Ladder Diagram | Schematic Diagram | Wiring Diagram

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Wiring Diagram Definition

Wiring Diagram
A wiring diagram, shown in figure 1, is an electrical print that shows connections of all components in a piece of equipment.

Wiring diagrams tend to show a close representation of the interior position of electrical components in a control cabinet and/or circuit. Sometimes wire diagrams can closely represent a picture. The only difference is components are represented by electrical symbols whether they are NEMA standard or IEC standard symbols, whereas a pictorial will have a more realistic representation of an electrical component.

Wiring diagrams can be very accurate for depicting equipment layout. All connection wires are shown and connected from one component to another. Wiring diagrams are used widely by electricians when connecting electrical or electronic equipment and by technicians when maintaining equipment.

fan wiring diagram

Figure 1. Fan Wiring Diagram

Schematic Diagram Definition

Schematic Diagram
A schematic diagram, shown in figure 2, is a type of drawing that illustrates the electrical connections and functions of specific circuit arrangements with graphic symbols.

Schematic diagrams do not often show the physical relationship or layout of the components in the circuit; however, schematic diagrams are useful for understanding the sequence of operations or the operation of the circuit.

Schematic diagrams are not intended to illustrate the physical size or appearance of the device nor the location. In troubleshooting, electrical schematics are essential because it enables a technician to trace the circuit and its function without regard to the actual location or physical size of the component.

Schematic Diagram

Figure 2. Schematic view of a Simple Circuit

Ladder Diagram Definition

Ladder Diagram
A ladder diagram, shown in figure 3, is a diagram that explains the logic of the electrical circuit or system using standard NEMA or IEC symbols.

A ladder diagram is used to point out relationships between circuit components, not the actual location of the components. Ladder diagrams provide a fast and easy understanding of the connection of electrical components in a circuit or operation.

Ladder Diagram

Figure 3. Ladder Diagram

The arrangement of symbols in ladder diagram should promote clarity and understanding. Graphic symbols, abbreviations, and device designations are drawn per industry standards. The circuit should indicate the most direct path of logical sequence. Lines between the symbols can be horizontal or vertical but should be drawn to minimize lines from crossing each other.

Ladder diagrams should not be confused with a one-line diagram. A one line diagram has only one line between individual components. Ladder diagrams; however, often show multiple lines leading to and from components whether they are series or parallel connections.


The ladder diagram, shown in figure 4, is easy to read since there are only two basic parts- the rails and the rungs. The rails are the two dark vertical lines that represent the power source to the control circuit. The control circuit voltage is usually rated at 12V-120V depending on the rated values of the loads connected in the circuit. The rungs are the horizontal lines that illustrate how the control devices and loads are interconnected to make up the control circuit.

1.A ladder diagram translates similarly to a book. Read the ladder diagram from left to right then up to down in order to understand the sequence of the operation.
2. The loads in a ladder diagram are always and shall be connected in parallel on the rungs.
3. The load is the last component connected to the right side of the rail unless there is a protective contact that opens the circuit in the case of an overload event.
4. Control devices are connected between the left side of the rail and the load or other control devices and the load. Control devices are never to be connected from the left rail to the right rail. The result of this type of connection will result in a short circuit.

Ladder Diagram

Figure 4.  Ladder Diagram

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