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4-1 Installing Conduit
Several general requirements apply to all types of conduit installation. All runs must be installed as a complete system before any conductors are pulled into them. In other words, a run of conduit (to include conduit, fittings, and supports) must be complete before the conductors are installed. A run of conduit should be as straight and direct as possible. When a number of conduit runs are to be installed parallel and next to each other, install them all at the same time. The minimum size raceway that can be installed is generally 1/2-inch electrical trade size. There are exceptions to this rule depending on specific locations. The exceptions for each type are outlined in the NEC.
All types of conduit must be reamed after they have been cut. Conduit threaded in the field must be threaded with a die that has a 3/4-inch taper per foot. Also, never use threaded couplings with running threads. Running threads weaken the conduit and may come loose. Threaded couplings and connectors used with any type of conduit must be made with tight connections. When the couplings or connectors are to be buried in concrete or masonry they must be the concrete-tight type. When installed in wet locations, they must be the watertight type.
Fittings for EMT are of two general types ¾ watertight fittings that may be used outdoors or in any location and fittings that provide strong mechanical and electrical connections, but may be used only in dry locations.
The watertight fittings join sections of tubing by means of a five-piece compression fitting. (Figure 4-1).
To put a watertight fitting together, use the following steps:
A similar fitting having only three pieces is used to make a watertight joint to metal boxes (Figure 4-2).
To make a watertight joint to a metal box, use the following steps:
Fittings for use in dry locations are simpler to use and less expensive. One type consists of a sleeve and two or four setscrews (Figure 4-4).
Another form of coupling is made by using a plain sleeve and an indenting tool (Figure 4-5).
To put on an indented coupling, use the following steps:
Fittings used for rigid-steel and PVC conduit are similar to those used for EMT. Threaded and threadless couplings and connectors are available for use with rigid-steel and PVC conduit and PVC. The threadless fittings are installed in the same way as those for EMT. The advantage of using threadless couplings and connectors is that threading the conduit is not required. Because EMT has a thin wall, it cannot be threaded, thus threaded couplings cannot be used with EMT (Figure 4-7).
On rigid steel conduit threaded couplings are screwed onto the threaded ends of the conduit and tightened with a pipe wrench (Figure 4-8).
Rigid-steel and PVC conduit is connected to electrical boxes by locknuts (Figure 4-9). The locknuts are tightened against each side of the box wall. The bushing is placed over the end of the conduit to provide the conductor with protection from physical damage.
Fittings for flexible metallic conduit are either internally or externally attached to the conduit. The internal type is designed to screw into the spiral of the conduit. This type of connector covers the end of the conduit completely, protecting the conductors from contact with the cut edge of the conduit. Externally attached connectors are secured to the conduit with clamping screws (Figure 4-10).
When using these connectors, make sure that the cut end of the conduit is pushed as far as possible into the connector, covering the cut end and protecting the conductors from damage.
A special type of metallic flexible metallic conduit is made for use in wet areas. It is called liquid-tight. Liquid-tight fittings are available for use with this conduit (Figure 4-11).
Connections are made in PVC conduit by cementing two pieces of PVC together (Figure 4-12). Joints must first be coated with primer. The cement used is actually a solvent that softens the plastic at the joint and allows the softened areas to flow together to form a weld. The resulting joint is watertight and strong. PVC conduit can be cut readily with any fine-tooth saw.
When you run conduit from one point to another, you often need to make more turns (total of 360°) than the NEC allows in a single run. When this is the case, you can use a fitting called a conduit body. A conduit body, as defined in the NEC, is "a separate portion of a conduit or tubing system that provides access through a removable cover to the interior of the system at a junction of two or more sections of the system or at a terminal point of the system." Figure 3-3, page 3-3, shows some of the more common conduit bodies and covers.
A conduit body is put in conduit between two outlets to keep the bends within NEC limits for a single run (Figure 4-13). As you can see, the run on the left has bends that total 360º, which is all the NEC permits. Therefore, a conduit body had to be installed so that the conduit could be continued to the box on the right.
Conduit must be supported by straps or hangers throughout the entire run (Figure 4-14).
On a wooden surface, nails or wood screws can be used to secure the straps. On brick or concrete surfaces, you must first make a hole with a star or carbide drill and then install an expansion anchor. Use an expansion tool to force the anchors apart, forming a wedge to hold the anchor in the hole. Secure the strap to the surface with machine screws attached to the anchor. On tile or other hollow material, secure the straps with toggle bolts. If the installation is made on metal surfaces, you can drill holes to secure the straps or hangers with machine or sheet-metal screws.
The number of supports needed depends on the type of conduit being used. Holes or notches in framing members may serve as supports. EMT requires supports within 3 feet of each outlet box, junction box, cabinet, or fitting and every 10 feet thereafter. Rigid-steel conduit must also be supported within 3 feet of a box. The distance between supports may be increased to 20 feet on direct vertical runs of rigid-steel conduit from machine tools and other equipment if threaded couplings are used and the riser is supported at each end. PVC must be supported as in Table 4-1.
In addition, PVC must be supported within 3 feet of each opening.
Flexible metallic conduit must be supported at intervals not to exceed 4 1/2 feet and within 12 inches on each side of every outlet box or fitting. Exceptions to this are runs of 3 feet or less where flexibility is needed or 6 feet when connecting light fixtures.
After all conduit has been installed, supported, and connected to the boxes, you are ready to install the conductors.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015