National Electrical Code (NFPA 70): Structure, Rules, and Application
How the Code is organized
NFPA 70 is arranged in nine chapters: 1 general, 2 wiring and protection, 3 wiring methods and materials, 4 equipment for general use, 5 special occupancies, 6 special equipment, 7 special conditions, 8 communications systems, and 9 tables. Article numbers track the chapter (210, 250, 310, 430), so knowing the chapter tells you what kind of rule you are reading. Article 100 definitions control the meaning of terms everywhere else. Chapter 9 tables cover conduit and tubing fill, conductor properties, and bending radii; the annexes are informational only.
Rules that decide questions
Shall is mandatory, shall not is prohibited, and informational notes are not enforceable. Section 110.3(B) requires equipment to be installed per its listing and labeling instructions, which is a favorite trap when an answer looks code-compliant but ignores the manufacturer. Article 90 gives the authority having jurisdiction approval power, not authority to ignore a requirement. Chapter 2 rules apply unless a later chapter modifies them.
Ampacity and overcurrent basics
Continuous loads are calculated at 125 percent for branch circuits, feeders, and overcurrent devices. Standard ratings come from 240.6, and 240.4(B) allows the next higher standard device only within its limits. Terminal temperature ratings are governed by 110.14(C); circuits of 100 A or less use the 60 C column unless the equipment is listed and marked otherwise. Ampacity is adjusted for ambient temperature and conductor count, then the smaller of the adjusted ampacity and the termination limit governs.
Worked example
A 40 A continuous load: 40 x 1.25 = 50 A, so conductors must have at least 50 A ampacity and the overcurrent device must be at least 50 A. Fifty amps is a standard size, so no rounding is needed. Round up at the breaker, never at the conductor.