For practical purposes, conduit is open to the environment unless you use special families of fittings.
While the original purpose of a metallic raceway was the protection of wires from mechanical damage, the metallic raceways of today serve a second purpose. Today, we expect them to carry fault currents — which are potentially lethal. That means we must pay special attention to component selection, preparation, and assembly. Whether you are installing electrical mechanical tubing (EMT), intermediate metal conduit (IMC) or galvanized rigid metal conduit (GRC), Sec. 110-3 of the NEC requires you to list and label your components.
Fittings come in several materials, including cast aluminum, cast malleable iron, cast zinc, and fabricated steel. Because all listed fittings meet minimum performance criteria, the Code doesn't specify what type of material you should use for those fittings. Therefore, you must base your decision on design considerations or personal preference.
Material composition is important, for example, when burying the conduit. In certain soils, you should use cast malleable iron. In others, cast aluminum works best. If you're going to bury conduit, find out what is customary for the locale. Your local distributor will know which material most installers use and may stock only fittings of that material. If you have any doubts, contact one of the manufacturers with the particulars of your application and get an engineering recommendation. Other factors to consider include local cathodic protection (which may either prohibit or require the use of aluminum in that facility's soil), the appearance of the conduit, the availability of all fittings in the same material, and customer specifications.
You have the option of ordering your fittings with insulated throat liners. Some fitting designs rely on throat liners as the primary means of protecting the conductors, so don't remove an existing throat liner from a fitting.
For practical purposes, conduit is open to the environment unless you use special families of fittings. For wet locations, use fittings listed for that use. Fittings listed as raintight may require a separate sealing ring and special assembly. However, raintight does not mean waterproof. For prolonged submersion in water, use NEMA 6 or 6P fittings. You should also use these in wet industrial environments, which include places heavy with coolants or machine oils — especially if those substances are present as aerosols or sprays.
What about concrete-tight fittings? UL does list some fittings specifically for this application. Neither threaded fittings nor gland fittings require a specific listing or marking for this purpose. If you adequately tape (per the manufacturer's recommendations) threadless GRC/IMC fittings, they pass the requirement for being concrete-tight. Don't assume a concrete-tight fitting is watertight or raintight — those require specific listings. However, fittings listed as raintight automatically qualify as concrete-tight.
Although many of us associate unthreaded metallic raceway with EMT, we also work with unthreaded conduit. Thus, unthreaded fittings are available for conduit. The unthreaded fittings fall into two general categories: gland (compression) type and set screw type. Gland type is the only type of unthreaded fittings listed for use in wet locations. Use only one type of fitting for a job. Otherwise, electricians will have to play mix-and-match games that slow down the work and increase the cost of labor. Let's look at the kinds of fittings available for both threaded and unthreaded conduit systems.
Bushings. These provide you with a smooth, rounded conduit entry to protect the conductors during wire pulls. Bushings are extremely important when you are transitioning from the conduit system into an enclosure — especially a bus box.
Combination couplings. Also known as transition couplings, they make the transition from a conduit (GRC/IMC) type of raceway to another type, such as EMT or flexible metallic conduit (FMC). Reducers are also included in this family.
Conduit bodies. Many electricians like to think of these as tiny pullboxes. When you need to make a splice somewhere between the two ends of a run, you can solve the problem by installing a conduit body. The Code permits you to use only conduit bodies listed for such a purpose and marked with an internal volume. Conduit bodies are also useful for changing direction in the raceway configuration and allowing access to the conductors in a completed system.
Couplings. These allow you to join conduit “sticks” together for a long straight run and join site-fabricated bends to conduit. However, consider using a conduit body where conduit passes through a wall or makes some other transition. If you aren't near a bend, use a conduit body instead of a coupling at the first conduit joint on the way into an enclosure and the first joint on the way out. This allows retraction of the conductors if someone needs to remove the enclosure. If a bend is nearby, use a pull elbow instead of a conduit body.
Drains. If your installation is in an area of widely varying temperatures — or if conduit runs from a warm area to a cool one — you can expect condensation. Install a drain at the lowest point of the run and at any point where water might accumulate. If the conduit runs underground (or to any place where a drain isn't feasible), create a low point ahead of the entry and place a drain there.
Elbows. These allow you to change the direction of the raceway by 90°. Elbows save significant labor costs and improve the appearance of the installation when used properly. This is especially true when you have several bends to make or are working in the larger trade sizes (in which case fabrication will be much more difficult).
Pull elbows. A pull elbow is a combination conduit elbow and conduit body. It allows you access to the conductors at a point where you are changing directions.
Expansion fittings. If you're going to run conduit between two areas where the temperature varies significantly, use expansion fittings. These will reduce stress on the raceway and its supports.
Grounding fittings. In some facilities, grounding considerations are so crucial that these special fittings are necessary. Their purpose is to allow you to “jumper” around grounding path discontinuities (or, potential discontinuities) in the conduit system. They provide a continuous path to ground, even if you break the conduit. In any conduit system, it's a good idea to install these wherever a crew may need to break a conduit run for maintenance, troubleshooting, repair, or any other foreseeable reasons.
Hubs. One of these will give you a threaded entry when a box (or enclosure) doesn't have one. Some hubs also provide trade size reduction. Use a hub when you run conduit to a box or enclosure that doesn't have an integral hub of its own. Never run conduit straight through a hole in a box.
Locknuts. Any time a fitting enters a box, secure it with a locknut. If you use a nut that has teeth facing only one side, be sure to install it with the teeth biting into the box.
Nipples. Like prefabricated bends, these save time and money in the field. This is especially true on short runs, where threading two ends of a conduit in the field can be challenging.
Reducers. This is a special kind of combination coupling that allows you to transition from one trade size of raceway to another. It is usually better to use a pullbox than a reducer — entering with one raceway size and exiting with another. Space will usually determine which you use.
Service entrance heads and fittings. You need to use these special fittings when transitioning overhead and underground services.
The National Electrical Manufacturer's Association (NEMA) stresses the importance of selecting the right fitting for the job. Combining your knowledge of how to do that with Sec. 110-12's required workmanship will ensure a safe, effective, and permanent installation.