National Electrical Code and Code Calculations
How the Code is built
NFPA 70 has nine chapters. Chapters 1 through 4 apply generally; Chapters 5 through 7 modify or supplement them; Chapter 8 (communications) stands alone; Chapter 9 holds the tables. Rules use shall for mandatory and may for permissive. Informational notes and fine print notes are explanatory, not enforceable. Article 90 covers scope and enforcement, Article 100 the definitions, Article 110 the general installation requirements.
The calculation articles to drill
Article 220 for branch-circuit, feeder, and service load calculations; Article 210 for receptacle spacing (no point on a wall more than 6 ft from a receptacle, so the 12 ft rule), GFCI (210.8) and AFCI (210.12) requirements; Article 240 for overcurrent protection, including the small-conductor limits of 240.4(D) and the next-size-up permission of 240.4(B); Article 310 for ampacity; Articles 250, 334, and 314 for grounding, cable, and box fill.
Traps
Forgetting that continuous loads are taken at 125 percent before sizing the overcurrent device; applying 240.4(B) where the circuit supplies multioutlet receptacles; forgetting that ampacity must be corrected for ambient temperature and adjusted for more than three current-carrying conductors; using the 90 degree column for a termination limited to 75 degrees.
Worked example
A continuous 40 A load: 40 x 1.25 = 50 A. A 50 A breaker is a standard size, and 6 AWG copper at the 60 degree column is rated 55 A, so 6 AWG with a 50 A device satisfies the load.
Rule of thumb
Noncontinuous load plus 125 percent of continuous load gives the minimum conductor and device rating. Check the small-conductor table before you trust a large breaker.