Wiring Methods and Materials
Chapter 3 Overview
Chapter 3 governs wiring methods: Articles 300 (general), 310 (conductors), 320-340 (raceways and cable), 350-362 (flexible and specialty raceways), 376-392 (wireways, busways, cable trays), and 394-398 (open wiring, outside conductors).
Concepts to Master
- Conductor ampacity: use Article 310 tables, apply correction factors for ambient temperature and adjustment factors for conductor count in a raceway.
- Raceway fill: Chapter 9 tables govern conduit and tubing fill; the 40% fill rule applies to over two conductors.
- Support and securement: each wiring method has spacing requirements for supports and boxes; know typical values and exceptions.
- Box fill: Article 314 governs box volume; count conductors, devices, and fittings.
- Cable and raceway types: understand uses permitted and not permitted for NM cable, MC cable, AC cable, EMT, IMC, RMC, PVC, and flexible conduits.
Traps
- Forgetting ambient temperature correction and conductor-count adjustment when determining ampacity.
- Applying the 40% fill limit to a single conductor or to a nipple (which has its own rule).
- Miscounting box fill by ignoring devices, clamps, or equipment grounding conductors.
- Assuming a wiring method permitted in one occupancy is permitted everywhere.
Worked Example
Six current-carrying conductors in a raceway require adjustment. With a base ampacity from the table, apply the adjustment factor for the conductor count, then any ambient correction, and compare to the overcurrent device. Rule of thumb: adjust for count, correct for temperature, then verify protection.
Rule of Thumb
Ampacity = table value x temperature correction x conductor-count adjustment; never skip either factor.