Chapter Three

ARTICLE 300. WIRING METHODS

300.1. Scope. Part (A) of this section indicates that the rules of Art. 300 applyto all installations of the wiring methods covered by this article and the remain-der of Chap. 3. As is generally the case, clearly not all the general requirementsin Art. 300 apply to specialized installations, such as remote-control circuits, tosignal circuits, to low-energy circuits, to fire-protective signaling circuits, andto communications systems. The wording “unless modified by other articles” isintended to convey that idea.

Part (B) establishes what amounts to the boundary line between the NEC and ULor other equipment testing labs, at least with respect to this article, which has no“construction” part. In fact the NEC contains requirements throughout that areeffectively aimed at the equipment manufacturers, in the process of setting policyaround how products need to perform in order to be used safely, based on a con-sensus vote of a code-making panel (CMP). For only one of countless examples,the reason there is foam-core (“nonhomogeneous”) PVC conduit available is thata code-making panel voted it in [352.10(G)], but then also limited its use to under-ground applications. The reason that UL, with well-earned pride, points to its rep-resentation on all the code-making panels is that it is then in a position to makesure that its product standards correctly implement the policy decisions reachedthrough the NFPA consensus process. This rule clearly states that there is no intent,or permission, to apply Code rules within Art. 300 to the interior wiring of equip-ment to be connected. That is the province of the testing labs. If a product is“listed,” it has been tested and found to be essentially free from hazards. Gener-ally, the equipment is tested for use in accordance with the Code, which includesany Code-prescribed construction or performance requirements.

In part (C) of this section, a table is provided to allow proper selection of stan-dard trade size raceways and tubing whether applying the Code in the United

537

States or abroad. Simply determine the needed raceway size, in either system,and use Table 300.1(C) to establish the correct metric or English system sizeequivalent. The table also establishes that the dimensions are not real dimen-sions, but only to be used for identification purposes. Generally throughout thisbook we are following the NEC style, and therefore if we are referring to what isknown on the street as “3-in. conduit” we refer to it here, in the customary NECpractice of listing the metric size first and the English unit following in paren-theses, as “metric designator 78 (trade size 3) conduit.”300.3. Conductors. Part (A) requires that single conductors described inTable 310.13(A) must be used only as part of one of the wiring methods coveredin Chap. 3. This basically means that the various insulated conductors recognizedin Table 310.13(A) may not simply be strung overhead, without benefit of beingincorporated into a cable assembly or otherwise protected and supported by con-duit or tubing. The Exception to part (A) recognizes the permission given in 225.6,where individual conductors are permitted to be run as “open conductors” inoverhead feeders and branch circuits installed outdoors, as well as in festoonlighting.

Part (B) requires that all conductors of the same circuit—including the neutraland all equipment grounding conductors—must be run in the same raceway,cable tray, trench, cable, or cord. Part (B)(1) recognizes the use of separate race-ways and cables, where circuits are made up of multiple (two or more) sets ofconductors or cables in parallel. The exception that follows correlates with300.5(I) Exception No. 2 for isolated phase installations. Part (B)(2) in that rulenotes those very specific and unusual sections of the NEC, where an equipmentgrounding conductor may be run separately from the other conductors of the cir-cuit such as for DC circuits and for retrofits under the provisions of 250.130(C).

Part (B)(3) requires taking steps to prevent induced currents where nonmetal-lic or nonmagnetic sheathed conductors are run through metallic enclosure wallswith magnetic properties as covered in 300.20(B). The second sentence gives asimilar warning for Type MI cables and references 332.31, which addresses theconcern for induced currents. Both of those other rules discuss methods for pre-venting induced currents.

Part (4) of 300.3(B) permits limited use of a pull box equipped with a terminalblock for the connection of the system neutral, as the point of origin for branchcircuit neutral conductors. That is, a properly sized neutral is run to the pullbox—which is connected by an auxiliary gutter to a column-width panelboard—from the panelboard, and the individual branch-circuit neutrals may be run fromthe pull box and need not go back to the panelboard where the hot conductorsoriginate. This saves space within the panelboard, but is only permitted forcolumn-width panelboards connected by an auxiliary gutter to a pull box that ismanufacturer-equipped with a neutral terminal block. Note that there is noinductive heating in this arrangement and it is arguably permitted even withoutthis permission. Every ampere that comes up the auxiliary gutter from the panelon a branch-circuit conductor is equaled by an ampere of current moving downthe same gutter space to the panel over one of the feeder conductors.

Part (C) covers system separations under ordinary conditions. The first topic isconductors operating on different voltages, but not over 600 V. This rule clearlyallows conductors of different voltage systems, and whether dc or ac, to occupy

538 CHAPTER THREE 300.3

the same raceway, cable assembly, or enclosure. The only limitation here is that allthe conductors must be insulated for the maximum voltage that will be present inthe common location. For example, nonpower-limited 6-V circuit conductors run-ning to a valve motor can run in the same raceway as 277-V circuit wiring to a ductheater, if both sets of conductors have 600-V (technically 277-V) insulation.

This brings up Fig. 300-1. Whoever wired this furnace must have read thissection, because when he wanted to get his 24-V thermostat circuit neatly downto the burner control, he carefully ran 600-V THHN down the EMT riser alongwith the 120-V circuit for the burner motor, making the transition to thermostatwire where the THHN poked out of the upper end of the riser. A nice neat job,and a serious code violation. The thermostat circuit is a Class 2 power-limitedcircuit, covered under the enhanced system separation rule found in 725.136(A).This comes up so often that the NEC now has a note at this location. A Class 2circuit is a special condition, covered in Chap. 7 of the NEC. As such, the rulesin Chap. 7 automatically supersede or modify information in Chaps. 1 through4 of the NEC, as per 90.3. And for Class 2 wiring, it is forbidden to ever rely oninsulation alone to define a circuit separation. These systems are presumed tobe incapable of fire and electrocution hazard by virtue of the limitations builtinto their power supplies and the only way to be really sure of this is to keepthem out of common raceways and enclosures, except under extremely well-controlled situations. There is even a special exception to the rule forbiddingcables to be attached to electrical raceways [300.11(B)(2)] that allows the Class2 wiring to be attached to the outside of the raceway, and that would have beenthe way to wire the oil burner in Fig. 300-1.

300.3 WIRING METHODS 539

Fig. 300-1. The 14 AWG THHN leaving the box from the EMT riser and connected to 18-2 ther-mostat wire for a 24-V Class 2 circuit does comply with 300.3(C)(1), but violates 725.136(A).

This raises the question as to why the exception for a Chap. 6 rule is in thislocation, when there is only a note for the Chap. 7 rule. This exception is forsolar photovoltaic applications covered in 690.4(B). Photovoltaic source andoutput circuits are DC circuits, often without overcurrent protection becausethe only possible current is the output of the module. Ever since Art. 690entered the NEC in the 1984 edition, these circuits have been segregated fromnormal power applications; and there is no problem with technical merit. Infact, however, this exception should be removed and converted to a note aswell. Back in the 1984 NEC it was commonly believed that these exceptions inChaps. 1 to 4 were necessary, and this quite simply has not, as of yet, beenattended to.

Part (C)(2) of 300.3 states that conductors operating at more than 600 Vmust not occupy the same equipment wiring enclosure, cable, or racewaywith conductors of 600 V or less. There are five conditions where this can berelaxed; however, note that none of these involve a common raceway. Race-ways remain fully segregated, but there are limited applications where havingboth systems in a common enclosure is essential. Part (C)(2)c is intended toapply to enclosures, not raceways, such as used for high-voltage motorstarters, permitting the high-voltage conductors operating at over 600 V tooccupy the same controller housing as the control conductors operating atless than 600 V (Fig. 300-2).

540 CHAPTER THREE 300.3

Fig. 300-2. Control wires for high-voltage starters may be used in the starter enclo-sure, but not in raceway with power conductors. (Sec. 300.3.)

The last sentence in part (C)(2) is intended to prohibit unshielded conductors(now limited by 310.6 to 2.4 kV) from occupying the same enclosure, raceway,or cable, unless the actual voltage carried is the same. Now that the allowablevoltage for unshielded conductors has been reduced from 8 kV, the applicationof this in new construction is limited, but there are a lot of higher voltageunshielded conductors still in use. Some feel that the normal voltage dischargeand leakage current will be increased where a difference of potential existsbetween two unshielded conductors. It is theorized that the result of theincreased discharge and leakage will be premature insulation failure. Althoughthe assertion of premature failure has not been determined empirically, there isno data to suggest otherwise. Any problem can be avoided by simply comply-ing with this requirement and using separate raceway systems.300.4. Protection Against Physical Damage. This rule presents a general require-ment for ensuring that conductors are properly protected were “subject tophysical damage.” Common sense must be applied and the normal operation ofthe facility in question must be considered. If it appears that, during normaloperations, a given conductor or cable is likely to be damaged, then physicalprotection in the form of a raceway sleeve, an enclosure, kickplate, etc., mustbe provided. This requirement is a catchall. That is, the specific situationsspelled out in the subsequent parts of 300.4 must be provided with the physi-cal protection prescribed, under the circumstances described. But, in any otherlocation where conductors or cables are exposed to damage, suitable remediesmust be employed to satisfy this most basic requirement.

Part (A) gives the rules on protection required for cables and raceways runthrough wood framing members, as shown in Fig. 300-3. Where the edge of ahole in a wood member is less than 11/4 in. (32 mm) from the nearest edge of themember, a 1/16-in. (1.6-mm)-thick steel plate must be used to protect any cableor flexible conduit against driven nails or screws. The same protection isrequired for any cable or flexible conduit laid in a notch in the wood. But, asgiven in Exception No. 1, rigid metal conduit, electrical metallic tubing (EMT),intermediate metal conduit (IMC), and PVC conduits do not require such pro-tection. And, Exception No. 2 allows for the use of steel plates that are less than1/16-in. thick where the plate is “listed and marked” as providing “equal or bet-ter protection” from nails, etc.

Clearance must be provided from the edge of a hole in a wood member to theedge of the wood member. The NE Code requires only 11/4 in. (32 mm). This per-mits realistic compliance when drilling holes in studs that are 31/2 in. (89 mm)deep. It also was taken into consideration that the nails commonly used toattach wall surfaces to studs are of such length that the 11/4-in. (32-mm) clear-ance to the edge of the cable hole affords entirely adequate protection againstpossible penetration of the cable by the nail.

Figure 300-4 shows typical application of cable through drilled studs, withholes at centers and adequate clearance to the edge of the stud. Figure 300-5shows an objectionable example of a drilled hole, violating the rule of part (2)of this section, which warns against “weakening the building structure.” Figure 300-6 shows an acceptable way of protecting cables run through holesin wood members.

300.4 WIRING METHODS 541

542 CHAPTER THREE 300.4

Fig. 300-3. Holes in wood framing must not weaken structure or expose cable tonail puncture. (Sec. 300.4.)

300.4 WIRING METHODS 543

Drilled holes at the center of the face of a joist do not reducethe structural strength of the joist.

Signal and alarm wiring is run through the same stud holes as the NM cables.The NEC does not prohibit use of more than one cable through a single hole.

Fig. 300-4. Holes or notches in joists and studs must not weaken the structure of abuilding. (Sec. 300.4.)

In part (B), the rules on installations through metal framing members applyto nonmetallic-sheathed cable and to electrical nonmetallic tubing (ENT). Part(1) of 300.4(B) applies to NM cable run through slots or holes in metal framingmembers and requires that such holes must always be provided with bushingsor grommets installed in the openings before the cable is pulled. But thatrequirement on protection by bushings or grommets in the holes does not applyto ENT where run through holes in metal framing members.

Part (2) applies to both NM cable and ENT and requires that the cable or tub-ing be protected by a steel sleeve, a steel plate, or a clip when run through metalframing members in any case where nails or screws might be driven into thecable or tubing.

Part (C) requires cables and raceways above lift-out ceiling panels to be sup-ported as they are required to be when installed in the open. They may not betreated as if they were being run through closed-in building spaces or fishedthrough hollow spaces of masonry block.

Part (D) requires cables and raceways run along (parallel with) framing mem-bers (studs, joists, rafters) to have at least a 11/4-in. (32-mm) clearance from the

544 CHAPTER THREE 300.4

Fig. 300-5. Excessive drilling of structural wood members can result indangerous notching (arrow) that weakens the structure, violating Sec.300.4(A)(2). (Sec. 300.4.)

nearest edge of the member; otherwise the cable or raceway must be protectedagainst nail or screw penetrations by a steel plate or sleeve at least 1/16-in. (1.6 mm)thick. Part (D) exists because of many persistent reports of nail and screw pen-etrations of both metallic and nonmetallic cables and raceway. The rule appliesto both exposed and concealed locations. Exception No. 1 excludes IMC, rigidmetal conduit, rigid nonmetallic conduit, and EMT from the rule. The rule willapply to Romex (Type NM), BX (Type AC), flexible metal conduit, ENT, TypeMC cable, and all other cables and raceways, except those excluded by Excep-tion No. 1. (Fig. 300-3, bottom). Exception No. 2 excludes from this rule con-cealed work in finished buildings and finished panels in prefab buildings,where cables may be fished. Exception No. 3 recognizes the use of a “listed andmarked” steel plate that is less than 1/16-in. thick as providing the requiredprotection.

Part (E), on wiring under roof decking, is new in the 2008 NEC and it requirescare in interpretation in order to achieve the intended outcome. It requires allwiring, whether cables or raceways (subject to an exception for rigid and forintermediate metal conduits) to be mounted far enough below “metal-corrugated sheet roof decking” so subsequent reroofing and repairs don’t endup impaling the wiring with the long screws commonly used to hold down thenew surface, as explained in the note. This change follows actual loss experi-ence. Review Fig. 300-7 to understand the interpretive issue.

300.4 WIRING METHODS 545

Fig. 300-6. Steel plates are attached to wood structure member to protect cable frompenetration by nail or screw driven into finished wall, where the edge of the cablehole is less than 1/4 in. (31.8 mm) from the edge of the wooden member. (Sec. 300.4.)

The literal text calls for 38 mm (11/2 in.) spacing between the “nearest” outsideedge of the wiring method (meaning nearest to the roof decking) and the “nearest”edge of the roofing (meaning nearest to an observer on the floor). This puts thewiring well below the lowest part of the corrugations, which is appropriatebecause subsequent roofing activities proceed blindly, with screws sized to pen-etrate to a bottom of a corrugation as easily as an upper portion. However, thelanguage chosen does not clearly convey the intent. If the wiring is placed in thehollow, especially on top of bar joists, it will likely be damaged by the drill-pointscrews used to apply the roofing. Review this carefully with the inspector.

Although the exception waives the rule for installations using heavy-wallsteel (rigid or intermediate) conduit, take care in using this exception. Tests runby UL on a variety of wiring methods to determine their susceptibility to pene-tration under conditions covered by the related rules in 300.4(D) showed thateven heavy wall conduits were no match for drill-point screws. Interestingly,only unrestrained steel-armored cables that could roll out of the way typicallysurvived the testing, a result not reflected in the Code.

Part (F) presents protection requirements where cables are run in “shallowgrooves.” This rule calls for the same 1/16-in (1.6-mm)-thick steel kick plate,sleeving, or the equivalent. Alternately, one may locate the cable with at least11/4 in. (32 mm) of “free space” for the cable’s entire concealed length. The “freespace” is to be measured from the top of the groove to the top of the cable, notto the bottom of the groove. This rule is aimed at applications with exposedbeam construction where grooves or channels are cut in the beams for supplyconductors to lighting fixtures or ceiling fans.

Part (G) requires insulated bushings on raceway entries to enclosures ifwiring 4 AWG or larger will be installed. The insulated surface must berounded. Nonmetallic conduit terminal adapters are insulating, but the edge ofthe open throat is not always as rounded as perhaps it should be, leaving roomfor interpretation. It is important to remember that this is a rule that applies towiring being pulled into raceways, and therefore there is no requirement to putan insulating bushing on the end of a SE cable connector, for example. If anenclosure has a molded hub, it will be sufficiently rounded on the inside andno additional protection is required.

546 CHAPTER THREE 300.4

Fig. 300-7. The new rule to avoid damage from roof repair requires care-ful interpretation in order to be applied to achieve the intended effect.[Sec. 300.4(E).]

NEC intent:“nearest surface”= surface closestto an observerstanding on thefloor

Wiring

method

Likely interpretation:“nearest surface” =surface closest to thewiring method; willresult in damagedwiring.

Roofing material, withscrews randomly appliedand long enough topenetrate any part of thedecking, including thelowest parts of thecorrugations.

11/2”

300.5. Underground Installations. This section is a comprehensive set of rules oninstallation of underground circuits for circuits up to 600 V. (Higher-voltage cir-cuits must satisfy 300.50.) Table 300.5 in the Code book establishes minimumearth cover needed for specific conditions of use. Figure 300-8 shows coverrequirements for rigid metal conduit and IMC. Figure 300-9 shows the basicdepth requirements for the various wiring methods. In all instances, “cover” isnot the depth of the trench. It is the depth of the trench plus the diameter of thewiring method, which equals the amount of dirt on top of the wiring methodwhen the job is complete.

Because Table 300.5 does not specifically mention EMT, it could be taken toindicate that the NEC does not recognize EMT for underground use. But358.10(B) does recognize EMT for direct earth burial if “protected by corrosionprotection and judged suitable for the condition”, and so does UL, with thisstipulation: “In general, electrical metallic tubing in contact with soil requiressupplementary corrosion protection.” Note that such protection is not literallymandatory in all instances, but it is close. The UL note means to indicate thatEMT might be buried without a protective coating (like asphalt paint) wherelocal experience verifies that soil conditions do not attack and corrode theEMT. On the other hand, UL does not evaluate supplementary corrosion protec-tion on this product with respect to the corrosive influences of soils, so theinspector is on his own in choosing to permit this. Supplementary corrosionprotection must always be applied unless there is a solid local record of posi-tive experience, which is very unusual. On balance, EMT should not be usedfor direct burial. Refer to the more extensive discussion at 358.10 in this bookfor additional information on this point.

Figure 300-10 shows modifications of basic cover requirements. If a 2-in.(50-mm)-thick or thicker concrete pad is used in the trench over an under-ground circuit other than rigid metal conduit or IMC, the basic burial depth inTable 300.5 may be reduced by 6 in. (150 mm). It should be noted that the intenthere is for the concrete pad to be in the trench, right over the cable or raceway.The wording must be taken to mean that it may not be a walk or other concreteat grade level. And the burial depth may not be reduced by more than 6 in.(150 mm) no matter how thick the concrete pad is. This rule is at odds withNote 3 to Table 300.50, where burial depth for high-voltage circuits may bereduced “6 in. (150 mm) for each 2 in. (50 mm) of concrete” or equivalent pro-tection in the trench over the wiring method (other than rigid metal or IMC).Remember this when installing underground runs of conductors and cablesthat are rated over 600 V. For those installations, compliance with 300.50, not300.5, must be ensured.

Note that in Table 300.5, rigid metal conduit or IMC that is buried in theground must have at least a 6-in. (150-mm)-thick cover of earth or earth plusconcrete—even if it has a 2-in. (50-mm)-thick concrete pad over it. But rigidmetal conduit, IMC, or other raceways may be installed directly under a 4-in. orthicker exterior slab that is not subject to vehicle traffic, without any need forearth cover. Given the fact that rigid metal conduit or IMC may be laid directlyon the ground (which supports it for its entire length) and would not necessar-ily require any concrete cover, there is no reason why it cannot be laid on the

300.5 WIRING METHODS 547

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300.5 WIRING METHODS 549

Fig. 300-9. These are the basic burial depths, but variations are recognizedin Table 300.5 for certain conditions. (Sec. 300.5.)

ground or flush with the ground and covered with at least 4 in. (102 mm) ofconcrete. (See Fig. 300-8.)

300.5 only applies to “underground installations” and is not applicable if theconduit is laid directly on the ground. No Code rule prohibits conduit laid onthe ground, provided the conduit is “securely fastened in place” (300.11) and isnot exposed to physical damage such as vehicular traffic, and many such instal-lations have been made for years. But, when conduit is installed in the ground,there is serious concern about damage due to digging in the ground, which300.5 addresses.

As shown in Fig. 300-11, Table 300.5 recognizes that raceways run underconcrete slabs at least 4-in. (102 mm) thick or under buildings have sufficientprotection against digging and are not required to be subject to the burial-depthrequirements given in the top line of Table 300.5. Where raceways are soinstalled, the rule requires that the slab extend at least 6-in. (150 mm) beyondthe underground raceway, as follows:

1. Any direct burial cable run under a building must be installed in a race-way, as required by 300.5(C), and the raceway may be installed in theearth, immediately under the bottom of the building, without any earthcover.

2. Any direct buried cable under a slab at least 4-in. (102 mm) thick and notsubject to vehicles is subject to the 18-in. (450-mm) minimum burial-depth requirement of Table 300-5. The reason for the equivalence in cover

550 CHAPTER THREE 300.5

Fig. 300-9. (Continued)

Fig. 300-10. Concrete pad “in trench” permits what amounts to a 6-in. (150-mm)reduction of burial depth for circuits up to 600 V for other than RMC, IMC, and appli-cations covered in column 5. (Sec. 300.5.)

between the 2-in. (50 mm) concrete-in-trench rule and the 4-in. (102 mm)concrete slab rule is that when the concrete is buried in the trench anexcavator will recognize it as a protective structure. The surface slab maynot be easily recognized as performing an additional protective function.

300.5 WIRING METHODS 551

Fig. 300-11. Table 300.5 eliminates burial-depth requirements for directburied raceways under specified conditions. (Sec. 300.5.)

Figure 300-12 shows the mandatory 24 in. (600 mm) of earth cover given inTable 300.5 for any wiring methods buried under public or private roads,alleys, driveways, parking lots, or other areas subject to car and truck traffic. Aminimum earth cover of 24 in. (600 mm) is required for any underground cableor raceway wiring that is installed under vehicle traffic, regardless of concreteencasement or any other protective measure. This requires the minimum 2-ft(600-mm) earth cover for wiring under the designated areas, including drive-ways and parking areas of private residences. The minimum earth cover forcables and raceways under driveways and parking areas for one- and two-family dwellings is only 18 in. (450 mm).

Fig. 300-12. All wiring methods must be at least 2 ft (600 mm)under vehicular traffic. (Sec. 300.5.)

Table 300.5 (second vertical column from right) gives limited use of lesserburial depth for the residential circuits described, as shown in Fig. 300-13. AnyGFCI-protected residential “branch circuit” not over 120 V and protected at 20 Aor less may be buried only 12 in. (300 mm) below grade, instead of, say, 24 in.(600 mm), as required for Type UF cable for any nonresidential use or for a res-idential “feeder.”

552 CHAPTER THREE 300.5

Fig. 300-13. This is OK only for a residential branch circuit rated not over20 A and protected by a GFCI circuit breaker. (Sec. 300.5.)

Figure 300-14 shows three other special conditions for burial depth. Table 300.5(vertical column at right) recognizes reduced burial depth for low-voltage land-scape lighting circuits and supply circuits to lawn sprinkler and irrigationvalves, as shown in Fig. 300-15. This recognizes the reduced hazards and safetyconsiderations for circuits operating at not more than 30 V. However, such sys-tems must be supplied by Type UF cable or other “identified” cable or raceway.Such identification is generally required to be marked by the manufacturer. Onthe cables and conductor, the marking should include a “W” indicating suit-ability for use in a “wet location” to ensure compliance with part (B).

Part (B) of 300.5 requires all electrical cables and conductors to be listed foruse in “wet locations” where installed underground, even if they are containedwithin a raceway or other enclosure (Fig. 300-16). This effectively applies to allraceways, cables, boxes, enclosures, etc., covered by Chap. 3.

Figure 300-17 shows the rule of part (C). Note that this rule has been clarifiedto avoid the former implication that the raceway always had to exit beyond thebuilding perimeter. It is now clear that a raceway can go from one point toanother below the floor grade of a building without needing to pass beyond thewall line. However, the requirement for all cables under a building to be inraceway still applies.

As shown at the top of Fig. 300-18, direct buried conductors or cables com-ing up a pole or on a building from underground installation must be protectedfrom the minimum required burial distance below grade (from Table 300.5, butnever required to be more than 18 in. [450 mm] into the ground) to at least 8 ft(2.5 m) above grade, as required by part (D) of this section. Where exposed tophysical damage, raceways on buildings and raceways on poles must be rigidconduit, IMC, PVC Schedule 80, or equivalent, and the raceway or other enclo-sure for underground conductors must extend from below the ground line up to8 ft (2.5 m) above finished grade. If a raceway on a building or on a pole is not

subject to physical damage, EMT or Schedule 40 PVC may be used instead ofother raceways.

Figure 300-19 shows the service lateral protection ribbon rule from300.5(D)(3), along with other requirements including the ground movementaccommodation rule in 300.5(J) and a warning on an important limitation onstraight underground USE cable not colisted with a building wire designation.

300.5 WIRING METHODS 553

Fig. 300-14. These applications are also covered in the table burialdepths. (Sec. 300.5.)

Figures 300-20 and 300-21 show other rules of 300.5. Part (E) covers directburial splices, which do not require boxes but do need to comply with110.14(B), which requires the splicing method to be listed for this application.There are twist-on wire connectors listed for direct burial, and they are anappropriate solution for single-conductor type USE; however, do not use theseconnectors for conventional Type UF cable. The direct-burial listing on TypeUF cable applies to the entire cable assembly and not to the individual conduc-tors within the jacket. Therefore, if a splice is made with the jacket strippedback and these connectors made up, the resulting exposure of individual con-ductors within the UF cable assembly to direct soil burial violates the UF cablelisting. The splicing method must provide an outer surface that is continuousfrom cable jacket to cable jacket.

Note that part (F) specifically requires that backfilled trenches must containany necessary protection for raceways or cables buried in the trench. It specifiesthat sand or suitable running boards of wood or concrete or other protection

554 CHAPTER THREE 300.5

Fig. 300-15. Reduced burial depth for low-voltage landscape lightingand lawn-sprinkler controls. (Sec. 300.5.)

Fig. 300-16. Conductors run underground must be listed for “wet loca-tions.” (Sec. 300.5.)

must be afforded in those cases where backfill consists of heavy stones or sharpobjects that otherwise would present the possibility of damage to the cable orraceway.

Part (I) of this section requires that an underground circuit made up of single-conductor cables for direct burial must have all conductors of the circuit run inthe same trench. That rule raises the question: When an underground directburial circuit is made up of conductors in multiple, must all the conductors beinstalled in the same trench? And if they are, is derating required for more thanthree current-carrying conductors in a trench, just as it would be for more thanthree conductors in a single raceway? The answer to both questions is yes.

The wording of the rule in part (I) clearly indicates that all the conductorsmaking up a direct burial circuit of single conductors in parallel must be run inthe same trench and must be “in close proximity.” The wording of 300.5(I) alsorequires that all conductors of a circuit be run in the same raceway if a racewayis used (with building wire suitable for wet locations, such as THW or THWN).But Exception No. 1 permits parallel conductor makeup in multiple raceways,with each raceway containing all hot, grounded, and (if used) grounding con-ductors of the circuit. And Exception No. 2 to 300.5(I) recognizes the use of“isolated phase” installations, provided the rules for paralleling conductors—given in 310.4—and the rules for reducing the effects of inductive heating inadjacent metallic materials—as given in 300.20—are satisfied.

300.5 WIRING METHODS 555

Fig. 300-17. Burial of cable in earth is not permitted under a building. (Sec. 300.5.)

When multiple-conductor makeup of a circuit is installed with all theparallel-circuit conductors in the same trench, it is necessary to observe therule of 310.15(B)(2) of the NE Code, which states:

Where single conductors . . . are installed without maintaining spacing for a contin-uous length longer than 24 in. (600 mm) and are not installed in raceways, the allow-able ampacity of each conductor shall be reduced as shown . . . ”

This means that the same deratings must be made as when more than threeconductors are used in a single conduit, as explained in 310.15(B)(2)(a). Cer-tainly, direct burial single conductors are covered by that requirement becauseTable 310.16 specifically covers direct burial conductors.

These Code rules often make for tricky and troublesome applications. Forinstance, as shown in Fig. 300-22, an underground circuit of Type USE insu-lated aluminum conductors might be used for a 3-wire, single-phase service toa multifamily dwelling. Because that is a residential service, 310.15(B)(6) mightbe considered to gain a higher-than-normal ampacity for the conductors; how-ever, the rule has been rewritten to exclude multifamily applications and itonly applies to applications where the conductors only supply a one-family

556 CHAPTER THREE 300.5

Fig. 300-18. Conductors from underground must be protected. (Sec. 300.5.)

dwelling unit. Assuming that a 400-A conductor ampacity is indicated by thecalculated demand load from Art. 220, each phase leg of the service feeder musthave an ampacity of 400 A. Refer to the 75°C column in Table 310.16: A 250kcmil THW aluminum has an ampacity of 205 A. Two such conductors per hotleg and two for the neutral would give the required 400-A capacity for the ser-vice. But how should the parallel circuit be run? All the circuit conductorsmust be run in close proximity in the same trench, as required by 300.5(I). Thatmeans all six USE conductors are in the same trench; and, because the neutralsdo not count as current-carrying conductors, the derating of these conductors

300.5 WIRING METHODS 557

Service lateral rising to ameter socket, where subject tosettlement or frost, must bearranged to prevent damageto itself and the socket.

NEC: 300.5(J) Unless protected with concrete, servicelaterals such as this, buried at least 450mm (18 in.) must use a warning ribbon atleast 300 mm (1 ft) above the cable (ornonmetallic conduit).

NEC: 300.5(D)(3)

Type USE cable, acceptable for direct burialand use outdoors. This wiring must never beused indoors unless dual rated as both TypeUSE and as building wire, such as Type RHW.Make a transition, if necessary, in the metersocket.

NEC: 338.12(B)(1)

Expansion fitting

Grade

“S” loop

Frost line

WARNING: BURIED ELECTRICAL LINE WARNING: BURIED EL

Fig. 300-19. Underground installations must accommodate ground movement. [Sec. 300.5(J).]