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National Electrical Code Articles and Information

Based on the 2020 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 646, Modular Data Centers

Why do we have Article 646? And what's a modular data center, anyhow? Let's answer that second question, first. Think of a modular data center as a self-contained "data center in a box." This box is pretty big, as far as boxes go. The normal size is a standard shipping container, which is what you see barreling down the interstate highway behind  a semi-tractor trailer. The box is that size because it has to be big enough to contain the standard IT racks and cabinets. But it doesn't get bigger than that, because it has to be small enough to be transported by truck.

Not all modular data centers have this form factor. The other two popular constructions are the pod (similar to the moving pod) and the "block" (which is normally made of steel and resembles a tool shed in size and shape).

So the idea behind it is you can have the modular data center wheeled up to your site and all you do is connect the power and communications to it. Of course, there are many "gotchas" in making those connections. It's a custom wiring job, rather than something as simple as, say, plugging in a vacuum cleaner. And that's why we have Article 646. It's new with the 2014 NEC.

Modular data centers have become increasingly popular as more facilities need some data center type capacity but don't need a full-blown traditional data center on site.

  1. This article covers modular data centers [646.1]. It provides some requirements for manufacturers, and some for installers.
  2. As with any other Code Article, it's not a step by step instruction set. It's a set of minimal requirements for protecting property and people from the hazards that may arise from using electricity, in this case where a modular data center is involved. The installation must be performed by qualified electricians.
  3. Further to the previous point, other Articles apply. For example, equipment used for electrical supply must conform with Parts I and II of Article 110 [646.10]. That's a specific mention. Where there isn't a specific mention, the relevant requirement from Chapters 1 through 4 will apply just as with any other installation.
  4. If it's a listed and labeled modular data center, you can skip all but one of the requirements presented in Article 646.3 [646.4]. If it's not listed, you've got a slew of requirements to meet. This means it is nearly always preferable to specify a preconfigured unit rather than one that's custom-built. Not only do you save on the engineering costs of specifying a custom system, you eliminate many costs of installation.

  5. A modular data center is a continuous load (expected to run more than three hours at a time), so you apply the 125% rule to sizing the supply conductors [646.6].
  6. You can use flexible cords for connecting between equipment enclosures of a modular data center (if not subject to physical damage), but you cannot use them as supply conductors [646.9].
  7. You must arrange illumination such that loss of any single lighting unit doesn't plunge the whole area into darkness [646.15]. This is a good practice, generally. With modular data centers, it's a required practice.
  8. Don't forget your emergency lighting. Article 646 provides requirements with not just one, but two sections: 646.16 and 646.17. Basically, you provide listed emergency lighting equipment such that a person can see the way out from anywhere in the modular data center and you don't connect other equipment to the emergency lighting branch circuits. This last requirement is waived if the emergency lighting system has a backup power source that supplies only the emergency lighting system upon loss of main power. But it's a good design practice to just keep the emergency lighting system separate from everything else.
  9. How much workspace is enough? Workspace about electrical equipment must comply with 110.26 [646.18]. Note that, contrary to popular misconception, this does not mean you can lock a tape rule at three feet and call the working space adequate because the tape can squeeze in there. The working space must "permit ready and safe operation and maintenance of such equipment." [110.26] Maybe three feet will be sufficient, maybe not.

    People have taken the three foot number from Table 110.26(A)(1) out of context and made it into both a minimum and maximum number. This table and its counterpart in OSHA's 29CFR1926 Part J provide the minimum clear distance under certain conditions, and those minimums are sometimes more than three feet. However, the real guide is not the tables but that statement in 110.26.
  10. Working space is a serious issue in Article 646. Not only does it reference 110.26, but it also provides additional requirements in 646.19. And in 646.20, 646.21, and 646.22.

How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.



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