Sizing a Protective Device for a Coaching-Centre Load
A coaching centre in Kankarbagh, Patna, runs its only classroom from a single 240 V supply line protected by a 6 A MCB. The connected appliances are: four 40 W tube lights, three 75 W ceiling fans, one 0.5 HP (373 W) water pump for the rooftop tank, one 1500 W room heater and one 1000 W water boiler. The owner wants to switch on everything at once during the January cold spell, because students complain of cold and the pump must run before class. The electrician says the MCB will not hold, but the owner argues that since the wiring is new, a 32 A MCB costing almost the same would solve the problem permanently. The supply is single-phase and the tariff is ₹6.50 per unit. On a working day the heater and boiler run for 2 hours each, the lights and fans for 6 hours, and the pump for 1 hour.
1. Calculate the total connected load and the current drawn when every appliance runs together. Will the 6 A MCB hold?
Read the worked answer
Total load = (4 × 40) + (3 × 75) + 373 + 1500 + 1000 = 160 + 225 + 373 + 1500 + 1000 = 3258 W.
Current I = P/V = 3258/240 = 13.6 A (13.58 A).
The 6 A MCB is rated far below 13.6 A, so it will trip within seconds of everything being switched on - the electrician is right. Two further points matter: the pump motor draws a starting current roughly 3–4 times its running current for a moment, and the running load alone already exceeds the rating. The conclusion is that the circuit cannot carry the full connected load; the MCB is not faulty.
2. Which is the smallest standard rating (6, 10, 16, 20 or 32 A) that would let all appliances run? Explain why the owner's plan of fitting a 32 A MCB is unsafe.
Read the worked answer
16 A is the smallest standard rating that clears 13.6 A: 16 × 240 = 3840 W > 3258 W, leaving about 18% headroom, enough for the pump's brief starting surge.
A 32 A MCB is unsafe because an MCB protects the cable, not the appliances. A 32 A device will pass 32 A continuously, while the 2.5 mm² wiring of such a lighting circuit is typically rated only about 18–24 A. If an overload or a partial fault of, say, 25 A occurs, the 32 A MCB never trips, the conductor overheats, the insulation degrades, and the risk of fire or shock rises sharply. The correct approach is to match the protective device to the wire; if a genuinely larger load is needed, a separate circuit in thicker cable with its own MCB must be run.
3. Compute the daily and monthly (30-day) energy use in units and the bill. Where can the owner actually economise?
Read the worked answer
Energy per day:
- Heater: 1.5 kW × 2 h = 3.000 kWh
- Boiler: 1.0 kW × 2 h = 2.000 kWh
- Lights: 0.160 kW × 6 h = 0.960 kWh
- Fans: 0.225 kW × 6 h = 1.350 kWh
- Pump: 0.373 kW × 1 h = 0.373 kWh
Total = 7.68 units per day.
Monthly = 7.68 × 30 = 230.5 units; bill = 230.5 × ₹6.50 ≈ ₹1498.
The lights and fans together use only 2.31 units a day, so the heater and boiler alone account for 5 units a day, i.e. about ₹975 of the monthly bill. Running the boiler only when needed and using a smaller heater would save far more than switching off the pump.