Symbols in One-Line Electrical Diagram

This article describes the most commonly used symbols in one-line electrical diagram. These symbols may vary significantly from one diagram to another because there is no specific standard dictating which symbols to use.

Therefore, it is important to include a symbol legend in the diagram or blueprint, explaining what each symbol represents. This ensures that anyone can understand the diagram.

Several commonly used symbols appear in electrical diagrams, but their usage is not mandatory. Below, we present some of the most widely used symbols in one-line electrical diagrams.


Transformer Symbol, Symbols in One-Line Electrical Diagram

Transformers are among the most frequently used components in one-line diagrams. The most common symbol is shown below:

This symbol is used for all types of transformers, including pole-mounted transformers, low-voltage dry transformers, pad-mounted transformers, etc.

The diagram should specify the transformer’s type, capacity, and operating voltages. Impedance is often included for short-circuit calculations, ensuring it does not exceed 10%.

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Fuse and Fuse Disconnect Symbol

The most common symbol for a fuse disconnect is as follows:

This symbol is widely used, especially in substation diagrams for fused cutouts (FCFs).

For other types of fuses, variations of the symbol are used, often featuring a rectangle with an “S” representing the fuse link. The following is an example of such variations:

It is important to specify the fuse’s capacity.


Circuit Breaker Symbol

The symbol for a circuit breaker is one of the most commonly used. However, it has numerous variations due to the variety of breaker types available. In highly detailed diagrams, especially in electronics, specific symbols are used for each breaker type.

In one-line diagrams, this level of detail is typically unnecessary. Variations in the symbol often depend on the designer’s preference. Below is a common example of a circuit breaker symbol:


Receptacle Symbols (Single-Phase and Three-Phase)

Symbols for receptacles or outlets, whether single-phase or three-phase, can vary. The following are common examples:

Single-Phase Receptacle:

The diagram should include the receptacle’s amperage rating.

Two-Phase Receptacle:

The symbol may differ, but it is crucial to indicate the number of phases, amperage, and operating voltage.

Three-Phase Receptacle:

As with other receptacles, the three-phase receptacle symbol should clearly specify its data. The symbol should be defined in the symbol legend.


Grounding Symbol

The grounding symbol is one of the easiest to identify, as it is universally recognized. It rarely varies across diagrams.

The symbol typically consists of horizontal lines of decreasing length stacked vertically, as shown below:

This symbol is used in one-line diagrams, electrical blueprints, and even circuit diagrams in electronics.


Load Center or Main Panel Symbol

Load centers or main panels can have various symbols. In one-line diagrams, it is common to use two long parallel lines to represent the copper or aluminum busbars.

This design allows for the straightforward addition of breakers. The busbars and breakers are enclosed within a rectangle, forming the panel or load center.

The main breaker, busbar capacity, the number of breakers, and their ratings can also be specified, along with circuit numbers.


Lighting Symbols in One-Line Diagrams

Lighting fixtures or luminaires often have the most varied symbols, as there are many types of lights. Here are a few common symbols:

It is important to describe the chosen symbol and specify details such as:

  • Type of light (e.g., LED, fluorescent).
  • Fixture style (e.g., reflector, spotlight, pendant, recessed, surface-mounted).
  • Power rating and operating voltage.

Motor Symbol

Motor symbols also vary, but a commonly used representation is shown below:

Motor Symbol

The diagram should include the motor’s capacity, operating voltage, frequency, power consumption, power factor, efficiency, and service factor if available.


Meter Base Symbol

Meter bases must be represented in one-line diagrams. As with other components, the key is to provide a clear description of the symbol used.

Below is an example of a meter base for 7-200A:

Meter Base Symbol

For a 13-20A meter base, the current transformers (CTs) may be depicted for clarity, adding more detail to the diagram.


These are some examples of common symbols in one-line electrical diagrams.

Since there are countless electrical devices, there is a corresponding variety of symbols. This article focuses on the most common and essential symbols.

As emphasized throughout, providing a clear description of each symbol used in the diagram is crucial for ensuring it can be easily understood by anyone.

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