Adding High-Load Appliances to an Old Lighting Circuit
A two-room house in Pune has one 6 A lighting circuit wired with 1.5 mm² copper, feeding 12 lamp points and four fans of 70 W each. The owner adds a 1.5 TR split air conditioner with an input of 1.7 kW, a 2 kW geyser, and a kitchen circuit with a 1.2 kW microwave and a 750 W mixer. The electrician proposes to tap the air conditioner and geyser from the existing lighting circuit and to change the 6 A MCB to 20 A, saying 'the wire will take it'. The main supply is 230 V single-phase with a 32 A service. The house has no RCCB; earthing is present at one pit. The existing wiring is ten years old, and the client wants minimum cost and no wall chasing. The air conditioner is a fixed-speed model.
1. Evaluate the electrician's proposal by calculating the air conditioner and geyser currents. Explain why changing the MCB to 20 A is unsafe on the existing 1.5 mm² circuit.
Read the worked answer
Air conditioner current = 1700/230 = 7.4 A.
Geyser current = 2000/230 = 8.7 A.
Together = 16.1 A, and adding even a small lighting/fan load takes the circuit to about 17.5 A.
The existing circuit is protected at 6 A and wired in 1.5 mm² copper, whose current-carrying capacity is roughly 14–18 A depending on installation method, and lower when enclosed or bundled. Two problems arise. First, the 6 A MCB will trip immediately at 16–17 A. Second, replacing it with a 20 A MCB does not turn 1.5 mm² cable into a 20 A cable: the conductor and old joints would overheat, insulation would degrade, and the lighting sockets are not rated for 16 A. The 20 A device would also remove the only overcurrent protection for the lamps and fans.
The proposal is unsafe. The protective device must protect the smallest cable and accessory in the circuit, so the correct answer is new dedicated circuits, not a larger MCB.
2. Recommend a circuit schedule with cable sizes, MCB ratings and protective devices for the added loads. Justify each choice.
Read the worked answer
A workable schedule is:
- Lighting and fans: 6 A MCB, 1.5 mm² copper, existing circuit retained.
- Kitchen power: 16 A MCB, 2.5 mm² copper, feeding 6 A/16 A sockets for microwave and mixer.
- Geyser: dedicated 16 A MCB, 2.5 mm² copper, with a double-pole switch outside the bathroom.
- Air conditioner: dedicated 20 A MCB, 4 mm² copper, with its own isolator.
- Main: 40 A, 30 mA two-pole RCCB ahead of the MCBs, plus a double-pole main switch.
Justification: each MCB rating is below the cable's ampacity and below the rating of the accessories it feeds. The air conditioner is fixed-speed, so its starting current is higher than its running current, and a 4 mm² cable gives margin and limits voltage drop. The geyser is a fixed resistive load of 8.7 A, so 2.5 mm² with a 16 A MCB is adequate. The kitchen sockets are rated 16 A, so the 16 A MCB protects them. The RCCB provides additional protection against earth leakage and electric shock.
3. Explain why an RCCB is needed even though MCBs are installed, and state what it does not replace.
Read the worked answer
An MCB responds to overload and short-circuit current, not to a small current flowing through a person. If a live part touches a metal body and the earth path is poor, the fault current may be only a few milliamps to a few amps, too low to trip a 16 A MCB, while the body remains live.
A 30 mA RCCB compares the current in the live and neutral conductors. If the difference exceeds 30 mA, it trips, typically within 30 ms at several times its rated residual current, before the current through a person becomes lethal. This is especially important in bathrooms and kitchens, where wet contact lowers body resistance.
The RCCB does not replace earthing. Earthing keeps the metal body near earth potential and provides a low-impedance path for fault current. Both are required, and the RCCB should be checked regularly with its test button.