Rack PDUs: A Look at Basic Concepts Involved in Power Distribution

Posted by Enconnex Team on May 4, 2021

| Categories: Power Distribution Unit

Given 24x7 operational requirements and growing concern about power efficiency, the power distribution unit (PDU) has become an increasingly important part of the data center infrastructure. Today’s rack PDUs do more than bring power to IT equipment. They provide monitoring, metering, and switched outlet capabilities that help administrators balance loads and remotely manage power requirements.

A wide range of rack PDU configurations are available, and which you choose depends upon the input power architecture (number of phases, current, input plug type, etc.), the power consumed by the equipment in the server rack, and other factors. Following is a primer discussing some of the concepts involved.


Branch Circuits

Branch circuits distribute power to server racks and cabinets from an electrical panel, switch, or distribution board. The wiring may run under a raised floor, in an overhead bus system, or both, terminating in an outlet near the server racks and cabinets. The load capacity of the branch circuit is given in volt-amps (VA) or kilovolt-amps (kVA) and is calculated by multiplying the rated voltage by the rated current.

The voltage depends in part on whether the circuit has single-phase or three-phase wiring. Single-phase wiring is what you would find in any office or home — one alternating current flows through the circuit, reversing direction 60 times a second. As a result, voltage drops to zero twice per cycle.

Three-phase circuits have three alternating currents that are out of phase with one another by 120 degrees. The voltage of each current drops to zero at different times, so the power supplied to the circuit remains consistent. Three-phase branch circuits deliver more power and require a rack PDU that is explicitly designed for them.

The current is determined by the thickness of the wire and the type of terminating receptacle. The rack PDU plug will need to match the outlet in the branch circuit receptacle. The power cord for a three-phase rack PDU is thicker and heavier than one for a single-phase, but a three-phase unit reduces the number of cords required to provide the same amount of power.


PDU Outlets and Overload Protection

Similarly, device plugs will need to match the appropriate outlet on the PDU. This protects the IT equipment by preventing a 120V device from being plugged into a 208V circuit, for example. Therefore, it’s essential to understand what type of equipment you have in the server rack or cabinet and the type and number of plugs. 

In many cases, you’ll need to power 120V networking devices as well as more demanding servers. A three-phase PDU can include 120V and 208V outlets to support both types of equipment for convenience and flexibility.

It’s important not to overload the circuit. In North America, rack PDUs and other electrical equipment must draw no more than 80 percent of the rated current of the plug. Additionally, PDUs that draw more than 20 amps must have built-in circuit breakers to protect the branch circuit wiring. However, most circuit breakers have a delay that allows them to handle brief overloads when IT equipment starts up.


Rack PDUs from Enconnex

Enconnex has power experts on staff who can help you understand your requirements select the right rack PDU for your use case. We offer single-phase and three-phase PDUs in a variety of configurations, ranging from basic PDUs to switched, metered, and universal units. Let us help you ensure safe, reliable, and efficient power distribution in your data center.

Posted by Enconnex Team on May 4, 2021

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