General Electrical Knowledge and NEC Fundamentals
How the Code is organized
The NEC is an installation code, not a design manual. Article 90.3 gives the hierarchy: Chapters 1 through 4 apply generally, Chapters 5, 6 and 7 modify or supplement them, Chapter 8 covers communications and stands largely alone, and Chapter 9 contains tables. When a general rule and a specific rule conflict, the specific rule governs. Informational notes are explanatory only and are not enforceable.
Terms and numbers to know cold
Article 100 definitions control meaning throughout the Code. Shall is mandatory, should is advisory, approved means acceptable to the authority having jurisdiction, listed means the equipment appears on a list published by a nationally recognized testing laboratory, and labeled means it bears that mark. A continuous load is one in which the maximum current is expected for three hours or more.
Rules of thumb
Continuous load is taken at 125 percent for branch circuits, feeders and overcurrent devices (210.19(A), 210.20(A), 215.3). Ampacity starts with Table 310.16, is corrected and adjusted per 310.15, and is then capped by the weakest termination rating per 110.14(C). Overcurrent device ratings come from the standard sizes in 240.6(A). Working clearance in front of equipment is 110.26.
Traps
The next-size-up rule in 240.4(B) applies only when the conductor ampacity does not match a standard rating and the next size up does not exceed 800 A. Candidates also apply 125 percent to the entire load instead of only the continuous portion, use nameplate current where the tables govern (motors), and treat a grounded conductor as a grounding conductor.
Worked example
A branch circuit supplies a 48 A continuous load with 75 degrees C terminations. 48 x 1.25 = 60 A, which is a standard rating. Select a copper conductor whose 75 degrees C ampacity is at least 60 A: 6 AWG copper at 65 A. Result: 60 A breaker, 6 AWG copper.