Refrigeration Service
Engineers Society
1666 Rand Road
Des Plaines, Illinois 60016
UNDERSTANDING ELECTRICAL SCHEMATICS
Part 1 (Revised)
by Howard L. Pemper, CMS
INTRODUCTION
The main schematic that you will be examining in this
chapter will be shown in its entirety first, and then will
be broken down into individual sections, which will be
considered separately. Before you begin looking at
wiring diagrams, though, remember that there are
always five basic components to any schematic:
Quite often, when you encounter a problem with a
piece of HVAC/R equipment, the problem turns out to
be electrical in nature. If you have a clear understanding of how to read wiring diagrams, you frequently can find the source of the trouble simply by
checking the wiring of the unit itself against the manufacturer’s wiring diagram. The purpose of this chapter is to familiarize service technicians with symbols
and conventions commonly used in schematic diagrams, thereby making it easier to identify components and their related circuits. The more you know
about the proper procedures for reading and interpreting electrical schematics, the more quickly you’ll
be able to find the solution to your problem. And
that’s the primary function of any wiring diagram—to
aid the technician in finding the problem and then
correcting it.
An intelligent service technician, before he or she
can solve a problem, must know how and where to
start looking for the source of the trouble. Although a
wiring diagram may appear very complex when
viewed as a whole, it can be simplified by breaking it
down into smaller parts. If you look at the individual
circuits, one at a time, the overall diagram and
machine functions become much easier to understand. Control systems, for example, usually are
designed so that an individual circuit controls only
one function of a piece of equipment—starting and
stopping a motor by means of a pushbutton, for
instance, or controlling a solenoid valve by means of
a limit switch. Naturally, many variations are possible,
but the main point to remember is that an individual
circuit usually is associated with a single, basic
apparatus function.
© 2003 by the Refrigeration Service Engineers Society, Des Plaines, IL
Supplement to the Refrigeration Service Engineers Society.
ª
a power supply
ª
a path for the power
ª
a load or component that operates from the
power
ª
a switch or component that interrupts the power
to the load
ª
a legend or key that explains what the various
symbols and abbreviations used in the schematic
mean.
TYPES OF WIRING DIAGRAMS
There are three basic types of wiring diagrams used
in the HVAC/R industry today. The first and most
common is the ladder diagram, so called because it
looks like the symbols that are used to represent the
components in the system have been placed on the
rungs of a ladder. From this point forward, ladder diagrams will be referred to as “schematic” diagrams, or
simply “schematics.” A typical schematic of a packaged air conditioner is shown in Figure 1 on the next
page. In electrical schematics, the symbols stand for
various components in the circuit, and the lines stand
for the wires connecting them. The intention of the
overall schematic is to show how the circuit functions,
not how it actually looks.
1
630-140-1
Section 4A n
This document is a revision of prior publication 630-140.
L1
Line voltage
208/230/1/60
L2
DISC
L1
L2
11
CAP
21
C1
COMP
12
22
C1
CAP
COND
1
CAP
2
IFR
IFM
CC HTR
C1
Thermostat
208/230 V
CB
ON
COOL
CA
WHT
C
TRSF
RED
A
Fan
24 V
C
BRN
AUTO
TC
R
OFF
TB
C1 IFR
C2
C1
C2
R
G
G
BLK
Y
Y1
ORN
C
BRN
LPS1
HPS1
C1
Figure 1. Typical schematic diagram
2
CC HTR
BLK
BLK
WHT
N/C
TB1
C1
RED
RED
11
21
12
22
BLK
Dual
capacitor
BLK
BLU
BLU
BLK
RED
BRN
C
S
C
S
YEL
R
R
YEL
RED
Compressor
BLK
WHT
Condenser
fan motor
CAP
BLK
Circuit
breaker
YEL
Transformer
YEL
WHT
Indoor
fan motor
BRN
RED
BRN
YEL
1
2
IFR
BLU
C1
LPS1
WHT
HPS1
C2
BRN
ORN
BLK
TB2
R
G
BLK
Y1
ORN
C
BRN
BRN
Figure 2. Typical line diagram
3
BRN
TB1
note the differences in the way that motors, switches,
and transformers are represented. Today it is not uncommon for some manufacturers to show both types
of diagrams on their equipment.
RED
Dashed lines
indicate field-supplied
wiring (use copper
conductors only)
BLK
BLK
The third and last type of diagram is the installation
diagram. This is a tool that is used primarily by the
installing contractor. It normally shows only what the
terminal board connections are, and very rarely will it
include any internal wiring of the unit. Figure 3 is a
typical installation diagram for a residential cooling
system.
Use 18-gauge
copper wire
R
SYMBOLS USED IN SCHEMATICS
TB2
R
When you look at a road map, you must understand
the symbols and signs used by the creator of the map
in order to find out where you are and how to get to
your destination. The same is true of an electrical
schematic.
G
Field-supplied
thermostat
G
BLK
Y1
ORN
C
BRN
Y1
If you think of a schematic as a road map, then the
“roads” are the wires that connect the various components and deliver power to the controls and to the
loads that make up the system. Let’s take a look at
some of the symbols used in electrical schematics.
C
Figure 3. Installation diagram
Power supplies
The second type of diagram is the line diagram. It
usually includes drawings that more closely resemble
the components themselves, rather than symbols.
Figure 2 on the previous page is an example of a typical line diagram. Compare Figure 1 and Figure 2 and
L1
208-1-60
YEL
Field
power
supply
Many different supply voltages are used in the
HVAC/R industry, ranging from 575-V, three-phase
power supplies to 24-V control circuit voltages. Power
supplies may be indicated by solid lines or by dashed
or dotted lines, as shown in Figure 4.
460-3-60
L1
L2
L3
L2
A
1
2
B
Ground
Figure 4. Power supply symbols
Figure 5. Wiring connections and crossovers
4
Wiring
A
Most schematics use straight lines
to represent the wires that connect
components to each other. If two
wires are connected internally, the
connection usually is shown as a dot
(a solid black circle), as illustrated at
those points marked “A” in Figure 5.
But note that there is no dot to indicate a junction or connection at
point “B.” This means that one wire
simply crosses over the other wire.
Now look at Figure 6. In this drawing, crossover wires are shown with
half circles or loops that “jump” over
other wires (see those points
marked “A”). Note also that in this
type of diagram, junctions are
shown without connection dots (see
those points marked “B”).
T1
T2
X
X
V
V
T3
B
Figure 6. More wiring connections and crossovers
BLK
WHT
The fact that not all manufacturers follow the same
schematic diagram practices can be confusing. You
will see several different styles of wiring diagrams in
your work, and you need to be aware that not all of
them will use exactly the same conventions. Remember: If dots are used to show junctions, then intersecting lines without dots mean that the two wires
cross without connecting. If loops or jumps are used
to depict crossovers, then wires that meet—even
without dots—are connected.
RED
11
BLK
12
WHT
BLK
YEL
BLK
Another variation you may encounter concerns the
weight of the lines themselves. Some manufacturers
use different line thicknesses to represent different
types of wires. Others also may use numbers or colors (or both) to help identify the various wires found
in a unit (see Figure 7). These usages should be
clearly indicated in the legend that accompanies the
drawing.
Figure 7. Wiring identification
draw switches and basic controls. A variety of simple
single-pole, single-throw (SPST) switches is shown
below. All are in the closed position.
Switches
One of the most basic components in any schematic
is the switch. A switch is a device that interrupts
power to the load. It may be manually operated, it
may be activated automatically by pressure or temperature, or it may be an electrically controlled switch
(relay). Again, there are several different ways to
Here are the same switch symbols shown in the open
position:
5
L1
S
L2
C
L3
R
Primary
Secondary
460 V
LO
230 V
MED
HI
208 V
24 V
COM
120 V
Motors
Common
Transformers
Horn
Bell
Lamp
Heaters
Figure 8. Load symbols
Loads
voltage and low-voltage sections. Normally the highvoltage section is placed at the top of the diagram,
and the low-voltage section is placed at the bottom of
the diagram (see Figure 10). The vertical lines at the
outer edges of the diagram represent the source of
electric power. All control devices and load devices
are located on the horizontal lines between these
outer vertical lines. An easy way to determine the different voltages in this type of schematic is to look for
the transformer. It normally is the “dividing line” for
voltage changes. (The dashed line used in Figures 9
and 10 to separate the sections does not appear in
actual schematics.)
Loads are devices that consume power and convert
it to some other form of energy, such as motion or
heat. They may be motors, heaters, lights, or other
pieces of equipment. A transformer is a type of
power-consuming device, but rather than converting
energy, a transformer changes the voltage or current.
Figure 8 shows typical symbols for several different
kinds of loads.
SCHEMATIC DIAGRAM CONFIGURATIONS
There are two basic configurations used in schematics today to show the approximate placement of
loads, switches, and different power or supply voltages. The first is a side-by-side arrangement, an
example of which is shown in Figure 9. Manufacturers usually place motors and other power-consuming
components on the right side of the diagram. This is
called the “load” side. The switches and other controllers are placed on the left side of the diagram. This
is called the “line” side.
LOCATORS
Think back to our discussion of roadmaps. Almost all
mapmakers place numbers and/or letters along the
vertical and horizontal edges of maps to help users
find particular cities, towns, landmarks, or other locations. Electrical schematics utilize a similar system.
Take a look at Figure 11 on page 8. This is the same
schematic of a packaged air conditioner that you saw
in Figure 1, but notice that now a column of small
numbers has been added, running down the lefthand side of the diagram. These numbers are used to
The second configuration is an up-and-down
arrangement that divides the schematic into high-
6
Line side
Load side
L1
L2
1
CAP
2
IFM
Indoor fan
Figure 9. Side-by-side configuration
L1
L2
High-voltage section
COMP
C
OFM
IFM
IFR
240 V
24 V
C
TC
IFR
Low-voltage section
Figure 10. Up-and-down configuration
7
L1
Line voltage
208/230/1/60
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
L2
DISC
L1
L2
11
CAP
21
C1
COMP
12
22
C1
CAP
COND
1
CAP
2
IFR
IFM
CC HTR
C1
208/230 V
CB
WHT
C
TRSF
RED
A
24 V
TB
C
BRN
C1 IFR
C2
28
C1
C2
7, 11, 35
R
G
BLK
Y1
ORN
C
BRN
LPS1
HPS1
C1
Figure 11. Numerical line locators
8
indicate the relative location of each horizontal line in
the diagram. (If a line falls between two numbers, the
number lower on the page generally is used as the
location reference.)
Now look at the lower portion of the wiring diagram in
Figure 11 and locate the relay coils “IFR” and “C1” on
lines 52 and 57. In the detail shown in Figure 12C,
note that there are small numbers along the righthand side of the diagram as well. These numbers
designate the line location of relay contacts. The
small number 28 in the right-hand margin tells you
the line location of the contacts associated with relay
coil “IFR.” Look back at line 28 in Figure 12B, and you
will find the “IFR” contacts.
This type of line-numbering system can be very useful in helping the reader identify the location of a specific component on the schematic, as well as its
controlling switch. In the detail shown in Figure 12A,
for example, “C1” contacts are located on lines 7 and
11. Similarly, in Figure 12B, you can find the highvoltage switches “IFR” and “C1” on lines 28 and 35,
respectively.
Detail A
Detail B
Detail C
3
4
5
6
7
8
9
10
11
12
13
14
15
11
21
CAP
C1
COMP
12
22
C1
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
49
50
51
52
53
54
55
56
57
58
59
60
61
62
Likewise, the numbers 7, 11, and 35 in the right-hand
margin of Figure 12C refer you to the lines where the
1
2
CAP
IFR
IFM
CC HTR
C1
TB
C1 IFR
C2
28
C1
C2
7, 11, 35
R
G
BLK
Y1
ORN
C
BRN
LPS1
HPS1
C1
Figure 12. Identifying line location of relay contacts
9
contacts associated with relay coil “C1” can be found.
Note that the 35 is underlined. An underlined number
signifies a normally closed contact (and, conversely,
a number that is not underlined signifies a normally
open contact). Accordingly, you will find that the “C1”
contacts located on line 35 in Figure 12B are shown
as normally closed, and that the “C1” contacts on
lines 7 and 11 in Figure 12A are shown as normally
open.
The legend and notes, if any, that accompany a
schematic diagram further explain the components
that make up the system and provide additional information where needed. When you look at a wiring diagram, always read the notes first, and make sure that
you know what the abbreviations used in the diagram
stand for.
LEGENDS
This concludes a general overview of the major parts
of any schematic or wiring diagram—namely, the
power supply, the pathways (wiring), the switches,
the loads, and the legend. With a knowledge of these
basics, you will be able to advance to Part 2 of this
topic and learn how to read more complex electrical
schematics.
SUMMARY
The schematic shown in Figure 1 normally would
include a legend like the one pictured below. Many of
the abbreviations, acronyms, and symbols used in
the schematic are listed in the legend, along with
their meanings.
LEGEND
Factory wiring
COMP . . . . . . Compressor
Field wiring
COND . . . . . . Condenser
Ground
DISC . . . . . . Disconnect
HPS . . . . . . High-pressure switch
IFR . . . . . . Indoor fan relay
C1 . . . . . . Compressor contactor coil
IFM . . . . . . Indoor fan motor
CA . . . . . . Cooling anticipator
LPS . . . . . . Low-pressure switch
CAP . . . . . . Run capacitor
TB . . . . . . Terminal board
CB . . . . . . Circuit breaker
TC . . . . . . Thermostat, cooling
CC HTR . . . . . . Crankcase heater
TRSF . . . . . . Transformer
10
Refrigeration Service Engineers Society
1666 Rand Road
Des Plaines, IL 60016
847-297-6464