Requirements for electrical wiring in hazardous (classified) locations are more restrictive than in the rules for wiring in general types of occupancies. Section 90.3 provides National Electrical Code (NEC) users with a road map that covers overall NEC arrangement and how it applies to installations and systems.
NEC Chapter 5 includes rules for special occupancies such as hazardous locations, healthcare facilities and so forth. The rules in Chapter 5 often modify or amend the general requirements in chapters 1 through 7. In the case of hazardous locations, these modifications are usually more restrictive. This column reviews a few important conduit seal requirements for wiring in Class I, Division 1 hazardous locations. Conduit and cable-sealing requirements for Class II locations are provided in Article 502 and are not covered in this column. Seals are not required in Class III locations.
There are two principal reasons for installing conduit seals. The first is to prevent explosions (flames) within the conduit and equipment from being communicated 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 to contain explosions and flames within the enclosure and prevent them from being rapidly transmitted through the conduit systems.
The second reason for conduit seals is to minimize the passage of gases or vapors from migrating from a hazardous location to other unclassified locations. Sealing fittings can only be performed 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, although some have used excuses such as not being able to draw wires in or out at a later date. Others have used inferior sealing methods such as filling a conduit seal with silicone or electrical-sealing putty.
The requirements for conduit seal in Class I, Division 1 locations are provided in Section 501.15(C). One of the most elementary rules requires sealing fittings to be listed. This ensures the fitting meets established product safety standards. Listed and labeled equipment has to be installed in accordance with installation instructions in accordance with 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 for the particular fitting. This compound must be durable for the surrounding atmosphere or liquids. The melting point cannot be less than 93°C (200°F) to withstand flames and heat from explosions. This compound has to be at least ⅝-inch in thickness and not less that the trade size of the conduit run in which it is installed. For example, if the seal is installed in a 2-inch conduit, the thickness of the compound in a completed seal cannot be less than 2 inches. 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. The reason for limiting the fill to 25 percent of the conduit cross-sectional area of the sealed raceway is to provide adequate room in the fitting for separating the conductors and achieving an effective seal. If the fill is in excess of 25 percent, the conductors can bunch up in the middle of the seal, creating space between them through which gases or vapors could migrate. For example, a ¾ rigid metal conduit with a conduit sealing fitting is generally limited to only (8) 12 AWG THHW conductors for 25 percent fill, where the same conduit without a sealing fitting allows for (12) 12 AWG THHW conductors for 40 percent fill. Some conduit seal manufacturers produce oversized sealing fittings can accommodate the 40 percent fill requirements.
Multiconductor cable sealing requirements are provided in Section 501.15(D). Conduits containing multiconductor cable assemblies, installed hazardous locations must also be sealed. Where multiconductor cables pass through conduit seals, there are two methods of achieving effective and compliant seals. The method used depends on whether or not the cable jacket can transmit gases or vapors through the core of the cable. If the cable jacket or sheath does allow passage of gases or vapors, the jacket has to be removed so the conductors in the assembly can be separated when installing the seal damming compound. For cables that do not transmit gases or vapors through the sheath to the cable core, the cable can be dammed and sealed as an individual conductor as it passes through the seal fitting. Multiconductor cable manufacturers should provide evidence of this suitability.
Remember the reasons for conduit seals is to prevent passage of explosion and flames beyond explosion-proof equipment, and minimize migration of gases or vapors from classified locations to unclassified locations.