Electrical Drawing, Estimation and Costing
Drawings you must read
Single-line diagrams show the supply, main switch, busbar, protective devices and feeders; wiring diagrams show actual terminal connections; layout drawings show the physical position of points. Symbols follow IS 732/IEC practice. A schedule of materials lists every item with size, quantity and specification.
Estimation and costing
An estimate is quantity × rate, plus labour, overheads, contingencies, profit and GST. Rate analysis breaks a rate into material, labour, transport and wastage. For wiring, measure conduit and cable run lengths on the layout, add 5–10% wastage, and count accessories point by point.
Worked example
A small extension has 10 light points. Material: cable 60 m at ₹35 = ₹2,100; 10 holders at ₹60 = ₹600; 10 switches at ₹45 = ₹450; DB and MCB = ₹1,200; conduit and boxes = ₹900. Material total = ₹5,250. Labour at 25% = ₹1,312. Subtotal = ₹6,562. Overhead and profit at 15% = ₹984. Total before tax = ₹7,546; add 18% GST = ₹8,904.
Traps
Forgetting labour and wastage, quoting a lump sum without a written scope, and omitting earthing, DB and testing charges. Using an old rate without checking the current market price also loses marks.