Chapter 3 — Wiring Methods: Cable Assemblies and Raceways

Chapter 3 — Wiring Methods: Cable Assemblies and Raceways

NEC Chapter 3 governs every wiring method used in building and construction work. Each article in Chapter 3 follows the same structure: permitted uses, prohibited uses, construction specifications, and installation requirements. Your job as an electrician is to open the correct article, confirm the method is allowed for the location and occupancy type, and then install it to code.

Why Wiring Method Selection Matters

Choosing the wrong wiring method is not simply a code violation — it is a safety hazard. A cable rated only for dry indoor locations will degrade in a wet or damp space, leading to ground faults, fires, or shock. Selecting the right method protects the conductors, protects people, and protects your license.

The Six Most Common Wiring Methods

NM Cable (Article 334 — Nonmetallic-Sheathed Cable)

NM-B is the standard cable in wood-frame residential construction. The "B" suffix means the insulated conductors are rated 90 °C. NM cable is permitted in normally dry locations inside one- and two-family dwellings and multifamily dwellings of the types the article specifies. It is not permitted in commercial garages, theaters, motion picture studios, storage battery rooms, hoistways, hazardous locations, embedded in concrete, or in any location exposed to physical damage. NM cable must be stapled or otherwise secured within 12 inches of every box and at intervals not exceeding 4½ feet. Bends must not damage the cable and the inside radius of any bend must be at least five times the diameter of the cable.

AC Cable (Article 320 — Armored Cable)

AC cable has a spiral metal armor that serves as an equipment grounding path when combined with its internal aluminum bonding strip. It is permitted in dry locations, exposed or concealed, and in some damp locations when listed for the purpose. It is not permitted in direct burial, in concrete, in areas subject to physical damage, or where exposed to corrosive conditions unless specifically listed. Support requirements are similar to NM: secure within 12 inches of every box and at intervals not exceeding 4½ feet.

MC Cable (Article 330 — Metal-Clad Cable)

MC cable looks like AC cable but carries its own separate equipment grounding conductor. Because it can be manufactured with a wider variety of inner conductors and outer jackets, MC cable has broader permitted uses: it may be installed in wet locations (when listed and the conductors are suitable), in cable trays, direct buried (when identified for that use), or outdoors. Always verify the listing — MC cable comes in many configurations, and the listing determines the permitted installation environment. Support must follow the manufacturer's instructions and the article's spacing requirements.

EMT (Article 358 — Electrical Metallic Tubing)

EMT, commonly called "thin-wall" conduit, is a raceway: conductors are pulled through it after installation. EMT is permitted in both exposed and concealed work, in concrete (when listed), in wet locations with listed fittings, and in most commercial and residential installations. It is not permitted where subject to severe physical damage, in hazardous locations where the code or listing does not allow it, or where subject to severe corrosive influences. EMT must be supported at least every 10 feet and within 3 feet of every box, cabinet, or fitting (unless structural members prevent it, with certain limits). The minimum number of bends between pull points is limited to four quarter-bends (360° total). Use EMT benders correctly — a kinked or flattened conduit is a code violation and must be replaced.

Rigid PVC Conduit (Article 352 — Rigid Polyvinyl Chloride Conduit)

Rigid PVC is permitted above and below grade, in wet and corrosive locations, encased in concrete, and direct buried when listed for that use. It is not permitted where exposed to physical damage (unless identified for such use), in hazardous locations (unless specifically permitted), or where ambient temperature or conductor heat would exceed the conduit's rating. Because PVC expands and contracts significantly with temperature changes, expansion fittings are required at specified intervals. PVC must be supported at intervals determined by trade size and listed in the article's table (generally every 3 to 5 feet for smaller sizes). Use only PVC-listed fittings; threaded metallic fittings require appropriate adapters.

RMC (Article 344 — Rigid Metal Conduit)

RMC is the heaviest and most mechanically protective raceway. It is permitted in all atmospheric conditions and in all locations, including hazardous locations, direct burial, concrete encasement, and areas subject to severe physical damage. RMC must be supported at least every 10 feet and within a code-specified distance of each termination. Threaded couplings and fittings are the standard connection method. Because of its weight and cost, RMC is typically reserved for service entrances, industrial applications, and areas needing maximum mechanical protection.

Reading an NEC Article: A Practical Habit

Every time you look up a wiring method, follow this sequence:

1. Find the article number (the comparison table in your materials lists them).

2. Read "Uses Permitted" — confirm your location and construction type qualify.

3. Read "Uses Not Permitted" — verify you are not in an excluded situation.

4. Read the installation requirements for bending radius, support spacing, and fitting type.

This three-step habit will catch errors before the inspector does.

Safety Reminders

Before handling any wiring method on an actual job, apply lockout/tagout (LOTO) to ensure the circuit is de-energized and cannot be re-energized. Wear appropriate PPE including safety glasses and gloves when cutting cable armor or conduit — metal burrs cause lacerations and contaminate conductor insulation. When pulling conductors through conduit, use listed pulling lubricant; never exceed conductor fill limits found in NEC Annex C and Table 1 of Chapter 9. A conduit or cable that is damaged, kinked, or improperly supported must be replaced, not reused.