Load Calculation for a Workshop Circuit
A trade workshop has a 230 V single-phase supply protected by a 16 A miniature circuit breaker. The following equipment is connected to one radial circuit: a 1.5 kW bench grinder, a 750 W drill press, a 500 W work lamp, and a 2.2 kW welding set used intermittently. The trainee wants to run the grinder, drill press and lamp together, then start the welder without switching anything off. The cable is rated for 20 A continuous. The workshop also has a second circuit on a separate 16 A breaker that is currently unused.
1. Calculate the total current drawn by the continuously running equipment and decide whether the welder can be added to the same circuit.
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Continuous load = 1500 + 750 + 500 = 2750 W.
Current = P / V = 2750 / 230 ≈ 11.96 A, so about 12.0 A.
Adding the welder: 2750 + 2200 = 4950 W, giving 4950 / 230 ≈ 21.5 A. This exceeds both the 16 A breaker rating and the 20 A cable rating. Even though the welder is intermittent, its starting surge would be higher still, so the breaker would trip.
Conclusion: the welder cannot be run on the same circuit with all three items. The trainee should move the welder to the unused second circuit, which has its own 16 A breaker, or switch off the grinder while welding. Running 21.5 A through a 20 A cable risks overheating and insulation damage.
2. Explain why the cable rating of 20 A matters even though the breaker trips at 16 A, and state one consequence of ignoring it.
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The cable rating is the maximum current the conductor can carry continuously without its insulation exceeding its temperature limit. The breaker protects the circuit, but a breaker rated above the cable's capacity would allow damaging current to flow without tripping; here the 16 A breaker is below the 20 A cable rating, which is the correct arrangement.
The risk arises if someone replaces the 16 A breaker with a larger one to stop nuisance tripping. The cable would then carry more than 20 A, heat up, and its insulation could degrade and eventually fail, causing a short circuit or fire. The correct response to tripping is to reduce the load, not to uprate the breaker.