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What are the NEC Requirements for Conduit Fittings in Explosion Proof Applications?

February 3, 2017
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4 Cable Seal Off

At Holland, we encounter many electrical system design challenges. Between large biopharmaceutical and food modules, smaller sanitary process skids, pump carts with VFD’s, etc., there are many chances to learn. Recently we had an interesting situation that required us to modify our design to meet a portion of the NEC standard for Hazardous Classified location equipment.

Section 500 of NFPA 70: National Electric Code defines the requirements for building equipment in Hazardous Locations. The standard is broken down into three “Classes”-Class 1 deals with flammable gases and liquids, Class 2 deals with combustible dusts, and Class 3 deals with ignitable fibers. For our purposes, we are concerned with Class 1. This classification defines (among many other things) the requirements for electrical enclosures (which must be explosion proof) as well as the conduit lines that carry electrical cables back to these enclosures.

The requirement of the standard is that all conduit lines must have a seal off located within 18 inches of the purged panel enclosure. Seal offs are (as there name implies) a barrier that is filled with a compound that “provides a seal against the passage of gas or vapors through the seal fitting”, and therefore into the enclosure potentially leading to a dangerous situation. Further, the standard requires that only 25% of the cross sectional area of the sealed fitting can be used up by the conductor wiring. Note that this is significantly less that the 40% allowable area that can be used in all the other fittings in the conduit assembly.

In our case, we realized during field installation that although we met the general 40% requirement, we did not meet the more stringent requirement in the seal off. This situation presented a few options that we had to think through. The first option was to increase the size of the seal off. The standard allows for a larger trade size seal when required to meet this requirement. Unfortunately, Crous-Hinds (as well as Gibson and Calbrite) doesn’t offer a seal off in stainless steel that is larger than 1 inch. There are other options for larger seal offs in other materials of construction (Robroy makes a PVS coated metal conduit (Plasti-Bond) in much larger sizes as well as EYX in galvanized iron), but the specification on this job required stainless steel.

Option two was to reduce the size of the cables. The initial choice was a 3 conductor + ground 18 gauge wire. There are some nice advantages to running multiple conductors, primarily being that it gives more flexibility down the road with device choices (think limit switches, more instrumentation, troubleshooting, etc.). However, in this case we needed to figure how to make this work. We opted to use a 2 conductor with 18 gauge wire, reducing the wire size by more than 30%. This choice met the requirement for cross sectional area of the seal off, effectively getting us in “just under the wire” (ok, I have been waiting to use that) to meet the standard.

Electrical design is a big part of our work at Holland. Hopefully this post will give you some ideas if you run into issues with seal offs and related Class 1 work. Contact one of our Holland Sales Engineers a call if we can help you with a project you are working on.

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