Electrical Drawing, Estimation and Documentation
Core Concepts
Understand electrical symbols and diagrams: schematic, wiring, and layout diagrams. Know how to interpret single-line diagrams, cable schedules, and bill of quantities (BOQ). Understand estimation: calculate quantities of materials (cable, conduit, switches, sockets), labor costs, and overheads. Know documentation: test certificates, schedules of test results, and as-built drawings. Understand scale drawing and take-off sheets.
Traps in Questions
- Schematic diagram shows electrical connections, not physical layout.
- Wiring diagram shows physical connections including terminal numbers.
- Estimation must include wastage allowance (typically 5-10% for cable).
- Cable length includes vertical drops and slack; do not just measure horizontal runs.
- Documentation must be completed after testing, not before.
Worked Example / Rule of Thumb
Example: A room has 6 double sockets, 4 lighting points, and 2 switches. Cable run per socket = 8 m, per light = 10 m, per switch = 5 m. Total cable = 6×8 + 4×10 + 2×5 = 48+40+10 = 98 m. Add 10% wastage = 107.8 m, round up to 108 m. Rule of thumb: For lighting circuits, allow 1.5 mm² cable; for socket circuits, 2.5 mm²; for cookers, 6 mm². Always check local regulations.