Estimation and Costing
Scope
This domain tests quantity take-off, rate analysis and preparation of estimates for domestic and small industrial installations, using schedule-of-rates practice.
Core rules
- Measure quantities from the drawing in running metres, numbers or square metres, and enter them in a standard take-off sheet with description, unit, quantity, rate and amount.
- Allow wastage: about 5% for cable and wire, 7–10% for conduit and accessories.
- Labour is usually 15–25% of material cost depending on the item; add overheads and contractor profit, then GST at the applicable rate.
- Circuit lengths include the run up walls and across ceilings, plus 0.3–0.5 m per point for looping and connection.
- Rates come from the prevailing CPWD or state PWD schedule, or from a market quotation; the basis must be stated.
Worked example
A 20 m single-phase circuit needs phase, neutral and earth - three runs. Conductor length = 20 × 3 = 60 m; with 5% wastage, 63 m. At ₹18/m the wire costs ₹1,134. Add 20 mm PVC conduit 20 m at ₹45 = ₹900, eight boxes at ₹30 = ₹240, two switches at ₹70 = ₹140, and one 6 A MCB at ₹180. Material total = ₹2,594. Labour at 20% = ₹519. Subtotal = ₹3,113; add 18% GST = ₹560, giving about ₹3,673. Rounding and a contingency of 3% would be added in a formal estimate.
Traps that cost marks
Forgetting the earth run, measuring only the horizontal length, applying wastage to accessories but not wire, and mixing material and labour rates. Never quote a lump sum without a take-off sheet behind it.