Wiring Systems, Cable Selection and Installation Methods
Cable selection is a chain of checks, not a single calculation. You must satisfy current-carrying capacity after derating, voltage drop, fault-loop impedance and disconnection time, mechanical protection, and environmental suitability. Understand the difference between thermoplastic (V-75, V-90) and thermosetting (X-90, X-HF) insulation, and between circular and flat TPS, armoured, and flexible cords.
Derating factors matter: ambient temperature, grouping (several circuits bunched), thermal insulation, and installation method (enclosed in conduit, in a wall, in free air, buried). The reference method determines the base current rating from the tables. A common trap is applying derating factors by adding them rather than multiplying, or forgetting that the protective device rating must not exceed the derated cable capacity.
Voltage drop limits in AS/NZS 3000 are typically 5 percent from the point of supply to any point of installation for low voltage, with tighter limits for some equipment. Calculate using mV/A/m tables or Vd = (mV/A/m × I × L)/1000.
Worked example: a 20 m run carrying 16 A using a cable rated 2.8 mV/A/m gives Vd = 2.8 × 16 × 20 / 1000 = 0.896 V, about 0.39 percent at 230 V - acceptable.
Other traps: confusing the protective earth conductor sizing rules, forgetting that buried cable needs deeper cover or mechanical protection, and using a cable in a location outside its rating (for example TPS in direct sun or wet areas).