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Sept. 4, 2012
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 system problems.

Understanding Electrical Schematics

This month, we begin a four-part series of excerpts from the Refrigeration Service Engineers Society publication, “Electricity for HVACR Technicians.” This publication won a Contracting Business.com Mechanical Systems Week Product Showcase Award in 2009, in the category of Education.

NOTE: this series is not intended to serve as a replacement for concentrated, formal classroom and/o r field training by a qualified electrical professional.

Types of Wiring Diagrams

There are three basic types of wiring diagrams used in the HVACR industry today. The first, and most common, is the ladder diagram. From this point forward, ladder diagrams will be referred to as “schematic” diagrams, or “schematics.”

A typical schematic of a packaged air conditioner is shown in Figure 1. 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.

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 shows a typical line diagram. Compare Figures 1 and 2, and note the difference in the way motors, switches, and transformers are depicted. Today, it’s not uncommon for some manufacturers to show both types of diagrams on their equipment.

The third type of diagram is the installation diagram. This is a tool that’s 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 shows a typical installation diagram for a residential cooling system.

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 look at some of the symbols used in electrical schematics.

Power Supplies

Many different supply voltages are used in the HVACR industry, ranging from 575 Volt, three-phase power supplies to 24 Volt control circuit voltages. Power supplies may be indicated by solid lines or by dashed or dotted lines, as shown in Figure 4.


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”).

The fact that not all manufacturers follow the same schematic diagram practices can be confusing. You’ll 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 without dots are connected.

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.


One of the most basic components in any schematic is the switch. A switch is a device that interrups 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 draw switches and basic controls. A variety of simple, single-pole, single-throw (SPST) switches is shown below. All are in the closed position.

Here are the same switch symbols shown in the open position.


Loadsare 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, including motors, heaters, and transformers.

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.

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.

The second configuration is an up-and-down arrangement that divides the schematic into high-voltage and low-voltage sections. 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. (NOTE: The dashed line used in Figures 9 and 10 to separate the sections doesn’t appear in actual schematics.)


The schematic shown in Figure 1 normally would include a legend such as the one shown here:

The legend and any notes, 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.

This material is excerpted from Electricity for HVACR Technicians, a publication of the Refrigeration Service Engineers Society. To learn more about this and other educational offerings from RSES, visit rses.org/training.aspx. Click the “eLearning” link for online versions of this course.” rses.org; 847/297-6464.