Overcurrent Protection, Grounding, and Bonding (NEC Articles 240, 250)
240 and 250 are the highest-yield pair on this exam
Article 240 protects conductors. 240.4 keeps the OCPD from exceeding conductor ampacity; 240.4(D) caps small copper conductors at 15 A (14 AWG), 20 A (12 AWG), and 30 A (10 AWG) unless an article listed in 240.4(G) applies (motors, A/C equipment, fire pumps). 240.4(B) permits the next standard size up when ampacity falls between standard ratings, the device is 800 A or less, and the load does not exceed conductor ampacity. 240.6 lists standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 …
Article 250: 250.24 covers service grounding and bonding; 250.50 requires all present electrodes to be bonded into one grounding electrode system; 250.52 defines electrodes (10 ft of metal water pipe in contact with earth, 8 ft rod, 20 ft of ½ in rebar or 4 AWG copper encased in concrete, 20 ft ground ring of 2 AWG); 250.53 requires a supplemental electrode for a water pipe and for a single rod unless 25 ohms or less is achieved. Use Table 250.66 for the grounding electrode conductor (sized from the largest service conductor) and Table 250.122 for the equipment grounding conductor (sized from the OCPD rating).
Worked rule: a 400 A service with 400 kcmil copper service conductors takes a 1/0 copper grounding electrode conductor (Table 250.66) but a 3 AWG copper equipment grounding conductor (Table 250.122). Swapping those tables is the classic error. Under 250.122(B), an EGC must be increased proportionally when circuit conductors are upsized for voltage drop.