Wiring Methods and Materials
Chapter 3 basics
Article 300 gives the general rules; Articles 320-362 cover specific methods such as AC, MC, NM, UF, rigid metal conduit, IMC, EMT, PVC, and ENT. For each method know where it is permitted, where it is prohibited, support spacing, securing requirements, and bending radii.
Common prohibitions
NM-B cannot be installed in wet locations, in masonry or concrete, or where physical damage is likely. UF is the underground and wet-location cable. EMT is not permitted where severe physical damage or corrosive agents exist unless protected. PVC expands and contracts, so expansion fittings are required for long runs and where it transitions from underground to above grade.
Fill and support
Conduit fill: one conductor 53%, two 31%, three or more 40% of interior cross-sectional area (Chapter 9). Boxes must be accessible; splicing in a pull box without a cover is a violation. Cable must be secured within 12 inches of a box and supported at intervals not exceeding 4.5 feet for common NM cable. Bends must not exceed 360° between pull points.
Traps
Questions love mixing up “support” and “secure,” applying the 40% fill to a two-conductor run (it is 31%), forgetting the 12-inch rule at boxes, and using NM in a damp crawl space where it is prohibited.
Worked example
A 1-inch EMT run has an interior area of about 0.864 in². At 40% fill the usable area is 0.346 in². A 12 AWG THHN conductor has an area near 0.0133 in², so about 26 conductors would fit by area - but ampacity adjustment and termination limits usually control long before fill does.