Electrical theory, wiring methods, and installation practice
What this domain tests
This domain tests electrical theory applied to installation practice: Ohm's law, power, series and parallel circuits, AC impedance, voltage drop, conductor ampacity, raceway and cable selection, support, fill, box fill, and working clearances. Exam questions often combine a calculation with a Chapter 3 wiring-method rule.
Core rules
- Ohm's law: E = I x R; power: P = I x E. Series circuits share current; parallel circuits share voltage. AC adds impedance and power factor.
- Conductor ampacity comes from Table 310.16, but the termination temperature rule in 110.14(C) limits the usable column. Derating under 310.15(C)(1) applies when more than three current-carrying conductors share a raceway or cable.
- Chapter 3 governs wiring methods: EMT, IMC, RMC, PVC, NM-B, MC, and AC cable each have support, securing, burial, and use restrictions. Table 300.5 gives minimum burial depths.
- Raceway fill uses Chapter 9 tables; box fill uses 314.16. Working clearance at equipment is 110.26.
Worked example
A 120 V single-phase branch circuit carries 16 A to a load 100 ft away, using 12 AWG copper with R = 1.93 ohm/kft. Voltage drop = 2 x I x R x L / 1000 = 2 x 16 x 1.93 x 100 / 1000 = 6.18 V. Percent drop = 6.18 / 120 = 5.15%. Reasoning: the factor 2 accounts for the outgoing and return conductors; the result exceeds the common 3% branch-circuit design target, so a larger conductor or shorter run should be considered. The NEC voltage-drop notes are generally informational, not a mandatory 3% rule.
Traps
Using the 90 C ampacity with 75 C terminations; forgetting continuous-load 125%; miscounting box fill; ignoring support spacing; using NM cable in wet or exposed-to-damage locations.