Reinforced thermosetting resin conduit is Listed in trade sizes ½ to 6 (metric designators 16 to 155) inclusive, in IPS, ID, RTRC 40, and RTRC 80 dimensions, and in trade sizes¾ to 6 (metric designators 21 to 155) inclusive, in XW dimensions, as marked on the product. Listing includes straight conduit, elbows, bends, and other fittings, unless otherwise noted.
XW-type reinforced thermosetting resin conduit is Listed for aboveground use and is suitable for use wherever IPS, ID, RTRC 40, and RTRC 80 conduit may be used. The marking “AG, XW, RTRC” identifies conduit suitable for use where exposed to physical damage in accordance with the NEC.
Reinforced thermosetting resin conduit has been investigated for use with conductors rated 90°C or less.
Reinforced thermosetting resin conduit is designed for connection to couplings, fittings, and boxes by use of a suitable epoxy-type cement or drive-on bell and spigot. Instructions supplied by the epoxy-type cement manufacturer describe the method of assembly and precautions to be followed.
Conduit marked “Below Ground” (or “BG”) has been investigated for underground use only—for direct burial, with or without being encased in concrete.
Conduit marked “Above Ground” (or “AG”) has been investigated for use above-ground, underground, and for direct burial with or without encasement in concrete. This conduit has been investigated for concealed or exposed work where not subject to physical damage.
Reinforced thermosetting resin conduit, elbows, bends, and other fittings investigated for direct exposure to reagents are identified by the designation “Reagent Resistant” and are marked to indicate the specific reagents.
356.2. Definition. This is a flexible nonmetallic raceway of circular cross section, available in three forms. Type LFNC-A has a smooth, seamless inner core and cover bonded together, with reinforcement between the core and cover layers. Type LFNC-B has a smooth inner surface together with reinforcement within the conduit wall. This is its most usual form. Type LFNC-C has a corrugated inner and outer surface, with no reinforcement in the wall. It is generally limited to 6 ft (1.8 m) lengths unless a longer length is required for the amount of flexibility called for at the point of use. However, the “B” style does not carry this length limitation.
356.10. Uses Permitted. Liquidtight flexible nonmetallic conduit may be used exposed or concealed and also may be used for direct burial in earth if “listed and marked for the purpose.” This extends similar permission to liquidtight flexible nonmetallic conduit that was given for liquidtight flexible metallic conduit in the 1987 NEC. And 356.10 recognizes this nonmetallic flex for “concealed” as well as exposed locations. This product [356.10(6)] is also authorized to be produced as a listed prewired assembly in the metric designator 16 through 27 (trade size ½ through 1) sizes.
356.20. Size. Although metric designator 16 (trade size ½) trade size is the smallest recognized size of liquidtight flexible nonmetallic conduit for general use, Part (A)(1) notes that metric designator 12 (trade size 
) liquidtight flexible metal conduit may be used for motor leads. This was added to coordinate with 430.245(B) for motors with detached junction boxes. The other part allows 1.8 m (6 ft) lengths for utilization equipment connections and where “part of a listed assembly for tap connections to luminaires as required in 410.117(C).” The upper size limit is metric designator 103 (trade size 4).
358.10. Uses Permitted. As is the case with most other raceways, the NEC only recognizes the use of listed EMT. EMT is a general-purpose raceway of the same nature as rigid metal conduit and IMC. Although rigid metal conduit and IMC afford maximum protection for conductors under all installation conditions, in many instances it is permissible, feasible, and more economical to use EMT to enclose circuit wiring rated 600 V or less. Because EMT is lighter than conduit, however, and is less rugged in construction and connection details, the NE Code restricts its use (Art. 358) to locations (either exposed or concealed) where it will not be subjected to severe physical damage or (unless suitably protected) to corrosive agents.
EMT distribution systems are constructed by combining wide assortments of related fittings and boxes. Connection is simplified by employing threadless components that include compression, indentation, and set-screw types.
Some questions have been raised about the acceptability of EMT directly buried in soil. The last sentence of 358.10(B) gives EMT exactly the same recognition for direct burial that 344.10(A) gives to rigid steel conduit. The wording of both sentences is identical, and therefore the relative acceptability of the different steel raceways will come down to the limitations placed on these listed products by the testing laboratories and local experience with soil conditions. In the UL listing on “Electrical Metallic Tubing,” a note says that “galvanized steel electrical metallic tubing in a concrete slab below grade level may require supplementary corrosion protection.” (That word may leaves the decision up to the designer and/or installer, subject to final review by the inspector.)
The next note says, “In general, galvanized steel electrical metallic tubing in contact with soil requires supplementary corrosion protection.” That sentence virtually requires that direct burial use include supplementary corrosion protection. Now compare the equivalent sentence from the guide card information on rigid ferrous metal conduit, “Galvanized rigid ferrous metal conduit installed in contact with soil does not generally require supplementary corrosion protection.” The identical wording goes with IMC, and both sentences create a presumption of acceptability for the heavy wall products that is not there for EMT. Further, UL does not evaluate supplementary corrosion protection on EMT for this use. Although EMT may be available with supplementary protection, the guide card information on this topic reads: “Galvanized electrical metallic tubing that is provided with a metallic or nonmetallic coating, or a combination of both, has been investigated for resistance to atmospheric corrosion” (emphasis supplied). This means that the inspector is entirely on his own if he or she recognizes any form of supplementary protection with respect to the corrosive influences of soil. And supplementary corrosion protection must always be applied unless there is a solid local record of positive experience, which is very unusual. On balance, EMT should not be used for direct burial.
Permission is given for use of aluminum fittings and enclosures with steel electrical metallic tubing.
358.20. Size. The whole concern and discussion regarding the differences of actual cross-sectional area between the various raceways has been rendered moot. That is, in recognition of the differences between actual csa from one conduit or tubing, the table in Chap. 9 covering csa—Table 4—and the tables covering maximum number of conductors of all the same size and insulation within a given size of raceway—now Tables C1 and C1A—have been completely rearranged and revised. The procedure remains the same, but the permitted fill is raceway-specific. And conductor dimensions have been corrected.
358.22. Number of Conductors. Conductor fill for EMT is the same as described under 344.22 for rigid metal conduit.
358.28. Reaming and Threading. Here, the rules clarify Code intent. Threading of electrical metallic tubing is prohibited, but integral couplings used on EMT shall be permitted to be factory threaded. Such equipment has been used successfully in the past and has been found satisfactory. The revised Code rule recognizes such use. But it should be noted that this applies to EMT using integral threaded fittings, that is, fittings which are part of the EMT itself as part of the manufacturing process. A listing still exists on this product, but apparently it has not been in actual production for many years.
358.42. Couplings and Connectors. Couplings of the raintight type are required wherever electrical metallic tubing is used on the exteriors of buildings. (See 225.22 and 230.53.) Note that the ability of conventional compression connectors and couplings to be actually raintight has been questioned and UL has revised the product standard, greatly toughening the rain tests on fittings that claim wet location suitability. The result was that every standard compression connector failed. Do not assume that because you are holding a compression connector, you have a fitting that meets the current test requirements for wet locations; look for specific labeling on the product carton. Wet-location suitable fittings now have special designs, usually including internal nonmetallic glands, to pass the new tests. One manufacturer developed a connector with a ferrule similar to what plumbers use to attach brass supply tubes to angle stops under sinks and toilets. This ferrule must engage steel EMT instead of soft brass, however, and the torque specification that comes with the fitting is impressive. It requires a crow-foot open-end wrench extended from a torque wrench, with the wrench reading adjusted for the extra reach of the crow foot, in order to properly seat this fitting.
314.17 requires that conductors entering a box, cabinet, or fitting be protected from abrasion. The end of an EMT connector projecting inside a box, cabinet, or fitting must have smooth, well-rounded edges so that the covering of the wire will not be abraded while the wire is being pulled in. Where ungrounded conductors of size 4 AWG or larger enter a raceway in a cabinet, the EMT connector must have an insulated throat (insulation set around the edge of the connector opening) to protect the conductors. See 300.4(G). For conductors smaller than 4 AWG, an EMT connector does not have to be the insulated-throat type. Using THW conductors, a circuit of 4 AWG conductors (a 2- or 3-wire circuit) requires a metric designator 27 (trade size 1) EMT (Table C.1, Annex C, NEC). Therefore, for THW or TW wire, there is no requirement for insulated-throat EMT connectors in the metric designator 16 and 21 (trade size ½ and¾) sizes. A circuit, say, of three 1 AWG THW wires would call for metric designator 35 (trade size 1¼) EMT, which would require use of insulated-throat connectors—or noninsulated-throat connector with a nonmetallic bushing on the connector end. In the larger sizes, the economics on the makeups can be significantly different. A metric designator 103 (trade size 4) insulated-throat EMT connector might cost $18, whereas a noninsulated-throat connector in that size might cost $10 and $2 for a plastic bushing (Fig. 358-1).
Fig. 358-1. Different-cost makeups for 4-in. (102-mm) EMT satisfy Code rules on EMT termination. (Sec. 358.42.)
When an EMT connector is used—either with or without an insulated throat to satisfy 300.4(G)—there is no requirement in Art. 358 that a bushing be used on the connector end. Note, however, that a bushing is required for rigid metal conduit and for IMC as covered in 344.46 and 342.46.
358.24. Bends—How Made. Refer to 344.24.
358.26. Bends—Number in One Run. Figure 358-2 shows EMT run from a panel-board to a junction box (JB) along the wall—with a total of exactly 360° of bend (from the panel: 45°, 45°, 90°, 90°, 45°, 45°). Note that assigning two opposing 45° angles to ordinary box kicks is unduly restrictive. Opposing 10° angles, so two such kicks equal about a 45° bend, is more realistic and reasonable.
358.30. Securing and Supporting. Figure 358-3 shows this rule applied to an EMT layout. As stated in the basic rule of this section, EMT must be supported every 10 ft (3.0 m) and within 3 ft (900 mm) of each “outlet box, junction box, device box, cabinet, conduit body, or other tubing terminations.” Prior to the 1993 NEC, this section referred to “each outlet box, junction box, cabinet, and fitting.” If the word fitting is taken to include couplings, then a strap must be used within 3 ft (900 mm) of each coupling. The definition of fitting, given in Art. 100, includes locknuts and bushings. That wording was changed to provide a “laundry list” of enclosures that are covered by this rule. The intent was to clarify that supports are not required within 3 ft (900 mm) of EMT couplings.
Fig. 358-2. EMT, like other conduit runs, is limited to not over 360° of bends between raceway ends. (Sec. 358.26.)
Fig. 358-3. EMT must be clamped within 3 ft (900 mm) of every enclosure or “fitting.” (Sec. 358.30.)
As permitted by Exception No. 1, clamps on unbroken lengths of EMT may be placed up to 5 ft (1.5 m) from each termination at an outlet box or fitting where structural support members do not readily permit support within 3 ft (900 mm). This exception allows the first clamp to be up to 5 ft (1.5 m) from a termination of EMT at an outlet box. This is like the comparable permission for heavy-wall steel conduits but with an important difference. The EMT between the support 5 ft back and the termination must be unbroken, without coupling. Exception No. 2 allows EMT to be fished; although this may seem odd, it has been done successfully where there is room to stage the unbroken length required.
Part (B), following the two exceptions, makes clear that no additional means of support or securing are needed where framing provides support for horizontal runs at least every 10 ft (3.0 m); however, in addition, the EMT must be secured within 3 ft (900 mm) of every termination.
Part (C) is new in 2008; refer to the extensive commentary at the same point in Art. 342 for essential information on the problems with this provision.
360.2. Definition. This section defines this NE Code raceway. The rule indicates that flexible metallic tubing is a raceway. Use of that term makes clear that all rules applying to “raceways” within the Code apply to Type FMT as well. The rule further indicates that flexible metallic tubing is intended for use where “not subject to physical damage” and gives use above suspended ceilings as an example. Although this wording does not limit its use to air-handling ceilings, it does raise some questions for electrical inspectors with respect to accepting flexible metallic tubing as a general-purpose raceway.
360.6. Listing Requirements. This rule makes it a violation of the Code to use any flexible metallic tubing that is not specifically listed for use with electrical conductors. Ensure that any flexible metallic tubing is listed and marked as Type FMT, which indicates its suitability for use with electrical conductors.
360.10. Uses Permitted. This section limits the use of flexible metallic tubing to branch circuits. In addition, branch-circuit conductors can only be installed in “dry locations”—either concealed or accessible—with systems rated no more than 1000 V. This product has particular utility for making connections in an air-handling ceiling because it completely excludes the transmission of air and it has no nonmetallic elements that could be a source of smoke or fumes. For these reasons it is permitted not just in other spaces for environmental air [300.22(B)], but in actual ductwork for connections, as covered in 300.22(B).
360.12. Uses Not Permitted. Here the Code states those applications for which Type FMT is not permitted. When the proposal was made to add flexible metallic tubing to the Code as a suitable raceway, it was indicated that it had been designed for certain specific applications and not for general use. It was specifically intended for use as the fixture whip on recessed fixtures where high-temperature wire is run from the branch-circuit junction box to the hot wiring compartment in lighting fixtures, an application long filled by flexible metallic conduit (Fig. 360-1). Today the cold-lead applications have largely disappeared with integrally wired thermally-protected luminaires, but this wiring method works equally well for conventional connections in hung ceilings. It should be noted that the limitation given in part (6) of this section limits the use of this raceway to lengths not exceeding 6 ft (1.8 m), which has the effect of effectively limiting this use of this product to the application for which it was originally intended—fixture whips.
Fig. 360-1. Flexible metallic tubing has limited application. (Sec. 360.12.)
360.20. Size. FMT is only available for general purposes in the metric designator 16 and 21 (trade size ½ and¾) sizes, with the metric designator 12 (trade size ) available for ductwork connections [300.22(B)], connections in other spaces for environmental air [300.22(C)], for luminaire connections, especially in accessible ceilings, and as part of listed assemblies.
360.24. Bends. The allowable bend radius for this product varies significantly based on whether it will be flexed, which should only be infrequently, after the initial installation. There are two tables that provide the required data.
362.2. Definition. One type of plastic raceway defined in the Code (Fig. 362-1) is ENT (electrical nonmetallic tubing), which is “a pliable corrugated raceway of circular cross section with integral or associated couplings, connectors, and fittings listed for the installation of electrical conductors. It is composed of a material that is resistant to moisture [and] chemical atmospheres and is flame retardant.” ENT can be bent by hand, when being installed, to establish direction and lengths of runs.
Fig. 362-1. ENT is a pliable, bendable plastic raceway for general-purpose use for feeders and branch circuits.
362.10. Uses Permitted. Electrical nonmetallic tubing is permitted to be used as a general-purpose, flexible-type conduit in any type of occupancy (Fig. 362-2). ENT is not limited to use in buildings up to three stories high. But, where the building does not exceed three stories above grade, ENT may be used in “exposed” locations. Where concealed throughout (and not just above the first three stories), ENT may be used in a building of any height—subject to conditions given in 362.10 and 362.12. ENT may be used:
1. Concealed in walls, floors, and ceilings that provide a thermal barrier with at least a 15-min fire rating from listings of fire-rated assemblies. In the case of walls, this is fairly easy to arrange, since most ½-in. drywall used in commercial construction carries this rating. The same holds true above a drywall ceiling. However, if there is a suspended ceiling (common in commercial occupancies), check with the building inspector. The support grid and the ceiling panels need to be identified as a combination for this duty. For example, having 15-min panels would do no good if the T-bars dumped those panels onto the floor after 11 min of fire exposure. Note that although this is an “exposed” use not normally permitted in high-rise construction, there is specific permission to use this procedure in 362.10(5).
Fig. 362-2. ENT may be used in residential and nonresidential buildings. (Sec. 362.10.)
As previously indicated, ENT may be used exposed without these limitations in a building that is not over three floors above grade. The first floor is defined as the one with at least half its exterior wall area at or above grade level; one additional floor level at the base is allowed for vehicle parking or storage, provided it is not designed for human habitation. This is limited permission that recognizes ENT for exposed use under the same limitations that were placed on use of Romex in the past, provided the building finish has a 15-min fire rating. In addition, where sprinklers are provided on all floors so as to provide complete occupancy protection, not just the areas in which the ENT is proposed, ENT may be used even if exposed.
2. In severe corrosive locations where suited to resist the particular atmosphere (but not “exposed”).
3. In concealed, dry, and damp locations not prohibited by 362.12.
4. Above suspended ceilings with at least a 15-min fire rating (see commentary above).
5. Embedded in poured concrete with fittings that are listed or otherwise identified for that use.
6. Metric designator 16 through 27 (trade sizes ½ through 1) sizes are authorized to be prewired as a listed manufactured assembly. Even if installed prewired, it is still a raceway and not a cable and must observe the four quarter bend rule, etc.
362.12. Uses Not Permitted. ENT may not be used in exposed locations, except above suspended ceilings of 15-min fire-rated material in buildings of any height above grade. This section excludes ENT from hazardous locations—except for intrinsically safe circuits per Art. 504—from supporting fixtures or equipment, from use where the ambient temperature exceeds that for which the ENT is rated, from direct burial, and from exposed use, with exceptions as noted. High-temperature wiring must not be used unless it is certain to be operating, due to limited loading, at temperatures below the rating of the ENT.
362.20. Size. ENT is Code-recognized in metric designator 16 to 53 (trade size ½ to 2) sizes. A full line of plastic couplings, box connectors, and fittings is available, which are attached to the ENT by mechanical method or cement adhesive (Fig. 362-3).
362.26. Bends—Number in One Run. ENT runs between “pull points”—boxes, enclosures, and conduit bodies—must not contain more than the equivalent of four quarter-bends (360°).
Fig. 362-3. Available in½-,¾-, and 1-in. sizes, ENT has a full line of couplings and box connectors. (Sec. 362.2.)
366.1. Scope. Auxiliary gutters differ from wireways only by the way they are applied in the field; they are usually listed for both purposes as they leave a manufacturer. They are available in both sheet metal and nonmetallic forms. They have hinged or removable covers that allow for conductors to be laid in place after the system is complete. Their function is to supplement wiring spaces at meter centers, distribution centers, switchboards, and similar locations in a wiring system (Fig. 366-1). Note that auxiliary gutters are not classified as raceways for the reasons discussed in the discussion in Art. 100 under the raceway definition, and as covered in this commentary.
Fig. 366-1. Typical applications of auxiliary gutters provide the necessary space to make taps, splices, and other conductor connections involved where a number of switches or CBs are fed by a feeder (top) or for multiple-circuit routing, as at top of a motor control center (right) shown with a ground bus in gutter (arrow). (Sec. 366.1.)
They are not wireways, which are unlimited in length and intended as a circuit wiring method that connects a line and a load. An auxiliary gutter with a rectangular opening cut to match a similar opening cut in a panelboard, and used to contribute to the wire bending space in the panelboard would be an excellent example. This concept is why the auxiliary gutter article has current-carrying limitations for busbars placed in the enclosure, but the wireway article does not address the topic. If a conductor needs to be pulled from the gutter through a nipple to a panel or switchboard, the use may be crossing over into the wireway article. That said, the basic field installation rules for conductor fill, derating thresholds, use as pull boxes, and distinctions between metallic and nonmetallic versions are similar. Refer to wireway topics in Arts. 376 and 378 for more information.
Auxiliary gutters are available in both metal and nonmetallic forms, but only the nonmetallic form requires listing, with specific listing requirements given that differ based on whether the use will be outdoors or not.
366.12. Uses Not Permitted. Auxiliary gutters are not intended to be a type of general raceway and are not permitted to extend more than 30 ft (9.0 m) beyond the equipment which they supplement, except in elevator work. Where an extension beyond 30 ft (9.0 m) is necessary, Arts. 376 or 378 for wireways must be complied with. The label of Underwriters Laboratories Inc. on each length of trough bears the legend “Wireways or Auxiliary Gutters,” which indicates that they may be identical troughs but are distinguished one from the other by their use. See comments following 376.2 in this handbook.
366.22. Number of Conductors. The rules on permitted conductor fill for auxiliary gutters are basically the same as those for wireways. Refer to 376.22. Note that part rule permits more than 30 current-carrying conductors—including neutrals in some cases, as described under 310.15(B)(4); but where over 30 such wires are installed, the correction factors specified in 310.15(B)(2)(a) must be applied to all the wires. One of the key differences between metallic and non-metallic gutters is the fact that the 30-conductor allowance before derating is imposed does not apply to nonmetallic gutters. For nonmetallic auxiliary gutters the derating factors apply after the first three current-carrying conductors, just like any tubular raceway. Metal gutters are much better at providing a heat sink and a surface that easily radiates heat away from itself.
No limit is placed on the size of conductors that may be installed in an auxiliary gutter.
Figure 366-2 shows a typical gutter application where the conductor sizes and fill must be calculated to determine the acceptable csa of the gutter. There are several factors involved in sizing auxiliary gutters that often lead to selecting the wrong size. The two main factors are how conductors enter the gutter and the contained conductors at any cross section. The minimum required width of a gutter is determined by the csa occupied by the conductors and splices and the space necessary for bending conductors entering or leaving the gutter. The total csa occupied by the conductors at any cross section of the gutter must not be greater than 20 percent of the gutter interior csa at that point (366.22). The total csa occupied by the mass of conductors and splices at any cross section of the gutter must not be greater than 75 percent of the gutter interior csa at that point [366.56(A)].
Fig. 366-2. Minimum acceptable gutter cross section and depth must be calculated. (Sec. 366.22.)
In the gutter installation shown in Fig. 366-2, assume that staggering of the splices has been done to minimize the area taken up at any cross section—to keep the mass of splices from all adding up at the same cross section. The greatest conductor concentration is therefore either at section x, where there are three 300-kcmil and one 4/0 THW conductors, or at section y, where there are eight 3/0 THW conductors. To determine at which of these two cross sections the fill is greater, apply the appropriate csas of THW conductors as given in Table 5, Chap. 9:
1. The total conductor csa at section x is 3 × 0.5281 sq in. plus 1 × 0.3718, or 1.9561 sq in.
2. The total conductor csa at section y is 8 × 0.3117 sq in. or 2.4936 sq in.
Section y is, therefore, the determining consideration. Because that fill of 2.4936 sq in. can at most be 20 percent of the gutter csa, the total gutter area must be at least 5 times this conductor fill area, or 12.468 sq in.
Assuming the gutter has a square cross section (all sides of equal width) and the sides have an integral number of inches, the nearest square value would be 16 sq in., indicating a 4- by 4-in. gutter, and that would be suitable if the 300-kcmil conductors entered the end of the gutter instead of the top. But because those conductors are deflected entering and leaving the gutter, the first two columns of Table 312.6(A) must also be applied to determine whether the width of 4 in. affords sufficient space for bending the conductors. That consideration is required by 366.58. The worst condition (largest conductors) is where the supply conductors enter; therefore the 300-kcmil cable will determine the required space.
Table 312.6(A) shows that a circuit of one 300 kcmil per phase leg (or wire per terminal) requires a bending space at least 5 in. deep (in the direction of the entry of the 300-kcmil conductors), calling for a standard 6- by 6-in. gutter for this application.
In Fig. 366-2, if the 300-kcmil conductors entered at the left-hand end of the gutter instead of at the top, 366.58 would require Table 312.6(A) to be applied only to the deflection of the No. 3/0 conductors. The table shows, under one wire per terminal, a minimum depth of 4 in. is required. In that case, a 4- by 4-in. (102- by 102-mm) gutter would satisfy.
366.56. Splices and Taps. Part (A) is discussed under 366.22.
Part (B) covers cases where bare busbar conductors are used in gutters. The insulation might be cut by resting on the sharp edge of the bar or the bar might become hot enough to damage the insulation. When taps are made to bare conductors in a gutter, care should be taken so as to place and form the wires in such a manner that they will remain permanently separated from the bare bars.
Part (C) requires that identification be provided wherever it is not clearly evident what apparatus is supplied by the tap. Thus if a single set of tap conductors are carried through a short length of conduit from a gutter to a switch and the conduit is in plain view, the tap is fully identified and needs no special marking; but if two or more sets of taps are carried in a single conduit to two or more different pieces of apparatus, each tap should be identified by some marking such as a small tag secured to each wire.
368.2. Definition. Busways consist of metal enclosures containing insulator-supported busbars. Varieties are so extensive that possibilities for 600-V distribution purposes are practically unlimited. Busways are available for either indoor or outdoor use as point-to-point feeders or as plug-in takeoff routes for power. Progressive improvements in busway designs have enhanced their electrical and mechanical characteristics, reduced their physical size, and simplified the methods used to connect and support them. These developments have in turn reduced installation labor to the extent that busways are most favorably considered when it is required to move large blocks of power to loadcenters (via low-impedance feeder busway), to distribute current to closely spaced power utilization points (via plug-in busways), or to energize rows of lighting fixtures or power tools (via trolley busways).
Busways classed as indoor low-reactance assemblies can be obtained in small incremental steps up to 6000 A for copper busbars and 5000 A for aluminum. Enclosed outdoor busways are similarly rated. In the plug-in category, special assemblies are available up to 5000 A, although normal 600-V AC requirements generally are satisfied by standard busways in the 225-to-1000-A range. Where power requirements are limited, small compact busways are available with ratings from 250 down to 20 A.
Plug-in and clamp-on devices include fused and nonfusible switches and plug-in circuit breakers (CBs) rated up to about 800 A. Other plug-in devices include ground detectors, temperature indicators, capacitors, and transformers designed to mount directly on the busway.
Busways that are listed by UL with the following general information:
This category covers busways and associated fittings, rated 600 V or less, 6000 A or less. Busways are grounded metal enclosures containing factory-mounted bare or insulated conductors, which are usually copper or aluminum bars, rods, or tubes. These enclosures and, in some cases an additional ground bus, are intended for use as equipment grounding conductors.
Some busways are not intended for use ahead of service equipment and are marked with the maximum rating of overcurrent protection to be used on the supply side of the busway.
Busways may be of one of the following designs:
Lighting Busway—Busway intended to supply and support industrial and commercial luminaires. Lighting busway is limited to a maximum current rating of 50 A.
Trolley Busway—Busway having provision for continuous contact with a trolley by means of a slot in the enclosure. Trolley busway may be additionally marked “Lighting Busway” if intended to supply and support industrial and commercial luminaires.
Continuous Plug-In Busway—Busway provided with provision for the insertion of plug-in devices at any point along the length of the busway. Continuous plug-in busway is intended for general use and may be installed within reach of persons. Busways of this design are limited to a maximum current rating of 225 A.
Short-Run Busway—Unventilated busway intended for a maximum run of 30 ft horizontally, 10 ft vertically and are primarily used to supply switchboards. Except for transformer stubs, short-run busway is not intended to have intermediate taps.
368.10. Uses Permitted. Figure 368-1 shows the most common way in which busways are installed—in the open. Note that the use of the term “concealed” in (B), as can be seen from context, squarely violates the definition of this term in Art. 100. For now, believe the text in the sentences that follow this erroneous use of terminology.
Wiring methods above lift-out ceiling panels are considered to be “exposed”—because the definition of that word includes reference to “behind panels designed to allow access.” This section calls for busways to be “located in the open and visible” and therefore does not allow them above suspended ceilings, except with the limitations given in (B). In such locations, the busway is permitted provided means of access are provided, the joints and fittings can be reached for maintenance, and the ceiling is either not air-handling, or if air-handling, the busway conductors are insulated and there are no provisions for plug-in connections. Figure 368-2 shows how other Code rules tie into this section. Note that the wording of 368.10(B)(2) directly correlates to the permission for such busway use in spaces used for environmental air—as stated in 300.22(C).
Special rules govern the routing of busways through floor slabs. Figure 368-3 illustrates the prohibition against the use of ventilated busways on the pass-through and up at least 1.8 m (6 ft) above the slab, as covered in 368.10(C)(2)(a). Figure 368-4 shows a close-up of a floor slab penetration. Depending on the occupancy, curbing is frequently required by 368.10(C)(2)(b).
Fig. 368-1. Ventilated-type (with open grills for ventilation) busways may be used only “in the open” and must be “visible.” Only the totally enclosed, nonventilating type may be used above a suspended ceiling. (Sec. 368.10.)
Other data limiting applications of busways are contained in the UL regulations on listed busways—all of which information becomes mandatory Code rules because of NEC 110.3(B). Such UL data are as follows:
Busways are intended for installation in accordance with Article 368 of ANSI/NFPA 70, “National Electrical Code” (NEC), and the manufacturer’s installation instructions.
Busways investigated to determine their suitability for
installation in a specified position,
for use in a vertical run, or for support at intervals greater than 5 ft,
for outdoor use
are so marked. This marking is on or contiguous with the nameplate incorporating the manufacturer’s name and electrical rating.
Fig. 368-2. Use of busways involves NEC rules on accessibility of overcurrent devices. (Sec. 368.10.)
A busway or fitting containing a vapor seal is so marked, but unless marked otherwise, the busway or fitting has not been investigated for passage through a fire-rated wall.
Busway marked “Lighting Busway” and protected by overcurrent devices rated in excess of 20 A is intended for use only with luminaires employing heavy-duty lampholders unless additional overcurrent protection is provided for the luminaire in accordance with the NEC. [See Fig. 368-5 for examples; refer in this handbook to the discussion at 210.21(A) and 210.23 with which this provision correlates for more information.]
Trolley busway should be installed out of the reach of persons or be otherwise installed to prevent accidental contact with exposed conductors.
Fig. 368-3. Ventilated busways may not be used through a floor slab and for 6 ft (1.8 m) above the floor. [Sec. 368.10(C)(2)(a).]
Fig. 368-4. Opening for a busway riser through a slab must be curbed in most cases, and fire-stopped if the floor is part of a required fire separation, as required by 300.21. [Sec. 368.10(C).]
Some busways have a number of short stubs and are marked for use with certain compatible equipment.
Busways and fittings covered under this category are intended for use with copper conductors unless marked to indicate which terminals are suitable for use with aluminum conductors. Such marking is independent of any marking on the terminal connectors and is on a wiring diagram or other readily visible location.
Unless the equipment is marked to indicate otherwise, the termination provisions are based on the use of 60°C ampacities for wire sizes 14 to 1 AWG, and 75°C ampaci-ties for wire sizes 1/0 AWG and larger as specified in Table 310.16 of the NEC. Termination provisions are determined based on values provided in Table 310.16 or Section 310.15(B)(6), with no adjustment made for correction factors.
Some fittings are suitable for use as service equipment and are so marked.
Note that fluorescent lighting fixtures can be fed by a 50-A lighting busway with each fixture individually fused at a few amps to protect its nonheavy-duty lampholders using the fuse in each fixture or in its attachment plug, as is permitted in the UL application information as well as by NE Code 368.17(C), Exception Nos. 2 or 3. In that case, the lighting busway for code purposes is halfway between a feeder and a branch circuit. Each fixture tap might be understood to be a branch circuit, except that the overcurrent protection would not qualify as a “branch circuit overcurrent device” as defined in Art. 100. On the other hand, if the busway is defined as the branch circuit, then there is at least a technical violation of 210.23(C). Since the permission is squarely granted in 368.17, it is surely safe to use it. Such is the subject of NEC proposals.
Fig. 368-5. These applications involve UL data and several Code sections. [Sec. 368.17(A).]
368.17. Overcurrent Protection. As given in part (A), Rating of Overcurrent Protection—Feeders, the rated ampacity of a busway is fixed by the allowable temperature rise of the conductors. The ampacity can be determined in the field only by reference to the nameplate.
The rule of part (B) covers Reduction in Ampacity Size of Busway. Overcurrent protection—either a fused-switch or CB—is usually required in each busway subfeeder tapping power from a busway feeder of higher ampacity, protected at the higher ampacity. This is necessary to protect the lower current-carrying capacity of the subfeeder and should be placed at the point at which the subfeeder connects into the feeder. However, the Exception to this section provides that overcurrent protection may be omitted where busways are reduced in size, if the smaller busway does not extend more than 50 ft and has a current rating at least equal to one-third the rating or setting of the overcurrent device protecting the main busway feeder (Figs. 368-6 and 368-7), but only at an “industrial establishment.” For all other installations, the basic rule for overcurrent protection at the point where the busway is reduced in size must be satisfied.
Where the smaller busway is kept within the limits specified, the hazards involved at industrial installations are very slight and the additional cost of providing overcurrent protection at the point where the size is changed is not considered as being warranted.
Fig. 368-6. A busway subfeeder may sometimes be used without protection. [Sec. 368.17(B).]
Fig. 368-7. Total length of a reduced busway is not over 50 ft (15.24 m). [Sec. 368.17(B).]
The rules of part (C) are interrelated with those of 240.24 and 404.8. The basic rule of this section makes it clear that branch circuits or subfeeders tapped from a busway must have overcurrent protection on the busway at the point of tap. And if they are out of reach from the floor, all fused switches and CBs must be provided with some means for a person to operate the handle of the device from the floor (hookstick, chain operator, rope-pull operator, etc.).
Although no definition is given for “out of reach” from the floor, the wording of 404.8(A) can logically be taken to indicate that a switch or CB is “out of reach” if the center of its operating handle, when in its highest position, is more than 2.0 m (6 ft 7 in.) above the floor or platform on which the operator would be standing. Thus, a busway over 2.0 m (6 ft 7 in.) above the floor would require some means (hookstick, etc.) to operate the handles of any switches or CBs on the busway.
Figure 368-8 relates the rules of 240.24 and Exception No. 1 to 368.17(C)—with the rule of 240.24 permitting overcurrent devices to be “not readily accessible” when used up on a high-mounted busway and 368.17 requiring such protection to be mounted on the busway. To get at overcurrent protection in either case, personnel might have to use a portable ladder or chair or some other climbing technique. Again, 6 ft 7 in. (2.0 m) could be taken as the height above which the overcurrent protection is not “readily accessible”—or the height above which the Code considers that some type of climbing technique (ladder, chair, etc.) may be needed by some persons to reach the protective device.
Then, where the plug-in switch or CB on the busway is “out of reach” from the floor (i.e., over 6 ft 7 in. [2.0 m] above the floor), provision must be made for operating such switches or CBs from the floor, as shown in Figs. 368-9 and 368-10. The plug-in switch or CB unit must be able to be operated by a hook-stick or chain or rope operator if the unit is mounted out of reach up on a busway. Section 404.8 says all busway switches and CBs must be operable from the floor. Refer to 404.8(A) Exception No. 1. Figure 368-10 shows a typical application of hookstick-operated disconnects.
Figure 368-11 shows an application that has caused controversy because 368.17 says that any busway used as a feeder must have overcurrent protection on the busway for any subfeeder or branch circuit tapped from the busway. Therefore, use of a cable-tap box on busway without overcurrent protection could be ruled a Code violation. It can be argued that the installation shown—a 10- or 25-ft (3.0- or 7.5-m) tap without overcurrent protection on the busway—is covered by Exception No. 1 of that section, which recognizes taps as permitted in 240.21—including 10- and 25-ft (3.0- or 7.5-m) taps. But, as is now clearly spelled out in Exception No. 1 to 368.17(C), busways may be tapped as would any feeder. It is to be viewed simply as a conductor. Then, the rules given in the referenced parts of 240.21 must be satisfied. Therefore, the application shown in Fig. 368-11 does satisfy the code.
Fig. 368-8. Protection must always be used on busway for these taps—regardless of busway mounting height. (Sec. 368.17.)
Fig. 368-9. Busway plug-in devices must be operable from the floor or platform where operator stands. (Sec. 368.17.)
Fig. 368-10. Disconnects mounted up on the busway (top arrow) are out of reach from the floor but do have hook-eye lever operators to provide operation by person standing in front of machines. Although the NEC does not literally require ready availability of a hookstick, it is certainly the intent of the Code that one be handy (lower arrow). (Sec. 368.17.)
Fig. 368-11. This use of unprotected tap from busway does not conflict with 368.17 as long as the tap meets the applicable requirements in 240.21(B) and thereby satisfies 368.17(C) Exception No. 1. (Sec. 368.17.)
368.17(D). Rating of Overcurrent Protection—Branch Circuits. Refer to data on busways on lighting branch circuits in 368.10 and Fig. 368-3.
368.30. Support. As shown in Fig. 368-12, busway risers may be supported by a variety of spring-loaded hangers, wall brackets, or channel arrangements where busways pierce floor slabs or are supported on masonry walls or columns. Fig. 368-4 shows an example of spring mounts for vertical busways which may be located at successive floor-slab levels or, as indicated in Fig. 368-12, supported by wall brackets located at intermediate elevations. Springs provide floating cradles for absorbing transient vibrations or physical shocks. Fire-resistant material is packed into space between the busway and the edges of slab-piercing throat.
Fig. 368-12. Vertical busway runs should be supported at least every 5 ft, unless designed and marked for another support interval. (Sec. 368.30.)
368.56. Branches from Busways. Busway branches can be made into a wide variety of Chap. 3 wiring methods. For cord connections, the rule here requires that a cord connecting to a plug-in switch or CB on a busway must be supported by a “tension take-up support device” with the swag not longer than 1.8 m (6 ft). Industrial occupancies can extend that limit indefinitely, provided the cord is supported at 2.5 m (8 ft) or shorter intervals. “Bus Drop Cable” has its own coverage in UL apart from most flexible cords. It is one of many in the category “Wire, Special Purpose (ZMHX)” and it is not covered under “Flexible Cord (ZJCZ)” where SO and W and other cords are covered. Fig. 368-13 shows a number of details regarding branches from buways.
368.320. Marking. Busway is also available for medium-voltage distribution, as covered in Part IV of Art. 368. The UL data that applies to this category follows:
This category covers metal-enclosed busways of the nonsegregated phase type, for use in accordance with Article 368 of ANSI/NFPA 70, “National Electrical Code.” Non-segregated phase busway is one in which all phase conductors are in a common metal enclosure without barriers between the phases.
These are assemblies of metal-enclosed conductors, together with associated interconnections, enclosures, and supporting structures.
These assemblies are intended for use on systems with nominal rated voltages from 601 V to 38 kV ac. Current ratings are from 600 to 10,000 A.
These assemblies may be intended for either indoor or outdoor applications. An assembly that has been investigated to determine that it is rainproof is marked “Rain-proof,” “Outdoor,” or “3R.”
Enclosures are of the ventilated or nonventilated type. A ventilated enclosure is provided with means to permit circulation of sufficient air to remove excess heat.
A nonventilated enclosure is constructed to provide no intentional circulation of external air through the enclosure.
These products are marked with the following electrical ratings: rated voltage, rated continuous current, insulation (BIL) level, frequency, rated frequency withstand voltage (dry), and rated short-circuit withstand current (momentary current). When shipped in sections, each section is marked.
Cablebus is an approved assembly of insulated conductors mounted in “spaced” relationship in a ventilated metal-protective supporting structure, including fittings and conductor terminations. In general, cablebus is assembled at the point of installation from components furnished by the manufacturer.
Field-assembly details are shown in Fig. 370-1. First, the cablebus framework is installed in a manner similar to continuous rigid cable support systems. Next, insulated conductors are pulled into the cablebus framework. Then the conductors are supported on special insulating blocks at specified intervals. And finally, a removable (ventilated) top is attached to the framework.
Fig. 368-13. Wiring details at busway connections must be carefully observed. (Sec. 368.56.) Circuits fed from busways may be run in any conventional wiring method—such as EMT or rigid conduit (left) or as “suitable cord,” such as “bus-drop” cable down to machines. And cable-tap boxes may be used (arrow at right) to connect feeder conductors that supply power to busway. SOME TYPE of tension-relief device must be used on bus-drop cable or other suitable cord where it connects to a plug-in switch or CB on busway, per 368.56(B)(4). Photo shows strain relief connector with mesh grip (arrow) on cord to bus-tap CB, which is equipped with hook-eye lever mechanism to provide operation of the CB by a hookstick from floor level—as required by 404.8(A), Ex. No. 1 and 368.17(C).
Fig. 370-1. Cablebus systems are field assembled from manufactured components. (Sec. 370.2.)
372.1. Scope. The term Precast cellular concrete floor, as mentioned in 372.2, refers to a type of floor construction designed for use in steel frame, concrete frame, and wall-bearing construction, in which the monolithically precast reinforced concrete floor members form the structural floor and are supported by beams or bearing walls. The floor members are precast with hollow voids which form smooth round cells. The cells are of various sizes depending on the size of floor member used.
The cells form raceways which by means of suitable fittings can be adapted for use as underfloor raceways. A precast cellular concrete floor is fire-resistant and requires no additional fireproofing.
372.5. Header. Connections to the cells are made by means of headers secured to the precast concrete floor, extending from cabinets and across the cells. A header connects only those cells that are used as raceways for conductors. Two or three separate headers, connected to different sets of cells, may be used for different systems (e.g., for light and power, signaling, and telephones).
Figure 372-1 shows three headers installed, each header connecting a cabinet with separate groups of cells. Special elbows extend the header to the cabinet.
Fig. 372-1. Headers, flush with finished concrete pour, carry wiring to cells. (Sec. 372.5.)
372.7. Junction Boxes. Figure 372-2 shows how a JB must be arranged where a header connects to a cell.
372.8. Markers. Markers used with this system are special flat-head brass screws which are installed level with the finished floor. One type of marker marks the location of an access point between a header and a spare cell reserved for, but not connected to, the header. A junction box can be installed at the point located by the marker if the spare cell is needed in the future. The screw for this type of marker is installed in the center of a special knockout provided in the top of the header at the access point. The second type of marker is installed over the center of cells at various points on the floor to locate and identify the cells below. Screws with specially designed heads identify the type of service in the cell.
Fig. 372-2. Junction box is used to provide conductor installation from header to cell. (Sec. 372.7.)
372.9. Inserts. A 1
-in. diameter hole is cut through the floor and into the center of a cell with a concrete drill bit. A plug is driven into the hole and a nipple is screwed into the plug. The nipple is designed to receive an outlet with a duplex electrical receptacle or an outlet designed for a telephone or signal system.
372.13. Discontinued Outlets. When an outlet is discontinued, the conductors supplying the outlet shall be removed from the raceway. The general practice is to loop wire intermediate receptacles between the header and the end of the run. This requirement assures that reinsulated conductors will not be resting in the raceway below an abandoned outlet. This in turn prevents a fish wire inserted afterward from a downstream location from getting caught on a reinsulated conductor, with very destructive consequences.
It is often advisable to wire each outlet on its own pair of conductors back to the header or other junction point. Then, when an outlet is abandoned, the associated pair of conductors can be withdrawn without disrupting other outlets on the run. Take care, however, to keep track of the total number of conductors at all portions of the duct, because the ampacity derating factors for mutual conductor heating (see 372.17) will apply. Some jurisdictions provide limited waivers to these derating factors in underfloor applications, however, in order to encourage this practice and thereby discourage the rein-sulation of conductors. When the conductors are withdrawn, leave a pull string in their place so the outlet can be easily reactivated in the future as necessary.
372.17. Ampacity of Conductors. This section makes clear that the rules given in 310.15(B)(2) requiring derating of conductors where there are more than three current-carrying conductors within a raceway, cable, or trench apply to conductors installed within cellar concrete floor raceways. Note the preceding comment.
374.2. Definitions. This is a type of floor construction designed for use in steel frame buildings in which the members supporting the floor between the beams consist of sheet steel rolled into shapes that are so combined as to form cells, or closed passageways, extending across the building. The cells are of various shapes and sizes, depending upon the structural strength required.
The cellular members of this type of floor construction form raceways. A cross-sectional view of one type of cellular metal floor is shown in Fig. 374-1.
Fig. 374-1. Cross section of one type of cellular-method floor construction. (Sec. 374.2.)
374.3. Uses Not Permitted. Connections to the ducts are made by means of headers extending across the cells. A header connects only to those cells that are to be used as raceways for conductors. Two or three separate headers, connecting to different sets of cells, may be used for different systems (e.g., for light and power, signaling systems, and public telephones).
Figure 374-2 shows the cells, or ducts, with header ducts in place. By means of a special elbow fitting the header is extended up to a cabinet or distribution center on a wall or column. A junction box or access fitting is provided at each point where the header crosses a cell to which it connects.
374.6. Splices and Taps. See 372.6.
374.7. Discontinued Outlets. When an outlet is discontinued, the conductors supplying the outlet shall be removed from the raceway. The general practice is to loop wire intermediate receptacles between the header and the end of the run. This requirement assures that reinsulated conductors will not be resting in the raceway below an abandoned outlet. This in turn prevents a fish wire inserted afterward from a downstream location from getting caught on a reinsulated conductor, with very destructive consequences.
It is often advisable to wire each outlet on its own pair of conductors back to the header or other junction point. Then, when an outlet is abandoned, the associated pair of conductors can be withdrawn without disrupting other outlets on the run. Take care, however, to keep track of the total number of conductors at all portions of the duct, because the ampacity derating factors for mutual conductor heating (see 374.17) will apply. Some jurisdictions provide limited waivers to these derating factors in underfloor applications, however, in order to encourage this practice and thereby discourage the reinsulation of conductors. When the conductors are withdrawn, leave a pull string in their place so the outlet can be easily reactivated in the future as necessary.
Fig. 374-2. Components for electrical usage in cellular metal floor must be properly applied. (Sec. 374.3.)
374.8. Markers. The markers used with this system consist of special flat-head brass screws, screwed into the upper side of the cells and with their heads flush with the floor finish.
374.9. Junction Boxes. The fittings with round covers shown in Fig. 374-2 are termed access fittings by the manufacturer but actually serve as junction boxes. Where additional junction boxes are needed, a similar fitting of larger size is provided which may be attached to a cell at any point.
374.10. Inserts. The construction of an insert is shown in Fig. 374-3. A 1
-in.-diameter hole is cut in the top of the cell with a special tool. The lower end of the insert is provided with coarse threads of such form that the insert can be screwed into the hole in the cell, thus forming a substantial mechanical and electrical connection.
Fig. 374-3. Typical insert for connecting from cell-to-floor outlet assembly. (Sec. 374.10.)
374.11. Connections to Cabinets and Extensions from Cells. This section establishes the acceptable methods for connection to equipment and enclosures supplied from a cellular metal floor raceway. Flex and liquidtight flex may be used, provided they are not installed in concrete. Where installed in concrete, RMC, IMC, EMT, or “approved fittings” are permitted. Where the equipment or enclosure has provisions for connecting and equipment grounding conductor, then nonmetallic conduit, ENT, and LFNMC are permitted.
As stated by the last sentence, where listed for installation in concrete, LFMC and LFNMC are recognized to supply equipment and enclosures from a cellular metal floor raceways.
374.17. Ampacity of Conductors. This section makes clear that the rules given in 310.15(B)(2) requiring derating of conductors where there are more than three current-carrying conductors within a raceway, cable, or trench apply to conductors installed within cellular metal floor raceways. See the comment at 374.7.
376.2. Definition. Metal wireways are sheet-metal troughs in which conductors are laid in place after the wireway has been installed as a complete system. Wireway is available in standard lengths of 1, 2, 3, 4, 5, and 10 ft (0.30, 0.61, 0.91, 1.22, 1.52, and 3.05 m), so runs of any exact number of feet can be made up without cutting the duct. The cover may be a hinged or removable type. Unlike auxiliary gutters, wireways represent a type of wiring, because they are used to carry conductors between points located considerable distances apart.
The purpose of a wireway is to provide a flexible system of wiring in which the circuits can be changed to meet changing conditions, and one of its principal uses is for exposed work in industrial plants. Wireways are also used to carry control wires from the control board to remotely controlled stage switchboard equipment. A wireway is approved for any voltage not exceeding 600 V between conductors or 600 V to ground. An installation of wireway is shown in Fig. 376-1.
376.10. Use. Figure 376-1 shows a typical Code-approved application of 4- by 4-in. wireway in an exposed location.
Fig. 376-1. Wireway in industrial plant—installed exposed, as required by 376.10 provides highly flexible wiring system that provides easy changes in the number, sizes, and routing of circuit conductors for machines and controls. Hinged covers swing down for ready access. Article 328 permits splicing and tapping in wireway. And 376.70 covers use of conduit for taking circuits out of wireway. (Sec. 376.10.)
376.22. Number of Conductors. Wireways may contain up to 30 “current-carrying” conductors at any cross section. (Signal circuits and control conductors used for starting duty only between a motor and its starter are not “current-carrying” conductors.) The total cross-sectional area of the group of conductors must not be greater than 20 percent of the interior cross-sectional area of the wireway or gutter. And ampacity derating factors for more than three conductors do not apply to wireway the way they do to wires in conduit. However, if the derating factors from 310.15(B)(2) are used, there is no limit to the number of current-carrying wires permitted in a wireway or an auxiliary gutter. But the sum of the cross-sectional areas of all contained conductors at any cross section of the wireway must not exceed 20 percent of the cross-sectional area of the wireway or auxiliary gutter. More than 30 conductors may be used under these conditions.
620.32 says that wireway used for circuit conductors for an elevator or escalator may be filled with any number of wires, occupying up to 50 percent of the interior cross section of the wireway, and no derating has to be made for fill.
The second sentence of the first paragraph has the effect of saying that any number of signal and/or motor control wires (even over 30) may be used in wireway, provided the sum of their cross-sectional areas does not exceed 20 percent of the wireway csa. And these conductors are not counted in the 30 allowable current-carrying conductors that can be installed before mutual conductor heating derating penalties are imposed.
Figure 376-2 shows examples of wireway fill calculations. The example at the bottom shows a case where power and lighting wires (which are current-carrying wires) are mixed with signal wires. Because there are not over 30 power and light wires, no derating of conductor ampacities is needed. If, say, 31 power and light wires were in the wireway, then the power and light conductors would be subject to derating. If all 49 conductors were signal and/or control wires, no derating would be required. But, in all cases, wireway fill must not be over 20 percent.
Fig. 376-2. Wireway fill and need for derating must be carefully evaluated. (Sec. 376.22.)
376.23. Insulated Conductors. Deflected conductors in wireways must observe the rules on adequate enclosure space given in 312.6. This section is based on the following:
Although wireways don’t contain terminals or supplement spaces with terminals, pull boxes and conduit bodies don’t either. This rule borrows language from both 366.58(A) and 314.28(A)(2), Exception, in an attempt to produce a consistent approach in the Code. Although in some cases the deflected conductors travel long distances in the wireway and are therefore easily inserted, in other cases the conductors are deflected again within inches of the first entry. The result is even more stress on the insulation than if they were entering a conduit body.
The next logical step, as in part (B) of this section, is to squarely address short wireway sections that are used as pull boxes. This part now assures that the usual requirements in 314.28 will be applied.
376.30. Securing and Supporting. Wireway must be supported every 5 ft (1.5 m). Wireway lengths over 5 ft (1.5 m) must be supported at each end or joint, unless listed for other support. In no case should the distance between supports for wireway exceed 10 ft (3.0 m). Vertical runs can use a 4.5 m (15 ft) interval, provided there is no more than a single joint between supports.
376.56. Splices, Taps, and Power Distribution Blocks. The conductors should be reasonably accessible so that any circuit can be replaced with conductors of a different size if necessary and so that taps can readily be made to supply motors or other equipment. Accessibility is ensured by limiting the number of conductors and the space they occupy as provided in 376.22 and 376.23. For power distribution blocks, the bending space requirements of 312.6(B) (the bigger distance table) apply even if a particular entry may happen to be arranged at a right angle. In addition, the block design must include an insulating cover so the uninsulated live parts are not exposed after the block installation is complete, even with the wireway cover open.
376.70. Extensions from Wireways. Knockouts are provided in wireways so that circuits can be run to motors or other apparatus at any point. Conduits connect to such knockouts, as shown in Fig. 376-1.
Sections of wireways are joined to one another by means of flanges which are bolted together, thus providing rigid mechanical connection and electrical continuity. Fittings with bolted flanges are provided for elbows, tees, and crosses and for connections to cabinets. See 250.118.
This article covers nonmetallic wireways. As given in 378.22, the maximum conductor fill is determined in the same manner as for metal wireways, but support requirements are slightly different. 378.30 requires support at 3-ft (0.9-m) intervals, instead of 5-ft (1.5-m) spacing. The most significant difference is found in 378.22 with respect to mutual conductor heating derating factors. Since nonmetallic wireways are not efficient heat sinks, the 30-conductor allowance prior to the imposition of derating factors that pertains to metal wire-ways does not apply to nonmetallic wireways. The derating factors begin to apply with the fourth current-carrying conductor, just like ordinary tubular raceways.
380.1. Scope. UL data are as follows:
This category covers metal raceways with factory installed conductors and attachment plug receptacles without provision for field installation of additional conductors except where the product is marked to indicate the number, type, and size of additional conductors which may be field installed. Also covered are non-metallic raceways with factory installed conductors and attachment plug receptacles either factory installed or separately listed as Multioutlet Assembly Fittings for field installation.
Separation of communication, signal, and data circuits from branch circuit wiring is provided in the assembly where the conductors are installed at the factory. Separate channels are provided in assemblies intended to be field wired with circuits requiring separation.
Multioutlet Assemblies are for installation in accordance with Art. 380 of the National Electrical Code.
380.2. Uses Permitted. These assemblies are intended for surface mounting, except that the metal type may be surrounded by the building finish or recessed so long as the front is not covered. The nonmetallic type may be recessed in baseboards. In calculating the load for branch circuits supplying multioutlet assembly, see 220.14(H).
A nonmetallic extension is an assembly of two insulated conductors within a nonmetallic jacket or an extruded thermoplastic covering. The assembly is mounted directly on the surface of walls or ceilings. Nonmetallic extensions are permitted only if (1) the extension is from an existing outlet on a 15- or 20-A branch circuit and (2) the extension is run exposed and in a dry location. Nonmetallic extensions are limited to residential or office buildings that do not exceed three stories above grade. This category also, as a new 2008 NEC category in this article, includes concealable nonmetallic extensions, which comprise two, three, or four insulated circuit conductors that mount on wall and ceiling surfaces in a flat configuration that is capable of concealment behind paint, joint compound, wallpaper, etc. In this form, if identified for the purpose, concealable extensions are permitted in buildings of more than three stories.
384.1. Scope. This article covers the installation and use of channel raceways. It provides requirements for the channel manufacturer as well as for the designer and installer. As given in 384.2, such raceways must be resistant to moisture or protected against corrosion. The next sentence specifically recognizes galvanized steel, stainless steel, and enameled or PVC-coated aluminum and steel as satisfying this requirement for moisture and corrosion resistance.
384.10. Uses Permitted. Channel raceways are permitted for a variety of applications. Analysis of the locations and conditions of permitted use spelled out here indicates that channel raceway may be used within buildings where exposed in dry locations—and for limited cases in “damp or corrosive” locations—for power poles to feed receptacles, electrified partitions, undercarpet or under-floor applications, and the like. Such raceways may be used only for systems rated up to 600 V, and in hazardous locations as described in 501.4(B).
384.12. Uses Not Permitted. Here the Code prohibits the use of channel raceway systems where concealed and permits their use only indoors when constructed with ferrous metal (e.g., steel or iron) and protected only by enamel coating.
384.22. Number of Conductors. To determine the number of conductors in a strut channel layout, first note whether you are using couplings that mount inside the channel or outside it. Then read out the area available for conductor fill from Table 384.22. For example, a standard 1 × 1 strut with an inside coupling (“joiner”) has 327 mm2 (0.507 in.2) available for wire fill. If all the wires you want to insert are the same size, divide this number by the csa of the wire from Chap. 9 Table 5. Otherwise, add the proposed fill and compare it with what you just determined was available.
example If 3 8 AWG THHN wires will be used in the above strut, how many 10 AWG THHN could be added?
Step 1: As determined above, the maximum possible fill area is 0.507 in.2.
Step 2: Subtract the known fill, 3 8 AWG = 3 × 0.0366 in.2 = 0.1098 in.2. Therefore, remaining fill area is 0.507 - 0.110 (three significant figures) = 0.397 in.2.
Step 3: Divide the remaining fill area by the csa of a 10 AWG wire based on Table 5; 10 AWG THHN csa is: 0.0211 in.2. 0.397 ÷ 0.0211 = 18.8 conductors; therefore, the answer is 18 10 AWG THHNs can be installed with the 3 8 AWG THHNs.
Note that there is a similar calculation to be done as part of a three-part procedure to determine if the installation qualifies for a waiver from the ampacity derating rules. This allowance depends on three factors being true, and then allows a similar waiver as applies to metal wireways. The second and third conditions are not difficult to meet. The first condition, however, apparently based on 386.22 for surface metal raceways, is impossible to meet. None of the raceways recognized by this article meet the 2500 mm2 (4 in.2) threshold in total cross-sectional area. The largest one, the 1½ × 3 size just misses at 2487 mm2 (3.854 in.2), and the others are all smaller. Therefore, this allowance cannot be used, all the wires are subject to the penalties in 310.15(B)(2)(a), and the exemption procedure should be ignored.
386.1. Use. At one time, this article was titled “Surface Metal Raceways and Surface Nonmetallic Raceways.” The article now covers only metallic surface raceways (Fig. 386-1).
Fig. 386-1. Surface raceway has become popular for new works as well as for modernization. (Sec. 386.10.)
386.21. Size of Conductors. Manufacturers of metal surface raceways provide illustrations and details on wire sizes and conductor fill for their various types of raceway. It is important to refer to their specification and application data.
386.22. Number of Conductors in Raceways. The rules of conductor fill may now be applied to surface metal raceway in very much the same way as standard wireway (Fig. 386-2). This rule applies wireway conductor fill and ampacity determination to any surface metal raceway that is over 4 sq in. in cross section. As with wireway, if there are not more than 30 conductors in the raceway and they do not fill the cross-sectional area to more than 20 percent of its value, the conductors may be used without any conductor ampacity derating in accordance with 310.15(B)(2). This allowance only applies to raceways with over 2500 mm2 (4 in.2) in cross-sectional area. Only as an example, in the Wiremold product line, that parameter applies to only two raceways, the 4000 (undivided only, 7.2 in.2) and the 6000 (16 in.2).
386.60. Grounding. In every type of wiring having a metal enclosure around the conductors, it is important that the metal be mechanically continuous in order to provide protection for the conductors and that the metal form a continuous electrical conductor of low impedance from the last outlet on the run to the cabinet or cutout box. A path to ground is thus provided through the box or cabinet, in case any conductor comes in contact with the metal enclosure, an outlet box, or any other fitting. See 250.118.
386.70. Combination Raceways. Metal surface raceways may contain separated systems as shown in Fig. 386-3. The separate compartments must be consistently distinguishable through the use of stamping, imprinting, or color coding of the interior finish. This is why the divider strips that are used to split these race-ways in the field are usually painted with two different colors. It is very important to apply that color code consistently. The NEC used to insist that the relative orientation of the divided segments be maintained throughout. That requirement has been deleted, and now only the color code identifies which divided segment is to be which.
Fig. 386-2. NEC rule permits conductor fill of metal surface race-way without ampacity derating of wires. (Sec. 386.22.)
Fig. 386-3. For separating high and low potentials, combination raceway or tiered separate raceways may be used with barriered box assembly. (Sec. 386.70.)
388.1. Scope. Although covered under the same article as surface metal race-way systems—which includes the associated fittings—this wiring method is now covered separately in its own article.
388.2. Definition. The Code describes the various characteristics that surface non-metallic raceways must possess. Obviously, for any nonmetallic raceway to have the necessary moisture and corrosion resistance, mechanical strength, flame retardance, and low-smoke-producing characteristics, etc., such characteristics must be designed into the raceway system, which means that the manufacturer is responsible for ensuring compliance with this rule. Designers and installers are required to select a manufacturer whose product meets the requirements given here; that is, a listed product must be used. Remember to look for the “LS” (low-smoke producing) marking on the selected nonmetallic raceway system.
390.10. Permitted Uses. Underfloor raceway was developed to provide a practical means of bringing conductors for lighting, power, and signaling systems to office desks and tables (Fig. 390-1). It is also used in large retail stores, making it possible to secure connections for display-case lighting at any desired location.
Fig. 390-1. Underfloor raceway system, with spaced grouping of three ducts (one for power, one for telephone, one for signals), is covered with concrete after installation on first slab pour. (Sec. 390.2.)
Fig. 390-2. The 1-in. cover is inadequate for raceways less than an inch apart. (Sec. 390.3.)
This wiring method makes it possible to place a desk or table in any location and it will always be over, or very near to, a duct line. The wiring method for lighting and power between cabinets and the raceway junction boxes may be conduit, underfloor raceway, wall elbows, and cabinet connectors.
390.3. Covering. The intent in paragraphs (A) and (B) is to provide a sufficient amount of concrete over the ducts to prevent cracks in a cement, tile, or similar floor finish. Figure 390-2 shows a violation. Two 1½-by 4½-in. underfloor race-ways with 1-in.-high inserts are spaced ¾ in. apart by adjustable-height supports resting directly on a base floor slab, as shown. After raceways are aligned, leveled, and secured, concrete fill is poured level with insert tops. But spacing between raceways must be at least 1 in. otherwise the concrete cover must be 1½ in. deep.
390.6. Splices and Taps. This section has a second paragraph that recognizes “loop wiring” where “unbroken” wires extend from underfloor raceways to terminals of attached receptacles, and then back into the raceway to other outlets. For purposes of this Code rule only, the loop connection method is not considered a splice or tap (Fig. 390-3).
Note: As noted in the exception, splices and taps may be made in trench-type flush raceway with an accessible removable cover. The removable cover of the trench duct must be accessible after installation, and the splices and taps must not fill the raceway to more than 75 percent of its cross-sectional area.
Fig. 390-3. “Loop” method permitted at outlets supplied from underfloor raceways. (Sec. 390.6.)
390.7. Discontinued Outlets. When an outlet is discontinued, the conductors supplying the outlet shall be removed from the raceway. The general practice is to loop wire intermediate receptacles between the header and the end of the run. This requirement assures that reinsulated conductors will not be resting in the raceway below an abandoned outlet. This in turn prevents a fish wire inserted afterward from a downstream location from getting caught on a reinsulated conductor, with very destructive consequences.
It is often advisable to wire each outlet on its own pair of conductors back to the header or other junction point. Then, when an outlet is abandoned, the associated pair of conductors can be withdrawn without disrupting other outlets on the run. Take care, however, to keep track of the total number of conductors at all portions of the duct, because the ampacity derating factors for mutual conductor heating (see 390.17) will apply. Some jurisdictions provide limited waivers to these derating factors in underfloor applications, however, in order to encourage this practice and thereby discourage the reinsulation of conductors. When the conductors are withdrawn, leave a pull string in their place so the outlet can be easily reactivated in the future as necessary.
390.17. Ampacity of Conductors. This section makes clear that the rules given in 310.15(B)(2) requiring derating of conductors where there are more than three current-carrying conductors within a raceway, cable, or trench apply to conductors installed within underfloor raceways. See the above comment.
392.2. Definition. Cable trays are open, raceway-like support assemblies made of metal or suitable nonmetallic material and are widely used for supporting and routing circuits in many types of buildings. Troughs of metal mesh construction provide a sturdy, flexible system for supporting feeder cables, particularly where routing of the runs is devious or where provision for change or modification in circuiting is important. Ladder-type cable trays are used for supporting interlocked-armor cable feeders in many installations (Fig. 392-1). Where past Code editions treated a cable tray simply as a support system for cables, in the same category as a clamp or hanger, the Code today recognizes a cable tray as a conductor support method, somewhat like a raceway, under prescribed conditions, and an integral part of a Code-approved wiring method. However, cable trays are not listed under the Code definition of raceway in Art. 100. Any “raceway” must be an “enclosed” channel for conductors. A cable tray is a support system, not a raceway.
392.3. Uses Permitted. The NEC recognizes a cable tray as a support for wiring methods that may be used without a tray (metal-clad cable, conductors in EMT, IMC, or rigid conduit, etc.), and a cable tray may be used in either commercial, industrial, or institutional buildings or premises (Fig. 392-2). Where cables are available in both single-conductor and multiconductor types—such as SE (service entrance) and UF cable—only the multiconductor type may be used in a tray. However, Sec. 392.3(B)(1) permits use of single-conductor building wires in a tray. Single-conductor cables for use in a tray must be 1/0 AWG or larger, listed for use in a tray, and “marked on the surface” as suitable for tray applications. In earlier NEC editions, single-conductor cable in a tray had to be 250 kcmil or larger. Sizes 1/0 through 4/0 AWG single-conductor cables may now be used but must be used in a ladder-type tray with rungs spaced not over 225 mm (9 in.) apart or in a ventilated trough cable tray. Sizes 250 kcmil and larger may be used in any kind of tray. This rule states that such use of building wire is permitted in industrial establishments only, where conditions of maintenance and supervision ensure that only competent individuals will service the installed cable tray system. This applies to ladder-type trays, ventilated troughs, solid bottom, or ventilated channel-type cable trays.
Fig. 392-1. Two basic types of cable tray. (Sec. 392.2.)
Fig. 392-2. Cable-tray use is subject to many specific rules in Art. 392. (Sec. 392.3.)
Single-conductor cables used in a cable tray must be a type specifically “listed for use in cable trays.” This is a qualification on the rule that was in previous codes permitting use of single-conductor building wire (RHH, USE, THW, MV) in a cable tray. The wording permits any choice of conductor types that may be used, simply requiring that any type must be listed. It adds thin-wall-insulated cables, like THHN or XHHW, to the other types mentioned. Present UL standards make reference to cables designated “for CT (cable tray) use” or “for use in cable trays”—which is marked on the outside of the cable jacket. Such cables are subjected to a “vertical tray flame test,” as used for Type TC tray cable and other cables. Only cables so tested and marked “VW-1” would be recognized for use in a cable tray.
A cable tray is not a wireway without a cover. This is one of the most abused provisions in the entire NEC. Single-conductor cable tray has been found in exhibition halls and hospitals, and just about every other nonindustrial occupancy, except dwellings. None of those occupancies qualify for single conductor tray. Single-conductor tray has also been seen over industrial motor control centers, a qualified occupancy to be sure, but filled with motor branch circuit and control conductors running from 12 AWG up to 3/0 AWG. Again, a cable tray is not a wireway without a cover. Single conductors in cable trays are only permitted in the very restricted circumstances itemized here. Specifically, single conductors are only permitted in industrial occupancies with qualified maintenance and supervision, in sizes 1/0 AWG and larger. Medium-voltage conductors run as Type MV cable are permitted to run as single conductors, provided they follow the same rules.
Part (C) specifically recognizes use of the metal length of a cable tray as an equipment grounding conductor for the circuit(s) in the tray—in both commercial, industrial, and institutional premises where qualified maintenance personnel are available to assure the integrity of the grounding path. Section 392.3(D) specifically uses the word “only” when referring to cable types that are permitted to be used in cable trays in hazardous locations. In previous Code editions, wording was more open-ended and permitted specific cables without limiting use to only such cables.
As covered in part (E), nonmetallic cable tray may be used in corrosive locations. This permits use of nonmetallic tray—such as fiberglass tray—in industrial or other areas where severe corrosive atmospheres would attack a metal tray. Such a tray is also permitted where “voltage isolation” is required.
392.4. Uses Not Permitted. Cable tray may be used in air-handling ceiling space but only to support the wiring methods permitted in such space by 300.22(C). This recognizes cable trays simply as supports for raceways or cables permitted in hung ceilings used for air conditioning. Section 392.6(H) requires cable trays to be exposed and accessible. Note that the two words exposed and accessible must be taken “as applied to wiring methods.” Cable trays may be used above a suspended, nonair-handling ceiling with any of the wiring methods covered by 392.3. If used with a wiring method permitted by 300.22(C), cable trays may be used above an air-handling ceiling.
392.6. Installation. Part (A) makes clear that cable trays must be used as a complete system—that is, straight sections, angle sections, offsets, saddles, and so forth—to form a cable support system that is continuous and grounded as required by 392.7(A). Cable trays must not be installed with separate, unconnected sections used at spaced positions to support the cable. Manufactured fittings or field-bent sections of tray may be used for changes in direction or elevation.
In the 1971 NEC, part (c) of Sec. 318-4 on Installation (now 392.6) read as follows:
(c) Continuous rigid cable supports shall be mechanically connected to any enclosure or raceway into which the cables contained in the continuous rigid cable support extend or terminate.
That wording clearly made a violation of the kind of hookup shown in Fig. 392-3, where the tray does not connect to the transformer enclosures—and is not bonded by jumpers to those enclosures. However, an FPN added in the 1993 Code—which was incorporated in the basic rule in 392.6(A) of the 1996 Code—indicates that use of “discontinuous segments” is intended to be acceptable—but, ground-continuity between the enclosure and the cable tray must be ensured. The discontinuity cannot exceed 1.8 m (6 ft), which correlates with the support interval for Type MC cable, and the cable has to be secured to the tray (or to the terminating location) at both ends. Refer also to 392.7.
Section 392.6(E) notes that any multiconductor cables rated 600 V or less may be used in the same cable tray. Section 392.6(F) points out that high-voltage cables and low-voltage cables may be used in the same tray if a solid, fixed barrier is installed in the tray to separate high-voltage cables from low-voltage cables. And where the high-voltage (over 600-V) cables are Type MC, it is not necessary to have a barrier in the cable tray, and MC cables operating above 600 V may be used in the same tray with MC cables operating less than 600 V or with nonmetallic-jacketed cables operating at not over 600 V. But for high-voltage cables other than Type MC, a barrier must be used in the tray to separate high-voltage from low-voltage cables (Fig. 392-4), or another tray must be used.
Figure 392-5 shows the rule of part (I). The first paragraph of part (J) covers the termination rules for cables and raceways arriving at the tray. In general, although raceways must always be secured to a tray they service, the tray is not to be used as a qualified point of support, and therefore the next support is the final one before the termination, and therefore must be, typically, within 900 mm (3 ft) of the tray. However, with qualified maintenance and supervision in industrial facilities, and with the knowledge that the tray system has been designed and installed to support the load, then the cable tray qualifies as being “permitted to support raceways and cables and boxes and conduit bodies.” In this case, the tray clamp or adapter for the raceway, which must be listed for this duty, becomes the support at a termination (within 900 mm [3 ft] of the actual raceway bushing) and therefore the next point of support could be 3.0 m (10 ft) away, or at whatever the support interval is in the article for the arriving wiring method. This is a much more flexible and realistic procedure for most industrial occupancies. The other two paragraphs address routing wiring method and boxes on the underside of a tray.
Fig. 392-3. This was clearly a violation of 318-4(c) in the 1971 NEC because the tray does not connect to the transformer enclosures. The tray continuity required by 392.6(A) of the present NEC does seem to be violated by lack of connection to the enclosures. (Sec. 392.6.)
Fig. 392-4. Cables of different voltage ratings may be used in the same tray. (Sec. 392.6.)
Fig. 392-5. Tray spacing must simply be adequate for cable installation and maintenance. (Sec. 392.6.)
392.7. Grounding. Part (A) requires cable trays to be grounded, just as conduit or other metal enclosures for conductors must be grounded. That rule combined with the last two sentences in part (A) of 392.6 makes cable trays comply with the Code concept that metal raceways constitute an equipment grounding conductor to carry fault currents back to the bonded neutral at a service, at a transformer secondary, or at a generator. Part (B) of 392.7 permits steel or aluminum cable tray to serve as an equipment grounding conductor for the circuits in the tray in much the same way as conduit or EMT may serve as the equipment grounding conductor—a return path for fault current—for the circuit conductors they contain, under the conditions specified in part (B). Even though metallic cable trays are permitted to act as an equipment grounding conductor in the same way that a metallic conduit, tubing, or cable sheath might be, it should be noted that a cable tray is not a “raceway” as defined in Art. 100. Therefore, other Code rules that apply to “raceways” (e.g., 200.7 on grounded conductor identification) do not apply to cable trays.
Note that paragraph (4) under part (B) requires all tray system components, as well as “connected raceways,” to be bonded together—either by the bolting means provided with the tray sections or fittings or by bonding jumpers, as shown in Fig. 392-6.
Fig. 392-6. Bare equipment bonding jumpers tie all tray runs together, with jumpers carried up to the equipment grounding bus in the switchboard above. (Sec. 392.7.)
392.8. Cable Installation. Although splices are generally limited to use in conductor enclosures with covers and are prohibited in the various conduits, part (A) permits splicing of conductors in cable trays. Splices are now permitted to project above the side rails, if not subject to damage.
Part (C) permits cables or conductors to drop out of a tray in conduits or tubing that have protective bushings and are clamped to the tray side rail by cable-tray conduit clamps to provide the bonded connections required by 392.7(B)(4). The clamps must also comply with 392.6(J), which requires listing for this equipment.
Figure 392-7 shows how single-conductor cables must be grouped to satisfy part (d) of this section for a 1200-A circuit made up of three 500-kcmil copper XHHW conductors per phase and three for the neutral. By distributing the phases and neutral among three groups of four and alternating positions, more effective cancellation of magnetic fluxes results from the more symmetrical placement—thereby tending to balance current by balancing inductive reactance of the overall 1200-A circuit.
Fig. 392-7. A parallel 1200-A circuit must have conductors grouped for reduced reactance and effective current balance. (Sec. 392.8.)
Part (E) prohibits stacking of single conductors 1/0 to 4/0 AWG in ladder or ventilated trough cable trays, unless bound together as a circuit group.
392.9. Number of Multiconductor Cables, Rated 2000 V or Less, in Cable Trays. These rules apply to multiconductor cables rated 2000 V or less. For cables rated 2001 V or higher, the number permitted in a cable tray is now covered in 392.12.
Section 392.9 is broken down into parts (A), (B), (C), (D), (E), and (F), each part covering a different condition of use. Section 392.9(A) applies to ladder or ventilated trough cable trays containing multiconductor power or lighting cables or any mixture of multiconductor power, lighting, control, and signal cables.
Section 392.9(A) has three subdivisions:
1. Where all the multiconductor cables are made up of conductors 4/0 AWG or larger, the sum of the outside diameters of all the multiconductor cables in the tray must not be greater than the cable tray width, and the cables must be placed side by side in the tray in a single layer, as shown in Fig. 392-8. If this applies, and 392.11(A)(3) is used to determine free-air ampacities for the multiconductor cables using the Neher-McGrath engineering calculation, then the cable tray must be wide enough to show that the cables can be spread out as required.
2. Where all the multiconductor cables in the tray are made up of conductors smaller than 4/0 AWG, the sum of the cross-sectional areas of all cables must not exceed the maximum allowable cable fill area in column 1 of Table 392.9 for the particular width of cable tray being used. The table shows, for instance, that if a 450 mm (18 in.) wide ladder or ventilated trough cable tray is used with multiconductor cables smaller than 4/0 AWG, column 1 sets 13,500 sq mm (21 sq in.) as the maximum value for the sum of the overall cross-sectional areas of all the cables permitted in that tray, as in Fig. 392-9.
Fig. 392-8. No. 4/0 and larger multiconductor cables must be in a single layer. (Sec. 392.9.)
3. Where a tray contains one or more multiconductor cables 4/0 AWG or larger along with one or more multiconductor cables smaller than 4/0 AWG, there are two steps in determining the maximum fill of the tray.
First, the sum of the outside cross-sectional areas of all the cables smaller than 4/0 AWG must not be greater than the maximum permitted fill area resulting from the computation in column 2 of Table 392.9 for the particular cable tray width. Then, the multiconductor cables that are 4/0 AWG or larger must be installed in a single layer, and no other cables may be placed on top of them (Fig. 392-10). Note that the available cross-sectional area of a tray which can properly accommodate cables smaller than 4/0 AWG installed in a tray along with 4/0 AWG or larger cables is, in effect, equal to the allowable fill area from column 1 for each width of tray minus 1.2 times the sum of the outside diameters of the 4/0 AWG or larger cables.
Another way to look at this is to consider that, for any cable tray, the sum of the cross-sectional areas of cables smaller than 4/0 AWG, when added to 1.2 times the sum of the diameters of cables 4/0 AWG or larger, must not exceed the value given in column 1 of Table 392.9 for a particular cable tray width.
For the installation shown in Fig. 392-10, assume that the sum of the cross-sectional areas of the seven cables smaller than 4/0 AWG is 16 sq in. and assume that the diameters of the four 4/0 AWG or larger cables are 3 in., 3.5 in., 4 in., and 4 in. The abbreviation “Sd” in column 2 of Table 392.9 represents “sum of the diameters” of 4/0 AWG and larger cables installed in the same tray with cables smaller than 4/0 AWG. In the example here, then, Sd is equal to 3 + 3.5 + 4 + 4 = 14.5, and 1.2 × 14.5 = 17.4. Then we add the 16-sq-in. total of the cables smaller than 4/0 AWG to the 17.4 and get 17.4 + 16 = 33.4. Note that this sum is over the limit of 28 sq in., which is the maximum permitted fill given in column 1 for a 24-in. wide cable tray. And column 1 shows that a 30-in. wide tray (with 35-sq-in. fill capacity) would be required for the 33.4 sq in. determined from the calculation of column 2, Table 392.9.
Fig. 392-9. Smaller than No. 4/0 cables may be stacked in tray. (Sec. 392.9.)
Section 392.9(B) covers use of multiconductor control and/or signal cables (not power and/or lighting cables) in ladder or ventilated trough with a usable inside depth of 6 in. or less. For such cables in ladder or ventilated trough cable tray, the sum of the cross-sectional areas of all cables at any cross section of the tray must not exceed 50 percent of the interior cross-sectional area of the cable tray. And it is important to note that a depth of 150 mm (6 in.) must be used in computing the allowable interior cross-sectional area of any tray that has a usable inside depth of more than 150 mm (6 in.) (Fig. 392-11).
Fig. 392-10. Large and small cables have a more complex tray-fill formula. (Sec. 392.9.)
Fig. 392-11. Tray fill for multiconductor control and/or signal cables is readily determined. (Sec. 392.9.)
Section 392.9(C) applies to solid-bottom cable trays with multiconductor power or lighting cables or mixtures of power, lighting, control, and signal cables. The maximum number of cables must be observed, as noted.
392.10. Number of Single Conductor Cables, Rated 2000 V or Less, in Cable Trays. This section covers the maximum permitted number of single-conductor cables in a cable tray and stipulates that the conductors must be evenly distributed on the cable tray. This section differentiates between (a) ladder or ventilated trough tray and (b) ventilated channel-type cable trays.
In Ladder or Ventilated Trough Tray
1. Where all cables are 1000 kcmil or larger, the sum of the diameters of all single-conductor cables must not be greater than the cable tray width, as shown in Fig. 392-12. That means the cable tray width must be at least equal to the sum of the diameters of the individual cables.
Fig. 392-12. For large cables, tray width must at least equal sum of cable diameters. (Sec. 392.10.)
2. Where all cables are from 250 up to and including 1000 kcmil, the sum of the cross-sectional areas of all cables must not be greater than the maximum allowable cable fill areas in square inches, as shown in column 1 of Table 392.10 for the particular cable tray width.
example 1 Assume a number of cables, all smaller than 1000 kcmil, have a total csa of 11 sq in. (7095 mm2). Column 1 of Table 392.10 shows that a fill of 11 sq in. (7095 mm2) is greater than that allowed for 6-in.-wide tray (6.5 sq in.) but less than the maximum fill of 13 sq in. permitted for 12-in.-wide tray. Thus, 12-in.-wide tray would be acceptable.
example 2 Assume four 4-wire sets of single-conductor, 600-kcmil RHH cables are used as power feeder conductors in a cable tray. Table 5 in Chap. 9 of the NE Code shows that the overall csa of each 600-kcmil RHH conductor (without outer covering) is 0.9729 sq in. The total area of 16 such conductors would be 16 × 0.9729, or 15.6 sq in. From Table 392.10, column 1, 15.6 sq in. is over the maximum permissible fill for 12-in.-wide tray, but it is well below the maximum fill of 19.5 sq in. permitted for 18-in.-wide tray. Thus, 18-in.-wide tray is acceptable.
3. Where 1000-kcmil or larger single-conductor cables are installed in the same tray with single-conductor cables smaller than 1000 kcmil, the fill must not exceed the maximum fill determined by the calculation indicated in column 2 of Table 392.10—in a manner similar to the calculations indicated above for multiconductor cables.
example 3 If nine 800-kcmil THW conductors are in a tray with six 1000-kcmil THW conductors, the required minimum width (W) of the tray would be determined as follows:
1. The sum of the csa of the nine 800-kcmil conductors (those smaller than 1000 kcmil) is equal to 9 × 1.2272 sq in. (from column 5, Table 5, Chap. 9, NE Code), or 11.04 sq in.
2. Each 1000-kcmil THW conductor has an outside diameter of 1.372 in. The sum of the diameters of the 1000-kcmil conductors is, then, 6 × 1.372, or 8.232 in.
3. Column 2 of Table 392.10 says, in effect, that to determine the minimum required width of cable tray it is necessary to add 11.04 sq in. (from 1 above) to 1.1 × 8.232 (from 2 above) and use the total to check against column 1 of Table 392.10 to get the tray width:
11.04 + (1.1 × 8.232) = 11.04 + 9.05 = 20.1 sq in.
From column 1, Table 392.10, the fill of 20.1 sq in. is greater than the 19.5 sq in. permitted for 18-in.-wide tray. But, this fill is within the permitted fill of 26 sq in. for 24-in.-wide tray. The 24-in.-wide tray is, therefore, the minimum size tray that is acceptable.
4. Where any cables in the tray are sizes 1/0 through 4/0, then all cables must be installed in a single layer. And the sum of the single-conductor cable diameters must not exceed the cable tray width as required for “Multiconductor Cables Rated 2000 V, Nominal, or Less” as covered in 392.9(A)(1).
Section 392.10(B) governs cable fill in ventilated channel cable trays. Where single-conductor cables are installed in 50-mm (2-in.), 75-mm (3-in.), 100-mm (4-in.), or 150-mm (6-in.) wide, ventilated channel-type trays, the sum of the diameters of all single conductors must not exceed the inside width of the channel.
When cable assemblies of more than one conductor are installed as required by 392.9, each conductor in any of the cables will have an ampacity as given in Table 310.16 or 310.18. Those are the standard tables of ampacities for cables with not more than three current-carrying conductors within the cable (excluding neutral conductors that carry current only during load unbalance on the phases). The ampacity of any conductor in a cable is based on the size of the conductor and the type of insulation on the conductor, as shown in Tables 310.16 and 310.18. For cables not installed in a cable tray, if a cable contains more than three current-carrying conductors, derating of the conductor ampacities must be made in accordance with 310.15(B)(2)(a). But the last sentence of 392.11(A)(1) flatly exempts cables in a tray from these penalties unless the cable itself includes more than three current-carrying conductors, and even then the penalties only apply based on the cable content itself and not on the basis of the total cable population in the tray.
Section 392.11(A)(2) reduces the above determination of conductor ampacities in the case of any cable tray with more than 6 ft (1.83 m) of continuous, solid, unventilated covers. In such cases, the conductors in the cable have an ampacity of not more than 95 percent of the ampacities given in Tables 310.16 and 310.18.
Section 392.11(A)(3) applies to a single layer of multiconductor cables with maintained spacing (one cable width) between cables, installed in uncovered tray. In such a case the ampacity can be calculated based on the Neher-McGrath method in 310.15(C).
The ampacity of any single-conductor cable or single conductors twisted together is determined as follows:
600 kcmil and larger—Where installed in accordance with 392.10, the ampacity of any 600-kcmil or larger single-conductor cable in an uncovered tray is not more than 75 percent of the ampacity given for the size and insulation of conductor in Table 310.17 or for the size and insulation of conductor in Tables 310.17 and 310.19. Note that this means 75 percent of the free-air ampacity of the conductor. And if more than 6 ft (1.8 m) of the tray is continuously covered with a solid, unventilated cover, the ampacities for 600-kcmil and larger conductors must not exceed 70 percent of the ampacity value in Tables 310.17 and 310.19.
No. 1/0 through 500 kcmil—For any single-conductor cable in this range, installed in accordance with 392.10 in uncovered tray, its ampacity is not more than 65 percent of the ampacity value shown in Tables 310.17 and 310.19. And if any such cables in this range are used in a tray that is continuously covered for more than 6 ft (1.8 m) with a solid, unventilated cover, the ampacities must not exceed 60 percent of the ampacity values in Tables 310.17 and 310.19.
Where No. 1/0 and larger single-conductor cables are installed in a single layer in an uncovered cable tray with a maintained spacing of not less than one cable diameter between individual conductors, the ampacities of such conductors are equal to the free-air ampacities given in Tables 310.17 and 310.19, as shown in Fig. 392-13. However, if the tray has a solid bottom, this procedure is not allowed because the cables will overheat, and the calculation must be done using the Neher-McGrath method of 310.15(C).
Fig. 392-13. With spacing, cables in tray may operate at free-air ampacity. (Sec. 392.11.)
If the single conductors are arranged in a triangle or diamond configuration, with a maintained free space not less than 2.15 times a conductor diameter between bundles, the ampacities of 1/0 AWG and larger conductors can be calculated on the basis of two- or three-conductors in a bundle of conductors run on a messenger, using Table 310.20.
Combinations of Multiconductor and Single-Conductor Cables—When both types are mixed, the ampacities for the single cables and the ampacities for the multiconductor cables are calculated independently using the applicable procedures above. However, this only applies if (1) the multiconductor fill per 392.9 and then expressed as a percentage of the total allowable fill in the cable tray, (2) the single conductor fill per 392.10 and then expressed as a percentage of the total allowable fill in the cable tray, and (3) total together not over 100 percent. In addition, the multiconductor cables must be installed in accordance with 392.9 and the single conductor cables must be installed in accordance with 392.10 and 392.8(D) and (E) with respect to binding parallel makeups and balanced, crossed phase groupings of conductors (A + B + C + N) + (A + B + C + N), etc.
392.12. Number of Type MV and Type MC Cables (2001 V or Over) in Cable Trays. This section applies only to high-voltage circuits in a tray. Type MV cable is a high-voltage cable now covered by new Art. 328. Type MC cable is the metal-clad cable operating above 2000 V—a cable assembly long known as inter-locked armor cable. (Type MC or other armored cable [e.g., ALS or CS] operating at voltages up to 2000 V must conform to 392.10 and 392.11 on number and ampacities of cables when used in tray.)
Type MV and Type MC high-voltage cables must conform to the tray fill shown in Fig. 392-14.
392.13. Ampacity of Type MV and Type MC Cables (2001 V or Over) in Cable Trays. This section covers the ampacities of MV and MC cables operating above 2000 V in cable trays—both single-conductor and multiconductor. 392.13(A)(2) recognizes the improved heat dissipation afforded by spacing of the cables and allows use of the free-air ampacity tables in loading multiconductor cables. The spacing of “one cable diameter” is also recognized for single conductors in 392.13(B)(3).
394.10. Uses Permitted. Note that this wiring method is restricted to use only for extensions of existing installations and is not Code-acceptable as a general-purpose wiring method for new electrical work. Under the conditions specified in (1) and (2), concealed knob-and-tube wiring may be used to extend an existing installation, and in all other cases only if special permission is granted by the local inspection authority having jurisdiction as noted in the second sentence of 90.4.
Fig. 392-14. Tray must be wide enough for all medium-voltage cables in a single layer. (Sec. 392.12.)
394.12. Uses Not Permitted. Of particular interest here is item (5), which forbids this wiring method to be used in joist or stud cavities that are insulated in a manner that envelops the conductors. This, for all practical purposes prohibits the use of this wiring method in exterior walls. Fiberglass, cellulose, blown foam will all envelop the conductors, in violation of this rule. Probably the only way to insulate a wall with this wiring method in it is to use rigid foam board, and that would require opening the wall. With the wall open, no one would be likely to try and save a wiring method that has no equipment grounding conductor, and therefore is almost incapable of extension without tripping over a number of other serious code issues.
394.23. In Accessible Attics. Where the wiring is installed at any time after the building is completed, in a roof space having less than 3-ft (900-mm) headroom at any point, the wires may be run on knobs across the faces of the joists, studs, or rafters or through or on the sides of the joists, studs, or rafters. Such a space would not be used for storage purposes, and the wiring installed may be considered as concealed knob-and-tube work.
An attic or roof space is considered accessible if it can be reached by means of a stairway or a permanent ladder. In any such attic or roof space wires run through the floor joists where there is no floor must be protected by a running board and wires run through the studs or rafters must be protected by a running board if within 7 ft (2.1 m) from the floor or floor joists.
394.104. Conductors. Conductors for concealed knob-and-tube work may be any of the general-use types listed in Table 310.13 for “dry” locations and “dry and wet” locations such as TW, THW, XHHW, RHH, etc.
396.2. Definition. This article covers a wiring system that has long been manufactured and widely used in industrial installations. The basic construction of the wiring method has been used for many years as service-drop cable for utility supply to all kinds of commercial and residential properties.
From long-time application, messenger-supported wiring is actually an old standard method, even though it has not been covered by the NE Code up until recent years. In 225.6(A)(1), the rule refers to “supported by messenger wire,” and that phrase has been in the Code for over 60 years. Figure 396-1 shows an example of triplex service-drop cable used for supplying floodlights at an outdoor athletic field. So coverage of this type of wiring is important—especially for the vast amounts of outdoor use where messenger-cable wiring offers so many advantages over open wiring. But messenger-supported wiring is recognized for both indoor and outdoor branch circuits and/or feeders. Refer to the discussion under 225.6 for outdoor use of messenger supported wiring.
396.10. Uses Permitted. Messenger support is permitted for a wide range of cables and conductors—for use in commercial and industrial applications. Part (B) covers ordinary building wires supported on a messenger and recognizes use of messenger supported wiring in “industrial establishments only ” with “any of the conductor types given in Table 310.13 or Table 310.62.” Tables 310.13 and 310.62 include single-conductor Types MV, RHH, RHW, and THW and also accept all the other single-conductor types, such as THHN, XHHW, TW. All such application is recognized either indoors or outdoors—provided that any conductors exposed to the weather are “listed for use in wet locations” and are “sunlight-resistant” if exposed to direct rays of the sun. The ampacity of the wiring method is easy to determine now that Table 310.20 has come out of Annex B and is now available for general use. Any of the cabled wiring methods in Table 396.10(A) can now, by right, be supported by a messenger, including tray cable, multiconductor service entrance cable, UF cable, etc., along with Type MV where consistent with other rules.
396.30. Messenger. Part (B) of this section, through a somewhat circular reference to 225.4, now recognizes the use of this wiring method under the exception to 250.32(B) where the neutral is also the equipment grounding conductor and therefore both a current-carrying conductor and a grounding conductor. Its normal function is as a grounding conductor that does not routinely carry current.
398.2. Definition. Conductors for open wiring may be any of the general-use types listed in Table 310.13 for “dry” locations and “dry and wet” locations such as THW, XHHW, THHN, and so on.
The conductors are secured to and supported by insulators of porcelain, glass, or other composition materials. In modern wiring practice open wiring is used for high-tension work in transformer vaults and substations. It is very commonly used for temporary work and is used for runs of heavy conductors for feeders and power circuits, as in manholes and trenches under or adjacent to switchboards, to facilitate the routing of large numbers of circuits fed into conduits.
Fig. 396-1. Messenger-supported cable, used here to supply pole-mounted floodlights, may be constructed in a number of different assemblies, such as this service-drop cable with an ACSR messenger cabled with insulated conductors.
398.10. Uses Permitted. This section limits open wiring on insulators to industrial or agricultural establishments, up to 600 V.
398.19. Clearance from Piping, Exposed Conductors, etc. The additional insulation on the wire referred to in this rule, is to prevent the wire from coming in contact with the adjacent pipe or other metals.
398.23 spells out such installations in unfinished attics and roof spaces.
398.30. Securing and Supporting. Methods of dead-ending open cable runs are shown in Fig. 398-1.
Fig. 398-1. Proven methods must be used for dead-ending open wiring. (Sec. 398.30.)
Where heavy AC feeders are run as open wiring, the reactance of the circuit is reduced and hence the voltage drop is reduced by using a small spacing between the conductors. Up to a distance of 15 ft between supports the 2½-in. spacing may be used if spacers are clamped to the conductors at intervals not exceeding 4½ ft. A spacer consists of the three porcelain pieces of the same form as used in the support, with a metal clamping ring.
In the rule of part (B), reference to “mill construction” is generally understood to mean the type of building in which the floors are supported on wooden beams spaced about 14 to 16 ft apart. Wires not smaller than No. 8 may safely span such a distance where the ceilings are high and the space is free from obstructions.
Fig. 398-2. Proper wiring support devices must be correctly mounted. (Sec. 398.30.)
Figure 398-2 illustrates the rules of subpart (D) on mounting of knobs and cleats for the support of No. 14, No. 12, and No. 10 conductors. For conductors of larger size, solid knobs with tie wires or single-wire cleats should be used.
