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Recently, i witnessed an electrical inspector and a group of electricians discussing the effectiveness of the bonding and grounding path where raceway fittings are used to connect raceways to threaded hubs, threaded bosses or conduit bodies. Although this is a fairly common practice in the field, the discussion revealed varied opinions on the acceptability of these installations. Bonding all nonenergized metal parts together and grounding these parts is an important part of any electrical installation. Proper bonding and grounding reduces the possibility of a shock hazard and also provides a low impedance path for ground-fault current. In addition to the grounding and bonding issues, raceway fittings connected to threaded hubs, bosses or conduit bodies may not provide a seal to prevent entrance of water through the connection, unless listed specifically for use with the particular raceway fittings. Thorough understanding requires the study of the National Electrical Code (NEC) as well as the listing of hubs and raceway fittings.
To understand this issue, one must first understand the purpose of a hub in an electrical installation. A hub is a type of device designed to transition from a conduit system into an enclosure. One-piece hubs are typically bolted to the top of an enclosure. Alternatively, a two-piece hub may be used and typically includes one piece that is designed to be inserted into a knockout in an enclosure from the outside, with the other piece inserted from inside the enclosure, and with the two pieces then screwed together. The two parts are fastened together, enabling the conduit to be connected to the enclosure.
Many electricians overlook the fact that hubs are only intended for use with threaded conduit, such as rigid metal conduit (RMC) or intermediate metal conduit (IMC). The hubs are designed with a tapered thread to match the tapered thread requirement of RMC or IMC of a 1-in-16 taper ( inch per foot taper) as covered in 342.28 for IMC and in 344.28 for rigid metal conduit. Connection to hubs using other than rigid metal conduit or IMC results in two issues that must be addressed. The first issue deals with connections in a wet location, the second deals with proper grounding paths, and while these are seemingly separate issues, the two issues are actually interrelated. We will deal with the wet location first and then cover the grounding issues second.
Electrical metallic tubing (EMT) fittings have straight threads and are designed to be installed through a knockout or hole in an enclosure, using a locknut to connect the fitting to the enclosure. In a wet location, the installation of a hub into a weatherproof enclosure requires a wet location rating of the connection. The wet location rating depends on the tapered thread of the raceway and the hub to provide a seal to prevent water from migrating into the enclosure. Screwing a fitting with straight threads into a hub may not provide the seal necessary to prevent water intrusion into the enclosure. In addition, an EMT fitting, marked for use in a wet location, has a gasket to provide a seal against water entering into the enclosure. An EMT fitting threaded into a hub may not seat the gasket properly into the hub, thus permitting water to migrate through the hub and into the enclosure. Section 358.42 requires EMT couplings and connectors to be made up tight and to comply with 314.15 of the NEC. Section 314.15 requires boxes, conduit bodies and fittings installed in damp or wet locations to prevent moisture from entering or accumulating within the box, conduit body or fitting.
The second issue deals with whether an EMT fitting threaded into a hub provides a reliable grounding path connection from the tubing to the hub and to the enclosure. Straight threaded EMT fittings inserted into tapered thread hubs may not establish a tight enough connection to provide a low impedance path for fault current. Where gaps occur between the straight threads of the fitting and the tapered threads in the hub, arcing currents can occur, causing higher impedance paths across the threaded connection. This may cause a delayed response of the overcurrent device protecting the circuit, resulting in a fire caused from ground-fault conditions. Some electricians install the EMT fitting into the hub with the locknut on the EMT fitting and, after installing the fitting, use the locknut to further tighten the fitting to the hub. This may or may not result in a lower impedance path. However, it is not the method used during the testing of the listed hub or fitting.
Care must be taken to install electrical components in accordance with the NEC, so do not use an EMT fitting with a hub, a boss or a conduit body. ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and firstname.lastname@example.org.