The National Electrical Manufacturer Association (NEMA) represents over 300 electrical equipment manufacturers in the USA and provides a forum for creating technical standards relating to electrical product function and safety. The NEMA standards are effectively a benchmark that guarantees compatibility, safety and proper function and are of benefit to both manufacturers and users.
While they publish over 600 standards, guides and technical papers relating to a variety of electrical product topics, the term “NEMA ratings” is commonly used to describe a specific set of standards which define the environments in which industrial control systems and other electrical system enclosures can be used. This rating system is used by manufacturers to specify the ability of an electrical enclosure to withstand certain environmental conditions.
NEMA Ratings for Non-hazardous Indoor Locations
The basic NEMA enclosure rating for non-hazardous indoor locations (no risk of fire or explosion) is the Type 1 rating. An electrical product enclosure with a NEMA Type 1 rating provides guaranteed user protection against electrical shock due to contact with live electrical parts. In addition, a Type 1 rated enclosure protects the electrical product from ingress of solid objects such as falling dirt. Type 1 enclosures are for general purpose indoor use.
Type 2 enclosures conform to Type 1 specifications but provide additional water drip or splash shields. These enclosures are suited to environments with severe condensation.
The NEMA Type 5 rated enclosure includes dust-proof seals for protection from fine particle ingress over and above the Type 2 protection.
The Type 12 and 12K rating extends the Type 5 rating by including corrosion resistance in the specification. The 12K rating provides for the inclusion of knockouts (partially stamped removable openings for cable / wire access) in the enclosure.
The final and most rigorous NEMA rating in this category, Type 13, conforms to Type 12 specifications but also requires protection against ingress of oil and non-corrosive coolant fluids.
NEMA Ratings for Non-hazardous Outdoor Locations
Outdoor electrical enclosure ratings for non-hazardous locations begin with Type 3 which requires protection from general atmospheric outdoor conditions. This standard specifies the enclosure should provide protection against falling dirt, windblown dust, rain, sleet and snow. The enclosure should also provide protection against damage due to external ice formation. Under the Type 3 banner, NEMA specifies Type 3R and 3S enclosures. The 3R specification excludes windblown dust protection, while the Type 3S specification requires the addition of external mechanisms that remain operable when coated in ice.
Type 4 enclosures must meet Type 3 requirements but provide extra water protection so that the enclosure can operate safely in a hose-directed water stream.
The addition of an “X” at the end of any Type 3 or 4 rating (e.g. Type 3SX) indicates a corrosion resistance requirement. 
The NEMA Type 6 and 6P ratings require enclosures to conform to Type 4 specifications but include additional measures to allow for safe temporary (6) or permanent (6P) submersion in water.
Ratings for Hazardous Locations
Locations with significant fire or explosion risk are classified as hazardous and enclosures for application in such areas require additional safety measures. NEMA provides three standards for enclosures meant for hazardous locations: Types 7, 8 and 9. Conformance to these NEMA ratings requires third party inspection unlike in non-hazardous environments.
Their Type 7 and 8 ratings for indoor and outdoor enclosures respectively require enclosures to be explosion proof. These ratings are for so-called Class I, Division 1 hazardous areas where ignitable concentrations of flammable gases may be present all or some of the time under normal operating conditions.
The Type 9 rating requires the enclosure to provide protection against dust ignition and provides specifications for safe use in Class II, Division 1 environments where ignitable concentrations of combustible dust may be present all or some of the time.
NEMA Ratings for Mining Applications
They provide a Type 10 rating that conforms to the requirements set out by the US Department of Labour’s Mine Safety and Health Administration (Code of Federal Regulations, 30, Part 18) for electrical motor-drive mine equipment and accessories.
By Michael Owen, PhD
Michael is a mechanical engineer working in academia. His research encompasses various aspects of fluid dynamics and heat transfer with a strong emphasis on industrial heat exchangers.
Sources:
- https://www.nema.org/pages/default.aspx
- https://www.nema.org/Standards/Pages/Industrial-Control-and-Systems-Enclosures.aspx
- https://www.nema.org/news/Pages/NEMA-Publishes-NEMA-250-2014-Enclosures-for-Electrical-Equipment-1000-Volts-Maximum.aspx
- https://iaeimagazine.org/magazine/2014/11/10/hazardous-classified-locations-nec-articles-500-through-517/
- https://en.wikipedia.org/wiki/Electrical_equipment_in_hazardous_areas#ANSI.2FNFPA_areas_description
- https://arlweb.msha.gov/readroom/coal%20handbook/CFR-2014-title30-vol1.pdf
