How to Choose a Rack PDU

With hundreds of models and configurations to choose from, choosing a rack PDU can be a more than a little confusing. Determining which rack PDU to use in a particular rack or data centre requires information about the equipment in the rack, the site power distribution, and preference for additional functionality.

When you look at all of the options available it’s difficult to know where to start, so here is a quick guide to help you make the right decision when choosing a rack PDU.

Step 1: Determine output plug type and quantity

IT equipment in racks can have several different plug types. The most common plug types in data centres are C-13 and C-19 connectors. C-13 connectors are usually found on servers and small switches and are similar in appearance to the type of plug you might find on a household appliance such as a kettle. While blades and larger networking equipment that draw higher amounts of power use the C-19 plug because of its higher current carrying capacity.

Selecting the proper combination of outlets for a rack PDU starts with a look at the IT equipment that will operate in the rack. Rack PDUs should have at least as many plugs of each type as the equipment inside the rack so that every piece of equipment can be connected, remember it is always better to have too many outlets than not enough!

Many data centre operators choose rack PDUs with more outlets of each type than are needed for the initial load. This allows additional outlets for future equipment. A popular outlet combination from several manufacturers has been (36) C-13 and (6) C-19 outlets because it allows for a mix of either high or low density equipment. From a data centre perspective, this means that a single, common rack PDU can serve practically any rack on the floor.

Step 2: Estimate power capacity

Several methods can be used to estimate the maximum power required per IT rack. Once the power requirements are established, a rack PDU can be selected that provides at least enough power to support the rack’s load.

1. Estimate the power usage of the equipment inside the IT rack. This method is commonly used when very high power loads are used such as enterprise class servers, blade servers, or high speed networking. The nameplate rating on this equipment is typically higher than actual usage because it is based on the power draw of the power supply at full load. Servers are generally not configured to draw the full power of the supply, so this approach is considered to be fairly conservative.
2. Several manufacturers including Cisco, Dell and HP offer online calculators that can more closely estimate the actual power draw accounting for the specific configurations (number and type of cards, drives, etc.). Schneider Electric offers a sizing tool to help estimate a realistic power usage based on specific models and configurations, but if you’re still not sure you can always call us and we will gladly help you work it out.
3. Assume a maximum power level of the IT rack based on an estimate of total data centre utilization. For example, if the data centre delivers 1 MW of power to the IT load, and the IT load consists of 100 racks, then a maximum power level could be estimated to be around 10 kW for the vast majority of the racks. This is easier to estimate and implement than calculating a maximum for each rack individually.

Step 3: Determine in- put plug type

With considerations inside the rack complete, decisions can now be made about the overall row power distribution. The site voltage (i.e. 240V, 400V etc) should be determined. Additionally, a decision must be made about delivering either single phase or 3-phase power to the rack.

The site power distribution scheme (voltage and phase) and the estimated power requirements will dictate the input power cord or whip amperage needed for the IT rack. Typically rack PDU’s will be available with ether 10amp, 16amp or 32amp input ratings. If more power is needed within a single rack, an additional set of PDUs on separate circuits should be installed.

10amp and 16amp input PDU’s are normally available with either a C14 or C20 plug type. Often in data centre environments a 16amp PDU will be available with an EC309 plug type, more commonly known as a commando connector. For higher loads requiring a 32amp input, rack PDU’s are almost always supplied with a 32amp commando connector.

Step 4: Select visibility and control options for branch circuits

Rack PDUs can also monitor power and provide visibility into the instantaneous power consumption in addition to overall power consumption trends over an extended period of time. There are three levels of visibility into rack-level power status: basic, monitoring, and metering.

A basic view provides no information about power consumption at all. In this view, the rack may be very close to tripping a breaker, but nobody is able to identify this as a problem.

A monitoring view provides a local screen that displays instantaneous information only. This can be useful in determining phase balancing and general circuit load status, but provides insight only into one instant. Decisions driven by this information will not take into account peak usage or cyclical trends. These devices cannot signal outside of the local display, and cannot alert IT staff about high-risk situations as they happen.

A metering view allows visibility when circuits approach the maximum capacity and when breakers are at risk for tripping. Through the use of a built-in network management card, metered PDUs can alert users when loads approach predetermined thresholds through email, text message, visual alerts on displays, and through other means before problems arise. IT staff can use stored power consumption history to analyze trends and make more informed decisions about where to add new devices so circuits can be balanced and never at risk of overload. This type of proactive planning is a solid approach to eliminating downtime from tripped breakers.

Some rack PDUs offer metering of individual outlets instead of just branch circuits. While most data centres are not prepared to take advantage of this deeper look into power con-sumption in the rack, it empowers IT professionals with the necessary tools for advanced energy management.

Many rack PDUs are also able to measure other dimensions of the operating environment. Commonly, temperature and humidity sensors can connect to metered rack PDUs. PDUs can then display that information on the local display and store or transmit the data across the network. After power availability, server temperature is the most likely cause of equipment failure and downtime. In-rack temperature sensors connected to rack PDUs are an easy way to ensure a proper operating environment for servers and other equipment.

Rack PDUs can also provide control as well as visibility. Switching functionality allows outlets to be cycled on/off remotely via a network connection. This is common for remote sites. Servers or switches occasionally need physical reboots, even in an office or facility that does not have on-site IT staff. A common way to do this involves calling the local administrator. The administrator walks into the unfamiliar network closet and looks for cues to identify the troubled device in order to restart it. Even with good intentions, the wrong equipment is sometimes rebooted. Switched rack PDUs allow knowledgeable IT staff to manage equipment cycling even if they are off site.

With colocated equipment, one of the most common calls is to request manual restarts of hung servers. In order to do this, the colo admin must walk to the rack, identify the correct server, restart it, and then verify with the customer that the task is completed successfully. This takes time, and there is also typically a fee associated with any work done by the colo to manage customers’ equipment. This lag time, risk, and expense can be eliminated with rack PDUs that can switch outlets remotely.

Step 5: Select form factor and mounting

Commonly, each piece of IT equipment has redundant power supplies intended to provide power in case the other fails. For data centre applications, these power supplies are generally connected to separate redundant rack PDUs which are, in turn, fed from separate sources or circuits. This prevents the entire load from dropping in the event of a fault along one power path.

Rack PDUs install into the back of a server cabinet and provide convenient outlets accessible to both IT equipment and users that must configure the equipment. Two primary mounting orientations are:

• Horizontal 482.6mm (19in) rack mountable PDUs mainly used with open frame racks and with audio/video equipment.
• Vertical 0-U PDUs that distribute outlets closer to the equipment they power. This style is the preferred orientation in data centres because they consume no rack U space and allow shorter power cords and require less cable management. This orientation pro vides a clearer and more visible power path for every cord.

Selecting a rack PDU should start with an understanding of the type of equipment that will be installed. IT equiment will dictate outlet type and quantity, as well as power consumption requirements. Using the power requirement estimates and information about the site voltage, whips and input cords can be appropriately sized.

Additional consideration should be given to preventing overloaded circuits and high temperature applications. Metered rack PDUs can alert administrators before circuits are overloaded and reduce the risk of downtime. They provide historical power usage data which can be used to make better decisions than instantaneous readings alone.

For most cases, it is possible to standardize on one or two rack PDUs that are sufficient for practically any cabinet in the data centre.

For more information abour our rack PDU range or to discuss your requirements, contact a member of our team today.

Call 01252 692559 or email: info@sourceups.co.uk