Load, MCB Rating and the True Cost of a 'Free' Geyser
A small tailoring unit in Guntur runs on a single-phase 230 V supply protected by a fixed 15 A MCB. The connected load is: an electric iron of 1,000 W, a water geyser of 2,000 W, three bulbs of 60 W each, one ceiling fan of 80 W and a television of 120 W. The owner's daily routine is: geyser 30 minutes, iron 15 minutes, bulbs 6 hours, fan 8 hours, television 5 hours. The tariff is ₹6.50 per unit (1 unit = 1 kWh) and the meter is read once every 30 days. The internal wiring is rated for 20 A, but the MCB is a fixed 15 A device. A relative offers her a used 3,000 W geyser, free of cost, to replace the 2,000 W one. Assume the supply stays at 230 V, treat every appliance as purely resistive for current calculations, and ignore the cost of the geyser itself.
1. Find the current drawn when every appliance is switched on together, and judge whether the 15 A MCB is adequate for the present load.
Read the worked answer
Total power = 1,000 + 2,000 + (3 × 60) + 80 + 120 = 3,380 W I = P/V = 3,380 / 230 = 14.70 A
On paper this is below 15 A, so the MCB should hold. In practice the margin is only 0.30 A, about 2%, which is a poor design point:
- An MCB is not a precision instrument; its actual trip threshold varies from unit to unit and with ambient temperature, so a 15 A device may trip below its nominal current when warm.
- Continuous loading at nearly the full rating is beyond the usual practice of keeping steady load under about 80% of the breaker rating (12 A here).
- Nothing extra can be added: a single 100 W appliance would push the current to about 15.1 A and the breaker would open.
Verdict: the routine is technically within rating but unsafe as a habit. The owner should not run the iron and the geyser simultaneously, since those two alone draw 3,000 W = 13.04 A, leaving almost nothing for the bulbs, fan and television that are on at the same time.
2. Calculate the energy consumed per day, the monthly consumption in units, and the monthly bill at the present load.
Read the worked answer
Convert each appliance to kWh per day (kW × hours):
| Appliance | Power | Hours/day | kWh/day |
|---|---|---|---|
| Geyser | 2.0 kW | 0.50 | 1.00 |
| Iron | 1.0 kW | 0.25 | 0.25 |
| 3 bulbs | 0.18 kW | 6 | 1.08 |
| Fan | 0.08 kW | 8 | 0.64 |
| Television | 0.12 kW | 5 | 0.60 |
Daily total = 1.00 + 0.25 + 1.08 + 0.64 + 0.60 = 3.57 kWh Monthly = 3.57 × 30 = 107.1 units Bill = 107.1 × 6.50 = ₹696.15 (about ₹696)
Note how the geyser alone, running only half an hour, is the largest single item - 1.00 kWh out of 3.57 kWh, i.e. about 28% of the bill from 2% of the day.
3. If the 3,000 W geyser replaces the 2,000 W one, will the MCB still hold? Quantify the change in the bill and advise the owner whether the 'free' geyser is worth accepting.
Read the worked answer
Current with the new geyser, everything on: Total = 1,000 + 3,000 + 180 + 80 + 120 = 4,380 W; I = 4,380/230 = 19.04 A That exceeds 15 A, so the MCB trips. It stays within the 20 A wiring rating, but the breaker, not the cable, is the limiting device.
A subtlety worth spotting: with the geyser, bulbs, fan and television on but the iron off, the load is 3,000 + 180 + 80 + 120 = 3,380 W = 14.70 A - exactly the old full-load current. So the trip happens only when the iron and the 3,000 W geyser overlap. The owner would have to run them strictly one after the other, or upgrade the MCB.
Bill impact: new geyser energy = 3.0 × 0.5 = 1.50 kWh/day, so daily total = 1.50 + 0.25 + 1.08 + 0.64 + 0.60 = 4.07 kWh. Monthly = 4.07 × 30 = 122.1 units; bill = 122.1 × 6.50 = ₹793.65. Extra cost = 793.65 − 696.15 = ₹97.50 per month (₹1,170 per year).
Advice: the geyser is not free - it adds about ₹1,170 a year in running cost and forces an inconvenient interlock between iron and geyser. Accept it only if the household genuinely needs faster heating and is willing to upgrade the MCB to 20 A (matching the wiring) or to switch the iron off whenever the geyser runs.