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Thinking about Flexible and Rigid Conduits?

Aug. 03, 2022

Flexible Conduit Systems are designed to provide the very same cable protection solutions as Rigid Conduit Systems- protecting cables, conductors and wiring within industrial, commercial and even residential applications. The main difference between Rigid and Flexible Systems is how they are applied in a variety of applications and installations.   



What are Flexible Conduits? 

The reference to “flexible” is associated with the Liquid-tight Flexible Metallic Conduit (LFMC), that provides the electric raceway solutions specified in Article 350 of the NEC. Product requirements for the LFMC are covered in UL 360, while Conduit Fittings meet the requirements of UL 514B.  Together UL 360 and UL 514B outline the product characteristics and test requirements for listed LFMC and conduit fittings. These standards are used to evaluate the products for performance characteristics such as impact, crush resistance, ingress ratings, temperature, strength, bending, flexibility, flammability and more. 

Liquid-tight Flexible Metal Conduit is constructed of a spiral core that typically consists of galvanized steel. Other conduit core types include brass, aluminum and stainless steel, each having specific characteristics for specific application requirements. The spiral wound core allows for flexibility while providing mechanical protection of the conductors and cables that it will carry. The core design, materials and trade sizes, all have influence on the conduit’s flexibility in either static or dynamic applications.  The jacket materials typically consist of PVC, but may include thermoplastic rubber, polyurethane or PA nylons.  

Flexible Conduit Systems are typically used in installations where flexibility is required to make connections, and where the installation may be subjected to shock and vibration. Product selection should take into consideration: 

● The specific application and industry requirements 

● System temperature ratings 

● System ingress ratings Indoor or outdoor installations

● Approvals/certifications 

● Reliability of performance over time.   


Considerations for Rigid Conduit 

When electrical wires and circuitry need mechanical protection and support in an industrial electrical installation between two points, one of the common methods used is rigid Conduit Systems. Typically, rigid metal conduit is heavy-duty galvanized steel tubing installed with threaded fittings. In some applications, rigid conduit may be aluminum. Rigid conduit is commonly used outdoors to provide solid protection from damage caused by the elements or mechanical impacts. They can also provide structural support for electrical cables, panels, and other equipment. An added feature of a rigid metal conduit system is that it can be direct buried and encased in concrete. It may also serve as a useful bonding conductor for grounding (NEC  250.118(2)).  

When transporting and protecting live electrical wires over long distances through various parts of an industrial facility, there are always two major concerns for the engineers designing the systems: (i) risk of fire and (ii) electromagnetic interference. In both instances, metal rigid systems provide an excellent choice offering exceptional fire resistance and reduction of electromagnetic fields by as much as 95%. 

In addition to use in ordinary environments, the rigid system solution is well suited for harsh environments. For instance, in an industrial environment where corrosive factors such as salinity and various chemical agents are prevalent, the metal rigid system can be fully coated with a layer of PVC, offering significantly increased protection and extension of the life of the electrical system. Similarly, in hazardous environments such as exists in many applications in the Oil & Gas facilities, Chemical Plants, Metal and Mining extraction, Pulp and Paper and other industries, rigid metal conduit will not permit the flow of flammable gases and vapors,  providing protection from fire and explosion hazard when installed with proper sealing fittings. The threaded ends of the rigid conduits offer both a flame path to prevent explosions and a proven way to cool off any gases that may escape the closed system. 



What if both Rigid and Flexible Cable Protection are needed? 

Many applications and installations will require transition solutions between Rigid and Flexible Cable Protection Systems. These transition solutions must deliver continuity of performance, ratings, approvals and specification requirements. When selecting transition solutions, these key factors should be considered: 

● System Materials & Corrosion Protection: Rigid and Flexible systems are specified by the installation requirements including environment, applications and installation environment. Steel, aluminum, stainless steel and coated solutions are available for both rigid and flexible components required for a complete installation solution.  

● System Ingress Ratings: Both Rigid and Flexible systems carry Ingress Ratings that meet UL, CSA, NEMA and IEC IP ratings. These ingress ratings ensure that the total system will perform for dust tight, liquid tight, rain tight and temporary submersions requirements. A clear understanding of the ingress requirements of the installation is needed to select products that have appropriate listed and certified ingress ratings. to help ensure the ingress integrity of the complete installation between cable protection systems, enclosures and control gear. 

● System Temperature Ratings: Rigid conduits systems inherently have very high temperature ratings for non-coated and coated products. Flexible conduit systems now offer dedicated High Temperature (HT) solutions, from -60 to +150°C (-72 to +302°F). 

● Transition of Trade Sizes: Based on the installation requirements, conduits and fittings may need to transition between different trade sizes.  For example, a system may need to transition from a 1” rigid body to a 3/4” flexible conduit and fitting.  These transition requirements can be easily resolved with the use of “enlargers” or “reducers,” available in many size combinations and materials.   

● Transition of Thread Types: In some applications, transitions may be required between different thread types including NPT, ISO Metric, PG and other thread types. When these applications are encountered, “thread converters” can be applied to make these transitions.   

● Approvals & Certifications: Selection of products that carry the appropriate listings, certifications and approvals will help provide an installation that performs safely and reliably.  


Key Points to Remember  

● Rigid and Flexible Conduit Systems can be used together, to provide a high performance and reliable cable protection solution over the life of the installation. 

● When specifying cable protection solutions, consider all application and installation requirements including: environment, materials, corrosion protection, temperature ratings, ingress ratings, approvals/certifications, etc. 

● Transitioning between Rigid and Flexible Systems can be accomplished with the correct reducers, enlargers and thread converter accessories.  

● All cable protection products must be applied according to national and local electrical codes and standards.