NEC ReferenceNEC 2023 · 2026 · Updated May 2026

NEC Article 300: Wiring Methods & Burial Depths

NEC Article 300: Wiring Methods covers Wiring Methods by Location through Types Compared. This guide explains each section with practical interpretation, common mistakes, and how these rules apply to real projects.

300.3 Wiring Methods by Location

The NEC permits different wiring methods depending on location and building type. Indoor dry locations: NM-B (Romex) is the standard residential method, MC cable provides physical protection without conduit, and individual conductors in EMT or PVC conduit are used for exposed runs.

Indoor wet or damp locations: UF-B cable or THWN/THWN-2 conductors in conduit. Outdoor above grade: weatherproof conduit (rigid PVC or RMC) with wet-rated conductors, or UF-B cable with physical protection. Underground: UF-B direct burial, USE-2 conductors, or THWN in buried conduit.

In commercial buildings (generally 4+ stories), NM-B is prohibited and conduit or MC cable is required.

300.5 Burial Depth Requirements

NEC Table 300.5 specifies minimum burial depth by wiring method. Direct burial cable (UF-B): 24 inches under normal conditions, 18 inches under a residential driveway with GFCI protection at 120V/20A or less. PVC conduit: 18 inches standard, 12 inches under a building or concrete slab at least 4 inches thick.

Rigid metal conduit (RMC) and intermediate metal conduit (IMC): 6 inches, the shallowest permitted option. Under concrete slab with at least 2 inches of concrete: any conduit type at zero additional depth. These are minimum cover depths measured from the top of the conduit or cable to finished grade.

Always verify local amendments, which may require deeper burial in areas with heavy vehicle traffic.

300.4 Protection Against Physical Damage

When running cables through wood framing, NEC 300.4(A) requires the edge of the bored hole to be at least 1-1/4 inches from the nearest edge of the wood member. If the hole is closer than 1-1/4 inches, a 1/16-inch steel nail plate must be installed to protect the cable from drywall screws and nails.

Cables in notches must be protected by a steel plate covering the entire notch. In accessible attics, cables within 6 feet of the scuttle opening must be protected by guard strips or run through bored holes. In unfinished basements, NM-B cable run along the bottom of joists must follow the surface of the joist (not run at an angle across joists) unless protected by a running board.

314.16 Box Fill Calculations

NEC 314.16(B) requires counting every item in the box against its volume capacity. Per Table 314.16(B), each conductor counts as: 14 AWG = 2.00 cubic inches, 12 AWG = 2.25 cubic inches, 10 AWG = 2.50 cubic inches. All equipment grounding conductors together count as one conductor volume (the largest EGC in the box).

Each device (switch, receptacle) counts as two conductor volumes. Internal cable clamps count as one conductor volume total. For example, a standard single-gang old-work box at 14 cubic inches can hold three 14 AWG hot/neutral conductors (6.0 cu in) + one device (4.0 cu in) + grounds (2.0 cu in) = 12.0 cu in, with 2.0 cu in to spare.

Use our conduit fill calculator for raceway sizing.

Conduit Types Compared

EMT (Electrical Metallic Tubing) is the most common raceway: lightweight, easy to bend, uses compression or setscrew fittings, and serves as an equipment grounding conductor per 250.118. EMT is the default choice for exposed residential and light commercial runs.

PVC conduit (Schedule 40 for most uses, Schedule 80 where subject to physical damage) is used in wet, corrosive, and underground applications. PVC requires a separate ground wire since it is non-metallic. RMC (Rigid Metal Conduit) is the heavy-duty option: provides excellent physical protection and serves as an equipment ground, but is expensive and labor-intensive.

FMC (Flexible Metal Conduit) and LFMC (Liquid-tight FMC) are used for final connections to equipment like AC units and motors where vibration or alignment adjustments are needed.

Key NEC Tables Referenced

Frequently Asked Questions

What does NEC Article 300 cover?

Article 300 covers wiring methods requirements. Key sections include 300.3 Wiring Methods by Location and 300.5 Burial Depth Requirements. It applies to all electrical installations in the United States.

Where can I find the full text of Article 300?

The full NEC is published by NFPA and available at nfpa.org. Many libraries provide free access. Your local building department also has copies available for reference during permit applications.

Does Article 300 apply to existing wiring?

The NEC applies to new installations, modifications, and additions. Existing wiring that was code-compliant when installed is generally grandfathered unless a renovation triggers an upgrade requirement. Check with your local AHJ (authority having jurisdiction).

When did the current Article 300 requirements take effect?

NEC 2026 is the latest published edition (effective September 2025). NEC 2023 remains the most widely adopted edition by states. Adoption varies - check your state adoption status on our state code pages.

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
This guide provides practical interpretation of NEC requirements. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction. WireRef is a reference tool and does not replace licensed professional judgment.

NEC references verified May 2026