NEC 300-398 — Wiring Methods (Washington)

NEC 300–398 — Wiring Methods (Washington Journeyman)

Learning Objectives

1.1 Article 300 in the 2026 Edition

Article 300 carries the general requirements for every Chapter 3 wiring method, and 300.1(B) lists the cases they do not reach. Section 300.4(A) limits Chapter 3 methods to 1000 volts ac, 1500 volts dc, nominal, or less unless a section says otherwise, and 300.4(C) requires conductors and wiring methods no longer suitable for use because of damage — overheating, fire, corrosive influences, or water — to be replaced.

The 2026 edition renumbered Article 300, so navigate by title: the rules practice material calls _300.3_ are 300.5, the underground rules and cover table are 300.7 and Table 300.7(A), physical damage is 300.6, corrosion is 300.8, and the same-circuit rule cited as _300.3_(I) or 300.9(I) sits in 300.5(B), while 300.9 covers raceways exposed to different temperatures.

1.2 Underground Installations — 300.7 and Table 300.7(A)

300.7(A) sets the minimum cover of Table 300.7(A), measured from the top surface of the buried item to finished grade, concrete, or similar cover; read the location row with the wiring method column.

Under a building, 300.7(C) requires underground cable and conductors to be in a raceway, but Exception No. 1 permits Type MI cable without one where embedded in concrete, fill, or other masonry in accordance with 332.10(6), and Exception No. 2 permits Type MC cable listed for direct burial or concrete encasement under 330.10; Type NM, SE, and UF cable are not permitted. The interior of enclosures and raceways installed underground is a wet location under 300.7(B), and conductors emerging from grade are protected at least 2.5 m (8 ft) above finished grade and not more than 450 mm (18 in.) below it.

1.3 Conductors and Containment — 300.5

300.5(A) permits single conductors specified in Table 310.4(1) only where installed as part of a recognized wiring method specified in Chapter 3, with an Exception for individual conductors installed as separate overhead conductors under 225.6. 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 in the same raceway, conduit body, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord. Paralleled conductors are permitted under 310.10(G), with the rule applying separately to each portion; equipment bonding conductors may be installed on the outside of raceways under 250.102(E), with related requirements in 250.109, 250.122, 250.130(C), and 250.134. 300.5(C) lets conductors of ac and dc circuits rated 1000 volts ac, 1500 volts dc, nominal, or less share an enclosure, cable, or raceway provided all conductors have an insulation rating equal to at least the maximum circuit voltage applied to any conductor in it — not 300 volts, not 600 volts, never the sum of the voltages.

1.4 Physical Damage, Corrosion, and Wet Locations

300.6 requires conductors, raceways, and cables subject to physical damage to be protected. Through bored holes in joists, rafters, or wood members the hole edge is not less than 32 mm (1 1/4 in.) from the edges; where that cannot be held, a steel plate, sleeve, or equivalent at least 1.6 mm (1/16 in.) thick protects the wiring.

300.8 handles corrosion by material: 300.8(A) requires ferrous raceways, cable armor, boxes, cabinets, fittings, and supports in concrete, in contact with the earth, or in severely corrosive areas to be of approved material or given approved protection, and requires outdoor equipment protected solely by enamel to be marked "Raintight," "Rainproof," or "Outdoor Type"; 300.8(B) adds supplementary protection for aluminum equipment embedded in concrete or contacting the earth; 300.8(C)(1) requires nonmetallic materials exposed to sunlight to be listed or identified as sunlight resistant; and 300.8(D) requires the exposed wiring system in indoor wet locations such as laundries and canneries to hold at least 6 mm (1/4 in.) of airspace from the wall, except that nonmetallic raceways, boxes, and fittings may sit on a concrete, masonry, tile, or similar surface.

1.5 Continuity, Bends, and Free Conductors

300.12 requires metal raceways, cable armor, and other metal enclosures to be metallically joined into a continuous conductor connected to all boxes, fittings, and cabinets, and forbids using wiring methods to support other cables, raceways, or equipment. 300.14 requires raceways, cable armors, and cable sheaths to be continuous between cabinets, boxes, fittings, and outlets; its Exception No. 1 is a favourite exam item — short raceway sections used only to support or protect cable assemblies from physical damage need not be mechanically continuous, with no bonding to framing and no listing condition.

300.15(A) keeps conductors continuous between outlets, boxes, and devices, with no splice or tap inside a raceway unless permitted elsewhere, and 300.15(B) requires in multiwire branch circuits that the continuity of the grounded conductor not depend on device connections such as lampholders or receptacles. 300.16(A) requires at least 150 mm (6 in.) of free conductor at each outlet, junction, and switch point; where the opening is less than 200 mm (8 in.) in any dimension, 300.16(B)(2) requires each conductor to extend at least 75 mm (3 in.) outside it. 300.17 requires a box at each outlet and switch point where splices or luminaire and device connections are made, and a box, conduit body, or fitting wherever a splice, pull, or termination point is needed. 300.24 caps the bends in any combination of conduit or tubing used for one run at 360 degrees between pull points.

1.6 Ducts, Plenums, and Induced Currents

300.25 covers ducts not used for air handling, ducts specifically fabricated for environmental air, and other spaces used for environmental air (plenums). 300.25(A) prohibits wiring in ducts used to transport dust, loose stock, or flammable vapors, and in ducts or shafts used for vapor removal or commercial cooking ventilation. 300.25(B) permits equipment, devices, and wiring in fabricated ducts only where necessary for direct action upon or sensing of the contained air, and only in the named methods — Type MI cable, Type MC cable with an impervious metal sheath, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, and rigid metal conduit without an overall nonmetallic covering; Type NM, Type SE, and Type AC cable and rigid PVC conduit are not permitted. Flexible metal conduit up to 1.2 m (4 ft) may connect physically adjustable equipment, and plenums follow the same rules.

300.22 limits induction heating of ferrous metal: 300.22(A) requires alternating-current conductors in ferrous metal enclosures or raceways to be arranged so the metal is not heated, which means grouping all phase conductors and, where used, the grounded conductor and all equipment grounding conductors together; 300.22(B) requires slots between the individual holes, or one insulating wall large enough for the whole circuit, where a single alternating-current conductor passes through magnetic metal.

1.7 Conductor Types, Minimum Size, and Marking

310.4(1) lists conductor types, temperature ratings, and permitted locations. THHN is rated 90°C and permitted in dry and damp locations; THW, THW-2, THWN, and THWN-2 are permitted in dry and wet locations; XHHW and XHHW-2 are dry, with the -2 types adding wet; TFE, FEP, and PFA are dry only; SIS is switchboard and switchgear wiring. 310.5(A) sets the minimum size for ratings up to 2000 volts at 16 AWG copper, 14 AWG copper-clad aluminum, or 12 AWG aluminum, except as permitted elsewhere. Options also quote 310.8 marking, 310.10(G) for paralleled conductors, 310.12 for single-phase dwelling services and feeders, 310.14 for conductors rated 0 volts through 2000 volts, 310.17 for single-insulated conductors in free air, 310.18 for insulated conductors in raceway or cable, 310.21 for bare or covered conductors in free air, and Table 315.60(C)(1) for conductors over 2000 volts.

1.8 Ampacity — Table 310.16, Correction, and Adjustment

Table 310.16 gives ampacities for not more than three current-carrying conductors in a raceway, cable, or earth at 30°C ambient. Copper, in the 60°C, 75°C, and 90°C columns: 14 AWG 15, 20, 25; 12 AWG 20, 25, 30; 10 AWG 30, 35, 40; 8 AWG 40, 50, 55; 6 AWG 55, 65, 75; 4 AWG 70, 85, 95; 2 AWG 95, 115, 130; 1/0 AWG 125, 150, 170; 3/0 AWG 165, 200, 225; 250 kcmil 215, 255, 290; 500 kcmil 320, 380, 430. Aluminum is lower at every size, so its values are standard distractors.

310.15(B) ambient correction. Table 310.15(B)(1)(1) assumes 30°C; multiply the table ampacity by the factor for the conductor's rating. Factors for the 60°C, 75°C, and 90°C columns: 31-35°C 0.91, 0.94, 0.96; 36-40°C 0.82, 0.88, 0.91; 41-45°C 0.71, 0.82, 0.87; 46-50°C 0.58, 0.75, 0.82; 51-55°C 0.41, 0.67, 0.76; and 56-60°C, 0.58 and 0.71 for the 75°C and 90°C columns. The result may not exceed the termination temperature ampacity of 110.14(C).

310.15(C)(1) adjustment. More than three current-carrying conductors without maintained spacing reduce ampacity to 80 percent for 4 to 6 conductors, 70 percent for 7 to 9, 50 percent for 10 to 20, 45 percent for 21 to 30, 40 percent for 31 to 40, and 35 percent for 41 and above. Multiply both factors with the Table 310.16 value: 115 A × 0.88 × 0.80 = _80.96_ A; 120 A × 0.82 × 0.70 = 68.9 A; 80 A × 0.88 × 0.80 = 56.3 A; 205 A × 0.94 = _192.7_ A; 225 A × 0.50 = _112.5_ A; 55 A × 0.70 = _38.5_ A; 70 A × 0.87 = 60.9 A; 85 A × 0.94 = _79.9_ A; 55 A × 0.71 = 39.1 A. Wrong answers repeat the same slips: _101.2_, _84.0_ and _120.0_ A for one factor or none; 70.4 and _64.0_ A for 80 A beside _80.0_ A uncorrected; uncorrected bases _85.0_ A, _70.0_ A and 55.0 A beside their corrected forms; _61.0_ A as a rounding variant; inverted division giving 80.5, 90.4, _77.5_ and 98.4 A; wrong-row factors giving 67.5, 76.5, 52.5, 44.0, 27.5, 180, 90, 15, 21 and 24 A.

1.9 Cabinets and Cutout Boxes — Article 312

312.4(A) requires surface-type enclosures in damp or wet locations to hold at least 6 mm (1/4 in.) of airspace from the wall, except that nonmetallic enclosures may be installed without the airspace on a concrete, masonry, tile, or similar surface; 312.4(B) requires raceways or cables entering a wet-location enclosure above uninsulated live parts to use fittings listed for wet locations. 312.5 requires the front edge of a cabinet in concrete, tile, or other noncombustible material to be set back no more than 6 mm (1/4 in.) from the finished surface, while in wood or combustible walls the cabinet is flush or projects from it. 312.7 requires broken noncombustible surfaces to be repaired so the front edge sits within 3 mm (1/8 in.) of the repaired surface, and 312.8 requires openings for conductors, cables, or raceways to be closed by listed means.

312.9(B)(2) sets minimum wire-bending space at terminals opposite the wall where conductors enter or leave an enclosure: a 500 kcmil conductor with one wire per terminal, not compact-stranded AA-8000 aluminum alloy, requires 356 mm (14 in.); a 3/0 AWG conductor with two wires per terminal requires 165 mm (6 1/2 in.); 152 mm (6 in.) serves 2/0 AWG with two wires; 178 mm (7 in.) serves 1/0 AWG with three wires; 190 mm (7 1/2 in.) serves 2/0 AWG with three wires; 254 mm (10 in.), 305 mm (12 in.), and 406 mm (16 in.) belong to other rows. 312.10 requires approved space to accommodate all conductors without crowding, and 312.11 governs wiring space for switches and overcurrent protective devices.

1.10 Box Volume and Box Fill — 314.16

Table 314.16(A) gives box volumes and Table 314.16(B) the allowance per conductor; the volume must be at least the calculated fill. Volumes: 4 × 1 1/4 round or octagonal 12.5 in³; 4 × 1 1/4 square 18.0 in³; 4 × 1 1/2 square 21.0 in³; 4 × 2 1/8 square 30.3 in³; 4 11/16 × 1 1/2 square 29.5 in³; 3 × 2 × 2 device 10.0 in³; 3 × 2 × 2 1/4 device 10.5 in³; 3 × 2 × 2 3/4 device 14.0 in³. Allowances: 14 AWG 2.00 in³, 12 AWG 2.25 in³, 10 AWG 2.50 in³, 8 AWG 3.00 in³, 6 AWG 5.00 in³.

Counting rules: (B)(1) each conductor originating outside the box that terminates or is spliced within it counts once, as does each pass-through conductor; (B)(2) internal cable clamps count as one allowance based on the largest conductor; (B)(3) one allowance for each type of support fitting, such as a luminaire stud or hickey; (B)(4) each yoke or strap containing devices gets a double allowance based on the largest conductor connected to it, and a device wider than a single 50 mm (2 in.) device box gets a double allowance per gang; (B)(5) up to four equipment grounding conductors count as one allowance based on the largest, plus a 1/4 allowance for each additional one; (B)(6) each terminal block assembly adds one allowance.

Worked examples: 4 × 2.00 in³ = 8.0 in³ (two 14 AWG circuits, one 14 AWG EGC, one pass-through); 5 × 2.00 in³ = _10.00_ in³ (four 14 AWG terminating conductors plus one 14 AWG EGC); 5 × 2.25 in³ plus 2 × 2.25 in³ = 11.25 in³ without a device and 15.75 in³ with one; 8 × 2.25 in³ = _18.00_ in³ (six 12 AWG current-carrying conductors, one clamp, one fixture stud); 7 × 2.25 in³ = 15.75 in³ (four terminating, two pass-through, one EGC); nine 12 AWG equivalents = _20.25_ in³; six 12 AWG current-carrying conductors = _13.50_ in³; 6 × 2.50 in³ = 15.0 in³ (four 10 AWG spliced, two pass-through); _10.00_ in³ + 4.50 in³ + 2.50 in³ = 17.0 in³; and a 100 in³ conduit body holding four 12 AWG and two 14 AWG conductors uses _13.00_ in³, leaving _87.00_ in³ or _38.67_ allowances.

Wrong answers repeat slips: 24.5 in³ from wrong dimensions, _24.75_ in³ from omitting the EGC with one clamp, _29.25_ in³ from two clamps and two receptacles, _22.50_ in³ from counting the EGC separately, 20.0 in³ from miscounting pass-through conductors, 16.0 in³ from dropping the EGC, and _12.00_ in³ from counting a device as one allowance.

1.11 Box Support, Depth, and Pull Box Sizing

314.23(E) permits a conduit body to be supported by a single raceway only where its trade size is not larger than the largest trade size of the conduit or tubing. 314.23(F) limits an enclosure supported by entering raceways to 1650 cm³ (100 in.³) with threaded entries or identified hubs, and requires each conduit to be secured within 900 mm (3 ft) of the enclosure, or within 450 mm (18 in.) where all entries are on the same side.

314.24(A) sets a minimum internal depth of 12.7 mm (1/2 in.) for outlet boxes that do not enclose devices or utilization equipment; the 25.4 mm (1 in.), 38.1 mm (1 1/2 in.), and 50.8 mm (2 in.) depths offered in options are not that minimum.

314.28 sizes boxes holding conductors 6 AWG and larger by the entering raceway. 314.28(A)(1) straight pulls: the box length is not less than eight times the trade size of the largest raceway, so a 3 in. raceway needs 24 in. 314.28(A)(2) angle or U pulls: the distance from each raceway entry to the opposite wall is not less than six times the trade size of the largest raceway in a row plus the sum of the other diameters in that row, so a 2 in. raceway alone needs 12 in. 314.28(B) requires any pull or junction box with a dimension over 1.8 m (6 ft) to have all conductors secured in an approved manner. 314.71(A) requires straight-pull boxes over 1000 volts ac, 1500 volts dc to be not less than 48 times the outside diameter of the largest shielded or lead-covered conductor and not less than 32 times that of the largest nonshielded conductor, with 314.71(B) setting the angle and U pull multipliers; 24 and 36 times belong to angle and U pulls.

1.12 Boxes, Covers, and Underground Enclosures

314.5 permits nonmetallic boxes only with open wiring on insulators, concealed knob-and-tube wiring, cabled wiring methods with entirely nonmetallic sheaths, flexible cords, and nonmetallic raceways — never with metal conduit, electrical metallic tubing, or metal-clad cable. 314.2 requires listed equipment, and 314.7 governs fasteners entering wiring spaces: a screw attaching a cover may extend no more than 10 mm (3/8 in.) beyond it, other fasteners penetrating a cover are limited to 8 mm (5/16 in.), and certain boxes exceeding 1650 cm³ (100 in.³) carry limits of 6 mm (1/4 in.) and 11 mm (7/16 in.). 314.17(B)(1) requires insulating bushings at individual openings where metal boxes or conduit bodies are used with conductors unprotected by flexible tubing. 314.30 covers handhole enclosures, whose covers weighing over 45 kg (100 lb) are considered suitable. 314.100(A) requires metal boxes, conduit bodies, and fittings to be corrosion resistant or well galvanized, enamelled, or otherwise coated inside and out, and 314.100(C) requires sheet steel boxes over 1650 cm³ (100 in.³) to be not less than 1.35 mm (0.053 in.) uncoated, while a smaller box requires 1.59 mm (0.0625 in.).

1.13 Cables: Type AC, MC, MI, NM, and Cable Trays

320.10 lists the uses permitted for Type AC cable: feeders and branch circuits in exposed and concealed installations. Underground use is not among them, so Type AC cable is not direct buried, not installed in underground raceways, and not permitted underground even when encased in concrete. 320.12 lists uses not permitted, including where subject to saturation with water, in concrete slabs in contact with the earth, and in ducts used for dust, loose stock, or vapour removal. 320.30 requires support and securement at intervals not exceeding 4 1/2 ft and within 12 in. of every box, cabinet, fitting, or other cable termination.

330.30 permits Type MC cable in a cable tray to be installed without being secured to the tray provided it is supported by the tray in accordance with 392.30, the 6 ft interval in distractors being the general exposed-cable interval. 332.10(6) permits Type MI cable under a building without a raceway where embedded in concrete, fill, or other masonry, and 334.4 covers nonmetallic-sheathed cable, which is not listed for direct burial. In Article 392, 392.20 covers cable and conductor installation in trays and 392.30 covers securing and supporting.

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Summary

Article 300 frames every Chapter 3 wiring method. Section 300.4(A) limits those methods to 1000 volts ac, 1500 volts dc, nominal, or less, and 300.4(C) requires damaged conductors to be replaced. Section 300.7 and Table 300.7(A) fix minimum cover by location and wiring method — 600 mm (24 in.) for direct-buried cable in locations not otherwise specified and under streets and parking lots, 150 mm (6 in.) for rigid metal conduit — and 300.7(C) requires a raceway under a building, with Type MI and qualifying Type MC cable as the exceptions. Section 300.5 keeps single conductors inside a recognized Chapter 3 method, requires all conductors of a circuit in the same raceway, cable, or trench, permits equipment bonding conductors outside raceways under 250.102(E), and rates mixed-system insulation at the maximum circuit voltage present. Article 310 supplies the conductor types of Table 310.4(1), the 16 AWG copper minimum of 310.5(A), the ampacities of Table 310.16, and the factors of 310.15 that multiply, never divide. Article 314 ties box volume to box fill through Tables 314.16(A) and 314.16(B), and sizes pull boxes at eight times the trade size for straight pulls, six times for angle pulls.