Understanding General Requirements of the NEC, Part 16

Article 110 provides general requirements for all installations. Part II provides the requirements for systems operating at 1,000V (nominal) or less. Section 110.28 provides the general requirements for enclosure types. The first sentence is a long one. It says an enclosure rated not over 1000V nominal must be marked with an enclosure type number per Table 110.28 if it is intended for any of X locations. And the list of locations is what makes it so long. It appears to be all-inclusive.

The second sentence says to use Table 110.28 for selecting these enclosures for use in specific locations other than hazardous (classified) locations. The third sentence says these enclosures are not intended to protect against what is basically water intrusion (including condensation and icing) that occurs within the enclosure or enters through raceway.

Table 110.28 is really one table stacked on top of another table. The top one covers enclosures for outdoor use, the bottom one covers enclosures for indoor use. It will show you which contaminants (e.g., dust) or conditions (e.g., prolonged submersion) a given enclosure number (e.g., 3S) will protect against (e.g., windblown dust).

NEMA 4 and NEMA 4X are commonly used enclosure types in manufacturing plants. The 4X provides additional protection against corrosive agents. The NEMA 4 protects against six conditions: incidental contact, falling dirt, falling liquids and light splashing, dust, and hosedown.

There’s a trap when choosing an enclosure for a manufacturing plant application. An equipment configuration change, such as relocating a finishing machine from one area of the plant to another area of the plant can mean introducing airborne dust into the new area. If the new area was formerly used just for assembly and all of its enclosures are NEMA 1, they will now have to be replaced with NEMA 4 or higher. For operational flexibility, it can make sense to treat an entire building as potentially having the same contaminants as the worst area of the building. But this can be expensive, and other solutions should be considered before just opting for this easy way out.

A plastics plant in Kentucky provides an example of another solution. Operations wanted to move a cutting machine closer to the next downstream process, to reduce wasted foot traffic and lift truck traffic on the floor. The cutting machine had been put in its current location due to the dust it generated. The solution was to install a dust collection system on that machine. This also protected the operators, who previously had to wear dust respirators while operating the machine and while cleaning the floor of accumulated dust several times per day.

So, while the NEC requires you to select the appropriate enclosure for the environment, it also does not stop you from making that environment less of a problem for whatever enclosures you install.