2)A commercial building has the following electrical loads.
A 10 kW water heater, a 10,000 watt lighting system, a 1.5 kW air conditioning unit, three 750 watt heating elements, and a motor load of 7.5 horsepower operating at an efficiency of approximately 80% and a power factor of 0.95.
a)Determine the power demand when all loads are energized.
b)Determine the reduced demand when the following conditions occur:
i)Summer loading: 50% motor load and no heating elements, with the other loads remaining the same.
ii)Winter loading: 75% motor load, and no air conditioning with other loads operating at maximum.
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For the 7.5 horsepower motor, you need to know that the rated horsepower is the mechanical output power. One horsepower is equal to 746 watts. Input power in watts is equal mechanical output power divided by efficiency. Therefore, at 100% load, the input power to the motor is 7.5 X 746 / 0.8. The power factor is not required to answer the questions. For 50% and 75% load, just assume that the efficiency remains the same and multiply the rated input watts by 0.5 and 0.75.
Answering the questions is just a matter of adding up the watts. One kW is 1000 watts.