Fittings Make the Connection

Conduit fittings come in a variety of types for specific applications. Here's how to properly install them.

Fittings Make the Connection
provided by: 


All conduit-based electrical systems—be they Rigid Metal Conduit (RMC), Intermediate Metal Conduit (IMC) or Electrical Metallic Tubing (EMT)—require the proper fittings before they are a completely functional system.
Before installing these fittings it is always wise to review the packaging labels containing specific applications. Do not take these applications for granted. Many fitting designs look the same but may contain subtle differences designed to enhance performance in particular applications. Listed fittings contain required, informative markings and any specific instructions for use. Fittings should only be used with conduit of the trade size indicated on the fitting or its smallest unit shipped container.
Special applications
Threadless fittings intended for use in wet locations are marked "Raintight" or "Wet Locations" on the fitting or its smallest unit shipping container. Raintight fitting designs that require a gasket or a sealing ring installed between the fitting and a box must be installed only with the specific component marked on the fitting's smallest unit shipping container. Raintight or Liquidtight fittings are not necessarily suitable for use in applications where submersion in water is expected.
RMC and IMC fittings for use in industrial applications involving sprayed mineral oils and coolants are marked "Liquidtight" on the fitting or its smallest unit shipping container. Threadless fittings intended for embedment in poured concrete are marked "Concrete-tight" or "Concrete-tight When Taped," "Raintight" on the fitting's smallest unit shipping container. Note that taping is adequate to prevent the entrance of concrete aggregate into the raceway or box. Fittings listed as "Raintight" are also "Concrete-tight."
Expansion fittings
Often conduit based electrical systems require that expansion fittings be installed where there are significant temperature differentials—such as when conduit is installed as outdoor raceway spans between buildings, attached to bridges, on rooftops, etc., where expansion and contraction would result from the direct heat of the sun coupled with significant temperature dips at night.
Threadless fittings
Threadless fittings must not be assembled to threaded RMC or IMC unless specifically recommended by the fitting manufacturer. Where threadless fittings are to be assembled to RMC, IMC and EMT conduit the ends must have squarely cut ends, free of internal and external burrs, and circular form as provided from the factory; be free from dirt or foreign matter on the surface of the conduit to be inserted into the fitting; have the ends of the conduit or tubing assembled flush against the fitting's end stop. Careful consideration must be given to the torque applied to the fitting's securement means.
Screw-type fittings
With screw-type fittings the length of screws provided vary from manufacturer to manufacturer. The appropriate torque for some designs is reached when the head of the screw touches a screw boss on the fitting. However, this cannot be universally relied upon. Screws on certain fitting designs, particularly larger trade sizes, can offer more than one tightening option including screwdrives or a bolt head from wrench applications. Greater mechanical advantage and torque can generally be achieved with a wrench. Where both screwdriver and wrench application options are offered, torque should be limited to that which can be applied by the screwdriver.
Compression-type fittings
Most compression or gland-type fittings achieve maximum securement after hand tightening, then wrench tightening one or two additional turns.
Prior to embedment in poured concrete, all threadless fittings, including those marked "Concrete-tight," shall be taped to prevent the entrance of concrete aggregate where they will be embedded more than 24 inches or where the pour area will be subjected to a concrete barrier. Tape must be applied after the fitting is assembled and secured to the conduit.
Threaded fittings
Threaded joints, both fitting to conduit and fitting to threaded integral box entries, shall be made up wrenchtight. Generally, a force equivalent to handtight plus one full turn with an appropriate tool is recommended. Conduit bodies generally have an integral bushing to provide a smooth surface for conductors when pulled. This bushing is often mistaken for a conduit end stop. It is not necessary that the conduit be inserted flush against this bushing to assure a secure joint.
Attachment
Prior to assembly to a box or enclosure or a threadless coupling, RMC, IMC and EMT must be supported at intervals required by the National Electrical Code, using raceway supports intended for the purpose secured by hardware acceptable to the local jurisdiction.
Properly align the raceway, fittings and knockouts to provide secure mechanical and electrical connections. Allow sufficient conduit length to complete engagement of the conduit and fittings at joints and entries.
Conduit bushings must not be used to secure threaded RMC or IMC to a box or enclosure. A locknut must always be assembled between a conduit bushing and the inside of the box or enclosure.
EMT connectors are permitted to be assembled into threaded entries of boxes, conduit bodies or internally threaded fittings having tapered threads. Threaded openings where these fittings are intended to be used are permitted to have either tapered (NPT) or straight (NPS) threads. Care should be taken to insure that the threaded entry will accommodate a minimum of three fully engaged threads.
Where a locknut is provided with a fitting as the means of securement to a box or enclosure, the locknut is to be secured by hand-tightening to the enclosure plus a quarter turn using an appropriate tool. Always remember while securing the locknut, take care to avoid excessive pressure where gripping the body of the fitting.
Do not rely upon locknuts to penetrate nonconductive coatings on enclosures. Such coatings must be removed in the locknut contact area prior to raceway assembly to assure a continuous ground path is achieved. Touch up bare areas after fitting assembly as needed.
Perhaps the most important aspect of constructing a conduit-based electrical system is inspection after construction. After the raceway is fully installed and supported, and prior to installing conductors, all fittings and locknuts must be re-examined for secureness.
William A. Wolfe is the Executive Director of the Steel Tube Institute of North America, steel tubing manufacturers dedicated to improving manufacturing techniques and informing customers about product usage. Learn more at www.steeltubeinstitute.org
author: By William A. Wolfe, Steel Tube Institute of North America




Local Articles
Industrial Goods & Services
Home