Electrical Theory, Circuits, and Calculations
Core concepts
A candidate must be fluent with Ohm's law and the power relationships that follow from it, series and parallel network reduction, AC impedance (resistance, inductive reactance, capacitive reactance, and their phasor combination), three-phase relationships, power factor, and the difference between real, reactive, and apparent power. Voltage drop, conductor ampacity derating, and box/conduit fill calculations recur in inspection scenarios, so the arithmetic must be automatic.
Traps in questions
- Mixing peak and RMS values; RMS is the default for AC quantities unless stated otherwise.
- Treating power factor as a loss rather than a phase relationship; kW stays fixed while kVA grows as PF falls.
- Forgetting that parallel branches share current, not voltage, and that total resistance is always less than the smallest branch.
- Applying single-phase formulas to three-phase loads; remember the √3 factor for line quantities.
- Confusing ampacity (thermal limit) with voltage drop (a separate design check).
Worked example
A balanced three-phase load draws 40 A per line at 480 V with a power factor of 0.85. Apparent power S = √3 × 480 × 40 ≈ 33.3 kVA. Real power P = S × 0.85 ≈ 28.3 kW. If PF improves to 0.95, P is unchanged but S falls to about 29.8 kVA, so current drops to roughly 35.8 A. Rule of thumb: at 480 V three-phase, 1 kVA ≈ 1.2 A.
Practical link to inspection
Inspectors use these calculations to judge whether a reported load is plausible, whether a conductor is protected, and whether a neutral is a current-carrying conductor for derating purposes.