Sizing a Geyser Circuit in a Two-Room House with Flickering Lights
A newly wired two-room house has a 230 V single-phase supply, a 32 A main switch and a 16 A MCB for the bathroom geyser. The geyser is rated 2000 W and is 30 m of cable route from the distribution board. The electrician has used 1.5 mm² copper cable for the geyser circuit and has taken the two bedroom lighting points from the same circuit. The owner reports that whenever the geyser switches on, the bedroom lamps dim noticeably for a few seconds, and the geyser takes longer than expected to heat water. Supply voltage at the board is 232 V. Copper resistivity is 0.0172 Ω·mm²/m. The house has no separate power circuit for the 5 A socket outlets.
1. Calculate the geyser current and the voltage drop on the existing 1.5 mm² cable. Is the circuit acceptable?
Read the worked answer
Geyser current I = 2000/230 = 8.70 A. For a single-phase circuit the drop is calculated over the go-and-return length, so Vd = 2 × I × ρ × L / A. Resistance per conductor = 0.0172 × 30 / 1.5 = 0.344 Ω; loop resistance = 0.688 Ω; Vd = 8.70 × 0.688 = 5.99 V. That is 5.99/230 = 2.6% of nominal, just inside the usual 3% limit but with almost no margin, and the supply is already 232 V. Thermally, 1.5 mm² copper in conduit is usually rated around 11 A, so 8.7 A is within its capacity, but the circuit is shared with lighting. The real defect is the shared circuit and the marginal drop, not a clear overload. The circuit is not acceptable as installed because a 2 kW fixed appliance should have its own circuit, and the drop plus shared lighting produces the visible dimming. Recalculate with 2.5 mm²: resistance 0.206 Ω per conductor, loop 0.413 Ω, Vd = 3.6 V (1.6%), which gives proper margin.
2. Explain why the lamps dim when the geyser switches on, and propose a corrected circuit arrangement.
Read the worked answer
When the geyser switches on, the sudden 8.7 A current flows through the shared cable and its resistance. The extra voltage drop appears instantly, so the voltage at the lamps falls from about 232 V to roughly 226 V. Incandescent lamps are sensitive to voltage and dim visibly; the effect lasts only until the thermostat opens or the current stabilises. A loose neutral or a poor terminal can make this much worse, so that must be checked first. Corrected arrangement: (1) a dedicated 16 A circuit in 2.5 mm² copper from the board to a 15 A/16 A switch-socket for the geyser, with a 16 A MCB; (2) separate 6 A lighting circuits in 1.5 mm²; (3) a separate 16 A power circuit for kitchen and 5 A sockets, not tapped from lighting; (4) proper earthing to the geyser body and a 30 mA RCCB at the board. This removes the shared drop and makes each circuit isolatable for maintenance.
3. The owner asks whether he can simply fit a 20 A MCB on the existing 1.5 mm² cable to stop the dimming. Give your reasoned answer.
Read the worked answer
No. A larger MCB does not reduce voltage drop; drop depends on current, length and conductor area. Fitting a 20 A MCB on 1.5 mm² cable is actively unsafe: the cable's current-carrying capacity is about 11 A in conduit, so a 20 A breaker would allow an overload that heats and damages the insulation before the breaker trips. The dimming would remain, because the same 0.688 Ω loop resistance is still carrying 8.7 A. The correct fix is to increase conductor size to 2.5 mm² and give the geyser a dedicated circuit. If a longer route or higher load were involved, the next step would be 4 mm². The MCB must always be selected to protect the cable, not to mask a symptom. I would also check terminals for tightness, since a high-resistance joint produces the same dimming and is a fire risk.