Electrical Blueprint Reading Revised
Introduction
by Taylor F. Winslow
Revisions and new illustrations by
E. Glenn Engineering
Chapter
One
ELECTRICAL
DRAWINGS
An
electrical blueprint is an exact copy or reproduction of an original drawing,
consisting of lines, symbols, dimensions, and notations to accurately convey an
engineer's design to workmen who install the electrical system on the job. The
student should keep in mind that the workmen must be able to take a blueprint,
and without further instructions, install or produce the electrical system as
the engineer or draftsman intended it to be accomplished. A blueprint,
therefore, is an abbreviated language for conveying a large amount of exact,
detailed information, which would otherwise take many pages of manuscript or
hours of verbal instruction to convey.
In every
branch of electrical work, there is often occasion to read an electrical
drawing. Electricians, for ex- ample, who are responsible for installing the
electrical system in a new building, usually consult an electrical drawing to
locate the various outlets, the routing of circuits, the location and size of
panelboards, and other similar electrical details, in preparing a bid. The
electrical estimator of a contracting firm must refer to electrical drawings in
order to determine the quantity of material needed. Electricians in industrial
plants consult schematic diagrams when wiring electrical controls for machinery.
Plant maintenance men use electrical blueprints in troubleshooting. Circuits may
be tested and checked against the original drawings to help locate any faulty
points in the installation.
TYPES
OF ELECTRICAL DRAWINGS
Electrical
Construction Drawings
Drawings
that represent the physical arrangement and views of specific electrical
equipment are called electrical construction drawings. These drawings give all
the plan views, elevation views, and other details necessary to construct the
job. Fig. 1-1 shows a sketch of an electrical panelboard "can,"
or housing. One side of the housing is labeled "front" and another
side, "top".
In
Fig. 1-2, the drawing labeled "top" is what you see when you look
directly down at the top of the housing, and in doing so, the sides, the bottom,
the front, and the back are cut from your view.
The drawing labeled "front" is what you
see when the block is directly in front of you. In this case, you cannot see the
top, bottom, back, or the two sides.
A "side" view is what would be seen if
the right side of the housing was turned toward you. This cuts from your view
the top, the bottom, the front, the back, and the left sides.
The width
of the housing is shown by the horizontal lines of the top view and the
horizontal lines of the front view. The height is shown by the vertical lines of
both the front and the side views, and the depth is shown by the vertical lines
of the top view and the horizontal lines of the side view.
These
three drawings-the top, front, and side views-tell all about the shape of the
housing, but they do not indicate the size of the housing. There are two common
methods to indicate the actual length, width, and height of the housing. One is
to draw these views to some given scale, such as 1" = 1'-0". This
means that 1 inch on the drawing represents 1 foot in the actual construction of
the housing. The second method is to give dimensions on the drawings; this
method can be seen in Fig. 1-3. Note that the gauge and type of material are
also given in this drawing. The drawing is also drawn to scale.
The
drawings of the panelboard housing that was just covered are typical of an
electrical construction drawing. They indicate how the equipment will look when
completed and show more clearly than any other electrical drawing the actual
outlines of equipment installed in their respective locations. All details of
the equipment and materials to be used are given on these drawings, and all
dimensions, notes, and references are shown. The complete construction drawing
gives all the physical information necessary for installing or erecting the
equipment.
Electrical
construction drawings, such as the drawing of the panelboard housing in Fig.
1-3, are used by electrical-equipment manufacturers. Electric utility companies
also use drawings, such as the one in Fig. 1-4 giving details on the
construction of a high-voltage trans- mission line. A consulting engineering
firm may use an electrical construction drawing to supplement building
electrical-system drawings for a special installation (Fig. 1-5). The latter is
often referred to as an electrical detail drawing.
Electrical
Diagrams
Electrical
diagrams are drawings that are intended to show, in diagrammatic form,
electrical components and their related connections. They are seldom, if
ever, drawn to scale, and show only the electrical association of the different
components. In diagram drawings, symbols are used extensively to represent the
various pieces of electrical equipment, and lines are used to connect these
symbols, indicating the size, type, and number of conductors (wires) that are
necessary to complete the electrical circuit.
Single-line
block diagrams are used extensively by consulting engineering firms to indicate
the electric service equipment. The power-riser diagram in Fig. 1-6, for
example, is typical of such drawings. The drawing shows all pieces of electrical
equipment as well as the connecting lines used to indicate the circuits. Notes
are used to identify the equipment, indicate the size of conduit necessary for
each circuit, and the number, size, and type of conductors in each conduit. A
panelboard schedule usually is included with single-line power-riser diagrams to
indicate the exact components (fuses, circuit breakers, etc.) contained in each
panelboard.
A
schematic wiring diagram (Fig. 1-7) is similar to a single-line block diagram
except that the schematic diagram gives more-detailed information and the actual
number of wires used for the electrical connections are shown.
ELECTRICAL
WORKING DRAWINGS
Electrical
working drawings that are prepared by architects and consulting engineers to
describe the electrical system in a building are very unique drawings. Most
drawings encompass all of the previously described types of electrical drawings
on each separate building project. For example, a complete set of working
drawings for an electrical system usually will consist of the following:
- A plot
plan showing the location of the building on the property and all outside
electrical wiring, including the service entrance. This plan is drawn to
scale with the exception of various electrical symbols which must be
enlarged to be readable. Fig. 1-8 shows a typical building plot plan with
related electrical wiring.
- Floor
plans showing the walls and partitions for each floor or level. The physical
location of all wiring and outlets
are shown for lighting, power, signal and communication, special electrical
systems, and related electrical equipment. Again, the building partitions
are drawn to scale as are such electrical items as fluorescent lighting
fixtures, panelboards, and switchgear. The locations of other electrical
outlets and similar components are only approximated on the drawings because
they have to be exaggerated. To illustrate, a common duplex receptacle is
only about three inches wide. If such a receptacle were to be located on the
floor plan of a building that was drawn to a scale of 1/8" =
1'-0", a small dot on the drawings would be too large to draw the
receptacle exactly to scale. Therefore, the symbol is used to indicate a
duplex receptacle and is clearly shown. When the electrician on the job is
locating these outlets, he usually measures to the center of the circle to
determine the distance between the outlets (Fig. 1-9).
- Power-riser diagrams to show the
service-entrance and panelboard components (Fig. 1-10).
- Control wiring schematic diagrams (Fig. 1-11).
- Schedules, notes, and large-scale details on
construction drawings (Fig. 1-12).
In order to be able to "read" an
electrical drawing, one must become familiar with the meaning of symbols, lines,
and abbreviations used on the drawings and learn how to interpret the message
conveyed by the drawings.
SUMMARY
A blueprint or an electrical drawing is an
abbreviated language for conveying a large amount of exact, detailed
information, which would otherwise take many pages of manuscript or hours of
verbal instruction to convey.
Types of electrical drawings usually fall into
the following categories:
- Electrical construction drawings.
- Single-line block diagrams.
- Schematic wiring diagrams.
Electrical working drawings for building
construction normally utilize all of the previously described types of
electrical drawings.
An electrical working drawing for building
construction usually will consist of: a plot plan, floor plans, sectional
drawings, various details, wiring diagrams, and schedules.
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