Estimating a 6 m × 9 m Machine Shop Installation
A 6 m × 9 m machine shop in a vocational institute is to be wired, and the institute wants an estimate before calling for quotations. The agreed requirements are: 10 LED fittings of 36 W each on two lighting circuits; six 16 A switched socket outlets on two circuits; one 3.7 kW, 415 V three-phase lathe; one 1.5 kW, 230 V compressor; and a 2 kW, 230 V water heater in the adjoining washroom. The supply is 415 V, 3-phase, 4-wire, 50 Hz with an assumed overall power factor of 0.85. Route lengths: DB to the farthest light point 24 m, DB to the lathe 18 m, DB to the heater 12 m. For the load schedule the institute instructs that each 16 A socket outlet be counted as 1000 W and each light point at its lamp wattage. A diversity factor of 0.7 is to be applied to lighting and 0.8 to socket outlets; the motors and the heater are treated as continuous. Single-phase loads are to be balanced across R, Y and B.
1. Prepare the connected-load and diversified-demand schedule and select the main incoming device, showing your working.
Read the worked answer
Connected load
| Load | Calculation | W |
|---|---|---|
| Lighting | 10 × 36 | 360 |
| Sockets | 6 × 1000 | 6000 |
| Lathe | 3.7 kW | 3700 |
| Compressor | 1.5 kW | 1500 |
| Water heater | 2.0 kW | 2000 |
| Total | 13 560 W |
Diversified demand
- Lighting: 360 × 0.7 = 252 W
- Sockets: 6000 × 0.8 = 4800 W
- Lathe: 3700 W (continuous)
- Compressor: 1500 W
- Heater: 2000 W
Total = 12 252 W ≈ 12.25 kW
Current at the main
kVA = 12.25 / 0.85 = 14.4 kVA
I = 14 400 / (1.732 × 415) = 20.0 A
Selection: a 32 A, 4-pole main MCB (or 32 A switch-fuse/isolator with a 4-pole MCB) in an 8-way TP&N distribution board. 32 A is chosen because 25 A leaves almost no margin once the lathe's starting current and future additions are considered, and because the next standard rating is only 40 A. The declared load to the utility should be rounded up to about 13.6 kW connected, since sanctioned load is declared in kW and the utility applies its own diversity.
2. Size the sub-circuits - cable cross-section, protective device and voltage drop for each - and check the single-phase balance.
Read the worked answer
Branch circuits (using Vd = 2ρLI/A for single-phase and √3ρLI/A for three-phase, ρ = 0.0175 Ω·mm²/m)
- Lighting, 2 circuits of 5 fittings: 180 W each → 0.78 A. Use 1.5 mm² copper with a 6 A MCB. Vd at 24 m = 2 × 0.0175 × 24 × 0.78 / 1.5 = 0.44 V (0.19 %). Negligible.
- Sockets, 2 circuits of 3 outlets: 3000 W each → 13.0 A. Use 2.5 mm² copper with a 16 A MCB and 30 mA RCD protection. Vd at 24 m = 2 × 0.0175 × 24 × 13.0 / 2.5 = 4.37 V (1.9 %) - acceptable if the main feeder drop is small; use 4 mm² if the main board is remote.
- Lathe: I = 3700 / (1.732 × 415 × 0.80 × 0.85) = 7.6 A. Use 2.5 mm² 4-core with a 16 A TP MCB and a starter with overload set at 7.6 A. Vd = 1.732 × 0.0175 × 18 × 7.6 / 2.5 = 1.66 V (0.4 %).
- Heater: 2000 / 230 = 8.7 A. Use 2.5 mm² with a 16 A DP MCB plus a 30 mA RCD for the washroom. Vd at 12 m = 1.46 V (0.6 %).
- Compressor: 1500 / 230 = 6.5 A on a 16 A circuit, 2.5 mm².
Balance: single-phase load = 360 + 6000 + 1500 + 2000 = 9860 W, about 3.3 kW per phase when the sockets, lights, compressor and heater are spread over R, Y and B. The lathe is the only three-phase load, so no phase carries more than roughly 3.3 kW of single-phase load - an acceptable balance. Record the allocation of each circuit to a phase on the DB schedule.
3. List the main bill-of-materials items with approximate quantities and state the allowances and checks you would show on the estimate.
Read the worked answer
Indicative bill of materials (quantities from route lengths plus 10 % wastage; site survey to confirm)
- 25 mm PVC conduit: room perimeter 30 m + drops to fittings and switches ≈ 60 m, plus bends, couplers and saddles.
- 1.5 mm² copper single-core (lighting): 3 runs × 55 m ≈ 165 m.
- 2.5 mm² copper single-core (sockets, heater, compressor): 3 runs × 70 m ≈ 210 m.
- 2.5 mm² 4-core armoured/PVC cable to the lathe: 20 m.
- 8-way TP&N distribution board with 32 A 4-pole main MCB.
- MCBs/RCDs: 6 A SP ×2, 16 A SP ×4, 16 A DP with 30 mA RCD ×1, 16 A TP ×1, 30 mA RCD protection for the socket circuits.
- Wiring accessories: 10 LED fittings, 10 batten holders or mounting plates, 6 × 16 A industrial sockets, 6 switches, plus 5 A sockets as required.
- Earthing: 6 mm² protective conductor throughout, two GI earth pipes with funnel and charcoal/salt, earth testing point.
Allowances and checks
Add 10 % on cable and conduit for wastage and routing, 5 % on sundries (saddles, glands, tape, ferrules), labour at the current schedule rate, and GST. Show on the drawing sheet the circuit schedule with cable size, MCB rating, phase allocation and the position of the RCDs, and mark that socket and washroom circuits are 30 mA RCD-protected, earthing is bondded to the DB, and all final circuit voltage drops are within the 3 % limit.