Electrical Wiring Diagrams for Air Conditioning Systems – Part One

In Article " Electrical Rules and Calculations for Air-Conditioning Systems – Part One ", which was the first Article in our new Course HVAC-2: Electrical Rules and Calculations for Air-Conditioning Systems, I explained the following points:

  • Introduction for Air-Conditioning Systems Types,
  • Introduction for Types of Motors/Compressors used in Air-Conditioning Systems,

Today, I will explain Electrical Wiring for different Air-Conditioning Systems types.

Third: Electrical Wiring for Air-Conditioning Systems 

1- Importance Of Electrical Wiring For Air Conditioning Systems

In the detailed design phase, the electrical designer must size and select the wires/cables, conduits, starters, disconnects and switchgear necessary for supplying power and control to HVAC equipment. This information designed by the electrical designer will be and must appear on the electrical drawings for proper installation by the electrical contractor.

So, to determine the electrical equipment and power supply required for the HVAC system proper operation, the electrical designer needs:

  • Knowing the size of the HVAC system (equipment types, locations, …),
  • Understanding how different HVAC equipment operates in a certain HVAC system. 

The above points can be fulfilled by understanding the electrical wiring diagram of individual HVAC equipment and of the whole system also.


Also the HVAC designer will need to know the size of the electrical loads to assess the impact of the heat generated by the electrical system on the HVAC load.

2- How to get the Electrical Wiring for Air Conditioning systems?

Usually, the electrical wiring diagram of any HVAC equipment can be acquired from the manufacturer of this equipment who provides the electrical wiring diagram in the user's manual (see Fig.1) or sometimes on the equipment itself (see Fig.2).



3- Types of Electrical Wiring Diagrams For Air Conditioning Systems

There are three basic types of wiring diagrams used in the HVAC/R industry today, which are:

  1. The Ladder Diagram,
  2. The Line Diagram,
  3. The installation diagram.

3.1 The Ladder Diagram

It is the most common type of wiring Diagrams. It is called ladder because the symbols that are used to represent the components in the system have been placed on the rungs of a ladder. ladder diagrams will be referred to as “schematic” diagrams, or simply “schematics.” A typical schematic of a packaged air conditioner is shown in Fig.3.


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.


A wiring schematic shows the condition of a piece of equipment when there is no power being applied to the unit. For example, if a switch is depicted as being normally open (N/O) or normally closed (N/C), remember that the position of the switch is shown as it appears when there is no power applied to that circuit. If there is any deviation from this practice, there will be an explanatory note on the schematic.

Before you begin looking at electrical schematic diagrams, though, remember that there are always five basic components to any schematic:

  1. A power supply,
  2. A path for the power,
  3. A load or component that operates from the power,
  4. A switch or component that interrupts the power to the load,
  5. A legend (see Fig.4)or key that explains what the various symbols and abbreviations used in the wiring diagram.


3.2 The Line Diagram

  • It usually includes drawings that more closely resemble the components themselves, rather than symbols.
  • Fig.5 on the previous page is an example of a typical line diagram. Compare Fig.3 and Fig.5 and 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.


3.3 The Installation Diagram

This diagram 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. Fig.6 is a typical installation diagram for a residential cooling system.


4- How to read Electrical Wiring Diagrams?

In order to read electrical Schematics, you need to be familiar with the following:

  1. Symbols Used In Schematics,
  2. Schematic Diagram Configurations,
  3. Schematic Diagram Locators.

4.1 Symbols Used In Schematics

The most important symbols used in electrical schematics are:

  • Power Supplies,
  • Wiring,
  • Switches,
  • Loads.

A- Power Supplies: (see Fig.7)

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.


B- Wiring:

  • 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 Fig.8.1. 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 Fig.8.2, 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”).


  • Not all manufacturers follow the same schematic diagram practices and you will see several different styles of wiring diagrams.
  • 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.

Wiring identification:(see Fig.9)

Every manufacturer can identify the wires used in electrical diagrams by one of the following methods:

  1. Using Different line thicknesses to represent different types of wires.
  2. Using numbers or colors (or both) to help identify the various wires found in a unit .



The used wiring identification method should be clearly indicated in the legend that accompanies the drawing.

C- Switches:

A switch is a device that interrupts power to the load. It may be:

  1. Manually Operated Switch,
  2. Activated automatically by pressure or temperature (Control Switches),
  3. Electrically controlled switch (Relays and Contactors).

C.1 Manually Operated Switch:

The switch can be in the closed position (Normally closed) (N/C) or in the open position (Normally open) (N/O) (see Fig.10). You must note that in electrical wiring schematic the position of the switch is shown as it appears when there is no power applied to that circuit. If there is any deviation from this practice, there will be an explanatory note on the schematic.


A switch is characterized by : (see Fig.11)

  1. The number of contacts (or poles): the number of poles can be considered as the number of circuits that the switch can control at one time or the number of contacts in the switch.
  2. The number of positions (or throws) it has: the number of throws can be considered as the number of paths a single circuit can take.



The dashed line in the switch symbols represents the mechanical connection that makes the contacts move together, but these contacts are not connected electrically.

C.2 Activated automatically by pressure or temperature (Control Switches):

Pressure and temperature controls are switches; they also may be configured with various combinations of poles and throws.

The position of the switch “arm” in the schematic symbol indicates the operation of the control. for examples: (see Fig.12)


  • The temperature switch (RS-2) is shown with the arm above the contacts. This signifies that the switch opens on a rise in temperature and closes on a drop in temperature.
  • The pressure switch (AFS-2) is shown with the arm below the contacts. This signifies that the switch opens on a drop in pressure and closes on a rise in pressure.
  • For SPDT limit switch (LS), When there is an increase in temperature, the contacts “C” to “N/C” move to the “N/O” position. When the temperature decreases, the contacts “C” to “N/O” move back to the “N/C” position.

C.3 Electrically Controlled Switch:

a- Relays:

  • Relays are electrically operated control switches. The schematic symbols used to represent relays are the same as those for manually operated switches, except that relay symbols often include a solenoid coil.
  • There are several possible ways of depicting the solenoid coil. Fig.13 shows two different schematic representations of a DPDT relay.

  • Note that multiple-pole Relays, like multiple-pole switches, are connected mechanically but not electrically.

b- Contactors:

  • A contactor is a type of heavy-duty relay that handles higher voltages and higher currents than a control relay. Contactors appear nearly identical to relays on schematic diagrams. 
  •  Some manufacturers employ contactors that use a single set of contacts. A “bus bar” is placed over the connection where the other set would be, as shown in Fig.14.


D- Loads:(see Fig.15)

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.



4.2 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. They are : (see Fig.16)

  1. Side-by-Side Arrangement,
  2. Up-and-Down Arrangement.

A- Side-by-Side Arrangement:

In this arrangement, 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.


B- Up-and-Down Arrangement:

In this arrangement, the schematic is divided into:

  1. High-voltage section,
  2. Low-voltage section.

Normally the high-voltage 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.

Fig.17 shows many of the schematic symbols used in the HVAC/R industry today.


4.3 Schematic Diagram Locators

As in 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 Fig.18. This is the same schematic of a packaged air conditioner that you saw in Fig.3, but notice that now a column of small numbers has been added, running down the left hand side of the diagram. These numbers are used to 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.

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. For examples:(see Fig.19)


A- Numbers on Left hand side of the diagram:

In Fig.19A, “C1” contacts are located on lines 7 and 11. Similarly, in Fig.19B, you can find the high voltage switches “IFR” and “C1” on lines 28 and 35, respectively. Now look at the lower portion of the wiring diagram in Fig.18 and locate the relay coils “IFR” and “C1” on lines 52 and 57.

B- Numbers on Right hand side of the diagram:

In Fig.19C, note that there are small numbers along the right hand 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 Fig.19B, and you will find the “IFR” contacts. Likewise, the numbers 7, 11, and 35 in the right-hand margin of Fig.19C refer you to the lines where the contacts associated with relay coil “C1” can be found.

C- Underlined Numbers:

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 Fig.19B are shown as normally closed, and that the “C1” contacts on lines 7 and 11 in Fig.19A are shown as normally open.

For more information about electrical diagrams, please review the following Articles:

In the next Article, I will explain Electrical Wiring Diagrams for different Air-Conditioning Systems Types and Equipment. So, please keep following.


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