From the two previous whitepapers published in this series we’ve learned about power selection in Data Center Power Series - Part 1 - Choosing the Right Voltage, and about power and cooling calculations in Data Center Power Series – Part 2 – Voltage, Amps, Watts and BTUs.  In this installment of the series, we’ll learn more about power connectors and cables, the various types typically used in data center environments, and specifications for each including diagrams to help you identify and choose the right connectors and cables for your environment. 


♦   Why Are There So Many Different Power Connectors?
Power connectors must be designed to meet many requirements including safety regulations, heat rating, efficient power delivery, functionality, intended use, wiring scheme, and circuit specifications.  Each cable and connector configuration is designed to meet these and other applicable requirements which determine the number of wires, wire jacket colors, wire and blade material and gauge, insulation, shielding, connector size, plug casing, etc. 

With the many variations in power configurations and requirements, standards organizations have developed to provide specifications for manufacturers to ensure cables, connectors, and devices are designed accordingly. The power connectors shown below are designated by standard codes as defined by the leading industry standards organizations, International Electrotechnical Commission (IEC), and National Electrical Manufacturers Association (NEMA).

The C13/14 10 amp connector is by far the most common found in data centers. The male plug features a hood that slips over the female plug to produce a stable connection with no exposed prongs. Typically used for PC/server-side power.
The C19/20 16 amp connector is similar to the C13/14 with a hood that slips over the female side. The C19/20 is often used where higher currents are required, such as high power servers, PDU, and UPS.

The NEMA 5-15 15 amp straight blade connector is commonly used for North American home and office outlets and power cords. In the data center, it is often used to connect a server to a PDU, however the more stable C13/14 is preferred.
NEMA L5-20 - Locking, 120 volt, 20 amp, single phase, 2 pole, 3 wire connector, single phase. NEMA locking connectors are often used for connections to the rack power bus for UPS and PDU devices. The locking plug has a curved, L-shaped blade that locks into the receptacle when twisted.
NEMA L6-20 - Locking, 240 volt, 20 amp, 2 pole, 3 wire connector, single phase. Often used for UPS and PDU connections to the rack power bus where 240 volt, 20 amp service is provided.
NEMA L6-30 - Locking, 240 volt, 30 amp, 2 pole, 3 wire connector, single phase. Often used for UPS and PDU connections to the rack power bus where 240 volt, 30 amp service is provided.
NEMA L15-20 - Locking, 240 volt, 20 amp, 3 pole, 4 wire connector, three phase. Often used for high-density rack system connections to the rack power bus where 240 volt, 20 amp, three phase service is provided.
NEMA L15-30 - Locking, 240 volt, 30 amp, 3 pole, 4 wire connector, three phase. Often used for high-density rack system connections to the rack power bus where 240 volt, 30 amp, three phase service is provided.
NEMA L21-20 - Locking, 120/208 volt, 20 amp, 4 pole, 5 wire connector, three phase. Often used for high-density rack system connections to the rack power bus where 120/208 volt, 20 amp, three phase service is provided.
NEMA L21-30 - Locking, 120/208 volt, 30 amp, 4 pole, 5 wire connector, three phase. Often used for high-density rack system connections to the rack power bus where 120/208 volt, 30 amp, three phase service is provided.

• Amperage and Wire Gauge

Wire gauge is determined by the amperage requirement and cable length. Basically, as cable
length increases, either the allowed amperage decreases or the wire gauge must increase to
compensate. The following table shows a sample of common cable and connector configurations.

• Power Cable Jacket Types

Jackets used in power cables are distinguished by a standard set of code letters, as defined in the
UL standard # 62 (UL62). Power cable jackets are stamped with appropriate letters to describe
the material used in the jacket, the voltage rating, the jacket's resistance to the elements, and
other factors.

• Wire Color-Coding

For safety and convenience reasons, color-coding standards are typically used for the jackets of the individual conductors inside power cords. The following list shows US and European color-coding standards. Please note that these apply to most power cords in the US and Europe, however, color-coding or non-standard cabling may used in certain applications.

♦   Conclusion
As you can see from the informatin presented here, there are many factors and options to consider when selecting power cables and connectors.  Selecting the wrong cable type or connectors can result in significant delays, unplanned downtime, and subsequent loss of revenue and/or productivity.  Fortunately, manufacturers generally make it reasonably straightforward to select the right combination of cables and connectors for your devices if you know the basics presented here and can clearly identify your requirements. 

Interesting Fact:  The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting.

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Upcoming whitepaper in this series:

Watts per Square Foot?  What?

About the Author

Don Small is a 20 year veteran of the IT and Operations service delivery field. Don has built Managed Services organizations that support 100's of customers worldwide including Fortune 500, government, and high security, regulated industries.  Currently Don Small holds the position of Chief Operations Officer for SilverBack Migration Solutions, Inc.

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