NEC Article 300 — Wiring Methods: The Complete Exam Guide

Article 300 is the foundation layer of the NEC's wiring method requirements. Before you pick up a piece of conduit or a cable, Article 300 tells you the rules that apply to every installation — how to protect conductors from physical damage, how deep to bury them, how much wire to leave at each box, and when a box is required at all.

These rules aren't tied to a specific wiring method. They apply whether you're running EMT through a commercial building or NM cable in a house.

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Article 300 Structure

Article 300 covers wiring methods for both low-voltage (up to 2000V) and some high-voltage systems. The most exam-relevant sections:

• 300.3 — Conductors of the same circuit together
• 300.4 — Protection against physical damage
• 300.5 — Underground installations (Table 300.5)
• 300.11 — Securing and supporting
• 300.12 — Mechanical continuity
• 300.13 — Electrical continuity of conductors
• 300.14 — Length of free conductors at outlets
• 300.15 — Boxes, conduit bodies, or fittings required
• 300.17 — Raceway sizing
• 300.22 — Wiring in ducts, plenums, and air-handling spaces

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300.3 — Conductors of the Same Circuit Together

All conductors of the same circuit — including the equipment grounding conductor — must be contained in the same raceway, cable, or cord, or run together in the same trench for underground installations.

Why this matters: Running conductors of the same circuit together minimizes inductive heating in metal raceways. If conductors of the same AC circuit are split across separate metal conduits, the unequal magnetic fields can heat the conduit through induction, which is both an energy waste and a fire hazard.

Exam tip: You cannot run the hot wire in one metal conduit and the neutral in another. All conductors of the same circuit — hot(s), neutral, and EGC — must share the same raceway.

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300.4 — Protection Against Physical Damage

This section governs how conductors must be protected when they pass through or alongside structural members — one of the most tested provisions in the entire article.

Bored Holes in Wood: 300.4(A)(1)

When a cable or raceway passes through a bored hole in a wood stud, joist, or rafter:

• The edge of the bored hole must be at least 1¼ inches from the nearest edge of the wood member
• If the 1¼ inch clearance cannot be maintained, a steel plate or bushing at least 1/16 inch thick must be installed to protect the wiring

Exam tip: 1¼ inches is the magic number for bored holes. If the hole is too close to the edge, a nail plate is required. This appears on nearly every residential exam question about rough-in wiring.

Notches in Wood: 300.4(A)(2)

When cables are run through notches in wood framing members, the cable must be protected from nails and screws by a steel plate at least 1/16 inch thick unless the wiring method itself is rigid metal conduit, IMC, or Schedule 80 PVC — which can protect themselves.

Parallel to Framing: 300.4(D)

Where cables or raceways are run parallel to framing members (studs, joists, rafters) and are closer than 1¼ inches from the edge, they must be protected by a steel plate.

Behind Drywall: 300.4(E)

Cables installed in concealed spaces in walls or partitions of wood or metal frame construction, where subject to penetration by nails or screws, must be protected by a steel plate or run at a distance greater than 1¼ inches from the framing edge.

Penetrating Metal Framing: 300.4(B)

Where cables pass through metal framing members, the cable must be protected by a listed bushing or grommet that fully covers the opening to prevent abrasion of the cable jacket.

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300.5 — Underground Installations

Table 300.5 specifies the minimum cover requirements for underground wiring. "Cover" is the distance from the top of the conduit, cable, or conductor to the finished grade.

Key Minimum Cover Depths (Table 300.5)

Exam tip: The most commonly tested depths are 24 inches (direct buried, general), 6 inches (rigid metal conduit), and 12 inches (residential branch circuits with GFCI protection). These appear on virtually every exam.

Additional Underground Requirements

• Conductors entering buildings must be protected by conduit from the point of emergence from the ground to the point of entry into the building (300.5(D))
• Splices underground are permitted only in listed underground splice kits or in approved enclosures (300.5(E))
• Backfill material must not damage or displace the raceways or cables; rocks and debris must not be used directly against the conduit (300.5(F))

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300.11 — Securing and Supporting

Raceways and cables must be supported independently of suspended ceiling grid wires and ceiling support wires.

The dropped ceiling T-bar grid supports ceiling tiles — it is not a permitted means of supporting electrical wiring. Independent support wires or conduit hangers must be used.

Exception: Wiring methods installed above a lay-in ceiling may use the ceiling grid support wires if the wiring is installed in a wiring method that itself does not require support (i.e., certain cable assemblies), subject to the specific conditions in 300.11(A)(1) and (A)(2).

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300.12 — Mechanical Continuity of Raceways and Cables

Raceways and cable armor or sheaths must be mechanically continuous between all boxes, fittings, and enclosures.

A raceway cannot simply dead-end in a wall — it must terminate in a listed fitting, box, or enclosure. Open-ended conduit is a code violation unless it is an intentional conduit stub-out (service entry, for example) with a listed rain-tight fitting.

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300.13 — Electrical Continuity of Conductors

Grounding Conductors: 300.13(A)

Metal raceways, cable armor, and enclosures must be electrically continuous. Equipment grounding conductors must not be used as the sole means of electrical continuity of metal raceways.

Neutral Continuity in Multi-Wire Branch Circuits: 300.13(B)

In a multi-wire branch circuit (two hots sharing a neutral), the neutral conductor must not be interrupted by a device — such as a switch or receptacle — in a way that would break the neutral continuity to downstream devices while the circuit remains energized.

Why this matters: If the neutral of a multi-wire branch circuit is broken at a device, the two ungrounded conductors can create a series circuit through connected loads, potentially driving full 240V across equipment rated for 120V. This is a safety hazard.

Exam tip: The 300.13(B) neutral continuity rule is a classic multi-wire branch circuit trap question. The neutral must be pigtailed — not run through the device terminals — so continuity is maintained even if the device is removed.

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300.14 — Length of Free Conductors at Outlets

At each outlet, junction, and switch point, conductors must be left long enough for proper connection.

The rule:

• At least 6 inches of free conductor must be available at the outlet
• Conductors must extend at least 3 inches outside the opening of the box

Exam tip: 6 inches free, 3 inches outside. These numbers appear frequently on exam questions. The 3-inch requirement ensures there's enough conductor outside the box to make connections without having to pull the device into the box.

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300.15 — Boxes, Conduit Bodies, or Fittings Required

A listed box, conduit body, or fitting must be installed at each point where conductors are:

• Spliced
• Connected to a device or fixture (outlet, switch, receptacle)
• Pulled (junction or pull point)
• Terminated in any wiring method

Exceptions: Listed conduit bodies containing splices, taps, or devices if the conduit body is at least twice the required cubic inch fill capacity. Some listed assemblies and fittings that provide equivalent protection are also permitted.

Exam tip: Splice = box required. If conductors are being joined, there must be a box. The only exceptions are specific listed fittings; these are narrowly defined and must be listed for the application.

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300.17 — Raceway Sizing

Raceways must be large enough to permit the installation of conductors without damaging the conductor's insulation.

This is the general rule that backs up the conduit fill calculations in NEC Chapter 9 Tables 1, 4, and 5. A raceway that is technically code-compliant by the fill tables but requires excessive force to pull conductors still violates 300.17.

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300.22 — Wiring in Ducts, Plenums, and Air-Handling Spaces

This section controls what wiring methods can be used in spaces that circulate air for HVAC — a critical safety provision since burning wiring in an air-handling space can spread toxic fumes throughout a building.

300.22(B) — Ducts for Dust, Loose Stock, or Vapor

No wiring is permitted in ducts used for transportation of loose material or flammable vapors. No exceptions.

300.22(C) — Other Space Used for Environmental Air (Plenum Space)

Wiring installed in the space above a suspended ceiling used as a return air plenum must use listed plenum-rated wiring methods only:

Permitted in plenums:

• Metal raceways (EMT, RMC, IMC, FMC, LFMC)
• Listed plenum-rated cable (Type CMP, Type OFNP, Type OFCP, etc.)
• Listed flexible metal conduit in lengths not exceeding 4 feet for connection to equipment

NOT permitted in plenums:

• NM cable (Romex) — never in plenums
• Standard PVC conduit — not listed for plenum use
• Most flexible cords and cables
• LFNC (liquidtight flexible nonmetallic conduit) — not listed for plenum

Exam tip: NM cable in a plenum is a code violation. This is one of the most commonly tested plenum questions. If the ceiling space is used as return air, only metal raceways or plenum-rated cable are permitted.

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Key Article 300 Numbers to Remember

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Common Exam Mistakes on Article 300

1. Forgetting the 1¼ inch rule applies to parallel runs — 300.4(D) covers cables running alongside framing, not just through it
2. Using the wrong burial depth — always check Table 300.5 for the specific wiring method and location; direct-buried and conduit depths differ significantly
3. Missing the multi-wire neutral continuity rule — 300.13(B) is a classic trap; the neutral must be pigtailed at devices in multi-wire circuits
4. Allowing NM cable in plenums — it's never permitted; even one piece of Romex in a plenum is a violation
5. Confusing "cover" with "depth to top of conduit" — cover is measured from the top of the raceway to grade, not to the center of the conduit

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Ready to Practice Article 300 Questions?

GetLicenseReady's practice questions include Article 300 scenarios covering physical protection, underground requirements, free conductor length, and plenum wiring.

Related reading: NEC Article 210 — Branch Circuits | NEC Article 240 — Overcurrent Protection | NEC Chapter 9 — Conduit Fill Calculations