Seal The Deal

Requirements for electrical wiring in hazardous (classified) locations are more restrictive than in the rules for wiring in general types of occupancies. National Electrical Code (NEC) Chapter 5 includes rules for special occupancies such as hazardous locations, healthcare facilities and assembly occupancies. Chapter 5 rules often modify or amend the general requirements in chapters 1 through 4. In the case of hazardous locations, these modifications are usually more restrictive. This article reviews important sealing requirements for wiring in hazardous locations, specifically locations that are designated Class I, Division 1 and 2. Conduit and cable sealing requirements for Class II locations are provided in Article 502 and are not covered in this article. Seals are not required in Class III locations.

Where required

In determining the need for conduit seals in hazardous locations, one must gain an understanding of why and where seals are required in Class I locations. There are two purposes for installing conduit seals. The first is to prevent explosions (flames) within the conduit and equipment from being transferred from one portion of the system to another. Generally, conduit seals are required within 18 inches of the point of entry to explosion-proof enclosures. This requirement seeks to contain explosions and flames within the enclosure and prevent them from being rapidly transmitted through the conduit systems.

Another reason for conduit seals is to minimize the passage of gases or vapors from migrating from a hazardous location to a nonhazardous location. Sealing fittings perform as intended if the seal is dammed and poured in an acceptable fashion. It is a Code violation and a risk to property when the seal is not completed. Some have used excuses—such as not being able to draw wires in or out at a later date—for not pouring the seals. Others have used inferior sealing methods, such as filling a conduit seal with silicone or electrical sealing putty. These remedial methods fall short of NEC compliance and achieving functional objectives.

Achieving Code compliance

Conduit seals in Class I, Division 1 and 2 locations must meet the requirements in Section 501.15(C). Sealing fittings must be listed, and listed equipment has to be installed in accordance with instructions in 110.3(B). Sealing fittings cannot contain splices and have to be located in accessible locations, without exception. Completing an effective conduit seal requires installing a damming fiber and sealing compound that is specific to the particular fitting. This compound must be durable for the surrounding atmosphere or liquids. The melting point cannot be less than 93°C to withstand flames and heat from explosions. This compound has to be at least ⅝ inch thick and not less than the trade size of the conduit in which it is installed. For example, if the seal is installed in a 2-inch conduit, the compound in a completed seal cannot be less than 2 inches thick. Conductor fill is restricted in conduit runs where sealing fittings are installed. The Code generally permits up to a 40 percent fill for conduits containing more than two conductors as provided in NEC Chapter 9, Table 1. Limiting the fill to 25 percent of the conduit cross-sectional area of the raceway provides adequate room in the fitting for separating the conductors and achieving an effective seal around each conductor as it passes through the fitting. If the fill is in excess of 25 percent of the conductors, it can bunch up in the middle of the seal, creating spaces between them through which gases or vapors could migrate. Some manufacturers produce listed sealing fittings that are oversized and can accommodate the 40 percent fill requirements.

Multiconductor cables

Section 501.15(D) provides cable sealing requirements. Conduits containing multiconductor cable assemblies used for voice, data or control must also be sealed. Where multiconductor cables pass through conduit seals, there are two methods of achieving an effective seal. The method used depends on whether the cable jacket can transmit gases or vapors through the core of the cable. If the cable jacket or sheath allows passage of gases or vapors, the jacket must be removed so the conductors in the cable assembly can be separated when installing the sealing compound. For cables that do not transmit gases or vapors through the sheath to the cable core, the cable can be dammed and sealed without removing the sheath. Multiconductor cable manufacturers should be able to provide evidence of sheath suitability.


Sealing is required for conduits and cables installed in Class I, Division 1 and 2 locations. Conduit seals must be listed and meet specific requirements to achieve an effective seal that performs as anticipated. Conduit seals prevent passage of explosion and flames beyond explosion-proof equipment and minimize the migration of gases or vapors from classified locations to unclassified locations. The seal fitting installation is not complete if the seal is not dammed and poured.

About the Author

Michael Johnston

Executive Director of Standards, NECA
Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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