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In this guide, learn the basics of reading and interpreting electrical wiring diagrams.
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Types of Electrical Diagrams For Wiring
Before we dive deep into how to read wiring diagrams, let's first start with the different types of electrical diagrams:
Schematic diagrams - schematic diagrams are used to show the flow of an electrical circuit. They primarily focus on functionality and use schematic symbols as representations of arrangements and connections between electrical components.
Pictorial diagrams - pictorial diagrams are much more simplified and great for those who aren't familiar with schematic symbols. As the name suggests, pictorial diagrams are more pictorial, using more graphic representations over symbols and abbreviations.
Block diagrams - block diagrams use blocks as representations of main electrical components, like batteries and resistors, for overall systems. Block diagrams are sometimes referred to as layout diagrams since their primary purpose is to show the arrangement of main parts of a system.
Ladder diagrams - ladder diagram often show the electrical components within a system and electrical connections between them. They also showcase switches and resistors that are interconnected in the system.
This guide will primarily dive into schematic and ladder diagrams, as they tend to be more complex. They are great for understanding electrical connections or troubleshooting circuits and include components like switches, resistors and conductors within the diagram.
Schematic Diagrams
Schematic diagrams include the wiring diagram itself, a notes section, and a legend for schematic symbols. Voltage sources, resistors, and conductors are examples of components represented in schematic diagrams. Schematic diagrams are used to show electrical components and their wiring connections in a circuit. They are especially useful when you are troubleshooting or wiring an electrical circuit.
The best place to start when reading a schematic wiring diagram is with the legend. The legend will explain what schematic symbols on the diagram represent.
For example, the schematic diagram pictured below shows abbreviations like CONT, CAP, and CH. It's difficult to understand what these abbreviations refer to, or which schematic symbols are a representation of what electrical components of the circuit. Since schematic wiring diagrams include various schematic symbols, it's important to get familiar with the legend section.
Now that you have an understanding of schematic symbols, the notes section of the schematic wiring diagram is what you'll want to see next. It includes important information about the electrical circuit. The notes section may have a note about the crankcase heater (CH) and may say something like “CH not used on all diagrams”. If you don't read the notes, you may have an incorrect understanding of the electrical circuit.
Finally, it's time to look at the schematic wiring diagram itself. Each electronic symbol on the diagram represents a component within the electronic circuit. The lines running between each component show how they are interconnected with a wire connection.
Representing Voltage in Schematic Diagrams
There are two sides of every circuit:
Line side
Control side
Components on the line side of the circuit will receive a higher voltage (for example, 230V). Components on the control side of the circuit will receive a lower voltage (or example, 24V).
Based on the image above, you can see that L1 and L2 power the line side of the circuit. L1 and L2 represent the two poles on an AC power supply. The line side of the circuit will also have bolder wiring connections than the control side. Note that the line voltage is labeled above L1 and L2.
The control side of the circuit can be powered by an external power supply. The control side of the circuit will have thinner wiring lines than the line side of the circuit.
How to Read a Schematic Diagram
Lines between components on a schematic diagram represent two main types of wiring on a circuit:
Field wiring
Factory wiring
Factory wiring is done by the manufacturer while field wiring is done by the technician. Factory wiring is represented by a solid line on the diagram; field wiring is represented by a dashed line on the diagram.
Most schematic diagrams will also tell you the color of wires. On each line in the diagram, you'll see a symbol like BLK or BLU; BLK stands for black wiring and BLU stands for blue wiring. Some legends will define the color symbols.
On a diagram, some electrical symbols will have a black border around them. A black border around a group of electrical symbols indicates the internals of a single component. This is meant to help you determine the type of contactor. A single pole contactor would only have one contactor switch.
A schematic diagram can also be used to determine how components are wired to each other. For example, view the image to the right. You can see that the image above shows a dual run capacitor, start capacitor, start relay, and a start thermistor.
Start by looking at the dual run capacitor. You can see that a blue (BLU) wire runs from the HERM terminal on the compressor to the start thermistor. You can also see that a yellow wire runs from the common terminal on the dual run capacitor to the start capacitor.
Ladder Diagrams
A ladder diagram is another type of wiring diagram that shows all of the electrical components, like switches and resistors, and wiring within a system. Like schematic diagrams, ladder diagrams will have both a diagram and a legend.
Ladder diagrams specialize in displaying the logic of a circuit. Circuit logic explains what happens to the circuit when a component switches its position, primarily in the case of gaining or losing power when a switch opens. Just like a schematic design, elements like resistors, voltage, and conductors are detailed in ladder diagrams.
The legend is the best place to start when reading a ladder diagram, since it will explain the meaning of electronic symbols on the ladder diagram. Each symbol on a diagram represents a component within the circuit. Ladder diagrams place each component in the circuit between two vertical power lines called L1 and L2. Each symbol is on a horizontal line between L1 and L2.
The horizontal lines are called rungs. Each rung represents one circuit within the electrical system. Recall that electrical systems have multiple circuits. Since systems have multiple circuits, the ladder diagram will have multiple rungs.
Lines between symbols represent the flow of current in the circuit. Note that lines between components do not represent the exact wiring and electrical connections you will find in the field.
Representing Voltage in Ladder Diagrams
Components that receive less voltage, like a 24V coil, will be placed on the control side of the diagram. Components that receive higher voltage, like a 240V compressor, will be placed on the line side of the diagram.
A ladder diagram will have all components on the line side of the electronic circuit between two vertical lines labeled L1 and L2. L1 and L2 represent the two poles on an AC power supply. L1 and L2 provide power to the components on the line side of the circuit.
The control side of the electronic circuit will not have lines for L1 and L2. The control side of the circuit will be below the line side on a diagram. A transformer usually marks the separation between the line and the control side.
It is important to note that the two sides of a circuit are not separate systems. The line side and control side refer to components that receive power from different sources.
A component can receive power from both the line and control voltage. This is common with contactors or switches. The 24V control voltage can energize the coil within a contactor. The switch within the contactor may reside on the line voltage at 240V.
For example, in the diagram to the right, the compressor contactor (CC) has components on the line and control side. You can see that there are contactor switches labeled CC on the line side, and coils labeled CC on the control side. Both symbols are a part of the same CC but receive different voltages.
How to Read a Ladder Diagram
Remember that there are three parts to the line side of the ladder wiring diagram: L1, rungs, and L2.
The current flows into the line side of the circuit through L1. Once L1 is energized, the current flows across each rung of the ladder diagram. This assumes that the rung is a closed connection. If the rung has an open switch, the current will not flow across the rung. You may also keep in mind other components connected within the circuit, like resistors or conductors.
Once the current crosses the rung, it arrives at L2. The current flows back to the top of L2, and then the process begins again. This process happens almost instantaneously.
Symbols on the same rung of the ladder diagram are connections on the same circuit. For example, look at the ladder diagram in the image to the right. You can see that the compressor and compressor contactor are on the same rung of the line side of the diagram.
Since the components are connections on the same rung, it becomes easier to understand the circuit logic. If the contactor on the rung is open, then the compressor will not receive power. The open contactor only prevents current from flowing through this rung.
Components in a circuit can be wired either in series or parallel circuit. Ladder diagrams make it easier to see if components are in a series or parallel circuit. For example, you can see that the compressor contactor (CC) is wired in series with the compressor. Between L1, CC, and the compressor, there is only one path for current to follow. One path between components means that the components are wired in series.
You can tell components are in parallel if there is more than one path for current to flow through a rung. In the diagram, the current could go from L1 through the EFR and then through the evaporator fan. The current could also go from L1 through the HR and then through the evaporator fan. Since current can take two paths from L1 to the evaporator fan, the EFR and HR contactors are in parallel.
All About Wiring Diagrams
Now that you've learned the fundamentals of wiring diagrams, you're ready to tackle them on the field! Wiring diagrams are a great tool for troubleshooting and cover how all electrical components - from resistors to conductors - can be connected within a circuit. Remember to always check the legend and notes section before you get started!
Quiz
Question #1: What is included in a schematic diagram?
A diagram
Notes
A legend
All of the above
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Answer: All of the above
A schematic diagram will include a diagram, a legend, and notes on the circuit.
Question #2: True or False - A schematic diagram displays the wiring between components.
True
False
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Answer: True
A schematic diagram will show you the components in a circuit and how they are wired.
Question #3: What do L1 and L2 represent on a schematic diagram?
Line voltage
Control voltage
Both
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Answer: Line voltage
L1 and L2 represent the line voltage side of the circuit.
Question #4: True or False - A black border around multiple symbols indicates that the symbols are a part of a larger component.
True
False
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Answer: True
A black border around multiple components on the diagram indicates that they are part of the same component.
Question #5: Horizontal lines between L1 and L2 on a ladder diagram are called...
Rungs
L1
L2
Symbols
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Answer: Rungs
The horizontal lines running between L1 and L2 are called rungs.
Question #6: Symbols between L1 and L2 are on which side of the circuit?
Line side
Control side
Free side
None of the above
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Answer: Line side
Components between L1 and L2 on a ladder diagram are on the line side of the circuit.
Question #7: True or False - In a ladder diagram, current flows from L1, across the rungs, and then through L2.
True
False
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Answer: True
True, in a ladder diagram current flows from L1 to the rungs and then from the rungs to L2. This process is instantaneous.
Question #8: Two components in parallel have...
More than one path for current to flow
One path for current to flow
No paths for current to flow
All of the above
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Answer: More than one path for current to flow
Components in parallel will have two paths for the current to flow into the next component in the circuit.