The requirements for the construction and installation of conduit bodies are mixed with those for boxes and fittings in Article 370 of the National Electrical Code (NEC).
A conduit body is defined in Article 100 as a separate portion of a conduit or tubing system that provides easy access through a removable cover to the interior of the raceway or tubing system. Conduit bodies are used primarily as junction or pull boxes and are usually installed in a raceway or tubing system to facilitate a change in direction. This access to the interior of the raceway can be used to satisfy the requirement in many raceway articles of no more than four-quarter bends or a maximum of 360 degrees between pull or junction points to prevent stress of the conductor insulation during installation. Conduit bodies marked with a cubic-inch capacity may also contain splices, taps, or devices.
The many different types of conduit bodies have designations such as “LB, LR, LL, Tee, X,” and “C.” The “L” Types are defined by holding the conduit body as if it were a pistol, gripping the short side as a handle and pointing the long portion of the conduit body forward, as you would a pistol barrel. If the removable cover is on top or on the back of the conduit body, it is an LB, with the “B” standing for “back.” If the cover is on the left side of the conduit body, then it is an “LL,” the “L” standing for “left.” If the cover is on the right side of the conduit body, then it is an “LR,” with the “R” standing for “right.”
A tee conduit body is formed in the shape of a “T” with a threaded raceway entry at the end of each leg and access through a removable cover on one side. The “X” conduit body is shaped as named and has four raceway entries with a cover on one side of the conduit body. The Type C is a straight conduit body with two threaded entries and a cover on the body to provide access to both entries.
Except for short radius bodies as covered in Section 370-5, Section 370-16(c) of the NEC provides specific information on the size and number of conductors permitted for conduit bodies containing No. 6 or smaller conductors. Regular conduit bodies must have a cross-sectional area not less than twice the cross-sectional area of the largest conduit or tubing to which it is attached. For example, Chapter 9, Table 4 specifies that a 1/2-inch rigid metal conduit has a 100 percent cross-sectional area of 0.314 (157/500) square inches. Therefore, a conduit body for use with this conduit must have an interior cross-sectional area of twice that cross-section, or at least 0.628 (157/250) square inch. The maximum number and sizes of conductors permitted in the conduit body is the same as that permitted by Chapter 9, Table 1 for the size of raceway connected to the conduit body.
If the conductors enclosed are larger than No. 6, the conduit body must also comply with Section 370-28(a) through (d). Section 370-28(a)(1) defines the minimum size of the box or conduit body where a straight pull is encountered, such as a Type C conduit body. The length of the conduit body from one entry to the other must not be less than eight times the trade diameter of the largest raceway i.e., a conduit body connected to 2-inch raceways must have at least a 16-inch dimension from entry to entry.
Section 370-28(a)(2) specifies requirements for conduit bodies where splices or angle or “U” pulls are being made. In this case, the distance between each raceway entry and the opposite wall or between raceway entries enclosing the same conductors cannot be less than six times the trade diameter of the largest raceway. For example, the minimum distance between the raceway entries in a 2-inch “LB” would be six times the 2-inch raceway, or 12 inches.
An exception in Section 370-28(a)(2) permits the distance between the raceway or cable entry and a removable cover that is located opposite to the raceway entry to be decreased to the allowable wire bending space for a single conductor, as specified in Table 373-6(a). In accordance with this exception, the distance from the back raceway entry to the removable cover of an “LB” conduit body may not be less than 4 inches for 3/0 or 4/0 conductors.
Section 370-28(a)(3) permits the use of conduit bodies with dimensions less than shown in the above examples for straight or angle pulls where the combinations of conductors are less than the maximum permitted conduit or tubing fill, as defined in Table 1 of Chapter 9. The conduit body used for this purpose must be approved and permanently marked for the maximum number and maximum size of conductors.
Understanding the requirements for conduit bodies can ease electrical system installation and minimize conductor insulation damage.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.