Wiring Methods and Materials
Selecting a Chapter 3 method
Chapter 3 governs raceways, cables, boxes, and fittings. The candidate must know when a method is permitted: EMT, IMC, RMC, PVC, FMC, LFMC, NM-B, AC, and MC each have location restrictions. NM cable is not permitted in wet locations or in most hazardous locations. PVC needs expansion fittings where temperature changes are expected. Raceways must be supported and secured within specified distances, protected from physical damage, and sized for conductor fill using Chapter 9 tables. Conductor insulation and ampacity come from Article 310 and must be coordinated with terminal ratings.
Support and protection traps
A frequent question asks for the maximum support spacing or the distance from a box to the first support. For EMT, the raceway must be securely fastened within 3 ft of a box or termination and supported at intervals not exceeding 10 ft. Other methods have different spacing, so do not apply the EMT rule to PVC or cable. Another trap is using the conduit trade size as if it proved fill compliance; fill must be calculated. Also watch wet, damp, and corrosive location restrictions.
Worked example
A run of 1/2 in. EMT leaves a junction box and travels 25 ft to a panel. It needs a support within 3 ft of the junction box, intermediate supports so no spacing exceeds 10 ft, and a support within 3 ft of the panel. If the run passes through a wall, protection from nails and screws is also required. Rule of thumb: EMT equals 3 ft from a box and 10 ft between supports.