Conduit vs Flexible Cable and Combustible Sites|
Article - November 19, 2018 By LarsonElectronics.com
Conduit vs Flexible Cable and Combustible Sites
In industrial facilities, electrical wirings must be protected to ensure reliable performance of equipment, machines and electronics. Conduits, or solid hollow pipes, are typically used to house wires, cables and other types of connections, which prevents exposure to moisture, flammable compounds (in explosive environments) and corrosive substances.
Conduits and cables come in many different sizes, materials and forms. The components are supported by elbows, junction boxes, seals, nipples and connectors, which form a raceway system. With this in mind, it is important to know how the various types differ and their respective applications. The National Electric Code (NEC) also provides several guidelines about conduits and cables, with provisions pertaining to combustible sites, wiring methods, appropriate lengths and conditions.
What’s the Difference?
By definition, a rigid conduit is a non-flexible pipe that can be non-metallic or metallic; and in most cases are threaded at the ends for ease of installation. Non-metallic, rigid conduits can take on plastic or vinyl construction (PVC) and come with lightweight, low-cost, corrosion-resistant and UV-resistant properties.
On the other hand, metallic, rigid conduits (RMC, RSC) are heavy-duty pipes that are constructed of aluminum, steel, stainless steel or brass – depending on the requirements of the installation. Metallic, rigid conduits can also be galvanized (GRC) or treated with an extra organic coating (IMC) to make the electrical accessory more resilient against heat, corrosion and other damaging elements at the work site.
Emphasizing the stiff characteristics of rigid conduits, these units are non-flexible. Challenges surrounding this feature are addressed using flexible metal conduits (FMC) or Greenfield – another term for the conduit (NEC Article 501.10 1-2). This type of conduit looks like a metallic cable protected with an external, spiral armor. Like metallic, rigid conduits, FMCs can be treated with special coatings, such as plastic (Liquid tight flexible metal conduit or LFMC), for additional protection in extreme environments (NEC Article 501.15 2).
Flexible Cables and NEC Regulations
Not all electrical installations in industrial buildings require conduits. Depending on the requirements of the location, device/machine and project, armored cables are sometimes used for connections. For instance, an MC-HL cable is a metal clad, continuously welded sheath cable used in hazardous locations, wet/dry conditions, directly buried and extreme environments (NEC Article 501.10c).
According to the NEC, this type of cable, as well as Type ITC-HL (NEC Article 501.10d) and optical fiber cables (NEC Article 501.10e), should only be utilized in industrial locations “with restricted public access” i.e. conditions that require qualified individuals for proper installation and maintenance. For combustible locations, the applicable cable type must be rated for Class I, Zone 1 or Class I, Division 1. For MC-HL cables, installations using this type of accessory should adhere to NEC Article 330, Pt II. Optical fibers are suitable for installation in raceways and should be sealed, based on practices outlined in NEC Article 501.10a and 501.15.
Uses and Applications
Starting with rigid conduits, this type of accessory can be used in underground, above ground, outdoor and explosive sites (see restrictions and recommendations above). Moreover, non-flexible units are suitable for stationary devices and machines that are fixed, contain non-moveable parts and are permanently mounted. The specific type of rigid conduit to be used will depend on the environment, as well as the damaging elements present during operation.
An explosion proof exit sign is an example of a unit that requires rigid conduit during installation. The light does not contain any moveable or rotating parts; and using a threaded, rigid metal conduit ensures compliance with NEC provisions in Article 501.10. Other types of units that benefit from rigid conduits include the following: stationary sensors, permanent-mount non-portable lighting systems, push-button control units, control stations and explosion proof horns.
Flexible metal conduits and flexible cables are utilized when fixed or portable equipment feature moving or rotating parts. Furthermore, the accessories are recommended for locations and machines that experience heavy vibration, such as motors, rotating fans and mixers. Installations with complex raceways may also use FMCs, such as vent systems and exhausts. Watertight variants are applicable to wet locations, while standard FMCs are found in dry installations.
Lastly, flexible, heavy-duty cables are recommended for use in explosive environments, with limitations surrounding access (NEC Article 501.10c). In installations wherein rigid conduits are not practical, FMCs are used (applications may vary, depending on building code and limitations related to length and size).