1. What is the difference between power and energy? State the units for each. (2 points)
2. A 20,000 ft2 commercial building receives 277/480 voltage 3-phase power. If 45 amperes is drawn by the higher voltage loads at a particular time and the building's power factor is 0.88 due to several motors, what is the real power used by the building at that time? (1 point)
3. For the building described in Question 2, assume power consumption of 5 W / ft2 during the entire month (30 days), except for a 2-hour period when the consumption was 16 W / ft2. The electricity rate for the building is $ 0.018 per kWh, and the demand charge is $14.03 per kW. Calculate the total electricity bill for that month. (3 points)
4. Equipment and appliances shown in the attached table will be used on a 120/240 V AC household circuit. Complete the table to determine the electricity bill for that month. Base analysis on a 120 V circuit and an energy charge of $0.15/kWh. Note: The total monthly bill will be unrealistically high for a residence. (3 points)
5. Explain the concept of power factor in an AC circuit by answering the following questions. (3 points)
- Why does a low power factor occur?
- What types of loads (provide examples) have a power factor of less than one?
- What can be done in a building to increase the power factor?
6. A utility company provides 750 kVA of 3-phase power to a large building at 13,440 volts, and the building requires 480 volts. What are the currents on the utility side and on the building side of the transformer? How many windings does the transformer need to step-down this voltage (i.e., what is the ratio of coil windings)? (2 points)
7. What is the service entrance of a building? What are its main components? (2 points)
8. Explain two reasons for grounding building electrical systems? (2 points)
9. A set of three motors each delivering an output of 15 horsepower at its shaft is known to be 94% efficient. What is the total kW electrical input required for these motors? (1 point)
10. What other concern, besides efficiency, might a building operator have with using motors in the building, in terms of electricity usage? What can be done to address this other concern? (1 point)
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