Electrical Design
Design questions ask you to apply AS/NZS 3000 principles before any calculation: safety, functionality, and compliance with the intended use. A compliant design must consider maximum demand, protection coordination, earthing, cable current-carrying capacity, voltage drop, and the environment (temperature, enclosure, corrosives, hazardous areas).
The design sequence is the examinable logic: determine loads and diversity, select protective devices, size conductors for current-carrying capacity, then check voltage drop and fault loop impedance, and finally verify protective earthing and bonding. Changing any one element forces re-checking the others - a favourite trap is a question where a longer run or higher ambient temperature invalidates an earlier cable choice.
Other traps: confusing design current (Ib), protective device rating (In) and cable capacity (Iz) - the rule is Ib ≤ In ≤ Iz, with derating factors applied to Iz; forgetting that derating for grouping, ambient temperature and thermal insulation compounds; and treating single-phase and three-phase voltage drop differently when they use different formulae and reference voltages.
Worked example (rule of thumb): For a 32 A single-phase submain, choose In = 32 A, then find Iz after derating. If ambient derating is 0.87 and grouping 0.80, required tabulated capacity = 32 ÷ (0.87 × 0.80) ≈ 46 A. Select a cable whose tabulated Iz ≥ 46 A, then confirm voltage drop over the run.