Kitchen Circuit, Fuse Rating and Overloaded Sockets
A family home runs on a 240 V supply. The kitchen is wired as a single circuit with 2.5 mm² twin-core cable rated at 20 A, protected by a 15 A fuse. The appliances are: refrigerator 150 W (compressor runs continuously, with a motor start-up surge of about three times its running power for a second), electric kettle 2000 W, rice cooker 700 W, microwave oven 1200 W, toaster 900 W, and LED lighting totalling 20 W. On a public holiday the family wants to boil water, cook rice, heat food in the microwave and make toast at about the same time, with the refrigerator running and the lights on. The fuse has already blown twice this month. The father proposes fitting a 30 A fuse, saying 'the problem will go away'.
1. Calculate the total power and the current drawn if every appliance in the kitchen runs at the same time. State what happens to the 15 A fuse.
Read the worked answer
Total power: 150 + 2000 + 700 + 1200 + 900 + 20 = 4970 W.
Current: I = P ÷ V = 4970 ÷ 240 = 20.7 A.
Conclusion: 20.7 A is well above the 15 A rating, so the fuse element melts and the circuit is broken - this is exactly what has happened twice already. Note also that 20.7 A exceeds the 20 A rating of the cable itself, so the cable is being asked to carry more than it is designed for even before the fuse acts.
Extra caution: the refrigerator motor draws about 450 W for a second at start-up, which adds a brief surge of roughly 1.9 A. This can blow a fuse even when the steady total is just below 15 A, so the safe working load should be kept comfortably under 15 A.
2. The family still wants to use the kitchen efficiently. Using the figures, determine the largest practical combination of cooking appliances that can run with the refrigerator and lights, and justify your choice.
Read the worked answer
Power budget: 15 A × 240 V = 3600 W maximum for the circuit. Refrigerator plus lights already use 150 + 20 = 170 W, leaving 3430 W.
Testing combinations:
- Kettle + rice cooker = 2000 + 700 = 2700 W; total 2870 W → 12.0 A. Safe, with about 3 A of margin for the fridge surge.
- Kettle + microwave = 3200 W; total 3370 W → 14.0 A. Under the limit on paper, but only 1 A of margin, so a fridge start-up or a slightly higher real kettle rating can blow the fuse. Not recommended as a routine habit.
- Kettle + rice cooker + toaster = 3600 W; total 3770 W → 15.7 A. Fuse blows.
- Microwave + rice cooker = 1900 W; total 2070 W → 8.6 A. Very safe.
Answer: the largest reliable combination is the kettle plus the rice cooker while the refrigerator and lights run. The other appliances should be sequenced - heat food in the microwave after the kettle has finished. Sequencing costs nothing; a blown fuse costs time and risks food spoiling.
3. Evaluate the father's proposal to replace the 15 A fuse with a 30 A fuse. Recommend what should actually be done and explain the principle behind your recommendation.
Read the worked answer
Evaluation: the proposal is dangerous and does not solve the problem.
Principle: a fuse protects the cable, not the appliance. It must be rated above the normal current but below the current the cable can safely carry. The cable here is rated 20 A. With a 30 A fuse, currents between 20 A and 30 A would flow through the cable without blowing the fuse: the cable overheats, the PVC insulation softens and melts, and a short circuit or fire can start inside the wall where nobody can see it. The fuse would only blow when damage may already have been done.
Recommended actions:
- Do not run the high-power appliances simultaneously (kettle and microwave especially).
- Have a qualified electrician install a separate circuit for the high-power cooking appliances, with its own correctly rated miniature circuit breaker and cable of adequate size.
- Keep the 15 A fuse or replace it with an MCB of the same rating - never a higher one on the same cable.
- Check sockets and plugs for overheating, and avoid daisy-chaining multi-way adaptors.
Marking insight: the answer earns credit for identifying that the fuse rating is tied to the cable's current-carrying capacity; simply saying 'it is unsafe' is not enough.