CHAPTER 13
POWER SUPPLY AND DISTRIBUTION
OVERVIEW
What we have included in this chapter represents the most common equipment types and systems
in building applications.
Figure 13-1 illustrates a 120/240 volt single phase system, typically used for residential, and a
comparison between 277/480 volt and 220/380 volt 3-phase, 4-wire systems, which are typically
commercial systems. The latter is commonly used in Europe and some Asian countries whereas
the equivalent in the U.S. is 120/208-volt system is provided. The 220/380volt system is chosen
in lieu of the popular 120/380 volt system in the U.S.A is to identify the text with international
readers.
Short circuit of an electrical system is the most important aspect of electrical engineering in
building systems. This is difficult to demonstrate, but protection equipment manufacturers, such
as Square-D, or Bussmann do have well documented videotapes or CDs on loan for use in the
class.
There is electrical distribution equipment in every school building. These are excellent
opportunities to demonstrate electrical power systems. With the help of the maintenance
department, the fronts of panels can be removed to review the interior of a panel. The jammed
wires within the panel always amaze students.
Wire and cables samples can also be found. The multi-conductor large 350 or 500 KCM (MCM)
cables are surprises.
Most important of all, the chapter provides the basic information on how to determine the size of
wire and the selection of equipment based on the rules given in the National Electrical Code
(NEC). The NEC contains several hundred pages and is revised every three years. Naturally,
excerpts from NEC rules and tables are incomplete. They serve only to illustrate the principle of
selecting wires and equipment ratings. The reader should use the full NEC as reference to design
your projects. Copy of the NEC can be ordered from National Fire Protection Association.
CHAPTER 13
POWER SUPPLY AND DISTRIBUTION
QUESTIONS AND ANSWERS
13.1 What electrical distribution systems are normally used to serve residences? Small
business buildings with load about 100 kVA?
13.2 What electrical distribution systems are normally selected to serve a combination of
single phase and 3-phase loads?
13.3 What is meant by voltage spread of any electrical distribution system? Voltage
drop?
13.4 What is the recommended maximum kVA rating of a single motor-driven
equipment on a 100 kVA, 120/240 volt, single-phase, 3-wire system? Why is the
recommended maximum considerably lower than the system rating?
11.5 What is the voltage to ground of a 120/208 volt, 3-phase, 4-wire
system?
11.6 What is the voltage to ground of a 480 volt, 3-phase, 3-wire system?
13.7 Why should the interior electrical distribution system be grounded?
13.8 Must all electrical systems be grounded?
13.9 Should the interrupting capacity (IC) of the switchboard be rated higher or lower
than the available short circuit current (Is)? Why?
13.10 Name a few NEC classified wires for building interior wiring systems and
temperature ratings.
13.11 What on-floor power system would you recommend for a high-rise office building
designed for flexibility and capability of using PC computers and electronic
equipment?
13.12 If you wish to control a bank of lights in the room from two different locations, how
many 3-way and 4-way switches shall be used?
13.13 Name several advantages of fuses. Advantages of circuit breakers?
The advantages of fuses over circuit breakers are:
1. Easily resettable
3. Adaptable for remote control
13.14 A 120/240 volt, single-phase system is a system with its center tap as the neutral
point. (See drawing below). If the load of circuit No. 1 connected between Line 1
and neutral is 1500 watts and that of circuit No. 2 between Line 2 and neutral is
1000 watts, what is the current in amperes carried by each circuit and the combined
neutral?
13.15 If the full load current of a transformer having 5% impedance is 500 amperes, what
could be the expected short-circuit current at the main switchboard: (Note: For
simplicity, a conservative approach is to assume that the utility power network
serving this transformer is sufficiently large enough to have practically zero
impedance during a fault.)
According to equation 11-1:
13.16 According to NEC, what is the allowable (maximum) current that can be carried by
a single #3/0, 90C rated conductor in conduit? Six conductors in conduit, and what
conduit size shall be used?
The allowable current carrying capacity is calculated as follows:
13.17 A 20,000 sq. ft. school building has the following estimated demand loads:
Lighting 20,000
watts 120 volt, 1-phase
Mechanical equipment 100 HP* 480 volt, 3-phase
Convenience power 1 W/sq. ft. 120 volt, 1-phase
What power distribution system or systems would you choose?
13.18 What is the standard frequency and the utilization voltage for branch circuits in
Europe?
13.19 If the line-to-neutral voltage of a 3 phase 4 wire system is 220 volt, what is the Line-
to-line voltage? Similarly, for a 2400-volt line-to-neutral voltage, what is the line-to–
line voltage?
13.20 Determine the maximum short current in amperes of an electrical power system
having one main transformer rated at 300 kva, 480 volt, 3 phase. The transformer
13.21 If the system secondary voltage is 240 volt in lieu of 480 volt, what will be the
maximum short circuit value?
13.22 What should be the interrupting capacity rating of the two systems in 13.20 and
13.21?
13.23 Describe the three alternate emergency power sources that can be used for
buildings.
13.24 What is the difference between K4 and K13 rated transformers?
13.25 Why large electrical cables are stranded?
13.26 What is the difference between standby power and uninterruptible power?
13.27 How do K-type transformers, UPS, and SPD differ in their functions with regard to
power-quality management?
13.28 What are the pros and cons of placing a generator indoors versus outdoors?