The last section in Part II of Article 110 in the National Electrical Code (NEC) covers enclosure types. The section for enclosure types has not always been 110.28. Enclosure types has only been 110.28 since the 2011 edition of the NEC. For the 2008 edition, the section for enclosure types was 110.20.


It is important to note that the section pertaining to enclosure types has not always been in Article 110. Before the 2008 NEC, these requirements and table were in Article 430, Motors, Motor Circuits and Controllers. Section 430.91, Motor Control Enclosure Types, was a part of Article 430 from the 1984 edition to the 2002 edition. 


Since the 2008 edition, these specifications apply to a lot more than just motor controllers. Section 110.28 also applies to enclosures of switchboards, switchgear, panelboards, industrial control panels, motor control centers, meter sockets, enclosed switches, transfer switches, power outlets, circuit breakers, adjustable-speed drive systems, pullout switches, portable power distribution equipment, termination boxes, general-purpose transformers, fire pump controllers and fire pump motors. Section 110.28 does not cover surrounding fences or walls covered in 110.31.


The enclosure types listed in Table 110.28 are the National Electrical Manufacturers Association (NEMA) enclosure types. When electricians, engineers, architects and inspectors reference enclosure types, they usually reference NEMA enclosure types. For example, instead of saying enclosure Type 1, Type 3R, Type 4X, etc., it is common to say NEMA 1, NEMA 3R, NEMA 4X, etc. The standard Enclosures for Electrical Equipment (NEMA 250 2014) covers enclosures for electrical equipment rated not more than 1,000 volts (V) and intended to be installed in non-hazardous (unclassified) locations. NEMA uses a standard rating system that defines the types of environments in which an electrical enclosure can be used and frequently signifies a fixed enclosure’s ability to withstand certain environmental conditions.


Enclosure types rated not over 1,000V nominal and intended for the locations described in Table 110.28, shall be marked with an enclosure-type number as shown in Table 110.28. When an enclosure is needed, use Table 110.28 for selecting the enclosure for use in a specific location other than a hazardous (classified) location. 


Table 110.28 is divided into enclosures for outdoor use and indoor use. Outdoor-use enclosure types provide a degree of protection against environmental conditions such as rain, sleet, snow and windblown dust. Although the top part of Table 110.28 states the enclosures are for outdoor use, these enclosures can also be used indoors. Enclosures for indoor or outdoor locations include Types 3, 3R, 3S, 3X, 3RX, 3SX, 4, 4X, 6 and 6P.


In the section for outdoor enclosures, sleet is listed twice as an environmental condition, but an asterisk next to one sleet references a footnote directly under Table 110.28, which states the mechanism shall be operable when ice covered. To find the enclosure needed, first determine if it will be needed for outdoor use or indoor use. Next, in the row on the left side of the table, find the environmental condition to which the enclosure will be subjected. Finally, look to the right to see which enclosure types can be installed. Enclosure types with an “X” shall be permitted to be installed, and enclosure types without an “X” shall not be installed for the condition listed (see Figure 1).


Indoor-use enclosure types provide a degree of protection against environmental conditions such as falling dirt; liquids and light splashing; circulating (and settling airborne) dust, lint, fibers and flyings. Enclosures for indoor locations include Types 1, 2, 5, 12, 12K and 13. Some environmental conditions listed in the section for outdoor use enclosure types are also listed in the section for indoor use enclosure types. Environmental conditions listed in both sections include hosedown, corrosive agents, temporary submersion and prolonged submersion. The other condition listed in both sections is incidental contact with enclosed equipment. All of these enclosures provide protection against incidental contact with enclosed equipment (see Figure 2).


There are two informational notes under Table 110.28. The first mentions the terms raintight, rainproof, watertight, driptight and dust-tight. “Raintight” is typically used in conjunction with Enclosure Types 3, 3S, 3SX, 3X, 4, 4X, 6 and 6P. “Rainproof” is typically used in conjunction with Enclosure Types 3R and 3RX. The term “watertight” is typically used in conjunction with Enclosure Types 4, 4X, 6 and 6P. The term “driptight” is typically used in conjunction with Enclosure Types 2, 5, 12, 12K and 13. “Dust-tight” is typically used in conjunction with Enclosure Types 3, 3S, 3SX, 3X, 5, 12, 12K and 13. The terms raintight, rainproof, watertight and dust-tight are defined in Article 100.


The most commonly installed enclosure for indoor locations is Type 1. In accordance with NEMA, Type 1 enclosures are constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts and to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt). Some equipment, such as some dry-type transformers, are in Type 2 enclosures, which are constructed for indoor use like Type 1 enclosures, and to provide the same degree of protection. Type 2 enclosures are also constructed to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing) (see Figure 3).


The most commonly installed enclosure for outdoor locations is Type 3R. In accordance with NEMA, Type 3R enclosures are constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts, to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt), to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow), and that will be undamaged by the external formation of ice on the enclosure (see Figure 4).


Two enclosures frequently installed in industrial plants are Type 12 and Type 4X. In accordance with NEMA, Type 12 enclosures are constructed (without knockouts) for indoor use to provide a degree of protection to personnel against access to hazardous parts, to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers and flyings), and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).


Type 4X enclosures are constructed for either indoor or outdoor use to provide a degree of protection against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (windblown dust), to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water and hose-directed water), that provides an additional level of protection against corrosion, and that will be undamaged by the external formation of ice on the enclosure.


As stated in the second paragraph of 110.28, Table 110.28 does not include hazardous (classified) location enclosures. In accordance with NEMA, enclosures for hazardous (classified) locations are Types 7, 8, 9 and 10. Use Types 7 and 9 enclosures for indoor locations, Type 8 enclosures for indoor or outdoor locations, and Type 10 enclosures for mining applications. As long as these enclosures are properly installed and maintained, Type 7 and 10 enclosures are designed to contain an internal explosion without causing an external hazard. Type 8 enclosures are designed to prevent combustion through the use of oil-immersed equipment. Type 9 enclosures are designed to prevent the ignition of combustible dust.


Next month’s column continues the discussion of electrical installation requirements.