For most electricians, the proper use of fittings is elementary. Fittings themselves are pretty straight forward in their use. A fitting is an accessory such as a locknut, bushing, connector, coupling, or other part of a wiring system that it is intended primarily to perform a mechanical rather than an electrical function. This simplicity becomes a matter of thought and caution when coupling different types of raceways together, especially, when compounded with the issues of wet locations.
The steel coupling transition between two raceways is probably used in some cases. In this application, two connectors are used in conjunction with a steel coupling to form a “from–to” (from this to that). This type of installation has several inherent weaknesses that should be understood and addressed. The most obvious weakness of this fitting assembly lies in its lack of a listing. Without a listing, the fitting violates the NEC codes.
The “from–to” fitting assembly is a violation of the NEC because of section 300.15 which states: a fitting must be used only in the application it is listed: “fittings … shall be used only with the specific wiring methods for which they are designed and listed.”
When referring back to the definition of a fitting, we understand that a fitting must meet two criteria. First, it must perform its mechanical duties in terms of connecting the raceways or cables together. Second, a fitting must also provide a low-impedance path for the continuity of the raceway, or its electrical function. Any fitting, used with metallic raceways, must not offer impediment to the flow of current. When constructing or evaluating any transition between raceways, these two factors can provide the guidelines for their acceptance. Does the transition offer good mechanical strength? Will the fitting or assembly impede the flow of current?
While the dry location transition sometimes offers good mechanical strength, it might not meet the second criteria of a fitting: its electrical function. This is where a “from–to” becomes problematic as the connection relies on the tightening of the connector against the steel coupling. In this case, the shoulders of the connector are in contact with the steel coupling in order to make the assembly tight. These shoulders and the threads must work together to create the system bonding path and offer little or no impedance to the flow of current. If the fitting is slightly corroded, the two small points of contact on the connector will provide the primary equipment bonding path and this “from–to’s” electrical function will likely be less than adequate. If installed loosely or if corroded, the fitting is in violation of the National Electrical Code as it does not perform both of its intended functions.
When transitioning between conduit systems, it is possible to use code-compliant products that are listed expressly for coupling different raceways.The combination coupling or the transition fitting is available for most of the applications in the field. The conversion of EMT to Rigid, EMT to FMC, Rigid to Liquidtight or any combination thereof can be accomplished using UL listed products.