Electrical Principles, Theory and Code Fundamentals
What the exam targets
Ohm's law and power relationships, series vs. parallel behavior, AC values (RMS vs. peak), single- vs. three-phase math, power factor, voltage drop, and the architecture of the NEC itself (Article 90 introduction, Article 100 definitions, Article 110 general requirements; Chapters 1-4 apply generally while Chapters 5-7 are special).
Concepts to own
- Ohm's law: E = I × R. Power: P = E × I = I²R for resistive loads; motors and transformers add power factor.
- Three-phase: P = 1.732 × E × I × PF; kVA = 1.732 × E × I ÷ 1000.
- Series: current is constant, voltage and resistance add. Parallel: voltage is constant, current adds, total resistance falls.
- RMS is the effective value: peak = 1.414 × RMS, RMS = 0.707 × peak. A 120 V RMS circuit peaks near 170 V.
- 120/240 single phase versus 208Y/120 three phase: 120 × 1.732 = 208, not 240.
Traps
Mixing watts and volt-amperes on transformer and generator questions; dropping power factor; applying 1.732 to single-phase loads; assuming 90 °C ampacity is usable when 110.14(C) terminal ratings usually hold you to 60 or 75 °C; using "next size up" overcurrent devices where the code does not permit it.
Rule of thumb
Balanced three-phase load: kVA = 1.732 × volts × amps ÷ 1000. Single-phase load: kVA = volts × amps ÷ 1000. Compare kVA to kVA, kW to kW.
Worked example
A 480 V three-phase motor draws 20 A at 0.85 PF. P = 1.732 × 480 × 20 × 0.85 = 14,130 W, about 14.1 kW. Apparent power is 1.732 × 480 × 20 = 16.6 kVA; the roughly 2.5 kVA difference is reactive power, which is why conductors and transformers are sized in amps/kVA rather than kW.