Adding an AC and a geyser to a 2 BHK flat on an existing 4 mm2 submain
A 2 BHK flat in Nashik has a single-phase 230 V supply, sanctioned load 4 kW, with the meter on the ground floor and the DB on the first floor 25 m away along the wall. The submain is 4 mm2 copper PVC in conduit, protected by a 32 A MCB; final circuits are lighting on 1.5 mm2 with a 6 A MCB, and three socket points on 2.5 mm2 with a 16 A MCB. There is no RCCB and the earth wire is 1.5 mm2, run to a single 8 ohm electrode. The owner now wants a 1.5 TR split air-conditioner (input 1.6 kW, starting current about 5 times running) in the bedroom and a 2 kW instantaneous geyser in the bathroom. The connected load after the addition is 7.4 kW and the owner insists nothing else needs changing because 'the MCB has never tripped'. The electrician must submit a short technical note with the quotation, because the housing society requires a load-sanction application to be supported by the internal wiring design.
1. Apply diversity, calculate the design current, and check whether the existing 4 mm2 submain and 32 A MCB are adequate, including the 3 percent voltage drop limit.
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Diversity. Take 100 percent of the two largest loads (AC 1.6 kW + geyser 2.0 kW = 3.6 kW) and 50 percent of the rest (lighting and fans 0.8 kW, sockets and kitchen 3.0 kW, i.e. 3.8 kW x 0.5 = 1.9 kW). Maximum demand = 5.5 kW.
Design current. 5,500 / (230 x 0.9) = 26.6 A, say 27 A.
Cable capacity. A 4 mm2 PVC copper conductor in conduit is rated about 30 A, so on current alone it is only just acceptable.
Voltage drop. Using 11 mV/A/m for single-phase 4 mm2: drop = 11 x 27 x 25 / 1000 = 7.4 V. Permitted is 3 percent of 230 V = 6.9 V. So the run already exceeds the limit at normal maximum demand.
Starting. The AC takes about 38 A for a fraction of a second, giving 10.5 V (4.6 percent) momentarily; a 32 A type C MCB needs 160 A or more to trip magnetically, so no nuisance trip from starting, but the compressor contactor and the owner's UPS will see a dip.
Conclusion. The 4 mm2/32 A submain is not adequate once the AC and geyser are added: it fails on voltage drop and has no margin on current.
2. Prepare the circuit schedule for the upgraded installation and justify each rating.
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Submain. Replace 4 mm2 with 6 mm2 copper (7.3 mV/A/m): drop = 7.3 x 27 x 25 / 1000 = 4.9 V = 2.1 percent, within limits. Protect with a 40 A MCB, which is below the 6 mm2 capacity of about 38 to 41 A in conduit, so the cable is protected.
Final circuits.
- AC: 2.5 mm2, 20 A MCB, dedicated circuit, isolator within reach of the indoor unit.
- Geyser: 2.5 mm2, 16 A MCB, dedicated circuit, 20 A DP switch outside the bathroom.
- Kitchen and general sockets: 2.5 mm2, 16 A MCB, maximum six points per circuit.
- Lighting and fans: 1.5 mm2, 6 A MCB.
Residual current protection. Fit a 40 A, 30 mA two-pole RCCB as the incomer (or RCBOs on socket, AC and geyser circuits). The geyser and bathroom sockets must be RCCB protected because the location is wet.
Earth. Upgrade the earth conductor to 2.5 mm2 throughout, keep it separate from the neutral, and add a second electrode to bring the value to 5 ohm or less. A 1.5 mm2 earth is not acceptable for a 20 A circuit.
Load sanction. Apply for enhancement from 4 kW to 8 kW; the internal design and the sanction application should quote the same maximum demand of 5.5 kW.
3. The owner offers to save money by keeping the 4 mm2 cable and simply fitting a 40 A MCB. Explain with numbers why this must be refused.
Read the worked answer
The cable becomes the fuse. The 4 mm2 PVC conductor is rated about 30 A in conduit. A 40 A MCB carries 40 A indefinitely and will only trip magnetically at roughly 200 to 400 A. A sustained 35 to 40 A load, which is exactly what an AC plus geyser plus kitchen produces, heats the conductor beyond its insulation rating. PVC softens and degrades above about 70 degrees C conductor temperature, and the MCB never sees this as a fault.
The risk compounds. Insulation that has been overheated cracks, moisture enters, and the next event is an earth fault on a circuit that has no RCCB. That is the classic sequence behind domestic wiring fires.
Nothing is gained. The voltage drop problem, 7.4 V against a 6.9 V limit, is unchanged by a bigger MCB; in fact the AC compressor will draw more current at lower voltage, worsening the drop and the heating.
Downstream danger. A single 40 A incomer also gives no protection to the 1.5 mm2 lighting circuit or the 2.5 mm2 socket circuits, so a short in a lamp holder relies entirely on the 6 A and 16 A MCBs being correct and functioning.
The correct message to the owner. The 6 mm2 upgrade costs a small fraction of the AC and geyser, is done in one day, and is the item the society's engineer will check before approving the load enhancement.