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A cabinet is an enclosure for housing your equipment with pre-drilled holes (or squares) along a 4-post support system for mounting equipment. When you look at the specs for a cabinet, it may list the inches or centimeters, but in the data center industry, the important specification is called U, or Rack Units. A rack is something different (see below), but the unit of measure is the same between a cabinet and a rack. The most common size for a data center cabinet is 42 rack units, or 42U. Not all 42U cabinets will have the same exterior dimensions - what the 42U refers to is the inside height, and there are various options that may extend the width or depth of a cabinet by varying amounts. Most cabinets today have an outside footprint of 24" wide by 42" deep, but just 10 years ago it was nearly unheard of to have a cabinet with a depth greater than 36". As hardware evolves, so does the infrastructure.

Breakers / Poles

Remote Distribution Unit

Breakers are placed in the panels based upon needs, and they will either occupy 1, 2, or 3 poles (slots) each. The voltage for a single pole is equal to the input 3-phase voltage divided by the Square Root of 3. So in the US, if we have 208 VAC 3-phase feeding the RDU, a single pole would give us a 120VAC circuit. When you use a 2-pole breaker, the voltage goes back up to the supply voltage, but it is now a single phase circuit - so in our example, 2 poles would give us a 208VAC single phase circuit. If you use a 3-pole breaker then you are back to 3-phase at the full voltage, but not the full amperage supplied to the cabinet. Amperage is clearly marked on each breaker and will be a multiple of 5, though in commercial settings it is very unusual to see any breaker with an amperage ending in a 5. Most tend to end in a 0.

Breakers are typically derated by 80% at the recommendation of NFPA70E / National Electrical Code. What this means is that you can run continuously at 80% without tripping the breaker, and you can go over 80% for periods of time before you will trip the breaker. You can even go over 100% for short periods of time. There is a curvature plotted by the manufacturer that will show the performance of the breaker at various loads (amperages), but the rule of thumb is that you always want to plan based on that 80% derating. That's why we automatically derate all amperages entered in openDCIM. If you define a CDU has being fed from a 208VAC panel on a 3-pole breaker at 30A, the capacity display will show 8.6 kW. However, if you perform the math based on 100% rating you will get a higher number
(208VAC * sqrt(3) * 30) = 10.6 kW

No, we're not being paid under the table by breaker manufacturers to make you buy more breakers. We're simply keeping you out of trouble by not letting you plan on that 100% rating unless YOU make the conscious decision to do so (in your head - there's no way to override it in openDCIM).

CDU / Cabinet Distribution Unit

In other words, a Power Strip. Many people use the term CDU because (a) it's cool to use acronyms and (b) it helps to differentiate it from what most people use at home - something picked up from the mega market when it was on sale for $6.99. Today's data center CDUs are typically IP addressable and intelligent enough to provide near real-time (we say near real-time, because there is a standard for energy billing precision, and most CDUs don't meet that standard) statistics on power consumption. Some models also have ports that allow you to connect environmental sensors to them for monitoring things like temperature and humidity at the cabinet level, and some even have remote on/off switching per port.

Point of Demarcation / Point of Entry

Depending on your fault tolerance design, this will be one or more locations in your facility where your service providers (electrical utility, ISP) enter and then terminate into some device that allows you, the customer, to extend that service as needed. In other words, it's the point where your utility no longer worries about the physical infrastructure. If you have designed your facility to be concurrently maintainable, there would be a minimum of two separate points of entry to your facility for all utilities.

Power Distribution Unit / PDU

Power Distribution Units

A good rule of thumb to remember the difference between a PDU and an RDU (Remote Distribution Unit) is that a PDU has a transformer. It may or may not have distribution panels embedded in them, and that's what most commonly confuses some people. In medium to larger commercial settings (and nearly always in industrial settings) a higher voltage than your data center equipment can operate at is typically provided by your utility. It is then the customer's (your) responsibility to use step-down transformers to get the voltage to something usable. In the US, a typical input voltage would be 480VAC 3-phase, but power supplies on your server can only operate in the 100-240VAC (single phase) range, so we have to step it down to something usable. We'll use a PDU, such as the one pictured here, and step it down to 208VAC 3-phase. If you don't understand electricity enough to know what is meant by 3-phase vs. single-phase, that's ok for now, but you should look into some electrical primers. The main thing is to realize when you need single phase versus 3-phase power (nearly all IT equipment uses single phase, but the CDUs will often take input from a 3-phase source and break it down into single phases at the plug level).


A rack is actually referring to infrastructure that was very commonly found in telecom closets and central offices - it is a 2-post system designed to house devices that don't need that additional support. As most hardware has gotten heavier since the early 2000s, it is becoming less and less common to see these racks in a data center other than at the point of demarcation.

Remote Distribution Unit / RDU / Remote Power Panel / RPP

Remote Distribution Unit

The Remote Distribution Unit (RDU) is sometimes called a Remote Power Panel (RPP), but they are one in the same. Certain vendors have tweaked the title to add their own flair to the term as well, but the thing to understand is that an RDU differs from a PDU in that there is no transformer. The RDU shown here is a model that has 4 panels (2 in the front and 2 in the back) of 42 poles each. Voltage has already been stepped down at the PDU, but I now need to distribute that power out to numerous circuits throughout the data center floor. Depending on your density, you could easily have one PDU feeding several RDUs in your facility. Think of this as simply a minor hub in a spoke and hub topology (to steal from networking vocabulary).