Article 393, Low Voltage Suspended Ceiling Power Distribution Systems
Based on the 2021 NEC
by Mark Lamendola
Please note, we do quote from copyrighted material. While the NFPA
does allow such quotes, it does so only for the purposes of education
regarding the National Electrical Code. This article is not a substitute
for the NEC.
These are the 10 NEC Article 393 items we deem most important, based
on the pervasiveness of confusion and the potential costs of same. Note that
Article 393 is new, with the 2014 NEC. Since it came out, the adoption of these systems has grown exponentially. The reason is the move toward "smart" buildings, interconnected systems, and the spread of LED lighting. How can LED lighting possibly be a reason? The fixtures are ideal for adding sensors, cameras, and controls; they become integrated with other systems. And if you have LEDs for your ambient lighting, you can add this and add that and so on. Why would LED be growing so rapidly? It's not the lower cost per lumen that is the main driver. They cost less in labor and materials to install and they provide the designer with a massive increase in flexibility of aesthetics and function.
- Low Voltage Suspended Ceiling Power Distribution Systems are, like
many items with their own Chapter 3 Article, a type of product. Just as EMT is a
type of raceway and thus a type of product, so are Low Voltage Suspended
Ceiling Power Distribution Systems a type of product.
- The defining characteristics of a Low Voltage Suspended Ceiling Power
Distribution Systems are that it distributes power from one or more Class 2
power supplies, serves as a support for a finished ceiling surface, and consists
of a busbar and busbar support system .
- Article 393 lists 4 permitted uses [393.10]. If you think about the types of
loads that Low Voltage Suspended Ceiling Power Distribution Systems support, it
quickly becomes apparent what types of uses make sense. This list has not changed since the previous Code cycle. Or the one before that.
- Article 393 lists seven prohibited uses for Low Voltage Suspended
Ceiling Power Distribution Systems [393.12]. If you think about the type of
Class 2 supplied equipment proliferating in modern office buildings (sensors,
communication systems, etc.), you see this list is almost a non-starter. You
would have to be misapplying the technology to violate almost any of these
prohibitions. This list has not changed since the previous Code cycle. Or the one before that.
- Both Part II and Section 393.14 (which is one of three Sections under Part
II) are titled "Installation." This error was in the previous edition, also. And the one before that. Only one of these can correctly have that title.
So which one is correct? You could think of the Part II title as "Uses and
Installation" as one way of clearing up the confusion (maybe just make that
correction manually in your NEC with a ballpoint pen). Another solution is to
take the same approach to 393.14 but call it "Installation Requirements".
- Another anomaly is the sequence of Article 393 requirements does not match
the logical sequence of your work flow. This is not really a defect in the Code,
as the Code is not an installation manual for the untrained. When you reference
a Code Article, read all applicable Parts and/or Sections before beginning the
work. The anomaly here is the requirements for securing and supporting the devices are in
393.30, and therefore come AFTER the 393.14(A) requirements for installing the
wiring. Don't let that throw you.
- The disconnect for the Class 2 supply must be accessible and within sight of
the Class 2 power supply [393.21(A)]. This "within sight" requirement is
standard for disconnects throughout the NEC, yet the NEC seems to state it as if
each installation is a special case in which this needs mentioning. It's normal,
not a special case.
- What we just mentioned about disconnects parallels the requirement for
wiring these in a neat and workmanlike manner [393.14(A)]. The "neat and
workmanlike" requirement applies to all installations [110.12]. Nowhere in the
NEC will you find any reference stating something like, "You can install this in
a sloppy manner, instead of a neat and workmanlike manner." It's puzzling why
Article 393 has redundancy, but the NEC has this redundancy issue
- The maximum branch circuit size for supplying one of these systems is 20A
[393.45(A)]. If your total load of Low Voltage Suspended Ceiling Power
Distribution Systems exceeds 18A, divide the Low Voltage Suspended Ceiling Power
Distribution Systems up over at least two branch circuits.
- The 2023 revision cleared up the confusion caused by the "grounding" requirement in 393.60(A). You do NOT
connect any of this equipment to a ground rod or to earth. In fact, 393.60(B)
makes this quite clear. But you DO bond all the metallic objects so as to
prevent a difference of potential between them. And thus the 2023 revision says to connect the supply side of a Class 2 power source to an equipment grounding conductor. This is actually an equipment bonding conductor, and if you look at the list of acceptable equipment grounding conductors [250.118] you'll understand that's the case. So why is this not called the equipment bonding conductor instead of the equipment grounding conductor? One reason is tradition (not a good reason), another is the equipment grounding conductor is eventually tied to ground. Not to a ground rod on the load side, but to the supply side ground.
There's also a school of thought that you "ground" current-carrying equipment such as power supplies but you "bond" non-current carrying equipment such as racks and panels. This is wrong thinking, because you bond to put all equipment at the same potential. Again, look at the list of acceptable equipment grounding conductors. When you connect EMT to a panel, that panel is then at the same potential as the EMT which is at the same potential as any other metallic object connected to that EMT system.
That rack and a Class 2 power supply need to have zero potential between them, so you bond both of them and the bonding means will also have the same potential.
These systems are often installed as add-ons, and that makes neat work more difficult to acheive. Never run cables or wiring at angles to the room. If existing cables or wiring are run that way, make the case for correcting that. Where possible, install hoops or gutters for running all wiring that goes in along the same path.