Adding a 2 kW geyser to an old 1.5 mm² lighting circuit
At a two-room house in a block town the owner wants a 2,000 W storage geyser fitted in the bathroom, 22 m of cable run from the distribution board. The installation is single-phase 230 V with a sanctioned load of 3 kW. The existing lighting circuit uses 1.5 mm² copper single-core PVC cable drawn in conduit and is protected by a 6 A MCB. Connected to it are five 60 W lamps, three 75 W fans, a 200 W refrigerator and a 120 W television. The main is a 32 A DP MCB behind a 63 A DP isolator. The conduit runs through the loft, where the ambient temperature in June reaches 44 °C. The owner says the previous electrician told him that 1.5 mm² is good for 15 A, and wants you to replace the 6 A MCB with a 16 A one and tap the geyser from the nearest light point. He wants the job finished the same evening.
1. Compute the existing connected load and the geyser current. Is the existing 1.5 mm² lighting circuit able to carry the geyser? Show every derating you apply.
Read the worked answer
Existing load: (5 × 60) + (3 × 75) + 200 + 120 = 300 + 225 + 200 + 120 = 845 W, i.e. 845 / 230 ≈ 3.7 A.
Geyser: 2,000 / 230 ≈ 8.7 A (a resistive, continuous load).
Combined: 2,845 W ≈ 12.4 A.
Capacity of 1.5 mm² copper PVC in conduit: about 11 A at a 30 °C reference ambient. Applying the ambient correction for 44 °C (factor ≈ 0.87) gives ≈ 9.6 A; grouping with the other circuits sharing the conduit takes it to roughly 8.5–9 A. A continuously running fixed heater should not be loaded above about 80 % of that, so the practical figure is close to 7–8 A.
The geyser alone (8.7 A) already sits above that, and the combined 12.4 A is far outside it. The 6 A MCB would trip on the geyser alone, so the circuit is not capable. Note also that the combined 12.4 A would be carried by a 16 A MCB without any problem for the MCB - which is exactly why the MCB must not be changed.
2. Design the geyser circuit properly: cable size, MCB rating, earth conductor, bathroom protection, and a voltage-drop check over the 22 m run. State what must change on the supply side.
Read the worked answer
Cable: 2.5 mm² copper single-core PVC in conduit, capacity ≈ 18–20 A at 30 °C, about 16 A after the 44 °C derating - comfortably above 8.7 A.
Protection: 16 A MCB, Type B or C (resistive load, so either is fine), in the DB, feeding a DP switch with indicator outside the bathroom plus a local isolator.
Earth conductor: 2.5 mm² copper, run to the same earth pit as the rest of the installation; continuity checked by test.
Extra protection: a 30 mA RCCB is required for the bathroom circuit. A bathroom is a wet location and the geyser is a fixed appliance in contact with water piping; the RCCB is the only protection that responds to a person's body current.
Voltage drop: for 2.5 mm² the drop is about 18 mV/A/m. Drop = 18 mV × 8.7 A × 22 m ≈ 3.4 V ≈ 1.5 % of 230 V - within the 3 % allowed for a final sub-circuit.
Supply side: total connected load becomes 845 + 2,000 = 2,845 W ≈ 12.4 A. The 32 A main and 63 A isolator are adequate, but the sanctioned load must be raised from 3 kW to 4 kW with the distribution company; otherwise the meter or load limiter will trip and the consumer risks a penalty.
3. The owner insists: 'Just put the 16 A MCB on the old wire and finish the job.' Explain, with the physics, why you must refuse, and what you will say and do.
Read the worked answer
The MCB protects the cable, not the appliance. The 1.5 mm² cable in that 44 °C loft can safely carry about 9–9.6 A. Standard MCB behaviour: it must not trip at 1.13 × rating (18 A) and is only required to trip within one hour at 1.45 × rating (about 23 A). So 12.4 A would flow through the cable indefinitely with no tripping.
What happens then: conductor heating rises as I²R. The PVC insulation is rated about 70 °C; once that is exceeded it softens, plasticiser migrates, the insulation chars and eventually breaks down to a short circuit or an earth fault. The damage happens inside the conduit, out of sight - a classic cause of concealed fires.
The joint is the second danger: a tap taken from a lighting point is a spur with a small connector, not rated for 8.7 A. A loose high-resistance joint heats, oxidises, heats more, and ignites.
What I tell him: the geyser needs its own 2.5 mm² circuit with a 16 A MCB and a 30 mA RCCB; it is about one extra hour and a modest amount of material. I will not energise a circuit I know to be unsafe, and I will note the refusal on the bill. The installation is not left half-done - it is left correct.