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CHAPTER 10
INRODUCTION TO ELECTRICITY
OVERVIEW
This chapter covers the fundamentals of electrical circuits and laws applicable to the
electrical equipment and systems commonly installed in buildings, but excludes what
may be employed in industrial process. In fact, most of the topics discussed in this
chapter have been covered in college physics. This chapter is merely a review of the
fundamental law of physics with emphasis on the power distribution quantities, such as
resistance, reactance, impedance, current, voltage, power, and energy.
Alternating current is the dominant system used for building power distribution systems,
although direct current is increasingly used for building transportation (elevators and
escalators) and communications (data, computer, telephone) systems. These DC systems
are converted from AC locally within or in proximity of the equipment. Thus, architects
and engineers need not be concerned with the conversion details.
CHAPTER 10
INRODUCTION TO ELECTRICITY
QUESTIONS AND ANSWERS
10.1 The flow of electrical current through a circuit is directly proportional to the
EMF (voltage) for both AC and DC systems. ( True ) ( False )
10.2 What is the relationship between current, voltage and impedance in an AC
circuit?
10.3 What is the relationship between energy, power, current, and voltage in a AC
system?
10.4 What is the standard voltage rating for convenience power (plug-in
receptacles) in the U.S.? In Europe?
10.5 What is Kirchhoff’s first law of electrical circuit? Is it possible to have all
currents flow into a junction?
10.6 What is Kirchhoff’s second law of electrical circuit? What will happen when
a 120 volt appliance is connected to a 240 volt system?
10.7 What is the relation between peak current and effective (rms) current of an
AC power system generated by commercial power companies in the U.S.?
10.8 What are the line-to-line and line-to-neutral voltages in a 120/240 volt, single
phase AC system? In a 120/208 volt, 3 phase system?
10.9 What is power factor? What is the power factor of a DC system?
10.10 Is it desirable to maintain a high power factor in an electrical system? Can
the power factor be improved? How?
10.11 What is the standard AC system frequency in the United States and in some
other countries?
10.12 Name the two major advantages of an AC system over a DC system in
building applications.
10.13 Can AC and DC systems be inter-converted, and if so, how?
10.14 What is the advantage of a 3-phase system over a single-phase system?
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and systems, such as those used in residences, are generally designed for 120 volt
or 240 volt single-phase systems.
Note: A single-phase system needs two wires per circuit. A 3-phase, 3-wire system
needs 3 wires to transmit 1.732X of power. A 3-phase, 4-wire system needs 4 wires
to transmit 3X of power. With the increased use of computers in office buildings,
there is an increase of “3rd Harmonics” in the common neutral wire of a 3-phase, 4-
wire system. Under these circumstances, separate neutral wire is recommended for
each circuit, and the cost advantage of a 3-phase, 4-wire system has been greatly
diminished.
10.15 Due to the internal resistance and/or reactance of the distribution equipment,
the voltage of an electrical system varies at different locations in the system.
The voltage at the end of the distribution system is always lower than the
beginning. A well designed system should limit the voltage drop to a
minimum. Answer the following:
(a) What is the recommended maximum overall voltage drop for a
combination of lighting and power system?
(b) For an electrical heating system?
(c) What is the nominal voltage rating of single phase appliances in the
United States?
10.16 An electrical load is rated for 1150 watts at 115 volts, single phase. What is
the load current if the power factor is 1.0 or 100 percent?
10.17 If the current of the load in question 10.17 is 12 amps, what is the power
factor?
10.18 If the same load as in question 10.17 (1150 watts) is 208 volt, three-phase,
what is the current with 0.8 PF?
10.19 What is the apparent power in VA, of the load in question 10.19?
10.20 What is the peak current of the load in question 10.9?
10.21 What is the peak voltage of a 120 volt, single-phase system? A 208 volt, 3-
phase system?
10.22 If the resistance is 2 ohms and reactance is 4 ohms, what is the impedance?
2 = X2 + R2 = 22 + 42 = 20
10.23 If three loads rated for 2, 3, and 4 kW are connected in parallel, what is the
total load?
10.24 If the system voltage for the loads in question 10.24 is 480 volts, 3-phase what
is the total current of the load? (Note: PF is not given, then it must be
assumed.)
First an assumption must be made for the power factor. In general:
Thus, the answers are:
10.25 Describe the difference between power and energy.
Power is the time rate of consuming (or transmitting) energy. It is easier to
10.26 If a single-phase electrical resistance load operates at 100 volts, how much
current is required to consume 800 watts of power?
10.27 If the load operates for 20 hours, how much energy has been consumed?
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10.28 If the power is a 3-phase system with 100 volt between each phase, what
energy will be consumed after 20 hours of operation of the load?
10.29 What shall be the current flow in the line supplying the single-phase load and
the 3-phase load? (Assuming the load is 100% resistance.)
With 100% resistance in the circuit, the power factor of the circuit is 1.0. From