General Power Engineering Fundamentals
Core concepts
General power engineering questions test whether you can move fluidly between real, reactive, and apparent power and apply the practical rules that govern equipment sizing. Know the power triangle (P, Q, S), the difference between kW, kVAR, and kVA, and that only real power does work while reactive power establishes magnetic fields. For balanced three-phase systems, S = √3·V_LL·I_L, with P = S·cosθ and Q = S·sinθ. Efficiency is useful output divided by total input; losses split into fixed (core, friction, windage) and variable (I²R) components. Load factor, demand factor, and diversity factor determine service and transformer sizing - not the arithmetic sum of connected nameplate loads. Also expect questions on power quality (harmonics and THD from non-linear loads) and on the codes governing installations, primarily the NEC for premises wiring and the NESC for utility lines.
Traps to expect
Mixing line-to-line and line-to-neutral voltage is the most common error: √3 appears only when using line-to-line voltage in a wye-connected calculation. Watch for a load described as "leading" versus "lagging," and remember that adding capacitors reduces kVA but never changes kW. Efficiency problems often hide the output-versus-input distinction.
Worked example
A 480 V, three-phase load draws 100 A at 0.8 power factor lagging. S = √3 × 480 × 100 = 83.1 kVA; P = 83.1 × 0.8 = 66.5 kW; Q = 83.1 × 0.6 = 49.9 kVAR. Correcting to unity power factor requires about 50 kVAR of capacitors, and line current falls to roughly 80 A - the kW stays the same.