You're correct to question this. The client's assertion is likely based on a correct interpretation of the NEC. Here's the breakdown: NEC 348.6, 350.6, and 344.6(C) and 344.42

These sections are key. They generally require that fittings and connectors used with conduit (including LFMC) must be designed and listed for that specific type of conduit. GRC couplings are explicitly mentioned for use with GRC (rigid metal conduit).

The Listing Requirement is Critical

The NEC requires that electrical equipment be installed according to its listing (NEC 110.3(B)). If a fitting is listed only for GRC, it's not compliant to use it with LFMC. The listing ensures the fitting has been tested and certified to provide proper mechanical and electrical connection, grounding, and environmental protection when used with the conduit it's intended for.

Why a GRC Coupling Isn't Suitable for LFMC



Different Thread Types/Designs

GRC (Rigid Metal Conduit) typically uses National Pipe Thread (NPT). While LFMC fittings can sometimes utilize NPT, the threads may not be cut to the same depth or tolerances. This could lead to a poor mechanical connection, increasing the risk of the conduit pulling out.

Environmental Sealing

LFMC is often used in locations where liquid-tightness is important. A GRC coupling isn't designed to provide a liquid-tight seal when used with LFMC, which defeats the purpose of using LFMC in the first place.

Grounding Path

While both GRC and LFMC are metallic, the grounding path provided by a GRC coupling when used with LFMC may not be as reliable as a fitting specifically listed for LFMC.

Finding the Specific NEC Section

The most relevant sections are:

NEC 110.3(B)

Installation and Use. This is the overarching requirement to follow listing and labeling instructions.

NEC 350.6

Flexible Metal Conduit (FMC). This section specifically addresses connectors.

NEC 348.6

Liquidtight Flexible Metal Conduit (LFMC). This section also addresses connectors. You'll need to cross-reference these sections with the listing information for the GRC coupling. The GRC coupling will be listed to UL 514B conduit fittings and UL 6 which covers rigid metal conduit.

What to Do


Consult the Fitting's Listing

Check the markings on the GRC coupling and try to find its listing information online (e.g., UL listing). This will definitively state what type of conduit it's approved for.2.

Use Listed LFMC Fittings

Replace the GRC coupling with a fitting specifically listed for use with LFMC. These fittings will be designed to provide a proper mechanical and electrical connection, as well as maintain the liquid-tight integrity (if required).3.

Insepctor Authority the AHJ (Authority Having Jurisdiction) has the final say. If you disagree with the contractor's interpretation, you can appeal to the inspector for clarification.

In Summary:

The client is likely correct. Using a GRC coupling with LFMC is generally a violation of the NEC because the coupling is not listed for that purpose. The NEC mandates that you follow the listing and labeling instructions for all electrical equipment, and a GRC coupling is specifically listed for GRC, not LFMC. Flag for review