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PDUs: the Data Cabinet Center of Power

The exponential growth of data center network devices and high-power computing equipment has led to an increased need for a device that delivers power efficiently, while conserving rack space.

A Power Distribution Unit (PDU) is a viable solution as it provides electrical power and protection to all connected networking equipment. It draws power from an uninterrupted power supply (UPS) and distributes equal voltage to multiple devices.

Power Distribution Units are considered the backbone of data centers and networks as they provide reliable power to connected servers.

Given that each data center and rack environment has unique requirements, there are several factors you should consider before buying a PDU. This guide explores the different types of PDUs to help you choose the right design for your racks and cabinets.

Types of Power

You can opt for either single-phase or three-phase power depending on the type of power you are offered at the Colo. Before you can even consider what models of network devices you can install, you must first resolve the power question.

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Single-phase power

Single-phase power systems deliver one “line” of electrical power.  Commonly used voltages at US data centers are 120 volts (120v) and 208 volts (208v). Also in the US, the AC current runs on a 60-hertz frequency. This means the current runs in alternate directions at a frequency of 60 cycles per second. It’s also important to note that this alternating current is distributed by two wires: an active conductor and a neutral line.

The amount of power that can be drawn for practical use is called apparent power. A single-phase power system with 20 amp of current and 120 volts has an apparent power of 2400W (using the well known formula of Volts x Amps = Watts, or power). Environments that have small workloads – like residential communities – use 120v power.  When you choose to locate at a data center, for single phase power, you will have an option of 20A (Amps) or 30A for each of the voltages offered.

Some common single phase APC PDUs would be the AP7930, the AP8841, and the AP8861.

Three-phase power

Three-phase power systems comprise three simultaneous waves that have a phase difference of 120 degrees. This eliminates zero voltage moments, unlike single-phase power. Three-phase is a better alternative to single-phase power as it reduces the physical bulk of cables and offers easy installation options.

The best way to look at three phase power is as three lines, operating at different phases of the 60 cycle.  An ideal way to take advantage of 3 phase is to have a 3-phase PDU that has three banks of power, one for each phase.  For each device with redundant power supplies that you might have, power one power supply from one phase, and one from another.  The unit will then not see any of the single phase issues. Three phase power is normally 208V. A three-phase 208V, 20A feed delivers 7200W of apparent power.

Using three-phase PDUs helps create neat, space-saving cabling solutions. It provides greater power capacity and flexibility, making it an ideal choice for data center  environments with heavy workloads needing stable and redundant power.

An example of a three-phase PDU is the AP7961.

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Normal amperage provided on a home circuit is 15A.  Data centers will likely offer 20A or 30A and 120V and 208V.  In the US, circuits are downrated only to manage up to 80 percent of their maximum capacity – known as the load capacity.

The amperage level required depends on the amount of power the PDU can handle.

For instance: if your PDU has a 30A fuse, the maximum current you can draw before it blows is 24A. This could be divided into six circuits, each capable of carrying up to 16A each.

A common 15A APC PDU would be the AP7900.

Some commonly ordered 20A 110V PDUs would be the AP7930 and the AP7901. Commonly ordered 20A 208V would be the AP7940, the AP7961, the AP8861, the AP8959 and the AP8961. Other commonly ordered 30A 120V would be the AP7932, the AP7902, the AP8832 and the AP8932.

The 208V 30A units would be the AP7941, the AP8941, the AP7911, the AP7811, the AP8641 and the AP8965.

Form Factor

Once you have determined the type of power, the voltage and the amperage that you will be using, your next step is to determine what form factor you wish to have in your rack.  For high-power networking organisations, one centimetre of extra space is worth its weight in gold. The amount of space your PDU takes up depends on its orientation.

A rack PDU can be mounted in two orientations (or form factors) – horizontally or vertically:

Vertical PDUs (0U)

Vertical PDUs (also called “Zero U” or 0u) look like tall power strips and are mounted on the upright rails to the side of a rack enclosure. They have become nearly omnipresent in high-powered computing centers, because they do not use rack space and can offer up to 40 outlets. They also tend to offer more receptacles per unit than their horizontal counterparts.

Most of the PDUs offered by APC are 0u or vertical PDUs.

Horizontal PDU

Horizontal PDUs come in 1U or 2U heights and can be mounted in EIA-310 standard equipment racks. They tend to have fewer outlets because of their smaller size. They can be placed above, below or in between the rack components.

Horizontal PDUs are used when power loads are lower or when power protection is required via in-row UPS equipment.

Commonly ordered horizontal PDUs are the AP7900, the AP7901, the AP7902, the AP7911, the AP7800, the AP7801, the AP7811, and the AP7911A.

APC recently released these with updated control boards under the original part number with a “b” suffix.

Plug Type/ Cord length

The most popular outlet combinations offered by manufacturers are the C-13 and C-19 outlets. In the United States, NEMA-style plugs are commonly used by PDUs. Other options like IEC-style plugs and California-style plugs (used for 50A circuits) are also found in some American data centers. Normal cord lengths are 10 or 12 feet.  Most Colo’s will be renting space on a raised floor and will offer power connectivity under the floor just below the rack or cabinet.  Power cord lengths can be customized if needed.  NetworkTigers recommends to keep the power source close to the connection points to limit power loss.

Additional Features

With plenty of types of available PDUs to choose from, it can be quite challenging to pick a PDU that fits perfectly with your rack design. To help solve the selection dilemma, here’s detailed analysis of each type of power distribution unit.

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There are four types of PDUs:

Basic PDU

Basic PDUs are cost-effective, reliable and space saving solutions that satisfy a wide range of requirements. They add to the benefits of a UPS system by transforming a single high-amperage UPS outlet to multiple low-amperage PDU outlets.

A basic PDU is the easiest way to supply power to your IT infrastructure. However, they cannot be accessed through a remote network or provide power usage metrics.

They are also the perfect fit for smaller server rooms, closer to output points.

APC basic rack PDUs follow this format AP75xx where xx are the various basic rack models. Commonly ordered basic rack PDUs are the AP7532.

Metered Rack PDUs

Metered PDUs offer the same functionalities as basic PDUs with added benefits. They can avoid potential overload and monitor load levels with an in-built current meter via a local display.

The local display shows power utilisation metrics to help the user analyse their power usage.

Metered PDUs are used by the power infrastructure of highly secure data centers that need to be isolated from other local networks.

APC metered rack PDUs normally follow this format AP78xx or AP88xx where xx are the various basic rack models. Commonly ordered basic rack PDUs are the AP7811, the AP8861 or the AP8861.

Monitored Rack PDUs

Monitored PDUs go one step further by offering benefits like real-time power consumption analysis through a protected web interface, which helps improve billing accuracy.

Users can monitor real-time analytics, regardless of location. Monitored PDUs can help prevent potential overloads by triggering alerts when they reach a specified threshold.

They also provide ‘daisy-chaining’ that ensures continuous data flow even during interruptions.

These rack power systems are generally recommended for huge colocation facilities that require live power usage reports or cloud centers that need to support fluctuating power inputs.

Switched rack PDUs

Switched PDUs are the zenith of power distribution systems.

The key feature of a switched PDU is the ability to remotely monitor and control the loads of the data centre.

Switched PDUs offer users complete control over their power usage and allow network managers to remotely reboot networking gear by turning on or off the associated power port.

These are highly valuable for any IT deployment, because they significantly reduce the need for hiring technicians to fix unresponsive servers or critical equipment.

APC switched rack PDUs normally follow this format AP79xx or AP89xx where xx are the various basic rack models. Commonly ordered basic rack PDUs are the AP7911, the AP7940 or the AP8941.

Network Tigers strives to engineer PDU solutions that supply efficient power and adapt according to the needs of the network operators.

data center, PDU, PDUs, power, Power Distribution Unit, single phase power, three phase power

Feba Maryann
Feba Maryann
Feba Maryann is a freelance journalist who writes for websites and magazines in Asia and North America. She is currently pursuing her Integrated Masters on Computer Science Engineering with a specialization in Data Science from VIT, Vellore.

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PDUs: the Data Cabinet Center of Power