In the U.S., there are two classification systems for the protection ratings of electrical equipment enclosures. One is the NEMA rating, and the other is the UL type rating. At first glance, the two do not seem to differ significantly. In this article, we explain why they are not interchangeable and what that means in practice.
Relation to UL
The NEMA and UL standards allow machines and systems to be designed for specific environments.
Why a NEMA Rating Cannot Replace a UL Type Rating
In electrical devices, enclosures serve two primary purposes: First, they protect the user from coming into contact with hazardous internal components; second, they protect those same components from potentially harmful external influences, such as water, oil, or dust. To determine the level of protection, there are two rating systems in the U.S.—the UL system and the NEMA system. Both use the same classification of protection classes. Nevertheless, a NEMA rating can never be an adequate substitute for a UL Type Rating. But why is that?
NEMA Rating
The National Electrical Manufacturers Association (NEMA) represents the interests of the U.S. electrical industry. It publishes several standards that define requirements for products in this sector. One of these is NEMA Standard 250, which addresses enclosures for electrical equipment with an operating voltage of less than 1,000 volts. It includes requirements regarding the protective performance of such enclosures against the ingress of dust, water, and oil, as well as corrosion protection, wall thickness, and much more.
Compliance with the NEMA standard is the sole responsibility of the respective manufacturer. It is the manufacturer’s responsibility to certify that its products comply with the standard. No practical tests are required for a manufacturer to provide this certification. In particular, no testing by NEMA or an independent institute is required.
UL Type Rating in accordance with UL 50E
The Underwriters Laboratories (UL) is an independent testing laboratory recognized by the U.S. Occupational Safety and Health Administration (OSHA) that tests products for safety. UL uses standards it has developed itself for this purpose. The standard for testing enclosures for electrical equipment is UL 50. This is supplemented by UL 50E, which governs environmental design requirements. The protection classes defined in UL 50E correspond in their classification to those of the NEMA 250 standard, although the exact requirements differ in some respects.
The key difference, however, is that compliance with the specifications for a UL Type Rating is not guaranteed by the manufacturer; rather, UL independently verifies it. Users can therefore be confident that a UL-certified enclosure actually provides the promised protection through the enclosure, interlocks, and so on.
What are the protection classes for NEMA and UL type ratings?
NEMA and UL define a total of 13 different protection classes. Classes 1, 2, 5, 12, 12K, and 13 apply to enclosures intended for indoor use, whereas Classes 3, 3R, 3S, 4, 4X, 6, and 6P govern the requirements for outdoor enclosures. The requirements for each protection class range from protection against the ingress of coarse dirt (Type 1) to reliable watertightness during continuous submersion (Type 6P). Common to all classes is that the enclosures must prevent contact with hazardous parts inside them to a certain degree.
Are there different requirements for NEMA and UL?
For most protection classes, the requirements for each class are the same under both the NEMA standard and UL 50E. The exceptions are Classes or Types 6, 6P, 12, and 12K. Let’s take a look at these differences using Classes 6 and 6P as examples.
While the NEMA standards for Type 6 enclosures require protection only against water from a hose, UL also includes snow, sleet, and rain. However, since both standards require protection against temporary (Type 6) or permanent submersion (Type 6P) for this protection class, these differences are largely formal in nature. The only practical exception here is corrosion protection, which is required only by UL.
What about the IP rating?
A third rating used to classify the degree of protection provided by an enclosure is the IP rating. IP stands for Ingress Protection and also addresses solid foreign objects—particularly dust—and liquids. The IP protection rating system is internationally recognized and is based on the international standard EN 60529 (or, in the European Union, on the standard IEC 60509:1989). As with the UL Type Rating, the assignment of an IP protection rating requires testing of the enclosure by an independent laboratory.
So, can an IP rating replace a UL Type Rating? No, because UL requires testing in accordance with its own standards. So it’s simple: Even if an enclosure already has an IP rating, but the finished end product is to receive UL certification, the enclosure must still be tested in accordance with UL 50 or 50E.
Zusammenfassung
NEMA Rating vs. UL Type Rating
The classification of protection classes does not differ between the NEMA 250 standard and the UL 50 standard. Differences exist only within the individual protection classes regarding the specific requirements placed on the enclosures. Nevertheless, UL does not recognize NEMA ratings, as Underwriters Laboratories awards its seal—a widely recognized label in the U.S. for product safety. To meet UL’s high requirements, a simple self-certification by the manufacturer, as required by NEMA, is not sufficient.
