General Electrical Knowledge, Theory, and Definitions
Fundamentals
Candidates must move fluently between Ohm's law, power relationships, and AC circuit behavior. Know that in a series circuit current is common and voltage divides; in parallel, voltage is common and current divides. For AC, distinguish peak, peak-to-peak, average, and RMS values; RMS is the effective heating value and is the default for nominal voltages. Impedance combines resistance and reactance, and power factor separates real power (kW) from apparent power (kVA).
Three-phase relationships
For balanced three-phase loads, use P = √3 × V_line × I_line × PF; single-phase uses P = V × I × PF. Line-to-line voltage is √3 times line-to-neutral in a wye system. Know definitions such as ampacity, continuous load, demand factor, diversity factor, and branch circuit versus feeder.
Traps
Common errors include plugging peak voltage into power formulas, mixing line and phase values, ignoring power factor on motor loads, and treating kVA as kW. Another trap is using 120 V when the question gives 208Y/120 or 480Y/277 and asks for line current.
Worked example
A 10 kW, 208 V, three-phase resistive load: I = 10,000 ÷ (1.732 × 208 × 1.0) = 27.8 A. The same 10 kW at 240 V single-phase draws 41.7 A. Rule of thumb: for three-phase amps, divide kW × 1000 by 1.732 × voltage.