Electrical Installations in Buildings
Building electrical design covers load estimation, circuit design, protective device selection, cable sizing, and layout. Start with diversity and demand factors: connected load is not the design load - apply diversity to estimate maximum demand, then size the supply and main switchgear.
Cable sizing is a recurring exam theme. The cable must satisfy three checks: current-carrying capacity (after applying correction factors for ambient temperature and grouping), voltage drop (typically limited to a few percent of nominal), and protective-device coordination (the device must protect the cable, and the earth-fault loop must allow disconnection within the required time). Understand that derating factors multiply - grouped cables in a hot environment derate more than either factor alone.
Know final circuit arrangements: lighting and power circuits, socket-outlet circuits, and the role of RCDs (residual current devices) for additional protection (commonly 30 mA for socket outlets). Understand segregation of circuits, isolation, and switching requirements.
Worked example: A cable rated 40 A is installed in a 40 °C ambient (derating factor 0.87) and grouped with three other circuits (factor 0.80). Effective capacity = 40 × 0.87 × 0.80 = 27.8 A. A 25 A protective device is acceptable; a 32 A device is not.
Traps: forgetting to apply both ambient and grouping factors; sizing cable on connected load rather than diversified demand; and neglecting voltage drop on long runs even when current capacity is adequate.