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.
- This article covers modular data centers [646.1]. It provides some
requirements for manufacturers, and some for installers.
- 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
- 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
- If it's a listed 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.
- 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
- 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].
- 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.
- 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
- 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. But
the real guide is not the tables but that statement in 110.26.
- 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 646.20.