5.1 Article 300 General Requirements and Underground Cover

Scope and the Grouping Rule

300.1(A) makes Article 300 the general requirements for wiring methods and materials for all wiring installations, except that it does not apply to conductors that form an integral part of equipment.

300.3(B) Conductors of the Same Circuit. All conductors of the same circuit — ungrounded, grounded, and all equipment grounding conductors and bonding conductors — shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted. The exceptions cover conductors in parallel (each raceway must still contain a complete set), grounding conductors as permitted by 250.130(C), nonferrous wiring methods in specific cases, and enclosures for certain equipment.

The reason is inductive: current flowing out through one conductor and back through another creates a magnetic field. If the two are in the same ferrous raceway, the fields cancel. Separate them into different steel raceways and the raceways heat like induction cooktops.

300.3(C)(1) Conductors of Different Systems. Conductors of AC and DC circuits rated 1000 volts nominal or less shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway, provided all conductors have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure. So a 480 V circuit and a 120 V circuit may share a raceway if all conductors are rated at least 600 V, which building wire is.

300.3(A) Single Conductors. Single conductors specified in Table 310.4(A) shall be installed only where part of a recognized wiring method of Chapter 3 — you cannot simply staple loose THHN to a joist.

Protection Against Physical Damage — 300.4

300.4(A)(1) Bored Holes. In both exposed and concealed locations, where a cable or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less than 1.25 inches (32 mm) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails by a steel plate at least 1/16 inch (1.6 mm) thick, of appropriate length and width, installed to cover the area of the wiring.

300.4(A)(2) Notches in Wood. Where there is no objection because of weakening the building structure, cables or raceways may be laid in notches, provided the cable or raceway is protected against nails or screws by a steel plate at least 1/16 inch thick.

300.4(B) Nonmetallic-Sheathed Cables and Electrical Nonmetallic Tubing Through Metal Framing Members. Where NM cable or ENT passes through factory or field punched, cut, or drilled slots or holes in metal members, the cable or tubing shall be protected by listed bushings or listed grommets covering all metal edges, securely fastened in the opening prior to installation.

300.4(D) Cables and Raceways Parallel to Framing Members and Furring Strips. Cables or raceways run parallel to framing members shall be installed so that the nearest outside surface is not less than 1.25 inches from the nearest edge of the framing member, or shall be protected by a 1/16-inch steel plate.

300.4(E) Cables, Raceways, or Boxes Installed in or Under Roof Decking. Cables, raceways, and boxes shall not be installed within 1.5 inches (38 mm) of the nearest edge of the roof decking, measured from the lowest surface of the roof decking to the top of the cable, raceway, or box.

300.4(F) Cables and Raceways Installed in Shallow Grooves. Where installed in a groove, to be covered by wallboard, siding, paneling, carpeting, or similar finish, cables and raceways shall be protected by 1/16-inch-thick steel plate, sleeve, or equivalent, or by not less than 1.25 inches of free space for the full length of the groove.

300.4(G) Insulated Fittings. Where raceways contain 4 AWG or larger insulated circuit conductors and these conductors enter a cabinet, box, enclosure, or raceway, the conductors shall be protected by an identified fitting providing a smoothly rounded insulating surface — an insulated bushing — unless the design of the box provides equivalent protection.

Underground Installations — 300.5

Table 300.5(A) Minimum Cover Requirements, 0 to 1000 Volts. Cover is measured from the top of the conductor, cable, or raceway to the finished grade.

Two rows carry most of the exam questions. Under a public street or parking lot everything goes to 24 inches, including rigid metal conduit that would otherwise need only 6. A residential driveway is 18 inches, which is a specific relaxation for one- and two-family dwellings.

300.5(D) Protection from Damage. (1) Direct-buried conductors and cables emerging from grade shall be protected by enclosures or raceways extending from the minimum cover distance required by 300.5(A) below grade to a point at least 8 feet above finished grade. (2) Conductors entering a building shall be protected to the point of entrance. (4) Enclosures or raceways with conductors emerging from the ground must be protected against physical damage.

300.5(E) Splices and Taps. Direct-buried conductors or cables shall be permitted to be spliced or tapped without the use of splice boxes, provided the splice or tap is made in accordance with 110.14(B) using a listed method.

300.5(F) Backfill. Backfill containing large rock, paving materials, cinders, large or sharply angular substances, or corrosive material shall not be placed where materials may damage raceways, cables, or other substructures, prevent adequate compaction of fill, or contribute to corrosion. Where necessary, granular material, suitable running boards, sleeves, or other approved means shall be used.

300.5(G) Raceway Seals. Where a raceway enters from an underground system, the end within the building shall be sealed with an identified compound to prevent the entrance of moisture or gases, or shall be arranged to prevent moisture from contacting live parts.

300.5(I) Conductors of the Same Circuit. All conductors of the same circuit, including the equipment grounding conductor, shall be installed in the same raceway or cable, or shall be installed in close proximity in the same trench.

The Frost-Heave Rule — 300.5(J)

This is the section that matters most in Alaska, and it is a real Code rule rather than a state amendment:

300.5(J) Earth Movement. Where direct-buried conductors, raceways, or cables are subject to movement by settlement or frost, direct-buried conductors, raceways, or cables shall be arranged so as to prevent damage to the enclosed conductors or to equipment connected to the raceways.

Informational Note: This section recognizes "S" loops in underground direct burial to raceway transitions, expansion fittings in raceway risers to fixed equipment, and, generally, the provision of flexible connections to equipment subject to settlement or frost heaves.

Note precisely what the rule does and does not do. It requires an arrangement that prevents damage and it recognizes three specific techniques in a non-enforceable Informational Note. It does not prescribe a burial depth below the frost line, and it does not mandate a particular fitting. The designer chooses the means; the AHJ judges whether damage is prevented. Alaska adopts the NEC without amending this section.

Sealing Against Condensation — 300.7(A)

A second rule with real Alaska consequence:

300.7(A) Sealing. Where portions of a raceway or sleeve are known to be subjected to different temperatures, and where condensation is known to be a problem, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve. An explosionproof seal shall not be required for this purpose.

A conduit running from a heated shop into an unheated arctic entry, or from a warm building out to a pad-mounted disconnect, is exactly the condition this addresses. Warm moist air migrates into the cold section, condenses, and fills the low point of the raceway with water that then freezes. Duct seal or an approved sealing compound at the temperature transition is the fix, and inspectors in Interior Alaska look for it.

Other Article 300 Rules Worth Tabbing

• 300.10 Electrical Continuity. Metal raceways, cable armor, and other metal enclosures shall be metallically joined together into a continuous electrically conductive system and shall be connected to all boxes, fittings, and cabinets so as to provide effective electrical continuity.
• 300.11 Securing and Supporting. Raceways, cable assemblies, boxes, cabinets, and fittings shall be securely fastened in place. Support wires that do not provide secure support shall not be permitted as the sole support. Independent support wires for electrical raceways in a ceiling assembly must be distinguishable by color, tagging, or other effective means.
• 300.12 Mechanical Continuity. Raceways and cable sheaths shall be continuous between cabinets, boxes, fittings, or other enclosures or outlets.
• 300.14 Length of Free Conductors at Outlets, Junctions, and Switch Points. At least 6 inches (150 mm) of free conductor, measured from the point in the box where it emerges from its raceway or cable sheath, shall be left for splices or connections. Where the opening to an outlet, junction, or switch point is less than 8 inches in any dimension, each conductor shall be long enough to extend at least 3 inches outside the opening.
• 300.19 Supporting Conductors in Vertical Raceways. Conductors in vertical raceways shall be supported at the intervals in Table 300.19(A) — 100 feet for 18 AWG through 8 AWG in aluminum or copper, 80 feet for 6 AWG through 1/0, 60 feet for 2/0 through 4/0, and shorter intervals for larger conductors.
• 300.20 Induced Currents in Metal Enclosures or Metal Raceways. Where conductors carrying alternating current are installed in metal enclosures or raceways, they shall be arranged so as to avoid heating the surrounding metal by induction — all phase conductors and the neutral must be grouped together.