Overview of Chapter 3
Chapter 3 addresses how conductors are physically installed, protected, and supported. It covers everything from general wiring requirements to specific conductor types, cable assemblies, raceway systems, and outlet boxes. This chapter is heavily tested because it governs the day-to-day work of electrical installation.
Exam Tip: Conduit fill calculations, conductor ampacity tables, and box fill calculations are among the most frequently tested topics from Chapter 3. Know how to use Tables 310.16, 314.16(A), and Chapter 9 Table 1.
General Wiring Methods (Article 300)
Article 300 provides requirements applicable to all wiring methods unless modified by a specific article. Key provisions include:
- Conductor protection: Conductors must be protected from physical damage. Where run through bored holes in wood framing members, the edge of the hole must be at least 1-1/4 inches from the nearest edge of the member, or a steel plate of at least 1/16 inch thickness must protect the cable.
- Underground installations: Minimum cover depths are specified in Table 300.5, varying by wiring method and location (e.g., 24 inches for direct-buried UF cable under a driveway).
- Securing and supporting: Raceways, cable assemblies, boxes, and fittings must be securely fastened in place per the specific article requirements.
- Conductor routing: Conductors of all phases and the neutral must be grouped together in the same raceway, cable, or trench to avoid inductive heating.
Conductors and Ampacity (Article 310)
Article 310 covers conductor materials, insulation types, and ampacity ratings.
Ampacity Tables
Table 310.16 (formerly 310.15(B)(16)) is the most referenced table in the NEC. It provides allowable ampacities for insulated conductors rated up to 2000 volts based on:
- Conductor size (AWG or kcmil)
- Insulation temperature rating (60 degrees C, 75 degrees C, or 90 degrees C)
- Conductor material (copper or aluminum)
Ampacity Adjustment Factors
- Ambient temperature correction: When ambient temperature exceeds 30 degrees C, ampacity must be reduced using correction factors from the applicable table.
- Conduit fill adjustment: When more than three current-carrying conductors are in a raceway or cable, ampacity must be adjusted per Table 310.15(C)(1): 4-6 conductors at 80%, 7-9 at 70%, and so on.
Cable Assemblies
Nonmetallic-Sheathed Cable (NM) - Article 334
NM cable (commonly called Romex) is the most widely used wiring method in residential construction.
- Permitted in one- and two-family dwellings and multifamily dwellings of certain construction types.
- Must be secured within 12 inches of every box and supported at intervals not exceeding 4.5 feet.
- Not permitted in commercial buildings, wet locations, or where exposed to physical damage.
Metal-Clad Cable (MC) - Article 330
MC cable features conductors enclosed in a metallic sheath, providing mechanical protection.
- Permitted in nearly all locations including commercial and industrial settings.
- The metallic sheath can serve as the equipment grounding conductor in certain configurations.
Armored Cable (AC) - Article 320
AC cable (commonly called BX) uses a spiral metal armor wrapping. An internal bonding strip is required to ensure grounding continuity.
Raceway Systems
Electrical Metallic Tubing (EMT) - Article 358
EMT is a thin-walled steel or aluminum raceway widely used in commercial construction.
- Maximum size is 4-inch trade size.
- Connected with compression or set-screw fittings; threading is not permitted.
- Must be supported at least every 10 feet and within 3 feet of every box or termination.
Rigid Metal Conduit (RMC) - Article 344
RMC is the heaviest-wall metallic raceway, providing excellent physical protection.
- Joints are typically threaded.
- Supported every 10 feet and within 3 feet of boxes.
- Suitable for all atmospheric conditions and occupancies.
PVC Conduit (Rigid Polyvinyl Chloride) - Article 352
PVC conduit is a nonmetallic raceway used where corrosion resistance is needed.
- Requires a separate equipment grounding conductor since PVC is nonconductive.
- Expansion fittings are required where temperature change may cause significant movement.
- Supported every 3 feet for trade sizes 1 inch and smaller.
Boxes and Conduit Bodies (Article 314)
Every splice, switch, receptacle, or junction point requires a box or conduit body.
Box Fill Calculations
Box volume must accommodate all conductors, devices, clamps, and fittings. Each conductor is assigned a volume based on its size (Table 314.16(B)). For example, a 14 AWG conductor counts as 2.0 cubic inches and a 12 AWG conductor as 2.25 cubic inches.
- Each device (switch or receptacle) on a yoke counts as two conductor volumes.
- Internal cable clamps count as one conductor volume (total, regardless of number).
- Equipment grounding conductors count as one conductor volume (total, regardless of number).
Calculation Reminder: Always add up all volume allowances and verify the box has sufficient cubic-inch capacity before installation.
