEC and UK plug PDUs are the most common.  However, for expansion, you can purchase PDUs with IEC60309 which enable you to daisy chain further PDUs (provided you do not exceed load limits).

PDU Form Factor

There are two common form factors – horizontal and vertical. The vertical PDUs are mounted in the rack vertically. There is a variation known as “zero-U” which is a low-profile version of the vertical PDUs. Vertical PDUs fit snugly within the rack using the minimal amount of rack space.

Deciding whether to use horizontal or vertical really depends on the preference of the user. Often customers will opt for two vertical PDUs, sometimes one on either side or often on the same side. Other users will opt for multiple horizontal PDUs spread throughout the height of the rack.

Why monitor rack power?

Electricity costs money. The more power you consume in a datacentre, the more it costs the provider and, accordingly, the more you will be charged each month. Technology vendors are constantly developing more modern equipment with far higher power efficiencies. It is for this reason that many datacentre tenants opt for the most modern, efficient ICT equipment saving money in the long run.

A modern Quad-Core Intel CPU when compared to a 2-year old CPU will use a fraction of the power and a far higher level of performance. It is worth using the tools mentioned in this article to estimate the power draw of your legacy equipment against some of the newer server models.

Next article

In the next article, I will explain datacentre cooling and how Wi-Manx uses Cold-Aisle containment to provide an efficient, environmentally controlled hosting environment.

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