How to choose PDUs for a data center

How to choose PDUs for a data center

Power distribution units (PDUs) are the power bars that distribute power within your server cabinets. PDUs come in all kinds of sorts and flavors. Choosing the right model is paramount to ensure your data center is clean, reliable and simple to operate. Here are our tips to choose the right product.

Power specs

Voltage

The first thing you need to consider is the voltage at which your servers will run at. Most data centers in North-America will operate at 208v, but some use 110v or even 240v. You cannot use a PDU made for X voltage on Y voltage unless the product specifically supports it.

Power density

Next, you need to figure out how much power you need in your cabinet. The amount of power you need will dictate how many amps your PDU needs to support. The most common types are 20,30 and 40 amps. For example, if you need 5 KW of usable power, you will probably get a 208v 30A PDU.

You can find PDUs from 1 to 20+ KWs of capacity. Always remember that you must never go over 80% of the PDU’s capacity for safety reasons.

Single-phase or three-phase

3 phase pdu

Low-density racks will always use single-phase power. However, medium or high-density racks will require 3 phase power. Of course, your PDUs will need to correspond with the power setup.

In the case of three-phase power, you will have the choice to choose from alternating phase power outlets or basic phase distribution. Alternating phase PDUs will alternate outlets in the order of phases (A-B-C), where are basic phase distribution PDUs will have a section for each (top section will be phase A, the middle will be phase B, and the bottom will be phase C). It is important to take this into account since all phases must be balanced. If you only have switches in the top of your rack, but power-hungry servers in the bottom, a basic 3 phase PDU will have unbalanced phases. An alternating phase PDU will allow you to plug any device into any phase no matter if the server is at the top or the bottom of the cabinet.

Input connector type

The input connector (the one that plugs the PDU into your bus bar or remote power panel) will mostly depend on the amount of current (amps_ your PDU needs and if your PDU is using a 1 or 3 phase power setup. Just make sure your power infrastructure can receive the type of power connector. This will vary from region to region.

Monitoring type

Basic

A basic PDU has no power monitoring type what so ever. Basic PDUs often have breakers to prevent power overload, but nothing else.

Metered

A metered PDU will have an LED screen that will show you the live power load of the PDU, either in amps or watts.

Smart metered

smart metered PDU

A smarted metered PDU will have an ethernet port that you can connect to your monitoring network and pull data from via SNMP to remotely monitor the power load of your PDU and set-up alerts if needed.

Metered by the outlet

Metered by the outlet PDUs will have the same smart interface as a smart metered PDU but will tell you which outlet is drawing how much power. This type of PDU is much more expensive but can help you track down power consumption by the server.

Switched

switched PDU

Switched PDUs will allow you to remotely turn on and off outlets via a web interface. This is very practical to remotely reboot a server that doesn’t have IPMI enabled or to prevent users from plugging devices in certain outlets.

Form factor

Rack-mounted

1u pdu

Rack-mounted PDUs will take 1-2Us of rack space and be installed just like a switch. They are practical for small racks but waste quite a bit of usable real estate.

Zero U

zero U pdu

Zero U PDUs will be installed at the back of the cabinet. Most quality server racks have the standard mounting points for these PDUs. Zero U PDUs allow you to keep all your server space for your IT equipment and have clean cable management.

Other considerations

Number of outlets

Of course, the number of devices you have will determine the number of outlets you need. Always get more than you need. It’s always nice to have space for more or to plug in a monitor for server troubleshooting.

Type of outlets

Depending on the voltage and amps your servers need, the output connector will vary, and your power cables will need to be purchased accordingly. 208V PDUs will often use C13 plugs for 15A and C19 plugs for 20A. Make sure you have the right cables.

Shared or dedicated outlets

PDUs will either have a shared or dedicated outlet setup. That explains why you’ll see some 11KW PDU’s with very few outlets and some with many. If you have just a few very power-hungry servers, you will want to get PDUs with dedicated outlets, whereas if you have multiple medium servers that don’t need the full 15A of power a C13 connector can deliver, a shared setup will be best.

Redundant PDUs

A+B pdu

True data center deployments will install 2 power chains for true redundancy (A+B or 2N). If you wish to install a redundant power deployment, you will need to reserve the necessary space in your cabinet for two PDUs and the necessary cables to connect both of your server power supplies on either one of two PDUs.

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