How Does a Power Distribution Unit (PDU) Work? The Basics

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

Those features are valuable for effective power management, but they really don’t impact how a PDU works at the most fundamental level. When selecting a PDU, it’s important first to consider the power requirements of the data center and IT equipment. Check out our PDU buying guide for more information on how to choose the right PDU.

The design of the branch circuits determines the voltage, the number of phases, the current, and the input plug type, and the PDU must match these characteristics. Additionally, the PDU must support the number and type of device plugs and the power consumed by the equipment in the server rack. We recently wrote a blog answering the questions of “What is a power distribution unit and what does it do?” The following paragraphs take this topic a bit further to explore basic concepts involved in how PDUs work.

How Does a PDU Work?

Although there is a lot of nuance involved with how rack power distribution units work, the basic concept is a little less complicated than you might think. Rack PDUs take power from the rack’s power source (branch circuits) and distribute it, as needed, to the various outlets on the PDU itself. In short, PDUs turn a single power input into multiple power outputs. The nuance comes as you consider variables such as PDU voltage, phases, plug and outlet types, and overload protection which we will discuss below.

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.

PDU Voltage and Number of Phases

The voltage depends in part on whether the circuit has single-phase or three-phase wiring. You would find single-phase wiring in any office or home — one alternating current flows through the circuit, reversing direction 60 times a second. As a result, the 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 designed explicitly for them (learn more about single-phase vs. 3-phase power).

Current and PDU Plug Type

The current is determined by the thickness of the wire and the type of terminating receptacle. The rack PDU plug must 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.

Number and Type of Device Plugs

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 and more demanding servers. A three-phase PDU can include 120V and 208V outlets to support both types of equipment for convenience and flexibility.

See blog about PDU power cord, plug, and outlet types for more information.

Overload Protection

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. 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 and select the right rack PDU for your use case. We offer single- and three-phase PDUs in various configurations, ranging from basic PDUs to switched, metered, and universal units. We are also getting ready to launch a new line of TAA-compliant, made-in-America basic PDUs. Browse our PDUs for sale and contact our power specialists to get the right PDU for your use case and to stay up to date on our latest offerings.