The Overloaded Extension Cord
A student sets up a display for the school science fair. She plugs a power strip into a single wall outlet and then connects four devices to the strip: a 60-watt lamp, a 40-watt fan, a 250-watt mini projector, and a 500-watt kettle. The wall outlet is on a circuit with a 15-amp breaker, and household voltage is 120 volts. During testing, the kettle and projector run together for several minutes. The student notices the cord of the power strip feels warm and the breaker eventually trips. She wonders whether the devices together drew too much current, and whether the warm cord is a sign of a problem or just normal operation.
1. Calculate the total current drawn when all four devices operate at once. Does this exceed the breaker's rating? Show your reasoning.
Read the worked answer
Power equals voltage times current (P = V × I), so I = P ÷ V.
Total power = 60 W + 40 W + 250 W + 500 W = 850 W.
Total current = 850 W ÷ 120 V ≈ 7.1 A.
This is below the 15 A breaker rating, so the breaker should not trip from current alone when all four run steadily. However, if the breaker tripped, something else is happening - perhaps an inrush surge when the kettle or projector starts, a fault, or another load on the same circuit. The calculation shows the steady-state load is not the full explanation, so the student should investigate further rather than assume the strip is simply overloaded.
2. Explain why the power strip cord might feel warm even when total current is below the breaker limit. Is warmth always dangerous?
Read the worked answer
Wires have resistance, so some electrical energy converts to thermal energy as current flows (P = I²R in the cord). A cord carrying ~7 A will warm slightly; this is expected. Warmth becomes a concern when it is excessive, when the cord is coiled (trapping heat), when the cord is undersized for the current, or when connections are loose (adding resistance). The breaker protects against sustained overcurrent, but it does not detect a hot, damaged, or overloaded extension cord at currents below 15 A. So warmth is not automatically dangerous, but a cord that is hot to the touch, smells, or is damaged signals a hazard and should be replaced or the load reduced.
3. The student wants to run all four devices safely for an hour. Identify one change she could make and explain how it reduces risk using energy or current reasoning.
Read the worked answer
Acceptable changes include: plugging the high-power kettle directly into the wall outlet (removing 500 W from the strip, dropping strip current to ~2.9 A), using a strip rated for the total wattage, uncoiling the cord, or not running the kettle and projector simultaneously. Any answer must connect the change to reduced current (I = P/V), reduced I²R heating, or keeping the strip within its rated capacity. Simply saying 'it will be safer' without the current or heat reasoning is insufficient.