One of the most versatile components of anelectrical raceway system is the conduit body. The National Electrical Codedefines a conduit body as “A separate portion of a conduit or tubing systemthat provides access through a removable cover(s) to the interior of the systemat a junction of two or more sections of the system or at a terminal point ofthe system. Boxes such as FS and FD or larger cast or sheet metal boxes are notclassified as conduit bodies.” This definition, however, doesn’t fully describeall of the uses permitted for a conduit body in the NEC, as a conduit or cable fitting, a splice box, a device box, a junction box and a pull box.
Knowing the exact use for which the conduit body will be employed is critical in applying the appropriate sections of the NEC.
First and foremost, a conduit body is part of the raceway system to which it is attached. A listed conduit body is always subject to the construction and performance requirements for a conduit fittingfor direct attachment to conduit or tubing. In addition, it is anticipated that a conduit body will be used as a pull box, and therefore they are subject to either specific dimensional or performance requirements to ensure adherence towire fill requirements and protection for electrical conductors during pulling according to NEC 314.28.
Metallic conduit bodies are provided withmeans for securing rigid or intermediate metal conduit by threading intointernally threaded hubs or electrical metallic tubing (EMT) by unthreaded hub shaving set-screws. Nonmetallic conduit bodies may have unthreaded hubs(sockets) into which rigid PVC conduit is to be glued or internally threaded hubs to accept threaded adapters that are listed for use with raceways. It hasalso been common practice to connect a wide variety of approved raceways,cables and cord to internally threaded hubs in conduit bodies using listed fittings.
NEC 300.15 also recognizes a conduit bodyas equivalent to a box for containing wire splices and wiring devices, such asa switch or receptacle. When intended for such use, a conduit body is subject to the requirements for a box. The simplest way to determine if a listed conduit body is suitable for use as a splice box or as an outlet or device box is to look for the internal volume marking required by NEC 314.16 in theconduit body. These same rules apply for determining the minimum size of the conduit body used as a box. Since a conduit body does not typically contain screw holes for wiring device mounting, specially designed and listed covers,for this purpose, are provided by the manufacturer of the conduit body. According to NEC 314.5, “Conduit bodies such as capped elbows and service-entrance elbows that enclose conductors 6 AWG or smaller, and are onlyintended to enable installation of the raceway and the contained conductors,shall not contain splices, taps, or devices and shall be of sufficient size toprovide free space for all conductors enclosed in the conduit body.”
The simplest conduit body has only one raceway attachment hub. The obvious use for this conduit body is at at ermination point of a wiring system. These conduit bodies typically aredesigned to contain wire splices or a wiring device so most often they will belisted for this purpose. Be sure, however, to look for the internal volume marked in the conduit body for verification.
Other conduit body configurations may havetwo, three or even four raceway attachment hubs of the same size or differentsizes. Two hub configurations are often referred to as straight, LB, or LL orLR, to designate the direction the raceway will be diverted [straight—straight through end-to-end; LB—90 degrees through the back of the body opposite thecover; LL or LR—90 degrees through the left or right side of the body]. Threehub configurations are referred to as “T” bodies, and four hub configurationsas “X” bodies
NEC 314.28 establishes certain minimum dimensions for pull and junction boxes and conduit bodies to ensure adherenceto wire fill requirements and protection for electrical conductors duringpulling. Conduit bodies of dimensions less than those required by 314.28(A)(1)and (A)(2) are permitted under conditions specified in 314.28(A)(3).
Understanding and enforcement of the requirements in Section 314.28 are sometimes difficult. In particular, sincemany of the conduit bodies offered today take advantage of the “smaller dimensions” provision in 314.28(A)(3), verifying the minimum and maximum sizeand number of conductors for which a conduit body is acceptable can be achallenge. Listed conduit bodies have been evaluated for their compliance withthese minimum dimensions or exceptions, and required markings. The following isintended to provide some clarity to the requirements in NEC 314.28.
314.28(A) Minimum Size. The smallest sizeconduit or tubing hub provided in a conduit body is metric designator 16 (tradesize 1/2). Since only one 4 AWG conductor consumes the maximum wire fill for 16(1/2) conduit or tubing, conduit bodies having only 16 (1/2) size hubs areexempt from the requirements in 314.28. Section 314.5 applies to such conduitbodies. The requirements in 314.28, therefore, apply to conduit bodies havinghub sizes 21 (3/4) or larger.
The so-called “6 and 8 times rule”addressed in 314.-28(A)(1) and (A)(2) respectively, establishes a seemingly simple guide for determining adequate space between entry hubs and walls andentry and exit hubs for through pulls. Since through pulls generally require bending of the conductors or cables, adequate space to accommodate the pulling operation, without over-bending, is an important and required design consideration.
Although traditional trade sizes (3/4, 1,1-1/4, etc.) for conduit and tubing have always been nominal (not representing actual inch dimensions), their use as the “units of measure” in inches has been prevalent, representing the conduit or tubing diameter for the purpose of calculating the required length according to the 6 and 8 times rule. The metric designators [NEC Table 300.1(C)] now the preferred reference in the NEC, if applied in millimeters as the “units of measure employed,” would require a greaterlength than that calculated using the traditional trade size/inch as the “unitsof measure employed.” The examples in figures 3 and 4 demonstrate there commended conversion where SI metric conversion is desirable (1 inch = 25.4mm).
Since cable wiring systems can only be connected to conduit bodies by use of listed cable fittings designed to thread into standard trade size threaded conduit entries, special care needs to betaken when transposing cable size into raceway size for these dimensional calculations. Present cable designs are often able to provide larger conductorsin cable assemblies having significantly reduced outside diameters. The conduitbody hub size into which listed fittings for these smaller cables are able tobe assembled may not represent the minimum metric designator (trade size)raceway required for the number and size of conductors in the cable. The minimum metric designator (trade size) raceway used for this calculation shallbe based on Table 1, Chapter 9.
(1) Straight Pulls. The length of the conduit body shall not be less than eight times the metric designator (tradesize) of the hub that will accommodate the largest raceway.
(2) Angle or U Pulls (Conduit body configurations LB, LL, LR, T). The distance between each raceway entry insidethe conduit body and the opposite wall of the box shall not be less than sixtimes the metric designator (trade size) of the largest raceway. This dimensionis not applicable when the opposite wall is the removable cover.
(3) Smaller Dimensions. Since many of the conduit bodies offered today take advantage of the “smaller dimensions”provision in 314.28(A)(3), verifying the maximum size and number of conductorsfor which a conduit body is acceptable, can be a challenge.
UL 514B, Conduit, Tubing and CableFittings, contains the requirements for listed conduit bodies. For conduit bodies designed to take advantage of this “smaller dimensions” provision, the manufacturer must specify the maximum size conductor and the number ofconductors of that size, and the conduit bodies are then subjected to a wirepull test to ensure damage to the conductors does not occur. If wire pulling lubricant is specified by the manufacturer for use during the test, this mustbe provided as a marking on the product.
The marking requirement indicating the maximum size conductor for which the “smaller dimension” conduit body has beenfound acceptable is straight forward. And, the maximum number of the same size maximum conductor can also be clearly indicated in the marking. Particularly inlarger size conduit bodies, however, often more than one size conductor ispulled into a conduit body and it is not always clear by the manufacturer’smarkings what combinations of conductors are included in the conduit bodylisting. The only sure way to make this determination is to utilize the samemethod as one would to calculate the conductor fill in a conduit where varioussize conductors will be installed (see NEC chapter 9 tables). Since the conduitbody manufacturer clearly provides the marking of the maximum size conductorand the maximum number of that conductor, one is able to calculate the percentage wire fill represented for the smallest conduit body hub size in athrough run. Any additional combinations of conductor sizes must comply withthat calculated conduit wire fill percentage for that size conduit.
NEC 314.15(A) provides the requirements forconduit bodies installed in damp and wet locations. Although gasketed coversare generally provided for use with conduit bodies, this does not necessarily mean that all conduit bodies are suitable for use in damp or wet locations.Listed conduit bodies are required to be marked “For use in wet locations” or “Wet locations” where determined by test to be suitable for this application.Field assemblies of conduit bodies with listed conduit or cable fittings threaded into conduit body hubs have not typically been evaluated for use in wet locations as part of their listing. Conduit and cable fittings listed and marked for use in wet locations, similarly have not typically been listed forsuch use when field assembled into the threaded hubs of a conduit body.
Conduit bodies are permitted to be supported by rigid and intermediate metal conduit, EMT, and rigid nonmetallicconduit, as appropriate, in accordance with the exceptions in NEC 314.23 (E)and (F).
All conduit bodies are required to beprovided with compatible covers that are suitable for the conditions of use. Because of industry standardization, covers are not always shipped from the factory with assemblies that include covers and/or gaskets. In dry locations,the covers and conduit bodies of different manufacturers are generally interchangeable. For wet locations, however, a conduit body and a cover specified by the listed manufacturer is required to be used.
