Wiring Systems, Cables and Accessories
Core concepts
Understand cable construction (conductors, insulation, sheath, armour), common cable types and their applications, and how installation method affects current-carrying capacity. Know the concept of derating: grouping, ambient temperature, thermal insulation and protective device type all reduce the tabulated current rating (I_t). Be familiar with standard circuit arrangements (radial, ring final), accessory selection, and the importance of matching cable and accessory ratings to the load and environment (e.g. IP ratings for wet or outdoor locations).
Traps in questions
Questions often present a cable that is adequate in free air but inadequate when enclosed in insulation or grouped with others. Another trap is selecting a cable by cross-sectional area alone without checking the installation reference method. Watch for ring final circuits where the protective conductor continuity and conductor sizing rules matter, and for accessories used outside their IP or temperature rating.
Worked example / rule of thumb
If a cable has a tabulated rating of 27 A but is grouped with three other circuits (factor 0.65) and run in thermal insulation (factor 0.5), the effective capacity is 27 × 0.65 × 0.5 ≈ 8.8 A. Rule of thumb: apply all derating factors multiplicatively, then confirm the result still exceeds the design current and coordinates with the protective device. Always state the reference method when justifying a cable choice.