Electrical Theory and Practice
Core concepts
Candidates need a working command of Ohm's law, Kirchhoff's laws, series and parallel networks, and AC quantities: RMS versus peak, frequency, phase relationships, impedance and reactance. Know the power relationships: true power (W or kW), reactive power (var or kvar) and apparent power (VA or kVA), and why power factor affects cable and transformer sizing. In New Zealand low voltage, the nominal phase-to-neutral voltage is 230 V and the phase-to-phase voltage is 400 V; understand star and delta relationships, including the √3 factor. Protection theory matters: fault loop impedance, prospective short-circuit current, disconnection times, protective earthing and the MEN system. Cable selection combines current-carrying capacity with derating for ambient temperature, grouping, installation method and thermal insulation, then checks voltage drop and short-circuit withstand. Motors, transformers, diversity and load balancing also appear regularly.
Traps
Mixing RMS and peak values (peak = √2 × RMS). Confusing kW with kVA, or assuming unity power factor. Using phase voltage where line voltage is required. Ignoring derating factors when checking a cable against its tabulated rating. Treating series and parallel resistance formulas as interchangeable. Forgetting that tabulated voltage drop values account for the go-and-return path, so the one-way route length is the correct length to use with mV/A/m.
Worked example
A single-phase 230 V, 20 A circuit uses 2.5 mm² copper with a tabulated mV/A/m of 18, over a 15 m route. Voltage drop = (18 × 20 × 15) ÷ 1000 = 5.4 V, which is about 2.3% of 230 V and acceptable against a common 5% limit. Rule of thumb: 1 mm² copper has a resistance of roughly 0.0175 Ω per metre at 20 °C.