Conductors, Cables, Boxes, and Outlet Devices
What you must understand
Candidates must know conductor ampacity tables, temperature ratings (60, 75, 90 degrees C), and the ampacity adjustment for ambient temperature and for more than three current-carrying conductors in a raceway. Understand termination temperature limitations: the ampacity used must not exceed the rating of the terminations, which for most equipment is 75 degrees C. Know receptacle ratings, grounding-type receptacles, GFCI and AFCI requirements, tamper-resistant receptacles, and the rules for receptacle spacing in dwellings. Understand box sizing, conductor identification (grounded conductor, grounding conductor, ungrounded conductor), and the uses permitted for flexible cords.
Traps
A classic trap is taking the 90-degree column ampacity when the terminations are only rated 75 degrees C. Another is forgetting that the neutral counts as a current-carrying conductor in some circuits (such as wye circuits with nonlinear loads). Receptacle spacing questions often hinge on the 6-foot and 12-foot rules along walls and the requirement for receptacles in kitchens and bathrooms. Watch for the difference between a grounding conductor and a grounded (neutral) conductor.
Rule of thumb and worked example
Rule of thumb: when more than three current-carrying conductors share a raceway, apply the adjustment table; four to six conductors is 80 percent. Example: a 90-degree C, 12 AWG copper conductor has a base ampacity of 30 amperes. With four current-carrying conductors, 30 × 0.80 = 24 amperes. If terminations are rated 75 degrees C, 12 AWG at 75 degrees C is 25 amperes, so the adjusted 24 amperes governs. Take the lowest applicable value.