Sealing Conduit in Class I Locations: The requirements and purposes in special occupancies

By Michael Johnston | Sep 15, 2022
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Requirements for electrical wiring in hazardous (classified) locations are more restrictive than for wiring in general occupancy types.

Requirements for electrical wiring in hazardous (classified) locations are more restrictive than for wiring in general occupancy types. 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 to unclassified locations.

Two reasons for seals

In determining the need for conduit seals in hazardous locations, one must know why and where seals are required in Class I locations. There are two purposes for installing conduit seals. One reason 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. These conduit seals contain explosions and flames within the enclosure and prevent them from being rapidly transmitted through the conduit systems.

The other reason for conduit sealing is to minimize the passage of gases or vapors from migrating from a hazardous to a nonhazardous location. Sealing fittings perform as intended if the seal is dammed and poured in an acceptable fashion. When a sealing process is not complete, it is a Code violation and a risk to property. The Code is specific about sealing conduits as directed by manufacturer’s instructions using listed damming fiber and sealing compound. Inferior sealing methods, such as filling a conduit seal with silicone or electrical sealing putty, are unacceptable. These remedial methods fall short of Code compliance and achieving functional objectives. The seal fitting installation is not complete if the seal is not dammed and poured.

What are the requirements?

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 is required to be installed in accordance with installation instructions. Sealing fittings cannot contain splices and have to be installed in accessible locations, without exception.

NEC Chapter 5 provides minimum requirements for special occupancies such as hazardous locations. Chapter 5 modifies or amends general requirements in chapters 1–4.

Completing an effective conduit seal requires installing damming fiber and sealing compound specific to the listed sealing 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 must be at least 5/8-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 40% fill for conduits containing more than two conductors as provided in NEC Chapter 9, Table 1. Limiting the fill to 25% 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 exceeds 25% of the conductors, it can bunch up in the middle of the seal, creating spaces between them through which gases or vapors could migrate. There are oversized listed sealing fittings that can accommodate up to 40% fill requirements.

Section 501.15(D) includes sealing requirements for cables. 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 cable’s core. 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.

NEC Chapter 5 provides minimum requirements for special occupancies such as hazardous locations and others. Chapter 5 rules often modify or amend the general requirements in chapters 1–4. In the case of hazardous locations, these modifications are usually more restrictive. Conduit and cable sealing requirements for Class II locations covered by Article 502 are not covered in this article. Seals are not required in Class III locations.

Header image by iStock.

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].






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