Lesson 1:  Wiring Diagrams and Line  Diagrams

The wiring diagram isn’t like the schematic diagram. There are strategic differences in their purposes. The wiring diagram, or connection drawing, shows an electrician exactly where each wire is supposed to be connected. In contrast, a schematic diagram simplifies the circuit connections to clearly demonstrate a circuit’s functionality. 

Schematics don’t contain enough information to show someone how to wire the circuit, and wiring diagrams don’t show how the circuit works. Schematics usually omit information irrelevant to the function of the circuit, such as the precise physical details of where a group of cables connects. What is shown as a single solid line joining together with a black dot on a schematic diagram may, in reality, be several different cables and wires connected together by jumper wires crossing between different sets of terminal blocks inside of a panel. In other words, schematics are more concerned with electrical characteristics than physical ones. 

For this reason, connection drawings are one of the final details to be completed. You should get a head start on them by drafting up preliminary versions as soon as you have enough information to do so, but usually the final document needs to include details like cable and conductor sizes, insulation colors, and conductor and cable numbers. Many of these details are established several steps into the design process, some closer to the end than others. 

Line Diagrams 

One-line diagram – a diagram that uses single lines and graphic symbols to indicate the path and components of an electrical circuit. 

One-line diagrams are used when information about a circuit is required but detail of the actual wire connections and operation of the circuit are not. 

Line Diagrams - A line (ladder) diagram is a diagram that shows the logic of an electrical circuit or system using standard symbols. „ A line diagram is used to show the relationship between circuits and their components but not the actual location of the components. 

Line diagrams provide a fast, easy understanding of the connections and use of components. 

The difference between single-line and schematic diagrams 

Single-line and schematic diagrams each have their respective places in the design process. But each serves different functions. The differences aren’t necessarily intuitive at first glance. 

Single-line diagrams show the overall conceptual layout of a circuit. They typically condense three-phase connections down to single lines for simplicity. Single-line diagrams are where upper-level details like generators, main transformers, and large motors are shown. 

Schematic diagrams show the functionality of more mid-level electrical circuits without getting bogged down in the details of individual connections (which are shown on wiring diagrams). Schematics typically show devices like the electrical power bus, breakers, fuses, electrical loads like relays and breakers, relay contacts, switches, and indicator lights. 

The schematic diagram is a symbolic presentation of a system’s control elements that makes it easier to understand an electrical system’s functional logic. For example, 120-volt relay logic is still in widespread use in nuclear plants. Schematic diagrams show the network of contacts, a network that can be extremely complex, and the relays and motors that those contacts actuate. 

The schematic diagram is a symbolic presentation of a system’s control elements that makes it easier to understand an electrical system’s functional logic. For example, 120-volt relay logic is still in widespread use in nuclear plants. Schematic diagrams show the network of contacts, a network that can be extremely complex, and the relays and motors that those contacts actuate. 

It’s not always true that one-line and schematic design will come first in the design process. There are often complicating factors that will interact with and govern certain aspects of design functionality, converting a straight-forward design process into a more iterative approach (think “Newton-Raphson method”). But in principle putting theoretical designs into single-line and schematic form will show you how they will operate within the existing system and help highlight any design considerations you may have overlooked.