Mechanical Engineering Chapter 14 Electrical Design And Wiring Overview After The Introduction Basic Physics Equipment

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CHAPTER 14

ELECTRICAL DESIGN AND WIRING

OVERVIEW

After the introduction of basic physics, equipment & systems, and life safety &

communications (auxiliary) of electrical systems, we are now ready to try designing and

preparing the electrical plans for building electrical systems. Naturally, we can only get

a flavor of the electrical design process. For the first time, students can apply their new-

found knowledge usefully. Well motivated students or group of students will find this

chapter interesting and challenging. You may want to assign individuals or group of

students home work, such as the documenting the wiring of the lighting and receptacles

of the classroom where the class in being held, or the wiring of their home or apartment.

They can try on their own to design and wire their studio projects. You may want

to coordinate the class progress with the instructor in the design studio or other related

classes. The assignment may either be as home work or as a semester paper.

The scope and arrangement of NEC are listed in this chapter for reference.

Section 14.7 contains a brief summary of the essentials of NEC . They serve to illustrate

the key principles of NEC which are derived from sophisticated engineering principles or

empirical formulae. Needless to say, the Disclaimer appearing at the front of this book

shall apply to the design and wiring of electrical systems.

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CHAPTER 14

ELECTRICAL DESIGN AND WIRING CONSIDERATIONS

QUESTIONS AND ANSWERS

14.1 Although electrical equipment does not require as much space as most

mechanical equipment, the design of electrical systems and the layout of

electrical components demand even closer interfacing with the architectural

elements in space. Why?

14.2 What are the sequential steps in electrical design? Must they be followed in a

logical order?

14.3 Determine the gross and the demand loads of a power distribution for a large

building complex with the following design data.

• Lighting 2000 kW connected, with 0.9 DF

• Convenience power 1500 kW connected, with 0.1 DF

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• Building systems 1000 kW connected, with 0.8 DF

• Estimated diversity Coefficient 1.2

The gross demand load of the system is the sum of the demand load of all load

groups:

Load group CL (kW) DF (%) DL (kW)

14.4 For safety reasons, adequate clearance must be maintained around the front

and any side with access to the interior of all electrical equipment. What is the

minimum clearance for a 120 volt wall-mounted panelboard? a free-standing

480 volt switchboard with rear access? and a 4160 volt controller which is not

totally enclosed?

14.5 Name several methods for providing convenience electrical power to the

middle of a space not accessible from wall outlets.

The commonly used methods are:

• Raised floor – This is most flexible and costly. It is used when there is a

concentration of plug-in type equipment, such as PCs, and telecommunication

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14.6 What is the difference between a schematic or elementary diagram and a

connection or wiring diagram?

14.7 A room has 8 fluorescent fixtures. Each fixture uses 3 fluorescent lamps

totaling 150 watts including the ballast. It is desired to switch on and off the

center lamp of each fixture for low level of illumination and the other 2 lamps

by another switch for median level of illumination. Switch on all 3 lamps will

provide the highest level. The switching shall be accessible from either of the

2 entries to the room. The room configuration, lighting layout and switching

locations are shown in the floor plan below. Determine the number of branch

circuits required, and prepare the branch circuit wiring plan.

• The total power of the lights is 1200 watts. It is within the normal rating of a

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devices (fixtures and switches). The routing of raceways (conduits) should

Step 1

Prepare an electrical background and determine the circuit and switching arrangement

Step 2

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Step 3

Indicate the number of wires in each segment of the conduit. (See 14.8d for wire

identifications.)

Step 4 (optional)

Identifies the wires in each segment of the conduit.

14.8 Complete the electrical floor plan for the wiring of convenience outlets in a

room as shown in the floor plan which follows. Section 14.8.1 provides the

suggested loads for general purpose outlets which is 1.5A per outlet, or 3A per

duplex outlet. If an outlet is identified to serve a specific equipment, then the

load for that outlet shall be based on the nameplate rating of the equipment,

for example, a TV, a computer, etc. For purpose of this problem, the load at

each receptacle is identified as follows:

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C – general purpose receptacle 3A

D – special equipment 5A

E – special equipment 10A

The electrical distribution system is a 120/208V, 3-phase, 4-wire system.

Figure 14.8a – Identify or estimate the

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Figure 14.8b – Determine the number of

branch circuits and to which circuit each

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14.9 Why is the system capacity for future loads important? What is the general

practice?

14.10 Name the advantages of three-phase system over single-phase system.

For the same electrical load, 3-phase delta or wye systems uses (4) wires including a

14.11 If a piece of the new equipment that will be installed in a building has a different

voltage rating or phase connections, what would you do?

14.12 Should National Electrical Code (NEC) be followed to be letters?

14.13 What is the minimum size wire for branch circuits in residential buildings and for

institutional buildings?

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14.14 If a branch circuit is rated for 30 amperes, what is the maximum rating of portable

equipment that can be connected to this circuit?

14.15 Is EMT conduit allowed for in-the-floor installations? In wet locations?

14.16 Should all 3 phase of a circuit be installed in the same conduit or in different

conduits?