Wiring Methods and Materials

Wiring Methods and Materials (Minnesota Journeyman)

Learning Objectives

1.1 Article 300 General Rules and Physical Protection (300.5, 300.6, 300.7, 300.9, 300.22)

Section 300.5(B) requires all conductors of the same circuit and, where used, the grounded conductor and all equipment grounding and bonding conductors to be contained within the same raceway, conduit body, auxiliary gutter, cable tray, trench, or cable unless permitted by 300.5(B)(1) through 300.5(B)(4); 300.7(I) repeats that rule for buried work, and removing an equipment grounding conductor from a trench defeats the effective ground-fault current path required by 250.4(A)(5). Section 300.5(C)(a) permits ac and dc circuits rated 1000 volts ac, 1500 volts dc, nominal, or less to occupy the same enclosure, cable, or raceway where every conductor has an insulation rating at least equal to the maximum circuit voltage applied to any conductor within it, and 300.5(C)(2) forbids circuits over 1000 volts ac, 1500 volts dc from sharing that space with circuits rated 1000 volts ac, 1500 volts dc, nominal, or less unless permitted in accordance with 305.13(B).

Section 300.22(A) requires ac conductors in ferrous metal enclosures or raceways to be arranged so as to avoid heating the surrounding ferrous metal by induction, which means grouping all phase conductors plus, where used, the grounded conductor and all equipment grounding conductors together; Exception No. 2 permits a single conductor only for skin-effect heating per 426.42 and 427.47. Section 300.22(B) resolves a single alternating-current conductor passing through magnetic metal by cutting slots between the individual holes, or by passing all the conductors of the circuit through an insulating wall large enough for all of them.

Section 300.6 keeps wiring 32 mm (1 1/4 in.) from the edges of wood and framing members where nails or screws are likely to penetrate, with steel plate(s) or bushing(s) at least 1.6 mm (1/16 in.) thick where that distance cannot be maintained, and requires rounded protection for 4 AWG or larger insulated conductors entering an enclosure. Section 300.9(A) requires sealing where condensation is a known problem, 300.9(B) requires expansion, expansion-deflection, or deflection fittings where necessary for thermal expansion and contraction, and 300.11 covers raceways in wet locations above grade.

1.2 Underground and Buried Installations (300.7, 305.15, 110.72, 110.75)

Table 300.7(A) is read by row before column. For All locations not specified below, direct burial cables or conductors require 600 mm (24 in.); rigid metal conduit and intermediate metal conduit 150 mm (6 in.); electrical metallic tubing and nonmetallic raceways listed for direct burial 450 mm (18 in.); and residential branch circuits of 120 volts or less with GFCI protection 300 mm (12 in.). Wiring methods under streets, highways, roads, alleys, driveways, and parking lots require 600 mm (24 in.) in every column, and a direct burial method under 50 mm (2 in.) of concrete takes 450 mm (18 in.).

Section 300.7(D)(1) requires direct-buried conductors and cables named in Columns 1 and 4 of Table 300.7(A) to be protected by enclosures or raceways extending from the minimum cover distance below grade to a point at least 2.5 m (8 ft) above finished grade, never required to exceed 450 mm (18 in.) below finished grade; 300.7(D)(3) requires a warning ribbon at least 300 mm (12 in.) above the installation. Section 300.7(E) permits direct-buried conductors or cables to be spliced or tapped without splice boxes where made in accordance with 110.14(B), and 300.17(C) drops the box requirement where cables enter or exit conduit used for support or protection against physical damage, requiring only a fitting on the ends.

Above 1000 volts ac, 1500 volts dc, nominal, Table 305.15(A) works like Table 300.7(A): for All locations not specified it requires 750 mm (30 in.) for direct burial cables or conductors rated 22 kV or less, 450 mm (18 in.) for raceways rated 22 kV or less, and 900 mm (36 in.) for direct burial cables rated over 22 kV to 40 kV, and Note c permits a raceway encased in not less than 50 mm (2 in.) of concrete to be 150 mm (6 in.) below grade.

Section 110.72 requires a manhole work space not less than 900 mm (3 ft) wide where cables are on both sides and 750 mm (2 1/2 ft) where cables are on one side, with headroom not less than 1.8 m (6 ft); 110.75(A) requires rectangular access openings not less than 650 mm × 550 mm (26 in. × 22 in.).

1.3 Boxes, Conduit Bodies, and Completing the Run (300.13, 300.15, 300.16, 300.17, 300.19, 300.20)

Section 300.17 requires a box or conduit body at each outlet point, switch point, conductor splice, junction, termination, wiring method transition, and pull point for conduit, tubing, Type AC, Type MC, and Type MI cable, nonmetallic-sheathed cable, and other cables, unless otherwise permitted in 300.17(A) through 300.17(L); 300.17(C) is the exception for cables entering or leaving conduit used for protection.

Section 300.13(A) requires raceways, cable assemblies, boxes, cabinets, and fittings to be securely fastened in place, and 300.13(C) permits a raceway to support other raceways, cables, or nonelectrical equipment only under 300.13(C)(1) through 300.13(C)(4), which include being identified as a means of support, supporting boxes or conduit bodies per 314.23 or luminaires per 410.36(E), or supporting an equipment bonding jumper. Section 300.15(A) requires conductors in raceways to be continuous between outlets, boxes, and devices, with no splice or tap within a raceway unless permitted by 300.17, 368.56(A), 376.56, 378.56, 384.56, 386.56, 388.56, or 390.56; 300.15(B) states that in multiwire branch circuits the continuity of a grounded conductor must not depend on device connections that would interrupt it when removed, which neither bonding under 250.148 nor terminal tightening under 110.14(D) cures. Section 300.16(A) requires at least 150 mm (6 in.) of free conductor, measured where it emerges from its raceway or cable sheath, at each outlet, junction, and switch point, and 300.16(B)(2) requires each conductor to extend at least 75 mm (3 in.) outside an opening less than 200 mm (8 in.) in any dimension.

Section 300.19(A) limits conductors and cables in a complete raceway to the percentage fill in Chapter 9 Table 1, with Table 300.19(B) holding metric designators and trade sizes. Section 300.20(A) requires raceways to be installed complete between outlet, junction, or splicing points prior to the installation of conductors unless permitted by 300.20(A)(1) through 300.20(A)(6), which cover busways, listed manufactured assemblies per 604.100, exposed raceways with hinged or removable covers, and raceways installed without terminating connections at the equipment.

1.4 Securing, Supporting, Fire, and Air-Handling Spaces (300.13, 300.21, 300.23, 300.25)

Section 300.13(B)(1) states that wiring above suspended ceilings must not be secured to or supported by the ceiling grid system, including the ceiling support wires, and 300.13(E)(1) requires cable ties used as a means of securement and support to be listed and identified for securement and support.

Section 300.21(A) requires conductors in vertical raceways to be supported if the vertical rise exceeds the values in Table 300.21(A), with at least one support method for each conductor at the top of the riser or as close to it as practical, and intermediate supports to limit supported lengths to: 30 m (100 ft) for copper conductors 6 AWG through 1/0 AWG, 60 m (200 ft) for aluminum or copper-clad aluminum conductors in that size range, 25 m (80 ft) for copper conductors 2/0 AWG through 4/0 AWG, and 18 m (60 ft) for copper conductors over 4/0 AWG through 350 kcmil.

Section 300.23 requires installations in hollow spaces, shafts, and air-handling ducts to be made so that the spread of fire or products of combustion is not substantially increased, requires openings around penetrations of fire-resistant-rated walls, partitions, floors, or ceilings to be firestopped by approved methods that maintain the rating, and notes the 600 mm (24 in.) separation that usually applies between boxes on opposite sides of such a wall.

Section 300.25(A) prohibits any wiring system in ducts used to transport dust, loose stock, or flammable vapors, or in a shaft containing only such ducts used for vapor removal or for ventilation of commercial-type cooking equipment. Section 300.25(B) permits only Type MI cable without an overall nonmetallic covering, Type MC cable with a smooth or corrugated impervious metal sheath, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit inside ducts fabricated for environmental air, with flexible metal conduit allowed in lengths not exceeding 1.2 m (4 ft) for adjustable equipment.

1.5 Conductor Use, Parallel Conductors, and Ampacity (Article 310)

Section 310.10(C) permits insulated conductors and cables in wet locations to be moisture-impervious metal-sheathed, to be Types RHW-2, THW-2, THWN-2, XHHW-2, or ZW, or to be listed for use in wet locations, and 310.10(D)(2) requires a covering such as tape or sleeving exposed to the sun to be listed as sunlight resistant. Temperature limitations of conductors are addressed in 310.14(A)(3) and 110.14(C).

Section 310.10(G)(1) permits circuit conductors to be connected in parallel, joined at both ends, only in sizes 1/0 AWG and larger, and 310.10(G)(2) requires each paralleled conductor, equipment bonding jumper, or supply-side bonding jumper to be the same length, material, circular mil area, and insulation type, and terminated in the same manner; 310.10(G)(3)(a) requires separately run cables or raceways to have the same number of conductors and the same electrical characteristics, while 310.10(G)(3)(b) states that one paralleled set need not match another set's physical characteristics.

Section 310.12(A) permits service conductors supplying the entire load of a one-family dwelling on a single-phase, 120/240-volt system to have an ampacity not less than 83 percent of a service rating of 100 through 400 amperes, with Table 310.12(A) usable where no adjustment or correction factors are required and the conductors are rated 75 °C (167 °F) or greater; the table gives 4 AWG copper and 2 AWG aluminum or copper-clad aluminum for 100 amperes, 1/0 AWG copper for 175 amperes, 2/0 AWG copper for 200 amperes, 3/0 AWG copper for 225 amperes, and 4/0 AWG copper for 250 amperes.

Table 310.15(B)(1)(1), based on 30 °C (86 °F), corrects the ampacities of Table 310.16 and Table 310.17: a 90 °C rated conductor takes 0.96 at 87 °F to 95 °F, 0.91 at 96 °F to 104 °F, and 0.87 at 105 °F to 113 °F, while a 75 °C rated conductor takes 0.82 at 105 °F to 113 °F; 310.15(B)(2) adds 33 °C (60 °F) where raceways or cables exposed to direct sunlight on or above rooftops are less than 19 mm (3/4 in.) above the roof. Section 310.15(C)(1) reduces the ampacity of each conductor where more than three current-carrying conductors are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.): 80 percent for 4 through 6 conductors, 70 percent for 7 through 9, and 50 percent for 10 through 20. Section 310.15(E)(1) counts a neutral as current-carrying in a 2-wire circuit, in a 3-wire circuit of two ungrounded conductors and the neutral of a 4-wire, 3-phase, wye-connected system, and in a 4-wire, 3-phase wye circuit whose load is mostly nonlinear; 310.15(E)(2) does not count a neutral carrying only unbalanced current, and 310.15(F) excludes grounding and bonding conductors.

1.6 Cabinets, Cutout Boxes, Meter Sockets, and Boxes (Articles 312 and 314)

Section 312.8(A) requires openings for conductors, cables, or raceways entering a cabinet, cutout box, or meter socket enclosure to be closed using listed means, and 312.8(C) requires each cable to be secured to the enclosure; Exception No. 1 permits entirely nonmetallic sheaths to enter the top of a surface-mounted enclosure through nonflexible raceways at least 450 mm (18 in.) but not more than 3.0 m (10 ft) long. Section 312.9(A) and Table 312.9(A) set the minimum wire-bending space at terminals, one wire per terminal, at 127 mm (5 in.) for 300 through 350 kcmil and 152 mm (6 in.) for 400 through 500 kcmil; parallel conductors are judged on the number in parallel, and bending space above 1000 volts ac, 1500 volts dc, nominal follows 305.13(C). Section 312.11(A)(1) limits all conductors at any cross section of the wiring space of enclosures for switches and overcurrent protective devices to 40 percent.

Section 314.15(C) addresses 3 mm (1/8 in.) to 6 mm (1/4 in.) drain openings for damp or wet locations. Table 314.16(B)(4) gives the volume allowance per conductor, including 36.9 cm3 (2.25 in.3) for 12 AWG, and 314.16(B)(5) requires one volume allowance for up to four equipment grounding conductors entering a box plus 1/4 allowance for each additional one.

Section 314.17(B)(2) requires a nonmetallic sheath entering through a cable clamp to extend at least 6 mm (1/4 in.) inside the box and 6 mm (1/4 in.) beyond the end of the clamp, and 314.17(B)(4) requires nonmetallic boxes and conduit bodies to be suitable for the lowest temperature-rated conductor entering. Section 314.20(A) requires boxes, plaster rings, extension rings, or listed extenders in noncombustible surfaces to be set back not more than 6 mm (1/4 in.) from the finished surface, 314.20(B) requires boxes in wood or combustible material to extend to the finished surface or project from it, 314.21 permits a gap of not more than 3 mm (1/8 in.) when repairing noncombustible surfaces around a box, 314.7 limits screw extension to 10 mm (3/8 in.), and 314.24(C)(2) requires 13 mm (1/2 in.) clearance for side-wiring entrances.

Section 314.23(E) permits a raceway-supported enclosure without devices or luminaires to be not larger than 1650 cm3 (100 in.3), with threaded entries or identified hubs and two or more conduits threaded wrenchtight into it, each secured within 900 mm (3 ft), or 450 mm (18 in.) if all entries are on the same side. Section 314.27(A)(1)(a) permits a luminaire or lampholder of 3 kg (6 lb) or less to be secured with no fewer than two No. 6 or larger screws, 314.27(A)(2)(a) requires a box at a lighting outlet to support a luminaire weighing at least 23 kg (50 lb), and 314.27(A)(2)(b) requires independent support above that weight.

Section 314.28(A)(1) requires a straight pull to have a box length at least eight times the metric designator, or trade size, of the largest raceway, and 314.28(A)(2) requires at least six times that dimension between each raceway entry and the opposite wall for angle or U pulls, increased by the sum of the diameters of the other entries in the same row; above 1000 volts ac, 1500 volts dc, nominal, 314.71(A) requires 48 times the outside diameter over sheath for shielded or lead-covered conductors and 32 times for nonshielded conductors in a straight pull, 314.71(B)(1) requires 36 times for angle or U pulls, reducible to 24 times for nonshielded non-lead-covered cables under 314.71(B)(2), while 314.71(E) covers handles to 45 kg (100 lb).

1.7 Cable and Raceway Wiring Methods (Articles 320 through 358)

Cable articles repeat three values: a bending radius as a multiple of the cable diameter, a support interval, and a distance from each box or termination. Type AC cable takes five times the diameter under 320.24 and support at intervals not exceeding 1.4 m (4 1/2 ft) under 320.30(B). Type MC cable takes ten times the sheath diameter for smooth-sheath cable not more than 19 mm (3/4 in.) in external diameter under 330.24(A)(1), twelve times under 330.24(A)(2) to 38 mm (1 1/2 in.), and fifteen times above that under 330.24(A)(3); 330.30(D)(2) supplies a 1.8 m (6 ft) unsupported length limit. Nonmetallic-sheathed cable takes five times the diameter under 334.24 and, under 334.30, support and securement at intervals not exceeding 1.4 m and within 300 mm (12 in.) of every cable entry into an enclosure; 334.80 limits its ampacity.

Type TC cable may be used in wet locations where resistant to moisture and corrosive agents under 336.10(8), direct buried where identified under 336.10(10), and as service-entrance cable where identified and marked Type TC-ER under 336.10(12), but 336.12(3) prohibits exposure to direct sunlight unless identified as sunlight resistant. Section 336.24 sets bending radii of four times the overall diameter for unshielded cable 25 mm (1 in.) or less, five times to 50 mm (2 in.), six times above that, and twelve times for shielded cable. Type SE cable used for interior wiring must comply with Article 334 Part II, excluding 334.80, under 338.10(B)(4)(a)(1), while 338.10(B)(4)(b)(2) applies Article 340 Part II to Type USE cable used as underground feeder cable; 338.24 requires a five-times-diameter bending radius for Types USE and SE, which 340.24 repeats for Type UF.

Raceway articles carry their own support intervals: 3 m (10 ft) for intermediate metal conduit and rigid metal conduit under 342.30(B)(1) and 344.30(B)(1), with field threading using a standard cutting die of 1 in 16 taper, that is 3/4 in. per ft, per ANSI/ASME B1.20.1 under 344.28; fastening within 300 mm (12 in.) of each box or termination and support at 1.4 m (4 1/2 ft) intervals for flexible metal conduit under 348.30(A), with 1.2 m (4 ft) unsupported permitted for trade sizes 1 1/2 through 2. Rigid polyvinyl chloride conduit requires securement within 900 mm (3 ft) of each box under 352.30(A) and support per Table 352.30(B), which gives 900 mm for metric designators 16 through 27, 1.5 m for 35 through 53, and up to 2.5 m for metric designator 155; 352.44 requires an expansion fitting where the length change is expected to be 6 mm (1/4 in.) or greater in a straight run between securely mounted items, and 355.44 does the same for reinforced thermosetting resin conduit. Liquidtight flexible nonmetallic conduit may not exceed 1.8 m (6 ft) under 356.12(3) unless approved as essential for flexibility, must be fastened at intervals not exceeding 900 mm (3 ft) and within 300 mm (12 in.) of each box under 356.30(1), and may run 1.8 m (6 ft) unsupported in an accessible ceiling under 356.30(4). Electrical metallic tubing requires cut ends to be reamed under 358.28(A), must not be threaded under 358.28(B) except for factory threaded integral couplings complying with 358.100, is fastened at intervals not to exceed 3 m (10 ft) and within 900 mm (3 ft) of each box under 358.30(A), and requires wet location listed fittings under 358.42.

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Practical Field Points

Summary

Chapter 4 covers wiring methods and materials, and the questions follow the code's order. Article 300 supplies the general rules every Chapter 3 wiring method inherits: circuit conductors stay together under 300.5(B) and 300.7(I), raceways are installed complete under 300.20(A), conductors are continuous between points under 300.15(A), boxes and conduit bodies are required at outlets, splices, terminations, and pull points under 300.17, and at least 150 mm (6 in.) of free conductor is left at each outlet and junction under 300.16(A).

Buried work is settled by Table 300.7(A) below 1000 volts and by Table 305.15(A) above it, reading the location row before the wiring method column, with the grade protection and warning ribbon rules of 300.7(D). Article 310 covers conductor use and ampacity: 310.10(C) and 310.10(D)(2) for wet and sunlit locations, 310.10(G) for parallel conductors, 310.12(A) for dwelling unit services, and 310.15(B), 310.15(C)(1), 310.15(E), and 310.15(F) for correction, adjustment, and counting.

Articles 312 and 314 complete the enclosure picture through 312.8, 312.9, 314.16(B), 314.17(B), 314.20, 314.21, 314.23(E), 314.27, 314.28, and 314.71. The cable and raceway articles add support, securement, and bending values, and the gutter, wireway, busway, surface raceway, and cable tray articles add fill percentages and heights.