Enclosures - What's in the Box?
Most gifts come in packages, and electricity can be a gift. In a way, it's the same with electricity in an enclosure. In this article, we will discuss the history of electrical enclosures, various types of electrical enclosures, and installation and use considerations. References given will be from NFPA 70.
History of Enclosures
In the early days of electrical power, switches were known as cut-outs. The name may derive from its use to “cut out” the power source. A simplistic term but effective and easily understood at the time. The cut-out was basically a simplified switch made of copper bars with an insulated “T” handle for the person to grasp for operation. The cut-out was mounted with bolts or screws to a wood, slate, or mica surface. It was visible and accessible as were its open live parts. There was no such thing as a qualified person because few people really knew the power and danger of electricity. The term cut-out is still used in the NEC today.
Cut-outs were installed in the open, and later placed in cut-out boxes. These are the switch enclosures of today.
In the early days of electricity, switchboards were exactly that: Switches mounted vertically on a board with open copper busbars. Live parts were open and exposed. Large fuses or Edison-base fuseholders were used to limit the amount of current flow. There were no circuit breakers as we know them. Electricity was used primarily for electric light and then for electric motors. The original electrical conduits were gas lines, which were already installed in buildings, and converted to carry wires for electrical circuits. There were lots of opportunities for fires or for people to fall into live parts.
Electrical power brought into homes in those days consisted of a knife-switch cut- out with cartridge and Edison-base fuses and open copper busbars and connections that were installed without an enclosure on a covered porch. People learned quickly that getting too close to the electrical service was uncomfortable and scary. It could even be deadly. Therefore, to improve safety, they installed the live parts in a wood cabinet. Hence the term “cabinet” that is still used in the NEC today.
The cabinet employed was similar to the cupboard where they placed dishes in the kitchen. This was a definite improvement over exposed live parts, but when other uses were discovered for electricity and taps were added to the existing system, loose connections and overheating sometimes occurred, which started fires. Cabinets made of dry wood were a fire hazard therefore metal cabinets were developed, which were more fire resistant.
A look at present day Articles 312, 314, 404, and 408 shows the correlation in the NEC. The cut-outs referred to in Article 312 are today’s switches, also known as disconnects, pull-outs, etc. The term cut-out today is commonly used for a utility-type switch on systems that operate at 5 KV or higher. Code Making Panel 9 accepted a proposal in the 2011 NEC cycle to add the term “enclosed switches” to Section 312.8 for clarification.
It is important to note that the switch or cut-out is the switch mechanism and that it is installed in an enclosure. The enclosure may be a cut-out box, cabinet, switchboard, or other type of enclosure. In Article 404, Section 404.3(A) tells us that generally, switches and circuit breakers “shall be of the externally operable type, mounted in an enclosure listed for the intended use” and refers to Section 312.6 for wire bending space. Exception 1 still permits open-knife switches on an open-face switchboard or panelboard without enclosures. The switch may be fused or unfused or could be a circuit breaker used as a switch or a molded case switch, which contains no overcurrent protection.
Panelboards are the same way. The panelboard is mounted in a cabinet or enclosure. The internal parts of the panelboard are the overcurrent devices – the circuit breakers or fuses – mounted on the busbars and support rails. A review of Article 408, in Section 408. 2, refers to Article 312 for requirements for panelboard enclosures.
Enclosures can also be a fenced area, as seen in the definition of enclosures found in Article 100: “The case or housing of an apparatus, or the fence or walls surrounding an installation to prevent personnel from accidentally contacting energized parts or to protect the equipment from physical damage.”
Enclosures often contain overcurrent devices. In Section 240.24(C), overcurrent devices are to be located where they will not be exposed to physical damage. But enclosures contain more than overcurrent protective devices. They also contain conductors and other electrical equipment such as relays, timers, motor controllers, and more. A fence or wall surrounding electrical equipment can be an enclosure, such as an electrical substation or switchyard that encloses live buss, transformers, switches, circuit breakers, reclosers, reactors, etc.
NEC Section 110.28 lists the types of enclosures: “Enclosures (other than surrounding fences and walls) 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, fire pump motors, and motor controllers.” Even standard device and junction boxes for snap switches, ground fault circuit interrupter or standard duplex receptacles, motion sensors, fire alarm pull stations, or a variety of other devices are enclosures, and the basic rules in Articles 312 and 314 apply to them.
Enclosures simply enclose electrical conductors and equipment. Table 110.28 is a useful tool for the electrical engineer, designer, or installer. It provides guidance for selection of enclosures for specified indoor or outdoor locations in standard National Electrical Manufacturers Association (NEMA) types. Requirements for enclosures located in hazardous or classified locations are found in Chapter 5 of the NEC.
Enclosures are required to be securely attached to their mounting surface in Section 110.13. In addition, equipment may require cooling to operate, such as a transformer enclosure –– which has clearances marked on the front of the transformer nameplate with instructions for spacing from obstructions that would impede air flow by convection past the transformer windings. Other electrical equipment, such as adjustable speed drives, have special requirements in the manufacturer’s instructions for installation and use, which are included with the equipment at time of sale.
Generally, in Sections 110.12(A) and 312.5, holes or openings in enclosures, other than those intended for operation, mounting, or required as part of the design of electrical equipment, are to be “closed to afford protection substantially equivalent to the walls of the equipment.” However, for conduit bodies, outlet, device, pull, and junction boxes installed in damp or wet locations, a provision has been placed back into the NEC that permits weep holes of ¼” maximum size if approved by the authority having jurisdiction.
Sharp edges of enclosures are a concern to electrical workers. Many of us have been cut when installing or maintaining electrical equipment such as panelboards or fluorescent luminaires. However, improvements to UL 1439 Standard For Sharp Edges of Electrical Equipment should help this. It’s wise for the worker to wear gloves for installation of electrical equipment where possible and appropriate personal protective equipment for working on energized equipment for protection.
Mike Weitzel is a member of the National Fire Protection Association, the International Association of Electrical Inspectors (IAEI), IEC, and NEC Code Making Panel 8. He holds all five certifications as an electrical inspector through IAEI and was formerly the Codes and Standards Specialist for IAEI. He has over 38 years of electrical experience, including electric utility, industrial plant construction and maintenance, UL 508 Industrial Control Panel Shop, construction electrician, electrical contractor, electrical inspector, and electrical instructor.