Wiring methods and materials
Scope
Wiring-methods questions test Chapter 3 plus Article 725/800 cable requirements. You must choose a method suitable for the location, support and protect it, and transition correctly at boxes and equipment.
Core rules
- Chapter 3 covers raceways and cables: EMT, IMC, RMC, PVC, FMC, LFMC, NM, MC, AC, and others. Each has permitted uses and restrictions.
- Article 300 requires conductors to be installed as a complete system, protected from physical damage (300.4), supported (300.11), and terminated in boxes or fittings (300.15).
- Class 2/3 cables are marked CL2, CL3, CL2R, CL3R, CL2P, or CL3P. The suffix controls use: P = plenum, R = riser, no suffix = general.
- Communications cables use CM, CMR, CMP. A plenum cable must be suitable for return-air space; low-smoke PVC is not a substitute for the correct listing.
- Raceway fill: 53% for one conductor, 31% for two, 40% for three or more (Chapter 9, Table 1). Pull points and box sizing still apply.
Worked example
A Class 2 thermostat cable must cross a return-air plenum above a ceiling. The installer proposes CL2. Because the space is used for environmental air, the cable must be plenum-rated: CL2P. A CL2R riser cable is not acceptable in the plenum, even if the run is short. The correct fix is CL2P or a Chapter 3 raceway method permitted in the plenum.
Traps
Using CL2 where CL3 is required by the circuit class; using CM in a plenum; mixing Class 2 cable with power conductors without separation; exceeding raceway fill; and supporting cable from ceiling grid components not listed for support.