Fig. 314-7. THIS MAY BE A CODE VIOLATION! A 4- × 4- × 1½-in. (102- × 102- × 38-mm) square metal box, generally referred to as a 1900 box, has four NM cables coming into it. At upper right is a 14/3 cable with 14 AWG ground. The other three cables are 14/2 NM, each with a 14 AWG ground. The red wire of the 14/3 cable feeds the receptacle to be installed in the one-gang plaster ring. The black wire of the 14/3 feeds the black wires of the three 14/2 cables. All the whites are spliced together, with one brought out to the receptacle, as required by 300.13(B). All the ground wires are spliced together, with one brought out to the grounding terminal on the receptacle and one brought out to the ground clip on the left side of the box. The wire count is as follows: nine 14 AWG insulated wires, plus one for all of the ground wires and two for the receptacle. That is a total of 12 14 AWGs. Note that box connectors are used instead of clamps and there is, therefore, no addition of one conductor for clamps. But Table 314.16(A) shows that a 4- × 4- × 1½-in. (102- × 102- × 38-mm) square box may contain only 10 14 AWG wires. Some think that this is not a violation. They say that because the area provided by the cover has not been considered. But unless the cubic-inch capacity is marked on the cover, it may not be considered. The box is only 49.2 cm3 (3 in.3) short of the required volume. Practical experience in working with this equipment strongly suggests that if this plaster ring (with at least a 13-mm (½-in.) rise is marked with a volume, that volume would be sufficient. Therefore, this comes down to the existence of a marking, which may well be on the other side. (Sec. 314.16.)
Fig. 314-8. Calculation of the proper minimum box size for the number of conductors used in ganged boxes must follow 314.16(A)(1), taking the assembly as a single box of the sum of the volumes of the ganged sections and filling it to the sum of the conductor count. (Sec. 314.16.)
Now sum the fill allowances. There are 15 current-carrying 14 AWG wires in this box, for a fill allowance of: 15 14 AWG conductors. The equipment grounding conductors count collectively as 1 wire, so add 1 14 AWG conductor. The internal cable clamps count collectively as 1 wire, so add 1 14 AWG conductor. The four devices (counted by the strap, not the individual devices) add 2 each for a total of 8 14 AWG conductors. The grand total is 25, and at 32.8 cm3 (2.00 in.3) for each, the total required volume is 820 cm3 (50 in.3). This is the back half of the calculation; compare it with the front half to determine compliance. The total required is far less than the total provided, so the layout is acceptable.
When different sizes of wires are used in a box, Table 314.16(B) must be used in establishing adequate box size. Using the same method of counting conductors as described in Sec. 314.16(B), the volume of cubic inches shown in Table 314.16(B) must be allowed for each wire, depending on its size. Where two or more ground wires of different sizes come into a box, they must all be counted as a single wire of the largest size used.
When allowances are made from the number of wires permitted in a box [Table 314.16(A)], as when devices, fixture studs, and the like are in the box, those allowances must “be based on the largest conductors entering the box” in any case where the conductors are of different sizes. This principle is true in such cases because the volume allowances equally affect all the conductors, regardless of size. However, when it comes to counting strap allowances, you only need to count based on the basis of the largest conductor arriving on a given strap, not the largest in the box.
Figure 314-9 shows a calculation with different wire sizes in a box. When conduit or EMT is used, there are no internal box clamps and, therefore, no addition for clamps. In this example, the metal raceway is the equipment grounding conductor—so no addition has to be made for one or more ground wires. And the red wire is counted as one wire because it is run through the box without splice or tap. As shown in the wire count under the sketch, the way to account for the space taken up by the wiring devices is to take each one as two wires of the same size as the largest wire attached to the device—that is, 12 AWG—as required in the end of the first sentence of part (A)(2). Note that the neutral pigtail required by 300.3(B) is excluded from the wire count as it would be under 314.16(A)(1).
Fig. 314-9. When wires are of different sizes, volumes from Table 314.16(B) must be used. (Sec. 314.16.)
From Table 314.16(B), each 12 AWG must be provided with 36.9 cm3 (2.25 in.3)—a total of 7 × 36.9 cm3 (2.25 in.3), or 258.3 cm3 (15.75 in.3) for the No. 12s. Then each No. 14 is taken at 32.8 cm3 (2.0 in.3)—a total of 4 × 32.8 cm3 (2.0 in.3), or 131.2 cm3 (8.0 in.3) for both. Adding the two resultant volumes—258.3 + 131.2 (15.75 + 8)—gives a minimum required box volume of 389.5 cm3 (23.75 in.3). From Table 314.16(A), a 100 × 100 × 54 mm 4- by 4-in. square box 2
in. deep, with 497 cm3 (30.3 in.3) interior volume, would satisfy this application.
For the many kinds of tricky control and power wire hookups so commonly encountered today—such as shown in Fig. 314-10—care must be taken to count all sizes of wires and make the proper volume provisions of Table 314.16(B).
Table 314.16(A) gives the maximum number of wires permitted in boxes. But the last sentence of the first paragraph of 314.16(A)(2) does indicate that boxes may contain more wires if their internal volumes are marked and are greater than shown in Table 314.16(A).
Fig. 314-10. Many boxes contain several sizes of wires—some running through, some spliced, and some connected to wiring devices. Calculation of minimum acceptable box size must be carefully made. The combination switch and receptacle here is on a single mounting strap, which is taken as two wires of the size of wires connected to it. (Sec. 314.16.)
Because the volumes in the table are minimums, most manufacturers continue to mark their products with the actual volume. This in many cases is somewhat greater than the volumes shown in the table. The last sentence of 314.16(A)(2) says that boxes that are marked to show a cubic-inch capacity greater than the minimums in the table may have conductor fill calculated in accordance with their actual volume, using the volume per conductor given in Table 314.16(A)(2).
Conduit bodies must be marked with their cubic-inch capacity, and conductor fill is determined on the basis of Table 314.16(C). Such conduit bodies may contain splices or taps. An example of such application is shown in Fig. 314-11. Each of the eight 12 AWG wires that are “counted” as shown at bottom must be provided with at least 36.9 cm3 (2.25 in.3), from Table 314.16(B). The conduit body must, therefore, be marked to show a capacity of not less than 8 × 36.9 cm3 (2.25 in.3), or 295.2 cm3 (18 in.3).
Fig. 314-11. Conduit bodies no longer must have “more than two entries” for conduit bodies to contain splices or taps. (Sec. 314.16.)
The point of the conduit body rule is simple. If you use a conduit body to add an access point to a raceway or change its direction, but do nothing to the enclosed wires, then its function is to be part of the raceway system, and its size is whatever Chap. 9 requires for the raceways. On the other hand if you use a conduit body to make a splice or support a device or a lampholder, then you are using it as a box and you will size it as if it were a box.
This leads to a practical consideration, and that is that it is almost inconceivable that a conduit body will actually work as a box in cases like this. This example, a very modest run of 4 12 AWG wires with two of them branching out at the tee fitting, comes out requiring 295.2 cm3 (18 in.3) in volume. The wires will fit easily in a metric designator 16 (trade size ½) raceway. Casual inspection of manufacturer’s data for conduit bodies strongly suggests that the smallest tee conduit body with this much volume is a metric designator 35 (trade size 1¼) conduit body. As a practical matter, who in their right mind would be so enamored of conduit bodies that they would redesign the raceway upward by three trade sizes just to make this splice? And be careful about suggesting bushing the larger conduit body down to the size of the raceway. That can be done on rigid metal conduit and intermediate metal conduit, but on no other wiring methods, and only when they are actually threaded into the enclosure, and only if the volume is below 1650 cm3 (100 in.3) which is true in this case. If those conditions are not met, such as the wiring method being EMT or PVC, then 314.23(E) will have the tee fitting independently supported to structure, instead of being able to rely on the entering raceways. This job would be done with a box in a heartbeat.
Part (C) of 314.16 contains a number of provisions which must be carefully evaluated. Figure 314-12 shows the first rule. For instance, in that drawing, if a conduit body is connected to metric designator 16 (trade size ½) conduit, the conduit and the conduit body may contain seven 12 AWG TW wires—as indicated in Table C8, App. C, for rigid metal conduit—and the conduit body must have a csa at least equal to 2 × 204 mm2 (0.314 in.2) (the csa of metric designator 16 [trade size ½] conduit), or 408 mm2 (0.628 in.3). That is really a matter for the fitting manufacturers to observe.
Fig. 314-12. For No. 6 and smaller conductors, conduit body must have a csa twice that of largest conduit. (Sec. 314.16.)
The second paragraph of part (C) covers the details shown in Fig. 314-13. The rule requires that where fittings are used as shown in the drawing, they must be supported in a rigid and secure manner. Because 314.23 establishes the correct methods for supporting of boxes and fittings, it must be observed, and that section refers to support by “conduits”—which seems to exclude such use on EMT because the NEC distinguishes between “conduit” and “tubing” (EMT), as in the headline for Table 1 of Chap. 9 in the back of the Code book. Figure 314-14 shows typical applications of those conduit bodies for splicing.
Fig. 314-13. Splices may be made in C and L conduit bodies—if the conditions shown in this illustration are satisfied. (Sec. 314.16.)
Refer to the discussion in the text associated with Fig. 314-11 for a full discussion of how unrealistic it is to expect to actually make use of the splicing allowances for conduit bodies. The only possible way out would be if manufacturers made oversize conduit bodies with much larger internal volumes, and that hasn’t happened in the 15 years since these same rules have been in the NEC. The applications in Fig. 314-14 are indeed common, and they speak to a failure in the industry to enforce the rules. The floodlight needs to go on a box, and the box needs support arranged for it. The connection at the motor should be in the terminal housing, and yes, the LB needs more support; one way is to use a conduit nipple and threaded coupling out of the LB to a straight Greenfield connector. That would allow the support that should have been provided. That said, there are many small devices out there, such as solenoids, which come with short leads, with no splicing enclosure, and located far from any support point. It is unfortunate indeed when conscientious installers are driven to 90.4 in order to provide workmanlike installations in this area of the trade.
Fig. 314-14. Splicing in C or L conduit bodies is common practice. (Sec. 314.16.)
314.17. Conductors Entering Boxes, Conduit Bodies, or Fittings. Part (A) requires “openings” to be adequately closed. Compliance with this rule can be achieved using a properly sized and listed enclosure, as is generally required by 300.15. Part (B) requires cables or raceways to be secured to all metal outlet boxes, conduit bodies, or fittings—such as by threaded connections, connector devices, or internal box clamps. It also addresses the rules for bringing loom into a box safely so the cable clamps don’t damage the wire. The first sentence of part (C) requires that a nonmetallic box must have a temperature rating at least equal to the lowest-temperature-rated conductor entering the box. This rule assumes that the lowest-temperature-rated conductor in a box must be suited to the temperature in the box. The box would, therefore, be properly applied if it has a temperature rating at least equal to that of the lowest-rated conductor. At least¼ in. of the cable sheath must be brought inside the box. Here too, it also addresses the rules for bringing loom into a box safely so the cable clamps don’t damage the wire.
Another very important limitation in this Code section applies to the need for clamping nonmetallic-sheathed cable at a KO where the cable enters anything other than a single gang box. The Code has always accepted the use of nonmetallic-sheathed cable without box clamps or any type of connector where the cable is stapled within 8 in. (200 mm) of the box. The cable is then brought into the box through an NM cable KO on the box, without any kind of a connector at the KO or any clamps in the box (Fig. 314-15). But the intention of the exception to part (C) is that boxes or enclosures other than single gang boxes must be provided with a clamp or connector to secure nonmetallic-sheathed cable to such boxes (Fig. 314-16). Only single gang nonmetallic boxes may be used without a cable clamp at the box KOs. Where the Code permits elimination of a cable clamp if the cable is clamped to the stud within 8 in. (200 mm) of the box, the rule specifies that the 8-in. (200-mm) length be measured along the cable and not simply from the point of the cable strap to the box edge itself. The idea is to reduce the likelihood that pushing the device back into the box after wiring will be able to force the cable assembly back out of the box.
Fig. 314-15. NM cable does not have to be clamped to single gang boxes. (Sec. 314.17.)
Fig. 314-16. NM cable must be clamped to all nonmetallic boxes that are not single gang boxes. (Sec. 314.17.)
When used with open wiring on insulators, knob-and-tube work, or nonmetallic-sheathed cable, nonmetallic boxes have the advantage that an accidental contact between a “hot” wire and the box will not create a hazard.
314.19. Boxes Enclosing Flush Devices. A through-the-wall box is a box which is manufactured to be installed in a partition wall so that a receptacle or switch may be attached to either side; therefore, it is not necessary to use two standard boxes, one facing each side, connected by a jumper. After the devices have been installed on both sides, and the required faceplates secured, the devices are enclosed on all sides and the intent of the rule is satisfied.
If the screws used for attaching the receptacles and switches to boxes were used also for the mounting of boxes, a poor mechanical job would result, since the boxes would be insecurely held whenever the devices were not installed and the screws loosened for adjustment of the device position. Hence the prohibition.
314.20. In Wall or Ceiling. For flush-mounting applications, a box must be flush with a combustible surface or even project forward; if the surface is not combustible the box can be recessed but not more than 6 mm (¼ in.). As used here, the term box includes the components that make up the final enclosure, including extension boxes and plaster rings. If the box is recessed more than is allowed by this rule, a “listed extender” is permitted to make up the difference.
314.21. Repairing Plaster and Drywall or Plasterboard. For boxes using flush covers and recessed in drywall or plaster surfaces, the plaster or drywall must be repaired so that the gap between the box and the surface does not exceed 3 mm ( in.). The spacing allowance provides some relief to installers, because for several generations the requirement was a complete repair. The dimension comes from the fact that when UL evaluates electrical box penetrations in firerated assemblies, they assume this gap will exist around the perimeter of the box being tested.
The purpose of 314.20 and 314.21 is to prevent openings around the edge of the box through which fire could be readily communicated to combustible material in the wall or ceiling. Both 314.20 and 314.21, and their counterparts in Art. 312, address issues related to when ordinary building construction will be allowed to complete an electrical enclosure. 314.20 is looking at the cut surface that is perpendicular to the building surface and extending into the opening and concluding that it is acceptable as long as it isn’t combustible and doesn’t extend more than 6 mm (¼ in.). 314.21 is looking at the plane of the building surface itself and asking, if the enclosure is behind the plane of the finished wall to some extent, how well does the cut-out need to be repaired to prevent a fault in the box from getting past the flush cover and into the wall along the side of the enclosure, and concluding the answer is 3 mm ( in.). 314.21 also concludes (implicitly) that it isn’t necessary to repair the wall edge if the wall is combustible, because in such cases the enclosure must come all the way out, and at that point the flush cover will pretty well seal to the enclosure walls, as extended if necessary. Therefore, it is not required to bring plastic wood filler to jobs in wood paneling.
None of this discussion is relevant to a surface-mounted style enclosure cover that telescopes over the enclosure walls or seats flat on all four sides, such as a raised cover on an outlet box designed for surface mounting. The box being recessed in the wall to a greater or lesser degree doesn’t change the fact that a surface cover will seal to the box and complete the enclosure. If the enclosure is half in the wall and half out, for example, and if the building surface isn’t repaired at all and looks ugly, so be it. If it is a penetration in a firerated partition, the repair will be required by 300.21, but many applications are not and it is not appropriate to write this rule as though a repair should always be required. The electrical enclosure is complete, and if the owner doesn’t like how the wall looks, he can repair it whenever he gets around to it. This is why these rules have been modified in recent code cycles to limit their application to flush-mounting situations where, as noted, the building finish is being called upon to complete an electrical enclosure.
314.22. Exposed Surface Extensions. The extension should be made as illustrated in Fig. 314-17. The extension ring is secured to the original box by two screws passing through ears attached to the box.
As noted in the exception, a surface extension may be made from the cover of a concealed box, if the cover mounting design is secure, the extension wiring method is flexible, and grounding does not depend upon connection between the box and the cover. This exception to the basic rule, which requires use of a box or extension ring over the concealed box, permits a method that provides a high degree of reliability.
Fig. 314-17. Extension ring must be secured to box for surface extension. (Sec. 314-22.)
The wording about the cover not falling off if the securing means loosens may need some explanation. An extension from a blank handy box cover or the like is not a problem because the 6-32 screws would have to come completely out of the box to release the cover. The rule is aimed at extensions from concrete rings or octagon boxes (or round plaster rings) on ceilings that have a usual keyhole-shaped slot for at least one of the 8-32 mounting screws. If those screws loosen, even a little, the cover can turn the required few degrees until the wide end of the slot lines up with the screw head and the cover falls free. To counteract this, some cover designs have a raised detent on the edge of the keyhole slot. The cover cannot turn past this point unless the screw is loosened at least another (roughly) three complete turns, and then the cover is pushed up against gravity and turned the rest of the way. The wording requires this or some other design feature that will keep the cover in place until it is really supposed to come down.
314.23. Supports. The Code rule strictly requires all boxes, conduit bodies, and fittings to be fastened in their installed position—and the various paragraphs of this section cover different conditions of box support for commonly encountered enclosure applications. The one widely accepted exception to that rule—although actually not recognized by the Code—is the so-called throw-away or floating box, which is a junction box used to connect flexible metal conduit from a recessed fixture to flex or BX branch-circuit wiring, in accordance with Sec. 410.117(C) (Fig. 314-18). In such cases, the connection of the fixture “whip” (the 18 in. to 6 ft length of flex with high-temperature wires, e.g., 150°C Type AF) is made to the branch-circuit junction box which hangs down through the ceiling opening, and then the junction box is pushed back out of the way in the ceiling space, and the fixture is raised into position. But with suspended ceilings of lift-out panels, there is no need to leave such a loose box in the ceiling space, because connection can be made to a fixed box before the ceiling tiles are laid in place. It is important to remember that this procedure is functionally obsolete for another reason. Most recessed luminaires are required to have thermal protection. This is arranged through a self-contained connection box mounted as part of the luminaire because the test labs need to know exactly where the device will be located so they can test it. Any high-temperature wiring ends in this box, and is not provided in the field to a remote box.
Figure 314-19 shows the rule of part (B)(1) of this section. Note that since everyone has screw guns these days, the rule now addresses screws where nails would have been used previously. Screws have sharp edges all along their shafts, which can damage wiring pushed blindly into the box. So, if screws are used with exposed threads in the box, some kind of sleeving has to be applied over the raw threads.
An outlet box built into a concrete ceiling, as shown in Fig. 314-20, seldom needs any special support, per 314.23(G). At such an outlet, if it is intended for a fixture of great weight to be safely hung on an ordinary
-in. fixture stud, a special fixture support consisting of a threaded pipe or rod is required, such as is shown in Fig. 314-21.
Fig. 314-18. Fixture supply flex is tapped out of junction box fed by flex or BX branch-circuit wiring in ceiling space. Box is later “thrown away,” unattached, into ceiling space. This is not permitted. (Sec. 314.23.)
Fig. 314-19. Box-mounting nails must not obstruct box interior space. (Sec. 314.23.)
Fig. 314-20. Box in concrete is securely supported. (Sec. 314.23.)
Fig. 314-21. Box in tile arch ceiling requires pipe-hanger support if very heavy lighting fixture is to be attached to the box stud. (314.23.)
In a tile arch floor (Fig. 314-21), a large opening must be cut through the tile to receive the conduit and outlet box.
The requirement of metal or wood supports for boxes applies to concealed work in walls and floors of wood-frame construction and other types of construction having open spaces in which the wiring is installed. In walls or floors of concrete, brick, or tile where conduit and boxes are solidly built into the wall or floor material, special box supports are not usually necessary.
As covered in part (C), in an existing building, boxes may be flush mounted on plaster or any other ceiling or wall finish. Where no structural members are available for support, boxes may be affixed with approved anchors or clamps. Figure 314-22 illustrates that. For cutting metal boxes into existing walls, “Madison Holdits” are used to clamp the box tightly in position in the opening. Actually, the local inspector can determine acceptable methods of securing “cut-in” device boxes because this provision provides appreciable latitude for such decisions.
Fig. 314-22. Part (C) of 314.23 refers to these types of clamping devices. (Sec. 314.23.)
And according to part (D), framing members of suspended ceiling systems may be used to support boxes if the framing members are rigidly supported and securely fastened to each other and to the building structure.
Figure 314-23 shows box-support methods that are covered by part (E) of 314.23 for boxes not over 100 cu in. (1650 cm3) in volume that do not contain devices or support luminaires. Pay close attention because a great number of rules are packed into this paragraph of code text. The rule there applies to “conduit” (rigid metal conduit and IMC only; see exception following for others) used to support boxes—as for overhead conduit runs. A box may be supported by two properly clamped conduit runs (rigid metal conduit or IMC) that are threaded into entries on the box or into field-installed hubs attached to the box. The words “threaded wrenchtight into” purposely exclude a set-screw or compression connector applied to the unthreaded end of a run of conduit. The conduit must thread into the box. If that can’t be done because of problems turning the conduit, then a short nipple and a union can be used, but the conduit system will thread into the box. These words, along with 352.10(H), also exclude PVC conduit as a support. The associated permission to use separate, field-installed hubs offers a relaxation of the previous demand for hubs manufactured with the box, such as FS boxes. This is the only permissible way to support a sheet metal box on the entering raceways: add hubs (the so-called “Myers Hubs”) to the box, and then thread RMC or IMC into the hubs, as before. Where locknut and bushing connections are used instead of threaded conduit connection to a threaded hub or similar connection, the box must be independently fastened in place. The orientation of the conduit entries also matters. They must enter on two or more sides because of the substantial distance allowed to the support point, 900 mm (3 ft). If that support distance is reduced to 450 mm (18 in.), then the conduit entries can be on the same side.
Fig. 314-23. Box may be supported by “conduit” that is clamped, but box must not contain or support anything. (Sec. 314.23.) Note that the vapor-tight luminaire installation, as shown in the center panel, probably does comply with 314.23(F), assuming the conduit supports don’t exceed 450 mm (18 in.) in distance from the box.
The rule as worded would not allow even a conduit body on EMT or PVC to be supported by its raceway entries, and calls for direct support to structure instead. This would be an absurd result and the exception is written to take care of these problems. The exception and the rule are very tightly integrated, and must be read together. The exception begins with a list of wiring methods which include RMC and IMC. Why? Because the rule does not allow for race-way supported enclosures above 1650 cm3 (100 in.3), and many conduit bodies for the larger conduit sizes well exceed this size threshold. However, a conduit body installed under this exception must be no larger than its largest entering raceway. The exception also recognizes the use of “E” fittings that have only a single conduit entry, for obvious reasons.
Note that this does vary from the principle that enclosures must never be supported by a single entry because an enclosure so mounted can easily untwist. “E” fittings are not much larger than the supporting conduit and so there is not much mechanical advantage presented by the fitting in terms of torque in comparison to a FS box or other comparable enclosure.
The question now arises, can a (for example) metric designator 27 (trade size 1) tee fitting be bushed down for all metric designator 21 (trade size¾) entries? This brings up an important rule of reading code text, namely, if you comply with a rule, it does not matter what a permissive exception following the rule happens to say. (Mandatory exceptions are extensions of the rule, must be followed if applicable, and in any list of exceptions must be listed first for this reason.) In this case, if the wiring is RMC or IMC and all the rules regarding support are followed, the installation is perfectly OK because the exception following is permissive. However, if you depart from the rule for any reason, then you have to abide by the terms of the exception.
The rules of part (F) of 314.23 are shown in Fig. 314-24. The basic rules and first exception closely follow the rules in (E), except that the support distance drops to 450 mm (18 in.) which is allowed to be all on one side. The exception does not list EMT or PVC because now there is considerable load on the conduit body. Figure 314-25 shows several installations that are in violation of these rules.
Fig. 314-24. Boxes fed out of the ground or a concrete floor, patio, or walk must observe these rules. (Sec. 314.23.)
Part (F) has a second exception to cover cantilevered applications where luminaires are on the ends of conduit stems, such as some commercial lighting over sidewalks, billboard lighting, etc., as shown in the upper left of Fig. 314-26. This exception and the work in Part (H)(2) below, was developed in conjunction with a task group with representation from the steel conduit makers, and focused in part on how much force typical threaded joints could be expected to withstand. Note that these are rules for field installations of conduit and boxes. If the conduit stem is provided with the luminaire and is covered by the luminaire listing, then these rules do not apply and the luminaire installation directions take precedence.
Fig. 314-25. A single rigid metal conduit may not support a box, even with concrete fill in the ground (left). Box may not be supported on EMT, even with several connections used (center). Method at right is a violation on three counts: EMT, not “conduit,” supports the box; only one hub on box is connected; box is more than 18 in. above ground. (Sec. 314.23.)
Part (H) permits a pendant box (such as one containing a START -STOP button) to be supported from a multiconductor cable, using, say, a strain-relief connector threaded into the hub on the box, or some other satisfactory protection for the conductors. The second part, (H)(2), covers the common practice, especially in industrial occupancies, of hanging HID luminaires from conduit stems, as illustrated in the lower right of Fig. 314-26. These rules also focus on limiting the breaking forces that could be imposed on the threads. Note that the maximum dimension for luminaire horizontal extension from the conduit entry, 300 mm (12 in.), precludes hanging 900 mm (3-ft) or 1.2 m (4 ft) fluorescent luminaires from a single conduit entry support. This is a significant issue with T5 fluorescent luminaire retrofits for previous HID applications.
314.24. Depth of Outlet Boxes. Sufficient space should be provided inside the box so that the wires do not have to be jammed together or against the box, and the box should provide enough of an enclosure so that in case of trouble, burning insulation cannot readily ignite flammable material outside the box. The 2008 NEC expanded the scope of this section considerably in response to proposals that substantiated fire alarm equipment in particular that was causing installation problems in even large boxes. The proposals wanted to recalculate the volume rules in 314.16. That would not have solved the problem because an item with small volume can still have a long reach backward. The solution was to add language here regulating box depth.
Fig. 314-26. Luminaires on conduit stems. [314.23(F) Exception No. 2 and 314.23(H)(2).]
Parts (A) and (B) were in the code previously and cover ceiling pans and the 25.4 mm (1-in.) deep boxes used in some special applications such as for door jamb switches. The fire alarm equipment problem is addressed in (C)(1). Equipment that has been listed for use in certain boxes may, of course, use those boxes.
314.25. Covers and Canopies. This rule requires every outlet box to be covered up—by a cover plate, a fixture canopy, or a faceplate, which has the openings for a receptacle, snap switch, or other device installed in a box. An electric discharge luminaire mounted over an outlet box is permitted to substitute for the cover normally required, but it must comply with 410.24(B) in the process, which means it will be punched to allow access to the interior of the box from the luminaire ballast channel without removing the luminaire from the wall surface.
Part (A) requires all metal faceplates to be grounded as required by 250.110. As noted in the commentary under that topic, the fact that any wiring method with an equipment ground must connect exposed metal surfaces to the equipment grounding conductor has transformed the practical implementation of the overall requirement. Instead of measuring to grounded surfaces, just look for the equipment ground, and make the connection. For faceplates (covered in Fig. 314-27) use a device with a grounding terminal. This rule is frequently violated for no good reason. For a cover a ground clip (Fig. 250-113) will easily attach to the side of a metal cover, creating a tail that is easily connected.
Fig. 314-27. Grounding switch must be used for metal faceplate on any non-metallic box. (Sec. 314.25.)
In part (B) of 314.25, if the ceiling or wall finish is of combustible material, the canopy and box must form a complete enclosure. The chief purpose of this rule is to require that no open space be left between the canopy and the edge of the box where the finish is wood or other combustible material. Where the wall or ceiling finish is plaster, the requirement does not apply, since plaster is not classed as a combustible material; however, the plaster must be continuous up to the box, leaving no opening around the box beyond the 3 mm ( in.) allowed in 314.21.
314.27. Outlet Boxes. Part (A) requires that any box used at luminaire (or lampholder) outlets in a ceiling be designed for this purpose and be capable of supporting a luminaire weighing not less than 23 kg (50 lb). Boxes used for the same purpose in a wall must also be designed for the purpose, but if they are not capable of supporting a 23 kg (50 lb) luminaire, they can be used, and must be marked on the inside of the box with the weight they can support if it’s anything other than 23 kg (50 lb) (some are more). This requirement, put together with the requirement in (B) that outlet boxes “designed for the support of luminaires” and supported per 314.23 is deemed capable of supporting a luminaire up to 23 kg (50 lb), means that any boxes with the marking “FOR FIXTURE SUPPORT” on the unit carton will have the required construction and be suitable for supporting a 23 kg (50 lb) luminaire. There have been some nonmetallic boxes that had been listed for intermediate support weights, such as 6.9 kg (15 lb). Although they were marked, most installers weren’t looking for the marking, believing that if it had the usual 8-32 screws in one of the usual mounting configurations, it was suitable for a 23 kg (50 lb) load. Boxes suitable for higher weights are marked accordingly.
The exception allows other boxes to support luminaires weighing up to 3 kg (6 lb) to be supported on other boxes not designed for luminaire support, such as device boxes or single-gang plaster rings with 6-32 mounting screw provisions. At least two 6-32 screws must be used to support the luminaire, and this exception only applies when the screws are in shear and not in tension, that is, for a wall- and never a ceiling-mounted luminaire. This exception does not apply to smoke detectors, etc.; refer to (E) for utilization equipment coverage.
Part (C) requires floor boxes to be completely suitable for the particular way in which they are used. Adjustable floor boxes and associated service receptacles can be installed in every type of floor construction. A metal cap keeps the assembly clean during pouring of concrete slabs. After the concrete has cured, this cap can then be removed and discarded and floor plates and service fittings added.
There are two types of floor receptacles, the usual ones and ones for the elevated floors of show windows, etc., where they will not receive the same floor traffic, as per the exception. These are classified by UL as “display receptacles.” Although they have a superficial similarity to floor receptacles, floor receptacles are sold with a box and the combination has passed additional tests, including a scrub-water test. Do not confuse the two.
As noted in part (D), a ceiling paddle fan must not be supported from a ceiling outlet box—unless the box is UL-listed as suitable as the sole support means for a fan (Fig. 314-28). The vibration of ceiling fans places severe dynamic loads on the screw attachment points of boxes. But boxes designed and listed for this application pose no safety problems. There are two weight limits in this section. No fan box may serve as the direct support of a paddle fan weighing over 32 kg (70 lb). All fan boxes need to be listed and marked that they are suitable for fan support, and the default weight limit for suitability is a 16 kg (35 lb) fan. Any manufacturer who chooses to design a box for a heavier fan may do so, provided (1) it will not be expected to hold more than 32 kg (70 lb) and (2) the designed fan weight, whatever it is between 16 kg (35 lb) and 32 kg (70 lb), must be marked on the box. A fan box capable of supporting a 16 kg (35 lb) fan does not require a weight marking, because that is the default suitability.
Fig. 314-28. Support for ceiling fans must be suited to the dynamic loading of the vibrating action. [Sec. 314.27(D).]
Part (E) is new for the 2008 NEC and covers utilization equipment mounted on outlet boxes. The rules in 314.27(A) and (B) carry over, but the 3 kg- (6-lb) exception differs in one key respect because there is no orientation limitation; the box can be mounted on the ceiling or on a wall.
314.28. Pull and Junction Boxes. As noted in 314.16, conduit bodies must be sized the same as pull boxes when they contain 4 AWG or larger conductors, if the conductors are required to be insulated. Grounding electrode conductors are routinely installed in conduit for protection, in raceways sized to carry the one conductor. Conduit bodies for the smaller raceways do not meet the dimensional requirements of this section. Since the size rules have primarily to do with protecting the long-term integrity of insulated conductors, the rules need not be applied in these cases.
For raceways containing conductors of 4 AWG or larger size, the NE Code specifies certain minimum dimensions for a pull or junction box installed in a raceway run. These rules also apply to pull and junction boxes in cable runs—but instead of using the cable diameter, the minimum trade size raceway required for the number and size of conductors in the cable must be used in the calculations. Basically, there are two types of pulls—straight pulls and angle pulls. Figure 314-29 covers straight pulls. Figure 314-30 covers angle pulls. In all the cases shown in those illustrations, the depth of the box only has to be sufficient to permit installation of the locknuts and bushings on the largest conduit. And the spacing between adjacent conduit entries is also determined by the diameters of locknuts and bushings—to provide proper installation. Depth is the dimension not shown in the sketches.
According to the rule of part (A)(2), in sizing a pull or junction box for an angle or U pull, if a box wall has more than one row of conduits, “each row shall be calculated separately and the single row that provides the maximum distance shall be used.” Consider the following:
Fig. 314-29. In straight pulls, the length of the box must be not less than 8 times the trade diameter of the largest raceway. (Sec. 314.28.)
A pull box has two rows of conduits entering one side (or wall) of the box for a right-angle pull. What is the minimum required inside distance from the wall with the two rows of conduit entries to the opposite wall of the box?
Row 1: One metric designator 63 (trade size 2½) and one metric designator 27 (trade size 1) conduit.
Row 2: One metric designator 16 (trade size ½), two metric designator 35 (trade size 1¼), one metric designator 41 (trade size 1½), and two metric designator 21 (trade size¾) conduits.
Fig. 314-30. Box size must be calculated for angle pulls. For boxes in which the conductors are pulled at an angle or in a U, the distance between each raceway entry inside the box and the opposite wall of the box must not be less than 6 times the trade diameter of the largest raceway in a row. And the distance must be increased for additional raceway entries by the amount of the maximum sum of the diameters of all other raceway entries in the same row on the same wall of the box. The distance between raceway entries enclosing the same conductors must not be less than 6 times the trade diameter of the larger raceway. (Sec. 314.28.)
Calculate as prescribed by this rule. Note that in accordance with the metrication rule in 314.28(A), these calculations must be done expressing the race-way size using the metric designator (trade size) units as actual dimensions in the system of measurement being employed. Based on the expected market for this book, the calculations going forward will be in the English system of units.
Calculating each row separately and taking the box dimension from the row that gives the maximum distance:
Row 1: 6 × largest raceway (2½ in.) + other entries (1 in.) = 16 in.
Row 2: 6 × the largest raceway (1½ in.) + other entries [ ½ in. + (2 × 1¼ in.) + (2 ×¾ in.)] = 9 in. + ½ in. + 4 in. = 13½ in.
Result: The minimum box dimension must be the 16-in. dimension from run No. 1, which “provides the maximum distance” calculated.
Figure 314-31 shows a more complicated conduit and pull box arrangement, which requires more extensive calculation of the minimum permitted size. In the particular layout shown, the upper metric designator 78 (trade size 3) conduits running straight through the box represent a problem separate from the metric designator 53 (trade size 2) conduit angle pulls. In this case the metric designator 78 (trade size 3) conduit establishes the box length in excess of that required for the metric designator 53 (trade size 2) conduit. After computing the metric designator 78 (trade size 3) requirements, the box size was calculated for the angle pull involving the metric designator 53 (trade size 2) conduit.
Fig. 314-31. A number of calculations are involved when angle and straight pulls are made in different directions and different planes. (Sec. 314.28.)
Subparagraph (3) of 314.28(A) permits smaller pull or junction boxes where such boxes have been listed for and marked with the maximum number and size of conductors and the conduit fills are less than the maximum permitted in Table 1, Chap. 9. This rule provides guidelines for boxes which have been widely used for years, but which have been smaller than the sizes normally required in subparagraphs (1) and (2). These smaller pull boxes must be listed by UL under this rule. The usual application of this provision is not to pull boxes but to conduit bodies marked for a fill that is smaller than the entering raceways could normally carry.
For example, a conduit body might be marked “3 4/0 XHHW MAX.” If you are using three XHHW conductors, then the marking means exactly what it says. Of course that doesn’t happen often. For any other insulation style or number of conductors, take the marking and translate it to a total cross-sectional area of wire fill, using the numbers in Table 5 of Chap. 9 at the end of the Code book. That is your maximum fill, regardless of what Table 4 might say the raceway can hold. Compare that maximum with what you need to install, and decide whether you need to look for a full mogul conduit body or whether you can use the smaller one. In the case of PVC, you will usually be stuck with a reduced size, and the only alternative will be to either increase the raceway size or use a pull box sized to the rules given here.
Figure 314-32 shows how the rules of 314.28(A) apply to conduit bodies. Important: The exception given in 314.28(A)(2) establishes the minimum dimension of L2 for angle runs, but this exception applies only to conduit bodies which have the removable cover opposite one of the entries, such as a Type LB body. Types LR, LL, and LF do not qualify under that exception, and for such conduit bodies the dimension L2 would have to be at least equal to the dimension L1 (i.e., 6 times raceway diameter). Be very careful in shopping for type “LB” conduit bodies that will enclose large conductors in big raceways. Many manufacturers have read 314.28(A)(2) and make so-called mogul conduit bodies that meet the 6 times rule. However only one manufacturer has read the entire rule all the way through, and made his large conduit mogul fittings with extended noses that accommodate Table 312.6(A) dimensions for large conductors. For example, a metric designator 78 (trade size 3) “LB” conduit body expected to carry 3 600 kcmil THW conductors must have 203 mm (8 in.) of distance between the conduit stop on the short end and the inside of the cover, dimension “L2” in Fig. 314-28. There is a “LB” fitting on the market designed exactly this way, but only one. Any other brand, at least as of this writing, violates these rules and should be rejected for larger sized conductors.
Fig. 314-32. Conduit bodies must be sized as pull boxes under these conditions. (Sec. 314.28.)
Figure 314-33 shows the racking of cable required by part (B) of this section.
Fig. 314-33. If a pull box has any dimension over 6 ft (1.8 m), the conductors within it must be supported by suitable racking (arrow) or cabling, as shown here for arcproofed bundles of feeder conductors, to keep the weight of the many conductors off the sheet metal cover that attaches to the bottom of the box. (Sec. 314.28.)
Figure 314-34 shows another consideration in sizing a pull box for angle conduit layouts. A pull box is to be installed to make a right-angle turn in a group of conduits consisting of two metric designator 78 (trade size 3), two metric designator 63 (trade size 2½), and four metric designator 53 (trade size 2) conduits.
Subparagraph (2) of 314.28(A) gives two methods for computing the box dimensions, and both must be met; here again for calculations we will use English units.
Fig. 314-34. Distance between conduits carrying same cables has great impact on overall box size. (Sec. 314.28.)
First method:
Second method:
Assuming that the conduits are to leave the box in the same order in which they enter, the arrangement is shown in Fig. 314-34, and the distance A between the ends of the two conduits must be not less than 6 × 2 in. = 12 in. It can be assumed that this measurement is to be made between the centers of the two conduits. By calculation, or by laying out the corner of the box, it is found that the distance C should be about 8½ in. (203 mm).
The distance B should be not less than 30½ in. (774.7 mm) approximately, as determined by applying practical data for the spacing between centers of conduits,
30½ in. × 8½ in. = 39 in.
In this case the box dimensions are governed by the second method. The largest dimension computed by either of the two methods is of course the one to be used. Of course, if conduit positions for conduits carrying the same cables are transposed—as in Fig. 314-30—then box size can be minimized.
The most practical method of determining the proper size of a pull box is to sketch the box layout with its contained conductors on a paper.
314.28 applies particularly to the pull boxes commonly placed above distribution switchboards and which are often, and with good reason, termed tangle boxes. In such boxes, all conductors of each circuit should be cabled together by securing them with tie-wraps so as to form a self-supporting assembly that can be formed into shape, or the conductors should be supported in an orderly manner on racks, as required by part (B) of 314.28. The conductors should not rest directly on any metalwork inside the box, and insulating bushings should be provided wherever required by 300.4(G).
For example, the box illustrated in Fig. 314-34 could be approximately 125 mm (5 in.) deep and accommodate one horizontal row of conduits. By making it twice as deep, two horizontal rows or twice the number of conduits could be installed.
Insulating racks are usually placed between conductor layers, and space must be allowed for them.
The rule in (C) requires covers to be provided on boxes that are compatible with the box construction and suitable for the conditions of use. The rule concludes by reiterating the grounding requirement of 314.25(A).
314.29. Boxes, Conduit Bodies, and Handhole Enclosures to Be Accessible. This is the rule that prevents boxes from buries in walls or otherwise behind building finish without suitable access to their enclosed conductors. For outdoor applications, buried boxes are permitted under the terms of the exception; these terms are both specific and intentionally restrictive. First the box must be below soil that can be easily shoveled, and second, the location of the box must be “effectively identified.” This might involve a map attached to the panel-board supplying the circuit(s) in the box, or some other way of assuring that those who come afterward don’t need to contend with a junction point that is entirely invisible.
314.30. Handhole Enclosures. A handhole enclosure is defined in Art. 100 as an enclosure for use in underground systems that is sized to allow personnel to reach into, but not enter, for the purpose of installing wiring and maintaining it afterward. They may contain electrical equipment, and they can be made with either an open or closed bottom. Most handholes are made with open bottoms, and are used as pull boxes for underground systems.
They must be identified for use on underground systems, and they must be designed and installed to withstand all loads likely to be imposed. The NEC identifies a standard (ANSI/SCTE 77-2002) that can be used to evaluate loading on these enclosures. This standard identifies a series of tiers that can be specified based on the expectation of vehicular loading. Tier 5 is for pedestrian use with a safety factor for occasional nondeliberate vehicle traffic; it carries a 5000 lb vertical design load rating. Tier 8 is for sidewalk use with a safety factor for nondeliberate vehicular traffic, with a comparable design load rating of 8000 lb. Tier 15 is for parking lots, driveways, and off roadway uses that are subject to occasional nondeliberate heavy vehicular traffic; here the vertical load rating is 15,000 lb.
Handhole enclosures have the same sizing rules related to wire bending space as for manholes. There is an instance where a different set of rules apply, however. If a raceway enters through the bottom plane of a bottomless enclosure, the measurements are taken from then end of the conduit or cable assembly. When such an entry is opposite a removable cover, as it would be in this case, the spacing rules change. For wires operating at 600 V and below, the minimum distance becomes the one-wire-per-terminal distance in Table 312.6(A) for the conductor sizes involved, as previously discussed. For medium-voltage wiring the distance is 12 times the shielded cable diameter and 8 times the diameter for unshielded conductors.
Handhole enclosures are renowned for filling up with sand and water, particularly for the ones without bottoms, which is usually the case. All wiring and splicing provisions must be listed for wet locations for this reason. In addition, if a handhole enclosure has a metal cover, it must be bonded to an equipment grounding conductor (usual case) or to a grounded circuit conductor if the application is on the line side of service equipment. This is a critical safety issue. There have been many fatalities, of both people and their pets, from energized handhole covers.
314.40. Metal Boxes, Conduit Bodies, and Fittings. This section through 314.44 cover construction of boxes. UL data on application of boxes supplement this Code data as follows:
1. Cable clamps in outlet boxes are marked to indicate the one or more types of cables that are suitable for use with that clamp.
2. Box clamps have been tested for securing only one cable per clamp, except that multiple-section clamps may secure one cable under each section of the clamp, with each cable entering the box through a separate KO.
Part (B) covers the thickness of metal for boxes in the usual outlet and device box sizes, and is primarily of interest to manufacturers and testing laboratories. However, it is important to recognize that boxes are not required to be listed, although almost all of these boxes are.
Part (C) of this section covers the pull boxes regulated by 314.28. UL data on such boxes are important and must be related to the Code rules. Listed pull and junction boxes may be sheet metal, cast metal, or nonmetallic, and all of these have a volume greater than 100 cu in. (1650 mm3). These boxes are frequently made in sheet metal shops to accommodate a particular field conditions and the decision not to require listings on these boxes is a very deliberate one in the NEC. Therefore, these construction requirements, together with the applicable UL standard (UL 50), have frequent field applications. Boxes marked “Raintight” or “Rainproof” are tested under a condition simulating exposure to beating rain. Raintight means water will not enter the box. Rainproof means that exposure to beating rain will not interfere with proper operation of the apparatus within the enclosure. Use of a box with either designation must satisfy 314.15, which notes that boxes in wet locations (such as outdoors where exposed to rain or indoors where exposed to water spray) must prevent moisture from entering or accumulating within the box. That is, water may enter the box if it does not accumulate in the box, where the box is drained. A box that is raintight or rainproof may satisfy that rule. Be sure, though, that any equipment installed in a box labeled “rainproof” is mounted within the location restrictions marked in the box. Boxes in wet locations must be fully listed for wet locations.
In part (D) of 314.40, connection provisions for a grounding conductor is required in metal box. This rule is intended to ensure a suitable means within a metal box to connect the equipment grounding conductor that is required to be used with such wiring methods to provide equipment grounding.
314.71. Size of Pull and Junction Boxes. Figure 314-35 shows the rules on sizing of pull boxes for high-voltage circuits. In addition, this part of Art. 314 includes spacing rules for a conduit entry opposite a removable cover that correlates with a similar provision in 314.28(A)(2) Exception. Here the requirement uses the minimum bend radius as given in 300.34.
Fig. 314-35. Minimum dimensions are set for high-voltage pull and junction boxes. For multiple angle pulls, increase the dimension by the size of the additional entries on the same wall. [Sec. 314.71.]
314.72. Construction and Installation Requirements. Part (E) requires that covers of pull and junction boxes for systems operating at over 600 V must be marked with readily visible lettering at least ½ in. (12.7 mm) high, warning “DANGER HIGH VOLTAGE KEEP OUT.”
All required warning signs must be properly worded to include the command “KEEP OUT.” While certain sections of the Code, such as this one, as well as 110.34(C), clearly require the inclusion of the command “KEEP OUT,” be aware that courts have held that warning signs that fail to include some sort of instruction or command with respect to an appropriate action that must be taken are inadequate and constitute negligence on the part of the individual posting the sign. Always include some phrase that will tell the individual what to do about the condition or hazard that exists.
320.2. Definition. Armored cable (Type AC) contains insulated conductors of a type accepted for general wiring applications in the NEC. The conductors are enclosed in an armor comprised of steel or aluminum interlocking tape. The armor is arranged with an internal bonding strip of aluminum or copper “in intimate contact with the armor for its entire length.” Type AC cable, which is commonly called BX, has largely been supplanted in the market by the interlocking-armor style of metal-clad cable. However, certain applications, especially patient care areas of health care facilities, require a wiring method where the outer margin of the wiring method, whether raceway or cable assembly, qualify as an equipment grounding return path, and Type AC cable inherently qualifies for this use. A recent addition to this list is the 210.12(B) Exceptions, which specifically enumerate the steel version of this cable as the only acceptable cabled wiring method acceptable under those provisions (Fig. 320-1).
Fig. 320-1. Type AC cable contains insulated conductors plus bonding conductor under the armor. (Sec. 320.2.)
Armored cable assemblies of 2, 3, 4, or more conductors in sizes No. 14 AWG to No. 1 AWG—such conductors may even incorporate an optical-fiber cable—conform to the standards of the Underwriters Laboratories. These standards cover multiple-conductor armored cables for use in accordance with the National Electrical Code, in wiring systems of 600 V or less, with conductors having insulation rated for temperatures of 90°C, as called for by 320.80 where the cable is embedded in thermal insulation.
320.10. Uses Permitted. Type AC armored cable can be used in all types of electrical systems for power and light branch circuits and feeders. Figure 320-2 shows use of three runs of 12/2 BX for the supply and two switch legs to a combination light-heat-fan unit in a bathroom. One 12/2 is the supply and the other cables control the appliance as shown in the wiring diagram. But the use of two 12/2 cables for the switch legs violates 300.20 because the neutral is not kept with all the conductors it serves. As a result, induction heating could be produced. A single run of 12/4 cable to the appliance would be necessary.
Fig. 320-2. Cable runs of 12/2 BX are used at junction box (above) which was then equipped with switches and pilot light (right) for light-heat-fan unit. Use of two 12/2 cables, with neutral in only one cable, is a violation of the concept covered in Sec. 300.20. A 12/4 cable could serve for all switch legs and satisfy the Code rule. (Sec. 320.10.)
As stated in the note following this section, the list of items in this “uses permitted” section is not intended to completely enumerate all permitted uses. These sections must be read in conjunction with “uses not permitted” section that follows to zero in on where particular issues lie that the NEC wants to address. For example, it is clear that this cable can be used embedded in a plaster finish on masonry construction, but only where those locations are classified as dry. It can also run in hollow block voids, but only if not subject to excessive moisture or dampness, which could well be all wet locations but probably not all damp locations; the local inspector would need to determine that in the field. Figure 320-3 shows an example of a permitted hazardous location use of the cable, which is also not (and need not be) itemized on the list.
320.15. Exposed Work. This section requires cables to “closely follow the surface of the building” where unexposed. Of course, this is to be correlated with the rules in 320.30. That is, where needed for flexibility [part (2)], or where used as fixture whips [part (3)] of 320.30, the requirement to “closely follow” the contour of the wall space is waived. In addition, use of joist bottoms is permitted where not subject to physical damage.
Fig. 320-3. The FPN following 320.10 indicates the list is not all inclusive and 504.20 recognizes BX cable for limited use in hazardous locations. (Sec. 320.10.)
320.23. In Accessible Attics. These rules also apply to Type NM cable installations because 334.23 specifically incorporates them by reference. Part (A) applies to all attic and roof spaces accessible by a permanent ladder or stair, and to other accessible attic locations within 1.8 m (6 ft) of the outside edges of the scuttle hole. Any cables run on top of floor joists, or across rafters or studs within 2.1 m (7 ft) of the floor or the tops of the floor joists must be protected by guard strips at least as high as the cable diameter. Cables run on and parallel to the sides of framing members do not require guard strips but must meet the spacing rules in 300.4(D).
320.30. Support. Armored cable must be secured and supported by approved staples, straps, or similar fittings, as shown in Fig. 320-4. Note that a requirement to secure a cable is more restrictive than a requirement to support the cable. For example, Type AC cable must be supported at intervals, a condition that can be met by routing it through successive holes in framing member, and it must be secured at terminations, a condition that requires preventing all movement such as by using a staple.
In exposed work, both as a precaution against physical damage and to ensure a workmanlike appearance, fastenings should be spaced not more than 600 to 750 mm (24 to 30 in.) apart. In concealed work in new buildings, the cable must be supported at intervals of not over 1.4 m (4½ ft) for Type AC to keep it out of the way of possible injury by mechanics of other trades. In either exposed work or concealed work, the cable should be securely fastened in place within 300 mm (1 ft) of each outlet box or fitting so that there will be no tendency for the cable to pull away from the box connector. Take care to observe the minimum bend radius for this cable, 5 times its diameter measured to the inner edge of the bend, as given in 320.24. Sharper bends can and often will break the convolutions, creating a hazardous condition.
Part (2) of 320.30(D) limits Type AC cable to not over a 600-mm (2-ft) unclamped length for flexibility where such a cable feeds motorized equipment (such as a fan or unit heater) or connects to any enclosure or equipment where the flexibility of the AC length will isolate and suppress vibrations. Similarly, part (3) of this section permits lengths of AC up to 1.8 m (6 ft) long to be used without any staples, clamps, or other support where used in a hung ceiling as a lighting fixture whip or similar whip to other equipment (Fig. 320-5). This permits use of AC in the same manner as permitted for flexible metal conduit (Greenfield) or liquidtight flexible metal conduit in lengths from 0.45 to 1.8 m (1½ to 6 ft) as a connection from a circuit outlet box to a recessed lighting fixture [410.117(C)]. This use of unclamped BX is an exception to the basic rule that it be clamped every 1.4 m (4½ ft) and within 300 mm (12 in.) of any outlet box or fitting. Part (1) recognizes the acceptability of fishing in Type AC cables, for which additional securing and supporting is not required.
Fig. 320-4. BX must be clamped every 4½ ft (1.4 m) and within 12 in. (300 mm) of terminations. (Sec. 320.30.)
Fig. 320-5. Armored cable (Type AC) may be used for 6-ft (1.8-m) fixture whips, without supports, in an “accessible ceiling.” (Sec. 320.30.)
Note that the requirements on clamping or securing of Type AC must be observed for applications in suspended-ceiling spaces, whether for air handling, as covered in 300.22(C), or non-air handling. The wording of part (C) in 320.30 recognizes that in horizontal runs the hole in the framing member is to be considered as satisfying the support requirements given in this section.
320.40. Boxes and Fittings. Note that a termination fitting—that is, a box connector—must be used at every end of Type AC cable entering an enclosure or a box (Fig. 320-6) unless the box has an approved built-in clamp to hold the cable armor, provide for the bonding of the armor to the metal box, and protect the wires in the cable from abrasion.
Fig. 320-6. Connectors for BX entering a panelboard cabinet or other enclosure must use approved fittings—some type of single-connector or duplex type (as shown, with two cables terminated at each connector through a single KO). (Sec. 320.40.)
A standard type of box connector for securing the cable to knockouts or other openings in outlet boxes and cabinets is shown in Fig. 320-7. A plastic bushing, as shown, must be inserted between the armor and the conductors. The plastic bushing, which can be seen through slots in the connector after installation, prevents the sharp edges of the armor from cutting into the insulation on the conductors and so grounding the copper wire.
Fig. 320-7. Every BX termination must be equipped with a protective bushing and a box connector or clamp built into the box. (Sec. 320.40.)
The box shown in Fig. 320-8 is equipped with clamps to secure Type AC cables, making it unnecessary to use separate box connectors. The other box shown is similar but has the cable clamps outside, thus permitting one more conductor in the box. See 314.16(A)(1).
Fig. 320-8. A box connector fitting is not required if box includes cable clamps for Type AC cable. (Sec. 320.40.)
As covered in the last sentence of 320.40, “a box, fitting, or conduit body”—such as a C conduit body—must be used where Type AC cable is connected to another wiring method. Figure 320-9 shows a typical application of this technique, in accordance with 300.15(F).
Fig. 320-9. This connection of BX to conduit or other cable is specifically recognized. Where there is no splice, a “from-to” connector may be used. (Sec. 320.40.)
320.80. Ampacity. This section makes it clear that the current-carrying capacity of Type AC cable must be established in accordance with the rules of 310.15. The last part of this rule places an additional limitation on the use of Type AC run in thermal insulation. As is the case with Types NM, NMB, and NMC, Type AC cable must be provided by the manufacturer with 90°C-rated insulation, but it must be loaded to no more than the 60°C value shown in Table 310.16. Although it is permissible to use the 90°C current value shown in Table 310.16 for the purposes of derating, the actual load carried must be no greater than the current value shown for the particular size and conductor material.
The wording of this section actually understates the true problem for cables run in thermal insulation. By allowing derating to occur from the 90°C column for such cables, the NEC allows users to ignore factors that increase the retention of heat in the cable until they reach the point of reducing the ampacity below the 60°C value, which in most cases is to allow them to be ignored altogether. Meanwhile, particularly for large, heavily loaded cables, even the 60°C numbers are too high. Review the discussion associated with 310.10 for detailed information on this topic.
320.100. Construction. Note that Type AC cable is recognized for branch circuits and feeders, but not for service-entrance conductors, which must be one of the cables or wiring methods specified in 230.43. Type MC (metal-clad) cable, such as interlocked armor cable or the other cables covered in Art. 334, is recognized by 230.43 for use as service-entrance conductors.
Because the armor of Type AC cable is recognized as an equipment grounding conductor by 250.118(9), its effectiveness must be ensured by using an “internal bonding strip,” or conductor, under the armor and shorting the turns of the steel jacket. The ohmic resistance of finished armor, including the bonding conductor that is required to be furnished as a part of all, except lead-covered armored cable, must be within values specified by UL and checked during manufacturing. The bonding conductor run within the armor of the cable assembly is required by the UL standard.
Because the function of the bonding conductor in Type AC cable is simply to short adjacent turns of the spiral-wrapped armor, there is no need to make any connection of the bonding conductor at cable ends in enclosures or equipment. The conductor may simply be cut off at the armor end.
Construction of armored cable must permit ready insertion of an insulating bushing or equivalent protection between the conductors and the armor at each termination of the armor—such as the so-called red head.
320.104. Conductors. As required by the second paragraph in 320.80, armored cable (BX) installed within thermal insulation must have 90°C-rated conductors (Types THHN, RHH, XHHW), but the ampacity must be taken as that of 60°- C-rated conductors. This requirement recognizes that the heat rise on conductors operating with reduced heat-dissipating ability (such as those surrounded by fiberglass or similar thermal insulation) requires that the conductors have a 90°C-rated insulation. That temperature might be reached even with the wires carrying only 60°C ampacities. Although the wires must have 90°C insulation, they must not be loaded over those ampacity values permitted for TW (60°C), as shown in Table 310.16.
320.108. Equipment Grounding. This section requires that Type AC cable provide “an adequate path” for fault current. This is accomplished by the No. 16 aluminum bonding strip that runs the length of the cable’s sheath. This bonding strip shorts out the high impedance of the coiled metal jacket and provides a UL-listed ground path.
322.2. Definition. Type FC cable is a flat assembly with three or four parallel 10 AWG special stranded copper conductors. The assembly is installed in an approved U-channel surface metal raceway with one side open. Then tap devices can be inserted anywhere along the run. Connections from tap devices to the flat cable assembly are made by pin-type contacts when the tap devices are fastened in place. The pin-type contacts penetrate the insulation of the cable assembly and contact the multistranded conductors in a matched phase sequence (phase 1 to neutral, phase 2 to neutral, and phase 3 to neutral).
Covers are required when the installation is less than 8 ft from the floor. The maximum branch-circuit rating is 30 A.
Figure 322-1 shows the basic components of this wiring method.
Fig. 322-1. Type FC wiring system uses cable in channel, with tap devices to loads. (Sec. 322.2.)
322.10. Uses Permitted. Figure 322-2 shows a Type FC installation supplying lighting fixtures. As shown in the details, one tap device provides for circuit tap-off to splice to cord wires in the junction box; the other device is simply a fitting to support the fixture from the lips of the channel.
Fig. 322-2. Limited application of Type FC cable system includes use as branch-circuit wiring method to supply luminaries. (Sec. 322.10.)
324.1. Scope. This article covers design and installation regulations on a branch-circuit wiring system that supplies floor outlets in office areas and other commercial and institutional interiors. (See Fig. 324-1.) The method may be used for new buildings or for modernization or expansion in existing interiors. FCC wiring may be used on any hard, sound, smooth floor surface—concrete, wood, ceramic, and so forth. The great flexibility and ease of installation of this surface-mounted flat-cable wiring system meet the need that arises from the fact that the average floor power outlet in an office area is relocated every 2 years.
Undercarpet wiring to floor outlets eliminates any need for core drilling of concrete floors—avoiding noise, water dripping, falling debris, and disruption of normal activities in an office area. Alterations or additions to Type FCC circuit runs are neat, clean, and simple and may be done during office working hours—not requiring the overtime labor rates incurred by floor drilling, which must be done at night or on weekends. The FCC method eliminates use of conduit or cable, along with the need to fish conductors.
Type FCC wiring offers versatile supply to floor outlets for power and communication—at any location on the floor. The flat cable is inconspicuous under the carpet squares. Elimination of floor penetrations maintains the fire integrity of the floor, as required by 300.21.
Fig. 324-1. Flat conductor cable (FCC) supplies terminal base for floor-outlet pedestal at exact location required for desk in office area. FCC cable is taped in position over an insulating bedding tape and then covered with a flat steel tape (not yet installed here) to protect the three conductors (hot leg, neutral, and equipment grounding conductor) in the flat cable. Carpet squares are used to cover the finished cable runs.
A typical system might use separate flat-cable circuit layouts for 120-V power to floor-pedestal receptacles, telephone circuits, and data communications lines for CRT displays and computer units. For 120-V power, the flat cable contains three flat, color-coded (black, white, and green), 12 AWG copper conductors for 20-A circuits—one hot conductor, one neutral, and one equipment grounding conductor. Telephone circuits use flat, 3-pair, 26 AWG gauge conductors. And data connection circuits use flat RG62A/U coaxial cable that is only 0.09 in. (2.25 mm) high.
324.2. Definitions. The various components of a Type FCC system are described here. Figure 324-2 shows typical components of an FCC system.
324.10. Uses Permitted. This section describes the acceptable uses for Type FCC. Part (A) recognizes the use of Type FCC for both general-use branch circuits and individual branch circuits, while (B)(1) and (2) regulate the maximum voltage rating (not more than 300 V between conductors) and current rating (not more than 20 A for general-purpose branch circuits, or 30 A for individual branch circuits) permitted for Type FCC.
324.18. Crossings. Not more than two FCC cable runs may be crossed over each other at any one point. To prevent lumping under the floor carpets, this rule permits no more than two Type FCC cables to be crossed over each other at a single point. This applies to FCC power cable and FCC communications and data cables.
Fig. 324-2. Typical components of a Type FCC system: (a) Bottom shield in place. (b) Connecting the conductor. (c) Coil of top shield.
In 324.41, “Floor Coverings,” the Code restricts the maximum size of carpet squares used to cover the Type FCC. Carpet squares used to cover Type FCC wiring must not be larger than 914 by 914 mm (36 by 36 in.). This rule eliminates questions that arose about the possibility of using single “squares” of broadloom carpet large enough to cover a floor from wall to wall. In addition, the carpet squares must be put down with a release-type adhesive.
In making an undercarpet installation, usual thinking would dictate installation of the cable layout first and then placement of the floor covering of carpet squares over the entire area. But some installers have found it easier and less expensive to first cover the entire floor area with the self-adhesive carpet squares and then plan the circuit layouts to keep the cable runs along the centerlines of carpet squares and away from the edges of the squares. After the layout is determined, it is a simple matter to lift only those carpet squares along the route of each run, install the cable and pedestal bases, and replace the self-stick carpet squares to restore the overall floor covering. That approach has proved effective and keeps carpet cutting to the middle of any square.
In 324.60, the Code calls for grounding of metallic shields—commonly employed in Type FCC assemblies to prevent interference on communication circuits. This rule further stipulates that those connectors be specifically designed for connecting the metallic shield to ground.
326.2. Definition. Type IGS cable is a “factory assembly of one or more conductors, each individually insulated and enclosed in a loose-fit nonmetallic flexible conduit.” The cable is for underground use—including direct earth burial—for service conductors, feeders, or branch circuits. It must not, however, be used as interior wiring or exposed in contact with any building. Sizes available for this product run are metric designators 53, 78, and 103 (trade sizes 2, 3, and 4). The introduction of this cable to the NEC was recommended on the following basis:
Underground cable costs are increasing at a high rate. A need exists for lower material costs and reduced cost for installation. Failures on underground cables are increasing, particularly direct-burial types.
The new cable system overcomes all the above problems. The new cable system has the advantage of low first cost for materials and low installation cost. It eliminates the need for field pulling of cables into conduits and eliminates the cost of assembly of conduit in the field. The new system may be directly buried, plowed in, or bored in for further savings. It is a cable and conduit system.
A tough natural-gas-approved pipe is used as the conduit. When it is pressurized, it will withstand much abuse. The gas pressure keeps out moisture and serves to monitor the cable for damage by insects or mechanical damage that can lead to future failure. The gas pressure can even be attached to an alarm to sound a loss of pressure or to trip a CB for hazardous locations. However, a loss of pressure in the cable will not cause it to fail. Even on dig-ins, the gas serves to warn the digger. The gas prevents combustion and burning on cable failure. The SF 6 gas is nontoxic, odorless, tasteless, and will not support combustion. It acts to put out a fire.
UL has tested a 3/C 250 MCM [kcmil] Type IGS-EC 600-V cable in 2-in. conduit. The UL test at zero gauge pressure shows a breakdown voltage between conductors of 14,000 V after numerous short-circuit, breakdown, and humidity tests. When the cable is single conductor, the breakdown voltage is even higher on loss of pressure, as the polyethylene pipe or conduit provides additional insulating value.
An award-winning installation was made in 1979 at 5 kV. Three installations of 3/C 250 MCM [kcmil] Type IGS-EC cable have been made for residential underground service entrances in Oakland, California. The first was made in May of 1979 and all have been successful.
326.80. Ampacity. This section indicates that Type IGS cable must have its ampacity determined in accordance with Table 326.80 for either single or multiconductor cable.
328.2. Definition. This is a very limited definition of a Code designation—Type MV. The description of this cable type is amplified in the Electrical Construction Materials Directory of the Underwriters Laboratories, as follows:
Medium-Voltage Cable (PITY)
Medium-voltage cables are rated 2400 to 35,000 volts.
They are single or multiconductor, aluminum or copper, with solid extruded dielectric insulation and may have an extruded jacket, metallic covering, or combination of both over the single conductors or over the assembled conductors in a multiconductor power cable.
All insulated conductors rated higher than 2400 volts have electrostatic shielding. Cable rated 2400 volts is nonshielded.
Nonshielded cables are intended for use where conditions of maintenance and supervision ensure that only competent individuals service and have access to the installation.
Shielded cable is marked either “MV-90” or “MV-105” and is suitable for use in wet or dry locations at 90 or 105°C.
Nonshielded cables is marked either “MV-90” indicating suitability for use in wet or dry locations at 90°C maximum or “MV-90” Dry Locations Only” indicating suitability for use only in dry locations at 90°C maximum.
Cables marked “oil resistant I” or “oil resistant II” are suitable for exposure to mineral oil at 60 or 75°C, respectively.
Cables marked “sunlight resistant” may be exposed to the direct rays of the sun.
Cables intended for installation in cable trays in accordance with Article 392 of the National Electrical Code are marked “For Use in Cable Trays” (or “For CT Use”).”
Cables with aluminum conductors are marked with the word “aluminum” or the letters “AL.”
Cables are marked with their conductor size, voltage rating, and insulation level (100% or 133%).
The basic standard used to investigate products in this category is UL1072, “Medium-Voltage Power Cables.”
The Listing Mark of Underwriters Laboratories Inc. on the product is the only method provided by UL to identify products manufactured under its Listing and Follow-Up Service. The Listing Mark for these products includes the name and/or symbol of Underwriters Laboratories Inc. (as illustrated in the Introduction of this Directory) together with the word “LISTED,” a control number and the following product name: “Medium-Voltage Cable.”
Medium-Voltage Cable, Classified in Accordance with UL 1072, with Metric Conductor Sizes (PIVW)
This category covers medium-voltage cables rates 2001 to 35,000 volts and in conductor sizes 10 through 500 sq mm.
The cable complies with all requirements specified in UL 1072 “Medium-Voltage Power Cables,” except that metric conductor sizes are used instead of AWG sizes. The cable is for use in jurisdictions where metric conductor sizes are required or permitted.
The cable is single or multiconductor, aluminum or copper, with solid extruded dielectric insulation. An extruded jacket, metallic covering, or combination of both may be provided over single conductors or over the assembled conductors in a multi-conductor power cable.
All insulated conductors rated 8001 volts and higher have electrostatic shielding. Cables rated 2001 to 8000 volts may be shielded or nonshielded.
Nonshielded cables are intended for use where conditions of maintenance and supervision ensure that only competent individuals service and have access to the installation.
Shielded cable is marked “MV-90” or “MV-105” and is suitable for use in wet or dry locations at 90 or 105°C.
Nonshielded cable is marked either “MV-90” indicating suitability for use in wet or dry locations at 90°C maximum, or “MV-90 Dry Locations Only.”
Cable marked “oil resistant I” or “oil resistant II” are suitable for exposure to mineral oil at 60 or 75°C, respectively.
Cable marked “sunlight resistant” may be exposed to the direct rays of the sun. Cable intended for installation in cable trays is marked “For CT Use” or “For Use In Cable Trays.”
Cables with aluminum conductors are marked with the word “Aluminum” or the letters “AL.”
Cables are marked with conductor size in sq mm, voltage rating and insulation level (100% or 133%).
The basic standard used to investigate products in this category is UL1072, “Medium-Voltage Power Cables.”
The Classification Mark of Underwriters Laboratories Inc. on the product, the attached tag, the reel, or the smallest unit container in which the product is packaged is the only method provided by UL to identify these products manufactured under its Classification and Follow-Up Service. The Classification Marking for these products shall only be as illustrated below:
Medium-Voltage Cable Classified by Underwriters Laboratories Inc® in accordance with UL 1072, with Metric Conductor Sizes Control No.
328.10. Uses Permitted. Because the Code has an article and cable designation (Type MV) for cables operating above 2000 V up to 35,000 V, it may be expected that electrical inspection authorities will insist that all cables in that voltage range must be Type MV to satisfy the NE Code.
Great care should be exercised in determining the attitude of local inspection authorities toward the meaning of this article. In particular, the relationship of 110.8 to Art. 328 should be determined. 110.8 states that “only wiring methods recognized as suitable are included in this Code.” The question to be answered is: Will electrical inspection agencies require all medium-voltage conductors to be Type MV? Or will inspection agencies accept medium-voltage conductors not specifically designated Type MV? In other words, because the Code has an accepted type of high-voltage cable, will it be permissible to use high-voltage cables that are not of this accepted type? The answer seems to be no. This is especially true when the OSHA insistence on listed equipment is considered. In addition, for circuits in common use up to 600 V, 110.8 has consistently been interpreted to require that any conductor or cable must be one of the types specifically designated in the Code—Table 310.13 or elsewhere in Arts. 300 to 398. That is, conductors must be Type TW, THW, or one of the other designated types, and cable must be Type AC, NM, MI, MC, or other designated cable. It would be a Code violation to use any non–Code-designated wire or cable for systems up to 2000 V. It would, therefore, seem to be similarly contrary to Code to use a non–Code-designated cable for higher-voltage circuits inasmuch as there is a Code-designated type (Type MV) for such applications.
Refer to Table 310.13(B) on Type MV conductors and to 392.13 for use of Type MV cables in tray.
330.2. Definition. This article covers “Metal-Clad Cable,” as listed by UL under that heading in the Electrical Construction Materials Directory (the Green Book). This section defines the type of cable assemblies covered by this article (Fig. 330-1). The definition for metal-clad cable—“a factory assembly of one or more insulated circuit conductors with or without optical fiber members enclosed in an armor of interlocking metal tape, or a smooth or corrugated tube”—makes this category the successor to former Type ALS and Type CS cables.
Aluminum-sheathed (ALS) cable had insulated conductors with color-coded coverings, cable fillers, and overall wrap of Mylar tape—all in an impervious, continuous, closely fitting, seamless tube of aluminum. It was used for both exposed and concealed work in dry or wet locations, with approved fittings. CS cable was very similar, with a copper exterior sheath instead of aluminum.
Because the rules of these three cable types have been compiled into a single article, use of any one of the Type MC cables must be evaluated against the specific rules that now generally apply to all such cables. The Code no longer contains the designations Type ALS and Type CS. They are included now as either smooth or corrugated styles as applicable along with interlocked armored cable as Type MC cables.
Type MC is rated by UL for use up to 2000 V. Cable rated 2400 to 35,000 V is listed as Type MV. Type MC cable is recognized in three basic armor designs: (1) interlocked metal tape, (2) corrugated, or (3) smooth metallic sheath.
Fig. 330-1. These are some of the constructions in which Type MC cable is available. (Sec. 330.2.)
330.10. Uses Permitted. Although this section clearly lists all the permitted applications of any of the various forms of Type MC cable, care must be taken to distinguish between the different constructions, based on the Code rules (Fig. 330-2). For a long time, the interlocked-armor Type MC and the corrugated-sheath Type MC have been designated by UL as “intended for aboveground use.” But part (5) of this section recognizes Type MC cable as suitable for direct burial in the earth, when it is identified for such use.
Note that item (11) includes the specific characteristics that must be met in order to qualify a particular cable style as suitable for wet locations. Item (12) recognizes single-conductor Type MC cable; care must be taken in such cases to track inductive heating issues 300.21(B), special ampacity calculations [330.80(B)], and termination issues [110.14(C)].
330.12. Uses Not Permitted. Type MC cables are permitted by 330.10 to be used exposed or concealed in dry or wet locations. But such cable must not be subjected to destructive, corrosive conditions—such as direct burial in the earth, in concrete, or exposed to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or of hydrochloric acids, unless protected by materials suitable for the condition.
Fig. 330-2. ALS (aluminum-sheathed) Type MC cable was used for extensive power and light wiring in refrigerated rooms and storage areas of a store. The ALS was surface mounted (exposed) on clamps in this damp location. Because the cable assembly is a tight grouping of conductors within the sheath, there would be no passage of warm air from adjacent nonrefrigerated areas through the cable which crosses the boundaries between the areas. It was therefore not necessary to seal the cables to satisfy Sec. 300.7(A). (Sec. 330.10.)
330.24. Bending Radius. Figure 330-3 shows the bending-radius rules for the “smooth-sheath” Type MC cables. Cable with interlocked or corrugated armor must have a bending radius not less than 7 times the outside diameter of the cable armor. To conform to ICEA rules on bending radius for shielded conductors in MC cable, the minimum value must be either 12 times the diameter of one of the conductors within the cable or 7 times the diameter of the MC cable itself, whichever is greater. For medium-voltage applications with shielded cables, the minimum bending radius is 12 times the cable diameter for a single-conductor makeup, and 7 times the overall diameter of a multiconductor makeup, as applicable.
Fig. 330-3. Minimum radius values prevent excessively sharp, destructive bending of ALS or CS cable. (Sec. 330.24.)
330.30. Securing and Supporting. Figure 330-4 shows the maximum permitted spacing of supports for the larger-sized Type MC cable. The interlocked-armor Type MC has commonly been used on cable tray, as permitted in part (A)(6) of 330.10 if it is identified for this use (Fig. 330-5). In addition to the 1.8 m (6-ft) interval, smaller cables—those with four or less conductors smaller than No. 8—must also be secured within 300 mm (12 in.) of “each box, cabinet, fitting, or other cable termination.” In addition, part (C) recognizes the common practice of entraining cables through successive holes in framing members.
Fig. 330-4. Surface mounting of Type MC cable must be secured. (Sec. 330.30.)
330.40. Boxes and Fittings. Only approved, UL-listed connectors and fittings are permitted to be used with any Type MC cable. Such fittings are listed in the UL Green Book under “Metal-Clad Cable Connectors.” Figure 330-6 shows typical approved connectors for interlocked-armor Type MC cable. As shown at left, 600-V terminations for interlocked-armor cable to switchgear or other enclosures in dry locations can be made with connectors, a locknut, and a bushing in the typical basic assembly shown. In damp locations, compound-filled or other protective terminations may be desired. Medium-voltage connectors (5 through 35 kV) are generally filled with sealing compound, and individual conductors are terminated in a suitable manner, depending upon whether the conductors are shielded. Or the IA cable may terminate in a pot-head for positively sealed and insulated terminations indoors or outdoors.
Fig. 330-5. Any Type MC cable is recognized for use in cable tray, and the interlocked-armor version has been widely used in tray, as shown here. (Sec. 330.30.)
Fig. 330-6. Terminations for interlocked-armor cables must be approved devices, correctly installed. (Sec. 330.40.)
Type MC cable must terminate in connectors designed for the particular cable involved. Pay particular attention to differences between MC and AC cable terminations, for example. The NEC doesn’t require anti-short bushings for Type MC cable, although one leading manufacturer does make them available for that purpose, and many contractors choose to use them anyway. They are not required because the throat designs of listed connectors keep the conductors away from the cut edges of the armor. In addition, since these connectors may have to handle ground-fault currents, they are tested with their designed cable types for this duty. Those tests have no validity beyond the cable types actually tested.
Before you run MC (or any other cable), look closely at the box or shipping carton for the connectors. Particularly in the case of Type MC cable, you’ll find very specific size ranges given. For example, the box might indicate suitability for smooth Type MC in a particular range of cable diameters, corrugated in a range of diameters, and interlocking with a range of conductor configurations. If you don’t find your particular application, select a different connector. The same principle applies to internal box connectors. Look at the shipping carton for the box, because the label (for a listed box) will always tell you what cables the internal clamps are designed for.
330.108. Equipment Grounding Conductor. Generally the smooth and corrugated types of MC cable have sufficient cross-sectional area of metal in their armors such that the armor itself qualifies as the equipment grounding conductor, although not always; if additional grounding conductors are incorporated, they must be used as part of the equipment grounding system, right along with the cable armor. Interlocking metal tape, without modification, is never suitable for equipment grounding because the spiral path adds too much impedance. Type AC cable solves this problem with the bonding tape and the paper filler, because the tape is held in firm contact with the armor, and as such it shorts the convolutions. Traditionally, Type MC with interlocking armor always carried an additional equipment grounding conductor, usually with green insulation and sized per 250.122. This conductor had to terminate in boxes and it counted for a required fill allowance in 314.16.
Recently one manufacturer solved the grounding continuity problem across interlocking MC cable armor by using a bare, fully sized aluminum equipment grounding conductor against the armor and on the outside of the plastic wrap that goes over the circuit conductors. This product can be used similarly to Type AC cable, without a separate grounding conductor entering the box. In addition, for instances where the connector and locknut cannot be relied upon for grounding continuity, such as for 480Y/277 system circuits running into cabinets with concentric knockouts, the bare wire can be left as long as the circuit conductors and brought to an equipment grounding terminal in the enclosure. Even Type AC cable isn’t capable of that trick because the little aluminum bonding strip does not qualify as an equipment grounding conductor. Here again, make certain to correlate the listing information for the connectors with the type of MC cable being used.
332.2. Definition. The data from the UL Green Book expand on the definition (Fig. 332-1) and cover application notes as follows.
Mineral-insulated metal-sheathed cable is labeled in a single-conductor construction from 16 AWG through 500 kcmil single conductor, two- and three-conductor from 16 AWG through No. 4 AWG, four-conductor from No. 16 AWG through No. 6 AWG, and seven-conductor Nos. 16, 14, 12, and 10 AWG. The exterior sheath may be of copper or alloy steel. There is also a signaling circuit style available with a 300 V rating and configured for 16 and 18 AWG in 2-, 3-, 4-, and 7-conductor put-ups. It is available with a copper sheath (the usual configuration) in which case the sheath is qualified as an equipment grounding conductor, and also with an alloy steel sheath that does not so qualify.
Fig. 332-1. Type MI is a single- or multiconductor cable that requires special termination. (Sec. 332.2.)
The standard length in which any size is furnished depends on the final diameter of the cable. The smallest cable, 1/C No. 16 AWG, has a diameter of 0.216 in. and can be furnished in lengths of approximately 1900 ft. Cables of larger diameter have proportionally shorter lengths. The cable is shipped in paper-wrapped coils ranging in diameter from 3 to 5 ft.
The original intent behind development of this cable was to provide a wiring material which would be completely noncombustible, thus eliminating the fire hazards resulting from faults or excessive overloads on electrical circuits. To accomplish this, it is constructed entirely of inorganic materials. The conductors, sheath, and protective armor are of metal. The insulation is highly compressed magnesium oxide, which is extremely stable at high temperatures (fusion temperature of 2800°C).
332.10. Uses Permitted. This section describes the general use of mineral-insulated metal-sheathed cable, designated Type MI. Briefly, it basically includes general use as services, feeders, and branch circuits in exposed and concealed work, in dry and wet locations, for underplaster extensions and embedded in plaster, masonry, concrete, or fill, for underground runs, or where exposed to weather, continuous moisture, oil, or other conditions not having a deteriorating effect on the metallic sheath (Fig. 332-2). The temperature rating of the cable for conventional power applications is limited by the rating of the end-seal fitting, which necessarily includes organic compounds for sealing purposes. UL sets the maximum rating of current end-seal designs as covered in the guide-card information that is current as of this writing at 90°C in dry locations and 60°C in wet locations. The cable itself, however, is recognized for 250°C in special applications. Permissible current ratings will be those given in Table 310.16 (or, under engineering supervision, in Table B.310.3 in Annex B in the back of the NEC book). Type MI cable in its many sizes and constructions is suitable for all power circuits up to 600 V.
There is no question that MI cable can be used “in underground runs” as indicated in 332.10(10). But there is the matter of the additional qualification that it be “suitably protected against physical damage and corrosive conditions.” Although the copper sheath of MI cable has good resistance to corrosion, acid soils may be harmful to the copper sheath. Direct earth burial in alkaline and neutral soils would generally be expected to create no problems, but in any direct burial application MI cable with an outer plastic or neoprene jacket would ensure effective application and provide compliance with the additional language in part (10) of 332.10. Such jacketed MI cables are available, and have been successfully used in direct burial applications.
Fig. 332-2. Type MI is recognized for an extremely broad range of applications—for any kind of circuit, indoors or outdoors, wet or dry, and even in hazardous locations, as where MI motor branch circuits supply pumps in areas subject to flammable gases or vapors. (Sec. 332.10.)
332.40. Boxes and Fittings. Part (A) calls for connections of Type MI cable to be carefully made in accordance with UL and manufacturers’ application data to ensure effective operation (Fig. 332-3).
Fig. 332-3. Termination fitting for Type MI cable must be an approved connector, with its component parts assembled in proper sequence. [Sec. 332.40(A).]
Part (B) of 332.40, “Terminal Seals,” presents a rule that is applied in conjunction with that of 332.40(A) to ensure both sealing of the cable end and means for connecting to enclosures (Fig. 332-4).
Fig. 332-4. MI cable termination must provide end sealing and connection means. [Sec. 332.40(B).]
332.80. Ampacity. The amount of heat that a run of MI cable can tolerate is governed by the temperature rating of the end-seal connector, which is 90°C in dry locations. For multiconductor configurations, look in Table 310.16 in that column and read out the ampacity. If the conductor size does not change prior to landing on a circuit breaker, then 110.14(C) applies and a lower-temperature ampacity column may need to be substituted. This is really no different than other multiconductor cables with 90°C insulation, such as Type MC cable.
However, this cable can be and very frequently is run as single-conductor cable, and 332.80(B) allows its ampacity to be calculated from Table 310.17. This permission is conditioned on a maintained spacing between adjacent cable groupings equal to not less than 2.15 times the outside diameter of the single largest cable in a group. For single conductor 4/0, that is 17.4 mm (0.684 in.) in diameter, the spacing would need to be about 37.4 mm (1.5 in.) between cable bundles. Even after the spacing is settled, however, there are other concerns that have to be addressed, as follows:
When it is run in this form, 332.21 requires that the cables be grouped to minimize the induced voltages on the cable sheaths. When this happens, the individual cables are not free to radiate their heat to the extent normally presumed for a conductor using free-air ampacities from Table 310.17. Consider a 3-phase, 4-wire makeup using the 4/0 AWG Type MI cable already mentioned, which comes out of the 90°C column of Table 310.17 at 405 A. Is this a way to avoid running 500 kcmil as pipe and wire? It might be, but read on.
When the cable is grouped, each cable does not radiate its heat as well. And in fact, loading 4/0 AWG MI cable to 405 A after it has been grouped or bundled results in a steady-state temperature within the bundle significantly higher than 90°C. This is a known fact; however, the MI cable manufacturers correctly pointed out that there are no end seals in the middle of one of these runs. The only thing there is copper and magnesium oxide. These items can withstand temperatures far higher than even temperatures over 90° indefinitely without damage. Therefore, it has been conclusively demonstrated that yes, the cable bundles will run hot, but Table 310.17 ampacities are not unreasonable for this type of cable applied in just this way.
The next issue is to bring this cable, in this heated state, to the panel, and then into the panel for terminations. This means getting the hot cables in the bundle to cool off enough so the end seal temperature operates within its temperature rating (90°C). This means spreading the cables out to a certain amount for a certain length, neither of which is specified in the NEC. That means requesting information from the cable manufacturer’s engineering department, and then following their recommendations, which should be documented. Strictly by way of illustration, informal conversations have resulted in informal suggestions in the 300 to 450 mm (1 to 1½ ft) range, but that will need to be determined properly.
The final step is to make certain that the terminations stay within the usual 75°C limits prescribed in 110.14(C)(1)(b). Here the MI cable manufacturers seem to be quite aware of this issue. They have a range of step-up sizing terminations that incorporate upsizing terminating tails that match typical ampacity combinations. For example, a step-up connector rated for 4/0 MI cable and providing a 500 kcmil tail for termination purposes is a stock item with the leading manufacturer of Type MI cable.
334.1. Scope. This section makes clear that nonmetallic-sheathed cable must be installed and manufactured as required here.
334.2. Definition. Nonmetallic-sheathed cable is one of the most widely used cables for branch circuits and feeders in residential and light commercial systems (Fig. 334-1). Such cable is commonly and generally called “Romex” by electrical construction people, even though the word Romex is a registered trade name of the General Cable Corp. Industry usage has made the trade name a generic title so that nonmetallic-sheathed cable made by any manufacturer might be called Romex. This generic usage of a trade name also applies to the term BX, which is commonly used to describe any standard armored cable, made by any manufacturer—even though the term BX is a registered trade name of General Electric Co. Type NM cable has an overall covering of fibrous or plastic material which is flame-retardant and moisture-resistant. Type NMC is similar, but the overall covering is also fungus-resistant and corrosion-resistant.
Fig. 334-1. Two of the three separate types of nonmetallic-sheathed cable are shown here. (Sec. 334.100.) Since these drawings were made, changes in the product standard have made conventional Type NM cable much smaller in overall size because the paper wrap has been eliminated (subject to a cold-weather pull-through-the-joists test) and now only the bare equipment grounding conductor has paper around it. Type NMC has largely been supplanted in the market by Type UF cable.
The letter C indicates that it is corrosion-resistant. Unlike most cabled wiring methods, Type NM cable must be listed.
334.10. Uses Permitted. This type of wiring may be used either for exposed or for concealed wiring (Fig. 334-2) in any kind of building or structure.
1. NM cable may be used in one-family dwellings.
2. NM cable may be used in two-family dwellings.
3. NM cable may be used in multifamily dwellings.
4. NM cable may be used in other structures.
But for multifamily dwellings or other buildings or structures, the use of NM is permitted only if these locations are permitted to be of Type III, Type IV, or Type V construction as given in NFPA 5000® (see Code Annex E). This is a critical point. The acceptability of NM cable depends not on how the building is actually constructed, but rather on what the applicable building code would permit the construction to be based on the occupancy. A small multifamily dwelling that meets the size and area tables of the local building code for Type V construction, and yet was constructed to Type II standards, may still use Type NM cable. For nonresidential applications, however, Type NM cable must be concealed within stud or joist cavities (or other protected areas) that have a finish construction that provides a minimum of a 15-min finish rating as defined in listings of fire-rated assemblies. This generally includes 12.7 mm (½ in.) drywall.
Fig. 334-2. Although NM cable is most widely used for branch circuits, the larger sizes (No. 8 and up) are commonly used for feeders, as run here from apartment disconnects to tenant panelboards. (Sec. 334.10.)
Type NM or NMC cable must be “identified” for use in cable trays. This requirement essentially calls for UL listing and marking on the cable to make it “recognized” as suitable for installation in cable trays. Type NM or NMC cable can also be fished into hollow voids of masonry block or tile construction.
Although NM cable is limited to use in “normally dry locations,” NMC—the corrosion-resistant type—is permitted in “dry, damp, moist, or corrosive locations.” Because it has been widely used in barns and other animals’ quarters where the atmosphere is damp and corrosive (due to animal vapors), NMC cable is sometimes referred to as “barn wiring.”
Part (B) says that Type NMC that is run in a shallow chase in masonry, concrete, or adobe and covered over must be protected against nail or screw penetration by a 1.59 mm (
-in.) (minimum) steel plate. This allowance does not extend to NM or NMS cables.
Part (C) specifies those permissible uses for Type NMS, which is a hybrid cable containing power, signaling, communication—voice/data/video—and even optical-fiber cable. This cable type was originally put in the Code to correlate with the former Art. 780 on closed-loop circuiting, where all outlets remained de-energized until a suitable plug was connected. This action would wake up a central controller and announce that a stipulated load was now connected, so a power draw could be provided that corresponded to the load. If the appliance malfunctioned and started to draw too much power, the circuit would be deenergized by the controller. The system was essentially foolproof, but never managed to gain any market share, and with this code cycle, Art. 780 has been deleted. Type NMS cable now relates to intelligent energy management systems that rely on signaling between an outlet and a central control system, and it can carry other systems as well, such as communications. The construction requirements in 334.116(C) assure that the system separation rules in Chaps. 7 and 8 of the NEC are respected in these cable constructions.
334.12. Uses Not Permitted. The first subpart of this rule eliminates the use of any nonmetallic-sheathed cable where it is not already permitted by 334.10, regardless of occupancy type (Fig. 334-3). Guidance regarding building type can be found on the building permit, and Table E.1 in Annex E of the Code, presented here as Table 334-1, can also be of assistance. Perhaps the best guidance can be provided by the local building department. Remember, the rule is based not on what the building is, but what it could be, and that is entirely within the purview of the building department. Note also that there is a new exception that allows NM cables to be pulled in to Chap. 3 raceways that would be allowed in the construction. This has a certain internal logic since if EMT is safe with THHN inside, why shouldn’t it be safe with NM cable built around THHN in the same location? Why anyone would buy and install NM cable where straight THHN would do is another question, but the exception appears to be harmless.
Fig. 334-3. Nonmetallic-sheathed cable is limited in application. [Sec. 334.12(A).]
The whole approach of using building construction types to define the permitted scope of a wiring method is unique in the history of the NEC, although some rules of transformer installations do include these references. The wording of the change originated in a special task group that met in the summer of 1999 to try and sort out where the real issues were in NM cable usage. The reversal of the three-story limitation was also implemented in a very unusual way; because it was imposed by the NFPA Standards Council in the 2002 NEC after it had been rejected at all steps in the usual code amendment process. The Council action was based on the entire record before it, including NFPA fire statistics that showed no association between Type NM cable and fire prevalence, and the report of the NFPA Toxicity Advisory Committee to the effect that in a fire the contribution of Type NM cable jacketing materials is a negligible fraction of the total smoke load.
It was also very clear at the time that major housing interests were prepared to expend significant resources litigating any continuing three-story limitation on grounds that would challenge the technical validity of the rule, and quite possibly the technical objectivity of the NEC panel. Furthermore, the rule had attracted the attention of housing activists, who identified it as a source of artificially inflated housing costs, and who were advocating state legislative attacks on the rule that would have undermined the traditional political independence of the local NEC adoption process in some major jurisdictions. The Standards Council decision seems to have been successful; those attacks are now moot, and the political challenges in this area have lessened.
The response from the NEC Committee after the Standards Council action has been a steady erosion of the reach of Type NM cable in nontraditional occupancies, while carefully leaving unchanged the basic approach of using construction types to define the applicability of the wiring method. No change is more emblematic of this process than 334.12(A)(2), which is a backdoor prohibition against the use of NM cable above any suspended ceiling in smaller commercial occupancies, a practice that had been done under the old one- to three-stories rule for decades. Since all of these spaces are accessible through removal of ceiling tiles, the areas meet the definition of “exposed” in Art. 100, and the prohibition is complete.
This 2005 NEC change replaced wording from the task group, implemented in the 2001 Standards Council action, which prohibited running NM cable in non-residential suspended ceilings where it was “in open runs.” The concept was that these ceilings were often used for random storage, and if NM cable ran from bar joist to bar joist on 1.2-m (4-ft) centers, the regular support rule would still be met but the cable would be subject to damage. The “in open runs” prohibition still allowed the cable in the ceiling, as long as it was protected with a running board or equal. The only substantiation behind making the change (2005 NEC) from “in the open” to “exposed” was that “exposed” was a defined term. A change that removes a wiring method from a good portion of a market, and in the teeth of public comments detailing the gravity of the change, does not turn on an editorial desire to use a defined term. This and comparable issues will undoubtedly fester for some time to come.
As covered in part (12)(B)(2), Types NM and NMS are effectively prohibited from embedding in plaster and so forth (Fig. 334-4). Given the robust nature of NMC, it is specifically permitted for such application, but it is also necessary to protect the cable against the possibility of being damaged by driven nails—such as nails used to hang pictures or add construction elements on the wall. Sufficient protection against nail puncture of the cable is provided by a cover of corrosion-resistant coated steel of at least 1.59-mm (-in.) thickness. Such metal protection must be run for the entire length of the cable where it is run “in shallow chase.” The metal strip protection must be run in the chase and then covered with the plaster or adobe (or similar) finish. But it must be carefully noted that NM, NMC, and NMS are prohibited by 334.12(A)(9) from embedment in cement, concrete, or aggregate—which is distinguished from plaster.
Fig. 334-4. This was permitted by previous editions of the Code and may still be acceptable. (Sec. 334.10)
Table 334.1A Fire Resistance Ratings for Type I Through Type V Construction (h)
334.15. Exposed Work. Figure 334-5 shows the details described in parts (A) and (B) of this section. The rules of this section tie into the rules of part (C),
Table 334-1B: Maximum Number of Stories for Types V, IV, and III Construction
Fig. 334-5. This applies to unfinished basements and other exposed applications. (Sec. 334.15.)
covering use in unfinished basements, which are really places of “exposed work.”
As covered in part (C) to 334.15, cables containing Nos. 14, 12, or 10 conductors must be run through holes drilled through joists, or installed on running boards. When running parallel to joists, any cable must be stapled to the wide, vertical face of a joist and never to the bottom edge. But, as shown in Fig. 334-6, larger cables may be attached to the bottom of joists when run at an angle to the joists. For mounting on the unfinished wall of a basement, there is now language in this location that allows NM cable to be sleeved in a raceway with a protective fitting at the upper end. Of course, this permission has already been in the NEC [300.15(C)] for over 30 years.
Fig. 334-6. Only large cables may be stapled to bottom edge of floor joists. [Sec. 334.15(C).]
334.17. Through or Parallel to Framing Members. This rule reiterates requirements on 300.4, and emphasizes that the grommets required for the use of NM cable in steel-stud construction must be designed to remain in place and be listed for the purpose of cable protection. There is one additional requirement in 300.4(B)(1) not mentioned here that needs to be emphasized in this regard: for NM cable, the grommet must encompass the entire cut opening. There is a V-shaped grommet design that is open at the top; these are not acceptable for NM cable. In the process of pulling through, especially if the holes don’t line up perfectly, the unprotected upper edge has been known to severely damage cables.
334.30. Securing and Supporting. Figure 334-7 shows support requirements for NM or NMC cable. Figure 334-8 shows a violation, both of this section and also 300.11(C) because cables tied together as shown amount to cables supporting cables. In concealed work the cable should, if possible, be so installed that it will be out of reach of nails. Care should be taken to avoid wherever possible the parts of a wall where the trim will be nailed in place—for example, door and window casings, baseboards, and picture moldings. See 300.4. NM cable is also permitted to be fished, and run in suspended ceilings [residential only per 334.12(A)(2)] with up to 1.4 m (4½ ft) of unsupported cable to a luminaire.
Connectors listed for use with Type NM, NMS, or NMC cable (nonmetallicsheathed cable) are also suitable for use with flexible cord or service-entrance cable if such additional use is indicated on the device or carton. Connectors listed under the classifications “Armored Cable Connectors” and “Conduit Fittings” may be used with nonmetallic-sheathed cable when that is specifically indicated on the device or carton. Connectors for NM, NMS, or NMC cable are also suitable for use on Type UF cable (underground feeder and branch-circuit cable—NE Code Art. 340) in dry locations, unless otherwise indicated on the carton. Each connector covered in the listing is recognized for connecting only one cable or cord—unless it is a duplex connector for connecting two cables or if the carton is marked to indicate use with more than one cable or cord.
Fig. 334-7. NM or NMC cables must be stapled every 4½ ft (1.4 m) where attached to the surfaces of studs, joints, and other wood structural members. It is not necessary to use staples or straps on runs that are supported by the drilled holes through which the cable is pulled. But there must be a staple within 12 in. (300 mm) of every box or enclosure in which the cable terminates. (Sec. 334.30.)
Part (C) covers a wiring device configured with a self-contained enclosure that does not require a separate outlet or device box. There must be not less than 300 mm (1 ft) of an unbroken cable loop (or 150 mm [6 in.] of free cable ends) left at the opening so the device can be serviced. This application is also covered in 300.15(E), and in the majority of cases the result is a flush installation in a building wall. See also 334.40(B). They are also noted in 334.40(C).
334.40. Boxes and Fittings. In part (A), the Code presents requirements for “Boxes of Insulating Material.” By using nonmetallic outlet and switch boxes, a completely nonmetallic wiring system is provided. Such a system has economic and other advantages in locations where corrosive vapors are present. See 314.3.
In 334.40(B), “Devices of Insulating Material,” note that use of switch and outlet devices without boxes is limited to exposed cable systems and for rewiring in existing buildings. These are primarily surface-mounted devices also covered in 300.15(H). They were far more common about 50 years ago, and were commonly used to provide snap switches and receptacles in summer cottages with NM cable surface mounted on unfinished framing. This reference must not be confused with that of subpart (C) as given in 334.30, which refers to approved wiring devices that incorporate their own wiring boxes, so they are devices “without a separate outlet box” and not devices “without boxes.”
Fig. 334-8. Absence of stapling of the NM cables within 12 in. (300 mm) of entry into the panelboards is a clear violation of Sec. 334.15. (Sec. 334.30.)
334.80. Ampacity. The second sentence requires that NM, NMS, and NMC cables always have their conductors applied to the ampacity of Type TW wire—that is, the 60°C ampacity from Table 310.16. However, the insulation on the conductors must be rated at 90°C. This provides an additional margin for error and helps prevent overheating of Type NM where installed, say, in the attic of a residential occupancy located in the New Mexico desert. The ambient temperature would soar in the summer, unless the attic was air conditioned.
The last paragraph of 334.80 correlates the use of NM cable to the rule of 310.15(B)(2), which says:
...where single conductors or multiconductor cables are stacked or bundled longer than 24 in. without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in the above table.
Bundled NM, NMS, or NMC cables will require ampacity derating in accordance with 310.15(B)(2) when the whole bundle is tightly packed, thereby losing the ability of the inside cables to dissipate the heat generated in them. An example of this is shown in Fig. 310-22. This is true of NM cables as well as any other cables. And the derating percentage from the table in 310.15(B)(2)(a) must be based on the total number of insulated conductors in the group. For instance, fourteen 3-wire cables would have to be ampacity derated to 35 percent of the conductor ampacity [14 × 3 = 42 conductors at 35 percent, from Table 310.15(B)(2)(a)], which, at 7 A (20 × 0.35 = 7 A) would immediately disqualify them from use, particularly for receptacle circuits. Also note that since this rule now goes directly to Table 310.15(B)(2)(a) and bypasses the heat-sink exception, there is no way around this except for drilling more holes. The same issue applies to the draft-stop limitation following, where if ever more than two NM cables are draft- (or fire-) stopped in the same framing hole, regardless of the length of penetration, the table derating factors apply.
The wording of this section overstates the problem for installations that are not embedded in thermal insulation. A home run of cables through a common set of holes in uninsulated basement joists should be allowed to use conventional ampacity calculation procedures. This is the snare that service entrance cables run indoors for branch circuits and feeders per 338.10(B)(4)(a) are now caught in. The larger issue, however, is that this wording actually understates the true problem for cables run in thermal insulation. By allowing derating to occur from the 90°C column for such cables, the NEC allows users to ignore factors that increase the retention of heat in the cable until they reach the point of reducing the ampacity below the 60°C value, which in most cases is to allow them to be ignored altogether. Meanwhile, particularly for large, heavily loaded cables, even the 60°C numbers are too high. Review the discussion associated with 310.10 for detailed information on this topic.
336.2. Definition. This article covers the use of a nonmetallic-sheathed power and control cable, designated Type TC cable (TC is the abbreviation for tray cable) (Fig. 336-1). The cable is an assembly of two or more insulated conductors, with or without associated bare or covered grounding conductors, under a nonmetallic jacket.
Fig. 336-1. This typical 3-conductor tray cable contains bare equipment grounding conductors. (Sec. 336.2.)
336.10. Use Permitted; 336.12. Uses Not Permitted. It is not permitted to be made with a metallic cable armor either under or over the nonmetallic jacket; however, metal shielding is permitted, and where employed, the minimum bending radius of the cable must not be less than 12 times the cable diameter. Otherwise, the minimum allowable bend radius is based on the cable diameter, with 4 times the diameter allowed for cables 25 mm (1 in.) or thinner, 5 times for cables up to 50 mm (2 in.), and 6 times for larger cables. In addition to cable tray usage, Type TC cable is permitted to run within raceways and on messenger wires, but for outdoor applications in direct sun it must be identified for that use. It may be directly buried, but also only if so identified.
Type TC cable is available with a more robust configuration that will meet the crush and impact tests that apply to metal-clad cable, Type MC. This form is identified as Type TC-ER. For industrial occupancies with qualified maintenance and supervision, Type TC-ER is permitted to exit a cable tray and run to utilization equipment or devices, provided also that it has continuous mechanical support, such as strut, angles, or channels. In addition, Type TC-ER is permitted to run unsupported between cable tray transitions, or from cable trays to utilization equipment or devices as long as the unsupported distance does not exceed 1.8 m (6 ft). Where the cable exits the tray, mechanical support must be provided so the required bend radius is maintained.
UL data on “Power and Control Tray Cable” include the following:
Power and Control Tray Cable (QPOR)
This category covers Type TC power and control tray cable intended for use in accordance with Article 336 of ANSI/NFPA 70, “National Electrical Code” (NEC). The cable consists of one or more pairs of thermocouple extension wires or two or more insulated conductors, with or without one or more grounding conductors, with or without one or more optical fiber members and covered with a nonmetallic jacket. A single grounding conductor may be insulated or bare and may be sectioned. Any additional grounding conductor is fully insulated and has a distinctive surface marking. The cable is rated 600 or 2000 V.
The cable is Listed in conductor sizes 18 AWG to 1000 kcmil copper or 12 AWG to 1000 kcmil aluminum or copper-clad aluminum. Conductor sizes within a cable may be mixed. Thermocouple extension conductors are Listed in sizes 24 to 12 AWG.
PRODUCT MARKINGS
Cable with copper-clad aluminum conductors is surfaced printed “AL (CU-CLAD)” or “Cu-clad Al.”
Cable with aluminum conductors is surface printed “AL.”
Cable employing compact-stranded copper conductors is so identified directly following the conductor size, wherever it appears (surface, tag, carton, or reel), by “compact copper.” The abbreviations “CMPCT” and “CU” may be used for compact and copper, respectively.
Tags, reels, and cartons for products employing compact-stranded copper conductors have the marking: “Terminate with connectors identified for use with compact-stranded copper conductors.” For termination information, see Electrical Equipment for Use in Ordinary Locations (AALZ).
If the type designation of the conductors is marked on the outside surface of the cable, the temperature rating of the cable corresponds to the rating of the individual conductors. When this marking does not appear, the temperature rating of the cable is 60°C unless otherwise marked on the surface of the cable.
Cable investigated for use where exposed to direct rays of the sun is marked “sunlight resistant.”
Cable investigated for direct burial in the earth is so identified.
Cable suitable for use between cable trays and utilization equipment in accordance with NEC 336.10(7) is surface marked with the suffix “-ER.”
Cable consisting of thermocouple extension wires is surface marked “THCPL EXTN,” “For thermocouple extension use only” or “Thermocouple extension wire only.”
Cable surface marked “Oil Resistant I” or “Oil Res I” is suitable for exposure to mineral oil at 60°C. Cable suitable for exposure to mineral oil at 75°C is surface marked “Oil Resistant II” or “Oil Res II.”
Cable that complies with the Limited Smoke Test requirements specified in UL 1685, “Vertical-Tray Fire-Propagation and Smoke-Release Test for Electrical and Optical-Fiber Cables,” is surface marked with the suffix “-LS.”
Cable containing optical fiber members is identified with the suffix “-OF.”
Regarding cable seals outlined in Article 501 of the NEC, Type TC cable has a sheath which is considered to be gas/vapor tight but the cable has not been investigated for transmission of gases or vapors through its core.
RELATED PRODUCTS
Connectors and fittings for use with this cable are covered under Power and Control Tray Cable Connectors (QPOZ).
Some connectors and fittings covered under Outlet Bushings and Fittings (QCRV), Nonmetallic-sheathed Cable Connectors (PXJV), and Service Entrance Cable Fittings (TYZX) are also suitable for use with this cable when specifically marked on the device or carton.
ADDITIONAL INFORMATION
For additional information, see Electrical Equipment for Use in Ordinary Locations (AALZ).
REQUIREMENTS
The basic standard used to investigate products in this category is ANSI/UL 1277, “Electrical Power and Control Tray Cables with Optional Optical-Fiber Members.”
UL MARK
The Listing Mark of Underwriters Laboratories Inc. on the product is the only method provided by UL to identify products manufactured under its Listing and Follow-Up Service. The Listing Mark for these products includes the UL symbol (as illustrated in the Introduction of this Directory) together with the word “LISTED,” a control number, and the product name as appropriate: Power and control tray cable that contains copper or copper-clad aluminum conductors has the product name “Power and Control Tray Cable Type TC”; power and control tray cable that contains aluminum conductors has the product name “Aluminum Power and Control Tray Cable Type TC.”
Note that this cable appears to be for cable tray only, but Type TC is recognized by 336.2 for use in raceway or with messenger support, in addition to use in tray.
Although item (4) of 336.12 has the effect of prohibiting the use of Type TC tray cable directly buried in the earth, the rule is modified by the phrase “unless identified for such use.” The result of this wording is to permit Type TC cable to be directly buried in the earth where the cable is marked or otherwise generally recognizable as suitable for this use. The product listing information (above) directly recognizes this possibility. This permission for direct burial was added because the cable assembly was designed to withstand such application and because Type TC cable has been successfully and effectively used directly for years in many installations (Fig. 336-2 with burial conforming to 300.5). Such cable is listed for direct earth burial by UL, and the performance record has been excellent.
Fig. 336-2. Type TC (power and control tray cable) is recognized for direct earth burial. (Sec. 336.12.)
338.2. Definition. The Code contains no specifications for the construction of this cable; it is left to Underwriters Laboratories Inc. to determine what types of cable should be approved for this purpose. The types listed by the Laboratories conform to the following data:
This category covers service-entrance cable designated Type SE and Type USE for use in accordance with Article 338 of ANSI/NFPA 70, “National Electrical Code” (NEC).
Service-entrance cable, rated 600 V, is Listed in sizes 14 AWG and larger for copper, and 12 AWG and larger for aluminum or copper-clad aluminum.
The cable is designated as follows:
Type SE—Indicates cable for aboveground installation. Both the individual insulated conductors and the outer jacket or finish of Type SE are suitable for use where exposed to sun. Type SE cable contains Type RHW, RHW-2, XHHW, XHHW-2, THWN, or THWN-2 conductors.
Types USE and USE-2—Indicates cable for underground installation including direct burial in the earth. Cable in sizes 4/0 AWG and smaller and having all conductors insulated is suitable for all of the underground uses for which Type UF cable is permitted by the NEC. Multiconductor Type USE cable contains conductors with insulation equivalent to RHW or XHHW. Multiconductor Type USE-2 contains insulation equivalent to RHW-2 or XHHW-2 and is rated 90°C wet or dry. Single- and multiconductor Types USE and USE-2 are not suitable for use in premises or aboveground except to terminate at the service equipment or metering equipment. Both the insulation and the outer covering, when used, on single- and multiconductor Types USE and USE-2, are suitable for use where exposed to sun.
Submersible Water Pump Cable—Indicates a multiconductor cable in which 2, 3, or 4 single-conductor Types USE or USE-2 cables are provided in a flat or twisted assembly. The cable is listed in sizes 14 AWG to 4/0 AWG inclusive, copper, and 12 AWG to 4/0 AWG inclusive, aluminum or copper-clad aluminum. The cable is tag marked “For use within the well casing for wiring deep-well water pumps where the cable is not subject to repetitive handling caused by frequent servicing of the pump units.” The insulation may also be surface marked “Pump Cable.” The cable may be directly buried in the earth in conjunction with this use.
For termination information, see Electrical Equipment for Use in Ordinary Locations (AALZ).
Based upon tests which have been made involving the maximum heating that can be produced, an uninsulated conductor employed in a service cable assembly is considered to have the same current-carrying capacity as the insulated conductors even though it may be smaller in size.
Figure 338-1 shows two basic styles of service-entrance cable for aboveground use. The one without an armor over the conductors is referred to as Type SE Style U—the letter U standing for “unarmored.” That cable is sometimes designated as Type SEU. The cable assembly with the armor is designated Type ASE cable, with the A standing for “armored.”
Fig. 338-1. Two types of aboveground SE cable. (Sec. 338.2.)
Figure 338-2 shows another type of SE cable, known as Style SER—the letter R standing for “round.” In a typical assembly of that cable, three conductors insulated with Type XHHW cross-link polyethylene are cabled together with fillers and one bare ground conductor with a tape over them and gray PVC overall jacket. For use aboveground in buildings, it is suitable for operation at 90°C in dry locations or 75°C in wet locations unless the insulation designation has the suffix “-2” in which case it is suitable for 90°C for both locations.
Fig. 338-2. Style SER cable contains individual conductors and no concentric neutral. (Sec. 338.2.)
For 3-phase, 4-wire grounded services, the three insulated conductors—a black, a red, and a blue—are used as the phase legs of the service, and the bare conductor is used as the neutral. For other applications that cable configuration can be used as a feeder or branch circuit to a power load with a bare equipment grounding conductor, and this form of cable is also available with black, red, and white insulated conductors for single-phase applications, including the supply of downstream panels, and black, red, blue, and white 5-wire configurations for 3-phase equivalent loads.
Figure 338-3 shows multiconductor Type USE cable for underground (including direct earth burial) applications of service or other circuits. Type USE may consist of one, two, or three conductors. If it has no building wire designation, its insulation will burn, and it is absolutely excluded from any interior wiring whatsoever by 338.12(B)(1). Where used aboveground it must terminate where it emerges from the ground, such as at a meter socket, as covered in 338.12(B)(2). It is recognized for use as aerial cable, provided it is in the multiconductor form, identified as suitable for aboveground use, and run on a messenger. Some cables have both a “USE” designation and also a building wire designation, such as “RHW.” This cable can continue into a building. The basic temperature rating for this cable is 75°C and that limit applies unless a different number is marked on the cable.
Fig. 338-3. Type USE cable may be multiconductor or single-conductor cable. (Sec. 338.2.)
Depending on whether USE cable is used for service entrance, for a feeder, or for a branch circuit, burial depth must conform to 300.5 and its many specific rules on direct burial cable.
338.10. Uses Permitted as Service-Entrance Conductors. As would be expected, where used as service conductors, Type SE must comply with Art. 230, “Services.” The wording in the second paragraph specifically permits Type USE to “emerge” from a trench and be run aboveground to terminate in meters or service equipment.
Part (B) recognizes use of service-entrance cable for branch circuits and feeders within buildings or structures, provided that all circuit conductors, including the neutral of the circuit, are insulated. Such use must conform to Art. 334 on installation methods—the same as those for Type NM cable.
Part (B)(2) covers permitted uses of service-entrance cable that contains a bare conductor for the neutral but limits such application to 120/240- or 120/208-V systems. When an SE cable has an outer nonmetallic covering over the enclosed bare neutral, this Code rule permits the use of SE cable for circuits supplying ranges, wall-mounted ovens, and countermounted cooking units (Fig. 338-4). And in such cases, the bare conductor may be used as the neutral of an existing branch circuit as well as the equipment grounding conductor (see 250.140).
Fig. 338-4. Although previously permitted, SE cable with bare neutral may be used for branch circuits to a range or other cooking units, bu t only “existing branch circuits.” (Sec. 338.10.)
SE cable is also permitted to be used as a feeder from one building to another building, with the bare conductor used as a grounded neutral, but now only for an existing premises wiring system in accordance with 250.32(B) Exception. Or an SE cable with a bare neutral may be used as a feeder within a building, if the bare neutral is used only as the equipment grounding conductor and one of the insulated conductors within the cable is used as the neutral of the feeder. (See Fig. 338-5.)
Part (B)(3) requires that SE cable used to supply appliances not be subject to conductor temperatures in excess of the temperature specified for the insulation involved. The insulated conductors of SE cables are 75 or 90°C, and if they are rated at 90°C, such marking will appear on the outer sheath. A cooking unit or oven that requires 90°C supply conductors would be an application for the use of SE cables, rated at 90°C. However, a review of UL listings for cooking units and ovens indicates that most such units do not require supply conductor ratings to exceed 60°C. The details in Fig. 338-4 show a method of connecting cooking units where the supply conductors are required to be 75 or 90°C.
In Part (B)(4)(a), “Interior Installation,” Type SE cable is permitted for use as interior branch circuits or feeders, but where so used, the installation must satisfy all of the general wiring rules of Art. 300. This section requires the installation of unarmored SE cable (which is the usual type of SE cable) to satisfy Art. 334 on nonmetallic-sheathed cable (Type NM). All the rules of Art. 334 that cover how cable is installed must be satisfied. This notably includes the building construction limitations in 334.10, and remember the discussion at that point in this book; these are limits based on what the building code requires the minimum characteristics of the buildings to be, and not necessarily on how the building is actually constructed. As a result, the use of SE cable as a feeder, as shown in Fig. 338-5, would be a violation in any building required to be constructed of Type I or II construction. This also brings in, for the first time in the 2008 NEC due to a subtle rewording here, the ampacity limitations in 334.80.
Because these limitations must apply, at the end of any calculations, such as that the 60°C column is not to be exceeded even in instances where the cable runs in free air, many customary applications of Type SE cable sizes have changed. There is no credible basis for applying 60°C temperature limitations to cable makeups using 90°C conductors in free-air applications, and this applies equally to Type AC or Type NM or Type SE [or Type UF per 340.10(4)] cables, but that is where the literal text of the NEC now stands. For example, 4/0 AWG aluminum SE cable has a 75° limit for terminations, which is 180 A in Table 310.16. Since very few feeders run in the window between 180 and 200 A for actual calculated load, this cable has been sold by the mile for 200 A services and, in SER configurations, as feeder cable to apartments and commercial 3-phase, 4-wire feeder cable to stores and in restaurants, etc. This cable is now 150 A cable, period.
Another issue, however, is that the applicable wording in 334.80 actually understates the true problem for cables run in thermal insulation. By allowing derating to occur from the 90°C column for such cables, the NEC allows users to ignore factors that increase the retention of heat in the cable until they reach the point of reducing the ampacity below the 60°C value, which in most cases is to allow them to be ignored altogether. Meanwhile, particularly for large, heavily loaded cables, even the 60°C numbers are too high. Review the discussion associated with 310.10 for detailed information on this topic. This was the reason why, deservedly, Type SE cable was brought within the orbit of 334.80. For the example just cited of 4/0 AWG aluminum SER cable, the true ampacity of this cable where actually embedded in thermal insulation is probably less than 120 A, and only a diligent application of 310.10 stands in the way.
Fig. 338-5. Typical application of SE cable with a bare neutral for use as a feeder within a building. (Sec. 338.10.)
340.2. Definition. Figure 340-1 shows a violation of the Code rule that a bare conductor in a UF cable is for grounding purposes only. Type UF cable must be listed.
Fig. 340-1. Bare conductor in UF cable may not be used as a neutral. (Sec. 340.2.)
340.10. Uses Permitted. As called for in part (A)(1), Figs. 340-2 and 340-3 show details on compliance of UF cable with 300.5. Where UF comes up out of the ground, it must be protected for 8 ft up on a pole and as described in 300.5(D).
The rules of part (A)(1) are shown in Fig. 340-4 and must be correlated to the rules of 300.5 on direct burial cables. The rule of (1) in part (A) corresponds to that 300.5(I). If multiple conductors are used per phase and neutral to make up a high-current circuit, this rule requires all conductors to be run in the same trench or raceway and therefore subject to the derating factors of 310.15(B)(2). Refer to the paragraph covering 310.15(B)(2). Also see discussion under 300.5(I).
Fig. 340-2. UF cable must conform to Sec. 300.5 on direct burial cables. (Sec. 340.10.)
UF cable may be used underground, including direct burial in the earth, as feeder or branch-circuit cable when provided with overcurrent protection not in excess of the rated ampacity of the individual conductors. If single-conductor cables are installed, all cables of the feeder circuit, subfeeder, or branch circuit, including the neutral cable, must be run together in close proximity in the same trench or raceway. It may be necessary in some installations to provide additional mechanical protection, such as a covering board, concrete pad, raceway, or the like, when required by the authority enforcing the Code. Multiple-conductor Type UF cable (but not single-conductor Type UF cables) may also be used for interior wiring when used in the same way as Type NM cable, complying with the provisions of Art. 334 of the Code. And UF may be used in wet locations.
Fig. 340-3. The second qualifier under “Location of Wiring Method or Circuit” in Table 300.5 permits a 6-in. reduction of UF burial depth. (Sec. 340.10.)
The effect of the wording in part (A)(4) where UF cable is used for interior wiring, is to require that its conductors must be rated at 90°C, with loading based on 60°C ampacity. This rule is a follow-up to the requirement that UF for interior wiring must satisfy the rules of Art. 336 on nonmetallic-sheathed cable (see 334.80). It is also subject to the construction type limitations in 334.10.
As noted in 340.12(8), single-conductor Type UF cable embedded in poured cement, concrete, or aggregate may be used for nonheating leads of fixed electric space heating cables, as covered in 424.43.
Application data of the UL are as follows:
This category covers underground feeder and branch circuit cable, rated 600 V, in sizes 14 to 4/0 AWG inclusive, copper, and 12 to 4/0 AWG inclusive, aluminum or copper-clad aluminum, for single and multiple conductor cables. It is designated as Type UF cable and is intended for use in accordance with Article 340 of ANSI/NFPA 70, “National Electrical Code” (NEC).
Some multiconductor cable is surface marked with the suffix “B” immediately following the type letters to indicate the usage of conductors employing 90°C rated insulation.
Such cable may also be installed as nonmetallic-sheathed cable, per Section 340.10(4) of the NEC. The ampacities of Type UF cable, with or without the suffix “B,” are those of 60°C rated conductors as specified in the latest edition of the NEC.
Fig. 340-4. UF cable may be used only as feeders or branch circuits. (Sec. 336.10.)
Submersible Water Pump Cable—Indicates multiconductor cable in which 2, 3, or 4 single-conductor Type UF cables are provided in a flat or twisted assembly. The cable is listed in sizes from 14 AWG to 4/0 AWG inclusive, copper, and from 12 AWG to 4/0 AWG inclusive, aluminum or copper-clad aluminum. The cable is tag marked “For use within the well casing for wiring deep well water pumps where the cable is not subject to repetitive handling caused by frequent servicing of the pump units.” The insulation may also be surface marked “Pump Cable.” The cable may be directly buried in the earth in conjunction with this use.
This cable may employ copper, aluminum, or copper-clad aluminum conductors. Cable with copper-clad aluminum conductors is surface printed “AL (CU-CLAD)” or “Cu-Clad Al.” Cable with aluminum conductors is surface printed “AL.”
Cable employing compact-stranded copper conductors is so identified directly following the conductor size wherever it appears (surface, tag, carton, or reel) by “compact copper.” The abbreviations “CMPCT” and “CU” may be used for compact and copper, respectively.
Tags, reels, and cartons for products employing compact-stranded copper conductors have the marking: “Terminate with connectors identified for use with compact-stranded copper conductors.” For conductor termination information, see Electrical Equipment for Use in Ordinary Locations (AALZ).
This cable may be terminated at boxes and other enclosures by using nonmetallic-sheathed cable connectors (see Nonmetallic-Sheathed Cable Connectors [PXJV]).
Cable suitable for exposure to direct rays of the sun is indicated by tag marking and marking on the surface of the cable with the designation “Sunlight Resistant.”
Only multiconductor Type UF cable may be used in cable tray, in accordance with 340.10(7).
This article covers a conduit with wall thickness less than that of rigid metal conduit but greater than that of EMT. Called “IMC,” this intermediate metal conduit uses the same threading method and standard fittings for rigid metal conduit and has the same general application rules as rigid metal conduit. Intermediate metal conduit actually is a lightweight rigid steel conduit which requires about 25 percent less steel than heavy-wall rigid conduit. Acceptance into the Code was based on a UL fact-finding report which showed through research and comparative tests that IMC performs as well as rigid steel conduit in many cases and surpasses rigid aluminum and EMT in most cases.
IMC may be used in any application for which rigid metal conduit is recognized by the NEC, including use in all classes and divisions of hazardous locations as covered in 501.4, 502.4, and 503.3. Its thinner wall makes it lighter and less expensive than standard rigid metal conduit, but it has physical properties from differences in the alloy that give it outstanding strength. The lighter weight facilitates handling and installation at lower labor units than rigid metal conduit. Because it has the same outside diameter as rigid metal conduit of the same trade size, it has greater interior cross-sectional area (Fig. 342-1). In the past this extra space was not recognized by the NEC to permit the use of more conductors than can be used in the same size of rigid metal conduit. However, with the elimination of Tables 2, 3A, 3B, and 3C, as well as the revisions of Tables 4 and 5 to more correctly reflect the interior area of raceways and the dimensions of conductors, the Code does permit greater fill in IMC (see Tables C4 and C4A in App. C).
342.10. Uses Permitted. The data of the UL supplement the requirements of part (A) on use of IMC, as follows:
Listing of Intermediate Ferrous Metal Conduit includes standard 10 ft. lengths of straight conduit, with a coupling, special length either shorter or longer, with or without a coupling for specific applications or uses, elbows, bends, and nipples in trade sizes ½ to 4 in. incl. for installation in accordance with Article 342 of the National Electrical Code.
Fig. 342-1. Typical comparison between rigid conduit and IMC shows interior space difference. This is now recognized by Tables C4 and C4A in Annex C. (Sec. 342.1.)
Fittings for use with unthreaded intermediate ferrous metal conduit are listed under conduit fittings (Guide DWTT) and are suitable only for the type of conduit indicated by the marking on the carton.
Galvanized intermediate steel conduit installed in concrete does not require supplementary corrosion protection.
Galvanized intermediate steel conduit installed in contact with soil does not generally require supplementary corrosion protection.
In the absence of specific local experience, soils producing severe corrosive effects are generally characterized by low resistivity less than 2000 ohm-centimeters.
Wherever ferrous metal conduit runs directly from concrete encasement to soil burial, severe corrosive effects are likely to occur on the metal in contact with the soil.
Although literature on IMC at one time referred to Type I and Type II IMC because of slight differences in dimensions due to manufacturing methods, the NEC considers IMC to be a single type of product and the rules of Art. 342 apply to all IMC.
Note that the wording in the UL data includes the word “generally” in stating that IMC does not need additional protective material applied to the conduit when used in soil. That is intended to indicate that local soil conditions (acid versus alkaline) may require protection of the conduit against corrosion. And the UL note about corrosion of conduit running from concrete to soil must be observed. Refer to comments under 344.2 covering these conditions.
At the end of 342.14(A) , the wording of the rule is significantly modified by the last sentence, which specifically permits use of aluminum fittings and enclosures with steel intermediate metal conduit (Fig. 342-2). This same wording is also given in Art. 344 on rigid metal conduit and Art. 358 on electrical metallic tubing. Tests have established that aluminum fittings and enclosures create no difficulty when used with steel raceways. The wording is intended to counteract the implication of that phrase that cautions against use of dissimilar metals in a raceway system to guard against galvanic action. This section prohibits the use of dissimilar metals, “where practicable.” This phrase is used frequently in the Code; in effect, it is saying, “You shall do it, if you can, or if the inspector thinks you can.” By using this phrase, the Code recognizes that the contractor may not always be able to comply. BUT, the last paragraph points out that use of aluminum fittings and enclosures is permitted by right.
Fig. 342-2. NEC warning against use of dissimilar metals does not apply to this. (Sec. 342.14.)
In part (B), wording of the rule intends to make clear that the galvanizing or zinc coating on the IMC does give it the measure of protection required when used in concrete or when directly buried in the earth. The last phrase, “judged suitable for the condition,” refers to the need to comply with UL regulations such as those contained in UL’s Electrical Construction Materials Directory, advising how and when steel raceways and other metal raceways may be used in concrete or directly buried in earth.
The UL data point out that there are soils where some difficulties may be encountered, and there are other soil conditions that present no problem to the use of steel or other metal raceways. The phrase “judged suitable for the condition” implies that a correlation was made between the soil conditions or the concrete conditions at the place of installation and the particular raceway to be used. This means that it is up to the designers and/or installers to satisfy themselves as to the suitability of any raceway for use in concrete or for use in particular soil conditions at a given geographic location. Of course, all such determinations would have to be cleared with the electrical inspection authority to be consistent with the meaning of Code enforcement.
For use of IMC in or under cinder fill, part (C) gives the limiting conditions. See 344.10.
342.20. Size. This is the only difference between this product and the ferrous versions of rigid metal conduit; RMC is available up to metric designator 155 (6 trade size) and IMC is only available up to metric designator 103 (trade size 4).
342.22. Number of Conductors in Conduit. The rules on conduit fill are the same for IMC, rigid metal conduit, EMT, flexible metal conduit, flexible metallic tubing, and liquidtight flexible metallic tubing—for conduits ½ in. size and larger although different tables are used. Refer to 344.20.
342.30. Securing and Supporting. The basic rule on clamping IMC is simple and straightforward (Fig. 342-3). Spacing may be increased to a maximum of 5 ft (1.5 m) where necessary because no structural member is available. But the distance must not be extended, except as permitted by the subsections. The subsections allowing wider spacing of supports are the same as those covered in 344.30 for rigid metal conduit.
Fig. 342-3. All runs of IMC must be clamped in this way. (Sec. 342.30.)
Spacing between supports for IMC (greater than every 10 ft [3.0 m]) is the same as the spacing allowed for rigid metal conduit. The subparts recognize the essential equality between the strengths of IMC and rigid metal conduit.
Part 30(C) is new in this article and in all the tubular rigid raceways in the 2008 NEC cycle, including also rigid metal conduit, PVC conduit, RTRC conduit, and EMT. It addresses the support of short nipples between enclosures. It requires direct support for all nipples regardless of length unless they meet three criteria. (1) They must not be over 450 mm (18 in.) in length (Fig. 342-4); (2) they must be unbroken (without couplings) (Fig. 342-5); and (3) they must not enter through a concentric knockout (Fig. 342-6). These requirements result in some absurd results, such as a support requirement for a 75-mm (3-in.) nipple if it enters a concentric knockout on just one end. Meanwhile, no change was made to the support requirements generally, so if one of these raceways is supported within 900 mm (3-ft), the run beyond that point, with no additional supports, can have an indefinite number of couplings (Fig. 342-7) and arrive at the center knockout of an indefinite number of concentric circles. Although the panel’s position is that all raceway segments always required direct support and this provision is simply relief from this universal requirement, the unsupported nipples in Figs. 342-4 through 342-6 reflect almost universal field experience. You will need to discuss this with the inspector to see the extent to which it is being enforced locally.
Fig. 342-4. Pipe nipples over 450 mm (18 in.) regardless of rigidity require independent support. [Sec. 342-30(C).]
Fig. 342-5. Pipe nipples with couplings regardless of size require independent support. [Sec. 342-30(C).]
Fig. 342-6. Pipe nipples encountering concentric knockouts require independent support, regardless of length. [Sec. 342-30(C).]
Fig. 342-7. Pipe termination rules, with 900 mm (3 ft) distance to a support, allow indefinite numbers of couplings and unused knockout circles. [Sec. 342.30(A).]
344.10. Uses Permitted. UL data on rigid metal conduit are similar to those on IMC and supplement the rules of this section, as follows:
Galvanized rigid steel conduit installed in concrete does not require supplementary corrosion protection.
Galvanized rigid steel conduit installed in contact with soil does not generally require supplementary corrosion protection.
In the absence of specific local experience, soils producing severe corrosive effects are generally characterized by low resistivity (less than 2000 ohm-centimeters).
Wherever ferrous metal conduit runs directly from concrete encasement to soil burial, severe corrosive effects are likely to occur on the metal in contact with the soil.
Conduit that is provided with a metallic or nonmetallic coating, or a combination of both, has been investigated for resistance to atmospheric corrosion. Nonmetallic outer coatings that are part of the required resistance to corrosion have been additionally investigated for resistance to the effects of sunlight.
Nonmetallic outer coatings of greater than 0.010-in. thickness are investigated with respect to flame propagation detrimental effects to any underlying corrosion protection, the fit of fittings, and electrical continuity of the connection of conduit to fittings.
Conduit with nonmetallic coatings has not been investigated for use in ducts, plenums, or other environmental air spaces in accordance with the NEC.
Rigid metal conduit with or without a nonmetallic coating has not been investigated for severely corrosive conditions.
For nonferrous rigid metal conduits, the UL application notes state:
Aluminum conduit used in concrete or in contact with soil requires supplementary corrosion protection.
As appropriate, a designation such as “Stainless Steel,” “Red Brass,” or “Aluminum” is appended to the product name or is substituted for the word “Metal” in the product name.
For direct earth burial of rigid conduit and IMC, the UL notes must be carefully studied and observed:
1. Galvanized rigid steel conduit and galvanized intermediate steel conduit directly buried in soil do not generally require supplementary corrosion protection. The use of the word “generally” in the UL instructions indicated that it is still the responsibility of the designer and/or installer to use supplementary protection where certain soils are known to produce corrosion of such conduits. Where corrosion of underground galvanized conduit is known to be a problem, a protective jacketing or a field-applied coating of asphalt paint or equivalent material must be used on the conduit. But, UL notes must be observed for resistance to corrosion under “severely corrosive conditions” because when steel conduits pass from concrete to direct earth burial, the juncture is classified as “severely corrosive” and nonmetallic coatings have not been investigated for these conditions. An agreement will need to be worked out with the inspector if severely corrosive effects are anticipated in any location. In the case of underground/concrete interfaces, supplementary protective coating on conduit at the crossing line can eliminate the conditions shown in Fig. 344-1.
Fig. 344-1. Protective coating on section of conduit can prevent this corrosion problem. (Sec. 342.2.)
2. Aluminum conduit used directly buried in soil requires supplementary corrosion protection. Exactly what that could be in this case will require some research and the approval of the inspector. At one time UL declared that such coatings “presently used” have not been recognized for resistance to corrosion, but that statement no longer appears in the guide card information for this category.
3. Red brass conduit is permitted by right for underground and swimming pool applications. Note that all rigid metal conduit is required to be listed per 344.6. This may be a difficult issue on this product. An admittedly nonscientific survey of all current listees in the nonferrous rigid metal conduit category found a number of producers of aluminum conduit, a few producers of stainless steel conduit, and no producers of red brass conduit. For swimming pool applications run to a forming shell this has been a known issue for some time, which is why 680.23(B)(2)(a) specifically allows brass conduit to be approved and not listed, thereby constituting a deliberate Chap. 6 amendment of the Chap. 3 rule in 344.6. Some plumbing supply houses carry heavy wall red brass pipe, often in 12-ft lengths, that takes a conventional pipe thread extremely well, and is a very robust product with an extremely smooth interior that, if anything, is somewhat more difficult to bend than IMC or RMC. Lack of heavy foot pressure with excessive force on the handle won’t kink the product, but will bend the bender handle. Approval is at the discretion of the local inspector, but this product should certainly be considered unless a listed alternative becomes more available.
As indicated by the rule of part (C), care must be taken where cinder fill is used. Cinders usually contain sulfur, and if there is much moisture, sulfuric acid is formed, which attacks steel conduit. A cinder fill outdoors should be considered as “subject to permanent moisture.” In such a place conduit runs should be provided corrosion protection as described, encased in 2 in. (50 mm) of concrete, or buried in the ground at least 18 in. (450 mm) below the fill. This would not apply if cinders were not present.
344.22. Number of Conductors. The basic NE Code rule on the maximum number of conductors which may be pulled into rigid metal conduit, rigid non-metallic conduit, intermediate metal conduit, electrical metallic tubing, flexible metal conduit, and liquidtight flexible metal conduit is contained in the single sentence of this section.
The number of conductors permitted in a particular size of conduit or tubing is covered in Chap. 9 of the Code in Tables C1 through C12A in Annex C for conductors all of the same size used for either new work or rewiring. Tables 4 and 5 of Chap. 9 cover combinations of conductors of different sizes when used for new work or rewiring. For nonlead-covered conductors, three or more to a conduit, the sum of the cross-sectional areas of the individual conductors must not exceed 40 percent of the interior cross-sectional area (csa) of the conduit or tubing for new work or for rewiring existing conduit or tubing (Fig. 344-2). Note 4 preceding all the tables in Chap. 9, in the back of the Code book, permits a 60 percent fill of conduit nipples not over 24 in. (600 mm) long and no derating of ampacities is needed.
When all conductors in a rigid metal conduit are the same size, Tables C8 and C8A in App. C give the maximum allowable fill for conductors depending on conductor type up to 2000 kcmil, for metric designator 16 to 155 (trade size ½ to 6) rigid metal conduit.
question What is the minimum size of rigid metal conduit required for six 10 THHN AWG wires?
answer Table C.8, Annex C, shows that six 10 THHN AWG wires may be pulled into a metric designator 16 (trade size ½) rigid metal conduit.
question What size conduit is the minimum for use with four 6 RHH AWG conductors with outer covering?
answer Table C.8, Annex C, shows that a metric designator 35 (trade size 1¼) minimum conduit size must be used for from four to six RHH AWG conductors with outer covering. If they lacked the outer covering, the answer would be a metric designator 27 (trade size 1) conduit.
question What is the minimum size conduit required for four 500-kcmil XHHW conductors?
answer Table C.8 shows that metric designator 78 (trade size 3) conduit may contain four 500-kcmil XHHW (or THHN) conductors.
Fig. 344-2. For three or more conductors the sum of their areas must not exceed 40 percent of the conduit area. (Sec. 344.20.)
When all the conductors in a conduit or tubing are not the same size, the minimum required size of conduit or tubing must be calculated. Table 1, Chap. 9, says that conduit containing three or more conductors of any type except lead-covered, for new work or rewiring, may be filled to 40 percent of the conduit csa. Note 6 to this table refers to Tables 4 through 8, Chap. 9, for dimensions of conductors, conduit, and tubing to be used in calculating conduit fill for combinations of conductors of different sizes.
example What size rigid metal conduit is the minimum required for enclosing six 10 AWG THHN, three 4 AWG RHH (with outer covering), and two 12 AWG TW conductors (Fig. 344-3)?
Fig. 344-3. Minimum permitted conduit size must be calculated when conductors are not all the same size. (Sec. 344.22.)
Cross-sectional areas of conductors:
From Table 5, Chap. 9:
10 AWG THHN0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.0211 sq in.
4 AWG RHH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1333 sq in.
12 AWG TW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.0181 sq in.
Total area occupied by conductors:
6 10 AWG THHN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 × 0.0211 = 0.1266 sq in.
34 AWG RHH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 × 0.1333 = 0.3999 sq in.
2 12 AWG TW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 × 0.0181 = 0.0362 sq in.
Total area occupied by conductors. . . . . . . . . . . . . . . . . . . . 0.5627 sq in.
Referring to Table 4, Chap. 9:
The fourth column from the left gives the amount of square-inch area that is 40 percent of the csa of the sizes of conduit given in the farthest column from the left. The 40 percent column in the table on rigid metal conduits shows that 0.355 sq in. is 40 percent fill of a 1-in. conduit, and 0.610 sq in. is 40 percent fill of a 1¼-in. conduit. Therefore, a 1-in. conduit would be too small and—
A metric designator 35 (trade size 1¼) rigid metal conduit is the smallest for these 11 conductors.
example What is the minimum size of conduit for four No. 4/0 TW and four No. 4/0 XHHW conductors?
From Table 5, a No. 4/0 TW has a csa of 0.3718 sq in. Four of these come to 4 × 0.3718 or 1.4872 sq in.
From Table 5 we find that four No. 4/0 XHHW have a csa of 1.2788 sq in.
1.4872 + 1.2788 = 2.766 sq in.
From Table 4, 40 percent of the csa of 3-in. rigid metal conduit is 3.000 sq in. A 2½-in. conduit would be too small. Therefore—
A metric designator 78 (trade size 3) conduit must be used.
Figure 344-4 shows how a conduit nipple is excluded from the normal 40 percent limitation on conduit fill. In this typical example, the nipple between a panelboard and a wireway contains 12 10 AWG TW wires, 6 14 AWG THHN wires, 3 8 AWG THW wires, and 2 2 AWG RHH wires (without outer covering). The minimum trade size of nipple that can be used in this case is metric designator 35 (trade size 1¼). (Nipple may be filled to 60 percent of its csa if it is not over 24 in. [610 mm] long. Area of conductors = 12 × 0.0243 sq in. [csa of each 10 AWG TW] plus 6 × 0.0097 sq in. [each 14 AWG THHN] plus 3 × 0.0437 sq in. [each 8 AWG THW] plus 2 × 0.1333, or a total of 0.7475 sq in. If we divide this number by 0.6, the result will be the minimum area that the nipple can be sized at; 0.7475 ÷ 0.6 = 1.246 sq in. The next higher sized rigid conduit raceway from Table 4, Chap. 9 is a metric designator 35 [trade size 1¼] conduit, so that is the size to select. Note that the Table 4 raceway sizes now include 60 percent columns done out, and entering the 0.7475 sq in. conductor summation in the 60 percent column shows it to be too large for the metric designator 27 [trade size 1] conduit but well within the next larger size.) And the conductors do not have to be derated in accordance with 310.15(B)(2)(a). If the nipple had been 25 in. long, calculation at 40 percent fill would have called for a metric designator 41 (trade size 1½) size, and all conductors would have had to be derated per 310.15(B)(2).
Fig. 344-4. Conduit nipples may be filled to 60 percent of csa and no derating is required. (Sec. 344.22.)
THWN and THHN are the smallest-diameter building wires. The greatly reduced insulation wall on Type THWN or THHN gives these thin-insulated conductors greater conduit fill than TW, THW, or RHH. And the nylon jacket on THWN and THHN has an extremely low coefficient of friction. THWN is a 75°C-rated wire for general circuit use in dry or wet locations; however, it is routinely being supplied in its THWN-2 variety, which has the same 90°C rating as THHN. THHN is a 90°C rated wire for dry locations only.
Although the same procedure applies, the tables in Annex C and the various parts of Table 4 must be correlated with the type of raceway to be used. This is a major departure from past Codes, but provides for more realistic fill. Remember, that spare fill capacity may be desirable in certain applications—such as long underground runs to outbuildings. The Code permits fill to 40 percent but no more. If a raceway is filled to the 40 percent maximum permitted in Chap. 9, a new raceway will be required if additional circuits are desired at a later date.
To fill conduit to the Code, maximum allowance is frequently difficult or impossible from the mechanical standpoint of pulling the conductors into the conduit, because of twisting and bending of the conductors within the conduit. Bigger-than-minimum conduit should generally be used to provide some measure of spare capacity for load growth; and, in many cases, the conduit to be used should be upsized considerably to allow future installation of some larger anticipated size of conductors.
344.24. Bends—How Made. The basic rule here provides general, common sense requirements regarding the bending of rigid metal conduits. The next part of the rule refers to Table 2 in Chap. 9. That table provides the same data that appeared in Table 344.24 in the 2002 NEC. Table 2 in Chap. 9 now shows the acceptable radii, depending on the type of bend that is made.
Table 2, Chap. 9 gives minimum bending radii for bends in rigid metal conduit, IMC, or EMT using any approved bending equipment and methods. (See Fig. 344-5.) However, the table headings in this rule permits sharper bends (i.e., smaller bending radii) if a one-shot bending machine or a full-shoe bender, including full-shoe hand benders, is used in making a bend for which the machine and its accessories are designed. The minimum radii for one-shot bends are given in the center column of Table 2, Chap. 9. The “other bend column applies to bends made with hickeys or with hydraulic machines that do not have shoes that support the walls of the conduit or tubing throughout the bend. All bending radii apply to any amount of bend—that is, 45°, 90°, and so on.
Fig. 344-5. Minimum bending radii are specified to protect conductors from damage during pull-in. (Sec. 344.24.)
344.26. Bends—Number in One Run. There must be not more than the equivalent of four quarter bends (360°) between any two “pull points”—conduit bodies and boxes, as shown in Fig. 344-6. In previous Codes, the 360° of bends was permitted between boxes and “fittings” and even between “fitting and fitting.” Because the word fitting is defined in Art. 100 and the term does include conduit couplings, bushings, and so forth, there could be very many bends in an overall run, totaling far more than 360° if the equivalent of four quarter bends could be made between each pair of conduit couplings. The present wording limits the 360° of bends to conduit runs between “pull points”—such as between switchboards and panelboards, between housings, boxes, and conduit bodies—all of which are “pull points.”
Note that the bends to be included in the summation include all deflections from a straight line, including a box kick at the end (about 20°), and bends of long radius, such as when conduits are joined outside of a trench and then forced in. There is no language that waives any bend of any radius, because pull force calculations show that the force to overcome a change in direction is independent of the radius; a longer radius results in more friction over the longer length involved. The sidewall bearing pressure is reduced on a longer radius bend, but that is only one of the forces to be considered.
Fig. 344-6. Rigid metal conduit—like all other types of conduits—is limited to not over 360° of bends between “pull points,” such as the panelboard and junction box shown here. (Sec. 344.26.)
The same concept of the number of bends permitted is given in all of the NEC Articles on raceways—ENT, EMT, IMC, rigid metal conduit, rigid nonmetallic conduit, and so on.
344.28. Reaming and Threading. As with IMC, rigid metal conduit always requires a bushing on the conduit end using locknuts and bushing for connection to knockouts in sheet metal enclosures (Fig. 344-7). But simply because a conduit can be secured to a sheet metal KO with two locknuts (one inside and one outside—as required by 250.97), it does not mean the bushing may be eliminated. Of course, no bushing is needed where the conduit threads into a hub or boss on a fitting or an enclosure.
Fig. 344-7. Conduit terminations, other than threaded connections to threaded fittings or enclosure hubs, must be provided with bushings for protection of the conductors. (Sec. 344.28.)
344.30. Securing and Supporting. As with Art. 342, the support distances to the first support can be increased to up to 1.5 m (5 ft) on a showing that there are no readily available support points at the default point of 900 mm (3 ft). In addition, where threaded couplings are used and the supports are arranged to prevent stress in the run and as may result from bends in the run from being visited on terminations, the greater support distances in the associated table can be used. 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.
344.42. Couplings and Connectors. Figure 344-8 shows a threadless connection of rigid metal conduit to the hub on a fitting. It is effective both mechanically and electrically if any nonconducting coating is removed from the conduit.
Fig. 344-8. Threadless connectors may be used on unthreaded end of conduit. (Sec. 344.42.)
A running thread is considered mechanically weak and has poor electrical conductivity.
Where two lengths of conduit must be coupled together but it is impossible to screw both lengths into an ordinary coupling, the Erickson coupling or a swivel-coupling may be used (Fig. 344-9). They make a rigid joint which is both mechanically and electrically effective. Also, bolted split couplings are available.
It is not intended that conduit threads be treated with paint or other materials in order to ensure watertightness. It is assumed that the conductors are approved for the locations and that the prime purpose of the conduit is for protection from physical damage and easy withdrawal of conductors for replacement. There are available pipe-joint compounds that seal against water without interrupting electrical conductivity.
Fig. 344-9. Fittings provide for coupling conduits where conduits cannot be rotated (turned). (Sec. 344.42.)
348.12. Uses Not Permitted. UL data supplement the Code data on use of standard flexible metal conduit—known also as “Greenfield” or simply “flex.” The UL data note:
This category covers flexible aluminum and steel conduit in trade sizes to 4 (metric designators 12 to 103) inclusive, flexible aluminum and steel conduit Type RW (reduced wall), flexible aluminum and steel conduit Type XRW (extra reduced wall) in trade sizes from to 3 (16 to 78) inclusive, for installation in accordance with Article 348 of ANSI/NFPA 70, “National Electrical Code” (NEC), for conductors in circuits of 600 V, nominal, or less. This product may also be used for installation of conductors in motor circuits, electric signs, and outline lighting in accordance with the NEC.
Flexible metal conduit (steel or aluminum) should not be used underground (directly buried or in duct which is buried) or embedded in poured concrete or aggregate, or in direct contact with earth or where subjected to corrosive conditions. In addition, flexible aluminum conduit should not be installed in direct contact with masonry in damp locations.
Flexible metal conduit no longer than six ft and containing circuit conductors protected by overcurrent devices rated at 20 A or less is suitable as a grounding means.
Flexible metal conduit longer than six ft has not been judged to be suitable as a grounding means.
To prevent possible damage to flexible aluminum conduit, flexible aluminum, and steel conduit Types RW and XRW, care must be exercised when installing connectors employing direct bearing set-screws.
PRODUCT MARKINGS
Flexible aluminum conduit is marked at intervals of not more than one ft with the letters “AL.”
Flexible aluminum conduit Type RW is marked at intervals of not more than one ft with the letters “AL” and “RW.”
Flexible steel conduit Type RW is marked at intervals of not more than one ft with the letters “RW.”
Flexible aluminum conduit Type XRW is marked at intervals of not more than one ft with the letters “AL” and “XRW.”
Flexible steel conduit Type XRW is marked at intervals of not more than one ft with the letters “XRW.”
Note that 348.12 is used to permit this wiring method in wet locations under several conditions; that permission has been revoked and now this wiring method is simply not permitted in a wet location.
348.20. Size. Part (A)(2) to this rule permits metric designator 12 (trade size ) flexible metal conduit to be used in lengths up to 6 ft (1.83 m) for connections to lighting fixtures. This provides correlation with 410.117(C), which includes 18 in. to 6 ft (0.45 to 1.8 m) of metal raceway for connecting recessed luminaires (generally the nonwired types). Figure 348-1 shows such application, and it is permissible to use 16 AWG or 18 AWG 150°C fixture wire, as shown in Fig. 360-1, for flex tubing.
The usual field application of trade designator 12 (trade size ) flexible metal conduit is covered in (A)(2)a, where the 1.8 m (6 ft) length (or shorter) can be used for any utilization equipment.
Part (A)(5) permits trade designator 12 (trade size ) flex if it is “part of a listed assembly,” which assumes it is supplied as part of UL-listed equipment, in lengths up to 6 ft (1.8 m) to connect “wired luminaire sections” as covered in 410.137(C).
Part (A)(3) permits flex in trade designator 12 (trade size ) to be used for the cable assemblies of modular wiring systems in hung ceilings [so-called manufactured wiring systems covered by 604.6(A)]. This is directed specifically to ceiling modular wiring. And the equipment grounding conductor run in such flex wiring assemblies may be either bare or insulated [see 604.6(A)(2)] (Fig. 348-1).
348.22. Number of Conductors. This section specifies that Table 1 of NEC Chap. 9 must be used in determining the maximum permitted number of conductors in ½-through 4-in. flex. Flexible metal conduit is permitted the same conductor fill procedure, although customized to the actual inner diameter, as other types of conduit and tubing. The number of conductors permitted in metric designator 12 (trade size ) flex is given in Table 348.22.
348.26. Bends—Number in One Run. Figure 348-2 shows the details of this section.
In this section, the limitation to no more than a total of 360° of bends between outlets applies to both exposed and concealed applications of standard metal flex and liquidtight metal flex.
Without restriction on the maximum number of bends in exposed and concealed work, bends could result in damage to conductors in a run with an excessive number of bends or could encourage installation of conductors prior to conduit installation, with conduit then installed as a cable system. A limit on number of bends for exposed and concealed work conforms to the requirements for other raceway systems.
Fig. 348-1. Flex of -in. size may be used for fixture “whip.” (Sec. 348.20.)
348.30. Securing and Supporting. Straps or other means of securing the conduit in place should be spaced much closer together (every 4½ ft [1.4 m] and within 12 in. [300 mm] of each end) for flexible conduit than is necessary for rigid conduit. Every bend should be rigidly secured so that it will not be deformed when the wires are being pulled in, thus causing the wires to bind. Note that there is some relief on the larger sizes, with an additional 300 mm (foot) to the first support for metric designator 41 and 53 (trade sizes 1½ and 2) and yet another 300 mm (foot) for larger sizes.
Fig. 348-2. Concealed or exposed flex must not have too many bends that could damage wires on pull-in. (Sec. 348.24.)
Figure 348-3 shows use of unclamped lengths of flex, as permitted by Exception No. 2. Figure 348-4 shows another example. Exception No. 3 is illustrated in Fig. 348-1. There is also a 1.8 m (6 ft) allowance for luminaire connections above hung ceilings.
Fig. 348-3. Lengths of flex not over 3 ft (900 mm) long may be used without clamps or straps where the flex is used at terminals to provide flexibility for vibration isolation or for alignment of connections to knockouts. (Sec. 348.30.)
Fig. 348-4. A length of flex not over 3 ft (900 mm) long connects conduit to pull box in modernization job, providing the flexibility to feed from fixed conduit to box. (Sec. 348.30.) Note that this run of flex already has almost 270° of bend before any bends in the rigid conduit are counted, and 300.18(A) requires raceway to be complete between pull points before wires are pulled in. Although there is no express language in the NEC that requires bends in succeeding raceway types joined end-to-end to adhere to the customary 360°rule, if there are several bends in the rigid conduit, the pull is likely to violate 300.17.
348.60. Grounding. As shown in the UL data under 348.12, flex in any length over 6 ft (1.83 m) is not suitable as an equipment grounding conductor, and an equipment grounding conductor must be used within the flex to ground metal enclosures fed by the flex. 250.118 permits flex as an equipment grounding conductor only under the given conditions—which would be the same as shown in Fig. 360-1 for flex tubing. Refer to 250.118 and to the discussion of grounding and bonding in 250.102.
The fourth paragraph of 348.60 essentially recognizes that an equipment bonding jumper used with flexible metal conduit may be installed inside the conduit or outside the conduit when installed in accordance with the limitations of 250.102. 250.118 and this rule make clear that use of flexible metal conduit as an equipment grounding conductor in itself is permitted only where a length of not over 1.8 m (6 ft) is inserted in any ground return path. The wording indicates that the total length of flex in any ground return path must not exceed 1.8 m (6 ft). That is, it may be a single 1.8 m (6 ft) length. Or, it may be two 900 mm (3 ft) lengths, three 600 mm (2 ft) lengths, or any total equivalent of 1.8 m (6 ft). If the total length of flex in any ground return path exceeds 1.8 m (6 ft), the rule requires an equipment grounding conductor to be run within or outside any length of flex beyond the permitted 1.8 m (6 ft) that is acceptable as a ground return path in itself. This rule also includes other flexible wiring methods in the ground return path; review 250.118(5) in this book for the complete story.
It should be noted that 250.118 is not applicable to the use of flex in a hazardous location. The rules in 501.30(B) and 502.30(B) simply require bonding for flex, with only a very narrow exception given in 502.30(B) (Fig. 348-5).
Fig. 348-5. Flex must always be bonded in Class I and Class II hazardous locations. (Sec. 348.60.)
250.118(5)d says that an equipment grounding conductor (or jumper) must always be installed for a length of metal flex that is used to supply equipment “where flexibility is necessary after installation,” such as equipment that is not fixed in place, or equipment that tends to vibrate, such as motors or transformers. That wording actually modifies the conditions under which a 6-ft or shorter length of metal flex (Greenfield) may be used for grounding through the metal of its own assembly, without need for a bonding wire. Because experience has indicated many instances of loss of ground connection through the flex metal due to repeated movement of a flex whip connected to equipment that vibrates, or flex supplying movable equipment, the last sentence requires use of an equipment bonding jumper, either inside or outside the flex in all cases where vibrating or movable equipment is supplied—for ensured safety of grounding continuity. The caption for Fig. 348-3 lists an example of each possibility. Lining up connections to a pull box does not involve flexibility after installation, but vibration isolation does.
350.1. Scope. This article covers metallic liquidtight flex. Liquidtight metal flex (often called “Sealtite” as a generic term in industry usage, although that word is the registered trade name of the liquidtight flex made by Anaconda Metal Hose Division) is similar in construction to the common type of flexible metal conduit, but is covered with an outer sheath of thermoplastic material (Fig. 350-1).
Fig. 350-1. Plastic jacket on liquidtight flex suits it to outdoor use exposed to rain or indoor locations where water or other liquids or vapors must be excluded from the raceway and associated enclosures. In lengths under 6 ft (1.8 m), UL-listed metal liquidtight flex does not require a bonding jumper. (Sec. 350.60.)
350.6. Listing Requirements. Most raceways are required to be listed, as is this one, but in this case it should be noted that unlike other raceways, enormous quantities of unlisted liquidtight flexible metal conduit were sold for many decades. Frequently supply houses would never stock the listed product and it was only available by special order. And the differences in this case are substantial. Only the listed product has copper wound into its convolutions that improve its ground-return effectiveness. In addition there are chemical differences in the stability of the nonmetallic jacket that bear on how well the product holds up in outdoor environments over time. In looking at existing installations, keep in mind that lengths of this product already in use may very well be substandard.
350.10. Uses Permitted. UL data on liquidtight metal flex present a variety of restrictions on the use of LFMC. And the basic wording of 350.10(A) essentially refers the reader to the limitations placed on the use of a particular type of LFMC by its specific listing.
As noted in the UL data, liquidtight flexible metal conduit is permitted for use directly buried in the earth if it is “so marked on the product.” The rule in 350.10 extends Code recognition to direct burial of liquidtight flexible metal conduit if it is “listed and marked” for such use. Based on many years of such application, liquidtight metal flex is recognized for direct burial, but any such use is permitted only for liquidtight flex that is “listed” by UL, or some other test lab, and is “marked” to indicate suitability for direct burial, to assure the installer and inspector of Code compliance. In the past, successful applications have been made in the earth and in concrete. Standard flexible metal conduit is prohibited from being used “underground or embedded in poured concrete or aggregate.” But that prohibition is not placed on liquidtight metal flex.
350.12(2) covers issues with operating temperatures that must be kept in mind. The UL guide card information provides the following points with respect to how to apply markings that may be found on the product:
Liquidtight flexible metal conduit suitable for direct burial and in poured concrete is marked “Direct Burial,” “Burial,” “Dir Burial,” or “Dir Bur.”
Liquidtight flexible metal conduit not marked with a temperature designation or marked “60 C” is intended for use at temperatures not in excess of 60°C (140°F).
Conduit intended for use in dry or oily locations at a temperature higher than 60°C (140°F) is marked “____ C dry, 60 C wet, 70 C oil res” or “____ C dry, 60 C wet, 70 C oil resistant” with “80” or “105” inserted as the dry-locations temperature.
Conduit marked “80 C dry, 60 C wet, 60 C oil res” or “80 C dry, 60 C oil resistant” is intended for use at 80°C (176°F) and lower temperatures in air, and at 60°C (140°F) and lower temperatures where exposed to water, oil or coolants.
Conduit that has not been investigated for use where exposed to oil is marked “OIL-FREE ENVIRONMENTS ONLY.”
350.20. Size. Refer to 348.20, which has the same rules. Figure 350-2 satisfies this subpart if the No. 12 wires are stranded, as required in 430.245(B). Table 348.22 accepts three No. 12 THHN plus an equipment grounding conductor of the same size in metric designator 12 (trade size ) Greenfield or liquidtight.
350.22. Number of Conductors. The maximum number of conductors must satisfy the rules of Table 1, Chap. 9. Refer to Tables C.5 and C.5(A) in Annex C for metric designators 16 to 103 (trade sizes ½ to 4), and to Table 348.22 for metric designator 12 (trade size ) data.
350.26. Bends—Number in One Run. Figure 348-2 shows this rule.
350.30. Securing and Supporting. As shown in Fig. 350-3, the rule permits a length of liquidtight flexible metal conduit not over 900 mm (3 ft) long to be used at terminals where flexibility is required without any need for clamping or strapping. Obviously, the use of flex requires this permission for short lengths without support. As in the case of flexible metal conduit, relief is now available for the distance to the first support point for the larger sizes.
Fig. 350-2. Both standard flexible metal conduit and liquidtight may be used here. (Sec. 350.20.)
Fig. 350-3. Unsupported length of liquidtight flex is okay at terminations. (Sec. 350.30.)
In addition, liquidtight metal flex is specifically recognized in lengths up to 1.8 m (6 ft) for fixture “whip,” without clamping of the flex. This covers a practice that has long been common. Either standard or liquidtight metal flex may be used to carry supply conductors to lighting fixtures—such as required by 410.117(C), where high-temperature wires must be run to a fixture terminal box.
350.42. Couplings and Connectors. As in the case of 348.42, it is a code violation to conceal an angle connector, because the eventual outcome will be some deciding they absolutely have to pull through it, and damaging insulation in the process. Angle connectors belong at the very end of a run, and where they can be disassembled and then reconnected in the field.
350.60. Grounding. According to the rule of 250.118, where flexible metal conduit and fittings have not been specifically listed as a grounding means, a separate grounding conductor (insulated or bare) shall be run inside the conduit (or outside, for lengths not over 6 ft [1.8 m]) and bonded at each box or similar equipment to which the conduit is connected. Refer to the discussion at 250.118(5 and 6) for full details of the circumstances under which this wiring method qualifies as an equipment grounding conductor. In addition, UL has some guide card information because it presents some of the same information in a mirror image from that in 250.118(6), because it clearly states the types of liquidtight flexible metal conduit is not to be considered as a qualified equipment grounding conductor, as follows:
1. The metric designator 41 (trade size 1½) and larger sizes.
2. The metric designators 12 and 16 (trade sizes and ½) sizes where used on circuits rated higher than 20 A, or where the total length in the ground return path is greater than 6 ft.
3. The metric designators 21, 27, and 35 (trade sizes ¾, 1, and 1¼) sizes where used on circuits rated higher than 60 A, or where the total length in the ground return path is greater than 6 ft.
As in the case of flexible metal conduit, where liquidtight flexible metal conduit requires flexibility as part of its normal usage pattern after the installation is complete, a separate equipment grounding conductor must be installed regardless of the size or length or protective ampere rating ahead of the flexible wiring method.
352.2. Definition. Rigid polyvinyl chloride conduit wiring systems include a wide assortment of products (Fig. 352-1). This article was formerly titled “Rigid Nonmetallic Conduit” but in the 2008 NEC, it was reserved to the PVC variety, with the fiberglass version (Type RTRC) given its own article (Art. 355). Note that since there is now no NEC article to cover nonmetallic conduit in the generic sense, and only three NEC articles cover rigid nonmetallic conduits (352 on PVC, 353 on HDPE, and 355 on RTRC), and since 110.8 spells out that only wiring methods recognized in the NEC as suitable are the ones enumerated therein, all other forms of rigid nonmetallic conduit, such as styrene, fiber, tile, asbestos cement, soapstone, and others that have been used in the past for underground use are no longer recognized as suitable under the NEC.
UL application data are detailed and divide rigid polyvinyl chloride conduit into two categories. Note that only the first, covering Schedule 40 and 80 PVC, involve wiring methods that can be used above grade (Fig. 352-2). Schedule 80 has a very heavy wall that subtracts from the inner diameter of the raceway, leading to special wire fill calculations (Fig. 352-3). The specific UL instructions on each category are as follows:
Fig. 352-1. Rigid nonmetallic conduit systems are made up of a wide variety of components—conduit, fittings, elbows, nipples, couplings, boxes, straps. (Sec. 352.2.)
Rigid Nonmetallic, Schedule 40 and Schedule 80 PVC Conduit (DZYR)
This category covers rigid nonmetallic PVC conduit (Schedule 40 and Schedule 80), including straight conduit and elbows in trade sizes ½ to 6 (metric designators 16 to 155) inclusive, intended for installation as rigid nonmetallic raceway for wire and cable in accordance with Article 352 of ANSI/NFPA 70, “National Electrical Code” (NEC).
Schedule 40 conduit is suitable for underground use by direct burial or encasement in concrete. Schedule 40 conduit marked “Directional Boring” (or “Dir. Boring”) is suitable for underground directional boring applications. Schedule 40 conduit is also suitable for aboveground use indoors or outdoors exposed to sunlight and weather where not subject to physical damage.
Schedule 80 conduit has a reduced cross-sectional area available for wiring space and is suitable for use wherever Schedule 40 conduit may be used. The marking “Schedule 80 PVC” identifies conduit suitable for use where exposed to physical damage and for installation on poles in accordance with the NEC.
Unless marked for higher temperature, rigid nonmetallic conduit is intended for use with wire rated 75°C or less including where it is encased in concrete within buildings and where ambient temperature is 50°C or less. Where encased in concrete in trenches outside of buildings it is suitable for use with wires rated 90°C or less.
Listed PVC conduit is inherently resistant to atmosphere containing common industrial corrosive agents and will also withstand vapors or mist of caustic, pickling acids, plating bath, and hydrofluoric and chromic acids.
PVC conduit and elbows (including couplings) that have been investigated for direct exposure to other reagents may be identified by the designation “Reagent Resistant” printed on the surface of the product. Such special uses are described as follows: Where exposed to the following reagents at 60°C or less: Acetic, Nitric (25°C only) acids in concentrations not exceeding ½ normal; hydrochloric acid in concentrations not exceeding 30 percent; sulfuric acid in concentrations not exceeding 10 normal; sulfuric acid in concentrations not exceeding 80 percent (25°C only); concentrated or dilute ammonium hydroxide; sodium hydroxide solutions in concentrations not exceeding 50 percent; saturated or dilute sodium chloride solution; cottonseed oil, or ASTM 3 petroleum oil.
Fig. 352-2. PVC and RTRC conduits are the only rigid nonmetallic conduits that may be used aboveground. And when enclosing conductors run up a pole (shown here feeding a floodlight at top), the PVC conduit must be Schedule 80 PVC conduit if it is exposed to physical damage, such as possible impact by trucks or cars. If the conduit is not so exposed, it may be Schedule 40 PVC conduit. See Sec. 300.5(D). (Sec. 352.2.)
Fig. 352-3. Extra-heavy-wall PVC conduit must have conductor fill limited to its reduced csa. (Sec. 352.2.)
PVC conduit is designed for connection to couplings, fittings, and boxes by the use of a suitable solvent-type cement. Instructions supplied by the solvent-type cement manufacturer describe the method of assembly and precautions to be followed.
Rigid Nonmetallic Underground Conduit, Plastic (EAZX)
This category covers plastic types of rigid nonmetallic conduit, including straight conduit, elbows, and other bends in sizes 1/2 to 6 (metric designators 16 to 155) inclusive, intended for installation underground as raceway for wire and cable in accordance with Articles 352 and 353 of ANSI/NFPA 70, “National Electrical Code” (NEC). This conduit may be: (1) polyvinyl chloride (PVC) Type A, Type EB, or Schedule 40, or (2) high density polyethylene (HDPE) Schedule 40, Schedule 80, EPEC A, EPEC B.
The conduit is intended for underground use under the following conditions, as indicated in the Listing Mark: (1) when laid with its entire length in concrete in any location (Type A), (2) when laid with its entire length in concrete in outdoor trenches (Type EB) and (3) direct burial with or without being encased in concrete (HDPE Schedule 40, Schedule 80, EPEC A, EPEC B, or PVC Schedule 40). The conduit is intended for use in ambient temperatures of 50°C or less.
Unless marked otherwise, Type A and HDPE Schedule 40, Schedule 80, EPEC A, EPEC B conduit is intended for use with wire rated 75°C or less. Type EB and Type A conduit, where encased in concrete in trenches outside of buildings, may be used with wire rated 90°C or less. HDPE Schedule 40, Schedule 80, EPEC A, EPEC B, or PVC Schedule 40 conduit, when directly buried or encased in concrete in trenches outside of buildings, may be used with wire rated 90°C or less.
Where conduit emerges from underground installation the wiring method shall be of a type recognized by the NEC for the purpose.
PVC conduit is designed for joining with PVC couplings by the use of a solvent-type cement. HDPE conduit is designed for joining by threaded couplings, drive-on couplings, or a butt-fusing process. Instructions supplied by the solvent-type cement manufacturer describe the method of assembly and precautions to be followed.
Note: As a result of the wording and intent of NEC 110.3(B), all the preceding application data constitute mandatory rules of the NEC itself—subject to the same enforcement as any other NEC rules.
When equipment grounding is required for metal enclosures of equipment used with rigid nonmetallic conduit, an equipment grounding conductor must be provided. Such a conductor must be installed in the conduit along with the circuit conductors (Fig. 352-4).
Fig. 352-4. Equipment grounding conductor must be used “within” the rigid nonmetallic conduit. (Sec. 352.60).
352.10. Uses Permitted. This section applies to use of the conduit for circuits operating at any voltage (up to 600 V and at higher voltages). The rules make rigid nonmetallic conduit a general-purpose raceway for interior and exterior wiring, concealed or exposed in wood or masonry construction—under the conditions stated. The Schedule 80 variety is acceptable where there would be a strong likelihood of damage to a less robust conduit.
Rigid nonmetallic conduit may be used aboveground to carry high-voltage circuits without need for encasing the conduit in concrete. That permission is also given in 300.37. Aboveground use is permitted indoors and outdoors.
Part (G) covers underground applications of all the types of rigid nonmetallic conduit—for circuits up to 600 V, as regulated by 300.5; and for circuits over 600 V, as covered by 300.50 (Fig. 352-5). Directly buried nonmetallic conduit carrying high-voltage conductors does not have to be concrete-encased if it is a type approved for use without concrete encasement. If concrete encasement is required, it will be indicated on the UL label and in the listing.
Figure 352-6 shows both underground and aboveground application. Referring to the circled numbers: (1) The burial depth must be at least 18 in. (450 mm) for any circuit up to 600 V. The buried conduit may be Schedule 40 or Schedule 80 (either without concrete encasement) or Type A or Type EB (both require concrete encasement). Refer to the opening coverage here and 300.5. (2) The concrete encasement where the conduit comes up from its 18-in. (450-mm) depth was required at one time by the NEC, but is no longer required. (See 300.5.) (3) The radius of the bend must comply with Table 2, Chap. 9 (minimum 18 in. [457.2 mm] if done in the field with a bending box; 330.2 mm [13 in.] if done in the factory with some form of one-shot bender). The conduit aboveground, on a pole or on a building wall, must be Schedule 80 if the conduit is exposed to impact by cars or trucks or to other physical damage. If the conduit is not exposed to damage, it may be Schedule 40.
In many cases where nonmetallic conduit is used to enclose conductors suitable for direct burial in the earth, inspectors and engineering authorities have accepted use of any type of conduit—PVC, polyethylene, styrene, and so on—without concrete encasement and without considering application of Code rules to the conduit. The reasoning is that because the cables are suitable for direct burial in the earth, the conduit itself is not required at all and its use is above and beyond Code rules. But temperature considerations are real and related to effective, long-time operation of an installation. Temperature effects must not be disregarded in any conduit-conductor application.
Fig. 352-5. All UL-listed rigid nonmetallic conduits are acceptable for use underground. PVC Schedule 40 and Schedule 80 and Type II fiber conduits do not require concrete encasement. Other types must observe UL and NEC rules on concrete encasement. (Sec. 352.10.)
Fig. 352-6. Schedule 80 PVC conduit may run up pole from earth to above-ground use. (Sec. 352.10.)
New in the 2008 NEC is the allowance here for the first time, for underground use only, of a foam-core PVC product. This conduit is significantly lighter than the customary solid product. It is referred to in the NEC as “nonhomogeneous” PVC and it can be used underground as both a direct-burial wiring method and where encased in concrete.
Although it is generally prohibited from supporting “equipment,” part (H) correlates with 314.23(E) Exception and recognizes the use of rigid nonmetallic conduit to support nonmetallic conduit bodies, provided the conduit body is no larger than the largest conduit that is providing the support.
352.12. Uses Not Permitted. It should be noted that nonmetallic conduit is not permitted in ducts, plenums, and other air-handling spaces. See 300.21 and the comments following 300.22.
Figure 352-7 shows a difference in application rules between rigid nonmetallic conduit and metal conduit with respect to supporting equipment. Part (B) allows limited use of rigid nonmetallic conduit for support of nonmetallic conduit bodies that do not contain devices or fixtures, as described in 352.10(H). This provision correlates with 314.23(F).
Fig. 352-7. This is okay for rigid metal conduit but not for rigid nonmetallic conduit. (Sec. 352.12.)
Parts (D) and (E) require care in use of the conduits so that they are not exposed to damaging temperatures. In using nonmetallic conduits, care must be taken to ensure temperature compatibility between the conduit and the conductors used in it. For instance, a conduit that has a 75°C temperature rating at which it might melt and/or deform must not be used with conductors which have a 90°C temperature rating and which will be loaded so they are operating at their top temperature limit. There is available PVC rigid conduit listed by UL and marked to indicate its suitability for use with all 90°C-rated conductors, thereby suiting the conduit to use with 90°C-rated conductors. The UL data described in 352.2 give the acceptable ambient temperatures and conductor temperature ratings that correlate to these NEC rules. Conductors with 90°C insulation may be used at the higher ampacities of that temperature rating only when the conduit is concrete encased (Fig. 352-8).
Fig. 352-8. UL data indicates that this violates Sec. 352.12(E). (Sec. 352.12.)
Probably the most important use of 352.12(E) Exception will be the use of PVC to enclose MV-105 conductors that have a 105°C temperature rating but that are being operated so as to not exceed 90°C. This is often done to increase reliability and to extend the expected project life of a job.
352.22. Number of Conductors. Refer to 344.22.
352.24. Bends—How Made. Refer to 344.24.
352.26. Bends—Number in One Run. Refer to 344.26.
352.28. Trimming. See Fig. 352-9.
Fig. 352-9. PVC conduit is designed for connection to couplings and enclosures by an approved cement, but leaving rough edges in the conduit end is a clear violation of Sec. 352.28. (Sec. 352.28.)
352.30. Securing and Supporting. In this section, Table 352.30(B), giving the maximum distance between supports for rigid nonmetallic conduit, permits greater spacing than some previous NEC editions. For each size of rigid non-metallic conduit, a single maximum spacing between supports, in feet, is given for all temperature ratings of conductors used in rigid nonmetallic conduit raceways (Fig. 352-10).
Fig. 352-10. Support rules on nonmetallic conduit are simple and direct. (Sec. 352.30.)
The wording in the paragraph of part (B) here is similar to subparts in Code articles covering other raceways and cables. This wording specifically recognizes holes in framing members as providing support for rigid nonmetallic conduit.
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.
352.44. Expansion Fittings. In applications in which the conduit installation will be subject to constantly changing temperatures and the runs are long, consideration must be given to expansion and contraction of PVC conduit. In such instances an expansion coupling should be installed near the fixed end of the run to take up any expansion or contraction that may occur. The normal expansion range of these fittings is about 6 in. (150 mm). The coefficient of linear expansion of PVC conduit is given in Table 352.44 of the NEC, and exceeds the expansion coefficient of steel by a factor of five. Without properly applied expansion fittings an aboveground installation subject to the usual range of outdoor temperatures will fail. If the job is installed on a cold winter day, and the conduit cannot freely move when the hot weather arrives, the result will look like an accordion. If the same job is installed on a hot day, come the winter months the conduit will contract to the point of pulling out of glue joints and exposing the conductors within.
In addition, when PVC conduit is exposed to direct sun, it absorbs even more heat than the surface wired over. A leading maker of PVC conduit recommends that at least 140°F be used as the upper temperature design parameter for this reason. If the minimum temperature were 0°F, that change in temperature would cause a 100-ft (30 m) length of conduit to expand and contract through a range of almost 6 in. (150 mm). The wiring system must be arranged with this in mind. Of course, some parts of the system will handle this movement without difficulty. For example, a service riser running straight up from a meter socket and ending at a weatherhead, if properly supported with clamps designed to allow movement, can expand and contract at will.
Expansion couplings are normally used where conduits are exposed. In underground or slab applications such couplings are seldom used because expansion and contraction can be controlled by bowing the conduit slightly. However, the rule of Sec. 300.5(J) now mandates that ground movement be addressed in underground installations. The FPN indicates methods—expansion joints—that may be used to satisfy the rule. Conduits left exposed for an extended period of time without expansion fittings during widely variable temperature conditions should be examined to see if contraction has occurred.
353.1. Scope. This article covers the requirements for the installation and use of Type HDPE conduit. It is a tubular raceway of circular cross section, and is available in discrete lengths, or in continuous lengths on a reel. It is available from metric designator 16 (trade size ½) up to and including metric designator 155 (trade size 6). This type of nonmetallic raceway has been used by utilities in various jurisdictions across the country for years. Its high durability and flexibility of application regardless of soil conditions makes it a very good choice for underground installations of electrical conductors. 353.2 provides a definition and 353.6 mandates the use of listed HDPE only.
353.10. Use Permitted. Part (1) indicates that Type HDPE is available in both individual cut-lengths or it may be supplied on a reel. It is suited for use in severely corrosive environments, cinder fill, and underground in direct contact with earth or concrete.
353.12. Uses Not Permitted. It must not be used above 50°C, either by reason of a high ambient temperature, or high operating temperatures of the enclosed conductors, or both. Polyethylene is flammable, and for that reason it is generally limited to direct burial applications. If not specifically prohibited, it is permitted above grade if encased in a concrete envelope not less than 50 mm (2 in.) thick. It is not permitted to be exposed, and it must not be used inside buildings.
353.22. Number of Conductors. Refer to 344.22.
353.24. Bends—How Made. NEC Table 354.24 specifies the minimum bending radius for this material, which is more restrictive than most tubular raceways. The table begins with a 250 mm (10 in.) radius for metric designator 16 (trade size ½) conduit, and rises to a 1.5 m (5 ft) minimum radius for the trade size 4 product. There is no table entry as of the 2008 NEC for metric designator 129 or 155 (trade size 5 or 6) conduit, so the manufacturer’s directions would need to be consulted for these sizes.
353.26. Bends—Number in One Run. Refer to 344.26.
353.48. Joints. All joints must be made by an approved method. The 2008 NEC added a note at this location suggesting the three methods of successfully splicing this wiring method, including heat fusion or electrofusion along with mechanical fittings.
This is the same product as Type HDPE conduit, and it follows the same installation rules, but conductors are preinstalled and shipped with the product by the manufacturer. Note that even though conductors arrive with the product preinstalled, it is still classified as a raceway, and the 360° maximum bends-in-the-run rule continues to apply. In this form, the upper size limit is metric designator 103 (trade size 4) conduit, also using the special bend radius Table 354.24.
355.2. Definition. This is the fiberglass entry in the nonmetallic conduit market. At one time it was only permitted for below-grade applications, but advances in chemistry have resulted in materials that meet above-grade fire resistance tests and it is now permitted for use in buildings where concealed in walls, floors, and ceilings, and also where exposed if identified for the application. It is stiffer than PVC and has a much lower coefficient of thermal expansion (about 45 percent of the value for PVC). Although the NEC uses the same support distance table as for PVC conduit, it does allow for longer support intervals if the product is listed for larger distances. It is more difficult to bend in the field, although an extensive range of different bend angle sweeps is available to accommodate field installation issues. It must not be used above 50°C unless listed for a higher temperature. The article is, in effect, a carbon copy of Art. 352 with two differences; first, there is no foam-core product mentioned for obvious reasons, and second, it has its own thermal expansion table, with a coefficient of expansion appreciably less (about 45 percent) of that for rigid PVC conduit. The “XW” variety of this conduit (see below) is its counterpart to Schedule 80 PVC. The UL guide card information follows:
This category covers reinforced thermosetting resin conduit and fittings intended for installation in accordance with Article 352 of ANSI/NFPA 70, “National Electrical Code” (NEC).
