Wiring Systems and Installation Practices
Core concepts
Understand the common wiring methods used in Australian installations: thermoplastic-sheathed (TPS) cable, tough rubber-sheathed cable, and conduit and trunking systems. Know how cables are selected for current-carrying capacity, derated for grouping, ambient temperature, and thermal insulation. Understand voltage drop limits, supporting and fixing requirements, protection against mechanical damage, and segregation of different circuits. Know the requirements for accessories, switchboard layout, and labelling.
Traps in questions
Questions often test derating factors applied in the correct order or ask whether a cable installed in a wall cavity with insulation requires a derating factor. Candidates confuse the current-carrying capacity tables with the voltage drop tables. Another trap is whether a circuit may share a neutral or an earth, and whether cables in a ceiling space need support. Be careful with minimum bending radius and with separation of communications cable from power cable.
Worked example
A 2.5 mm² TPS cable is clipped to a timber stud in a roof space with an ambient temperature of 40 °C and grouped with two other circuits. Start with the base current-carrying capacity from the relevant table, then apply the ambient temperature correction factor and the grouping factor. If the corrected capacity falls below the design current, select the next cable size or alter the installation method. Always record each factor and its source.
Rule of thumb
Design current must not exceed the derated cable capacity, and voltage drop must remain within the limit for the circuit length. When in doubt, increase cable size rather than rely on a marginal calculation.