What Is a Single Line Diagram & How to Draw a Circuit Diagram (2024)

What Is a Single Line Diagram & How to Draw a Circuit Diagram (1)

Need a site electrical single-line diagram?

Wondering how to draw an electrical circuit diagram?

Then this article is for you!

Inside I will answer these questions about single-line diagrams and more!

  • What is a schematic diagram?
  • Why do you need it?
  • How to draw a circuit diagram?
  • Which electrical symbols do you use?
  • What info do you need to include?
  • Or maybe you’re just interested in how to read electrical blueprints…
  • Ready?

Let’s go!

FREE DOWNLOAD: GUIDE TO MAKE ANINDUSTRIAL SINGLE LINE DIAGRAM[ACCESS NOW]

What is a single line diagram?

A single-line diagram (SLD) is a high-level schematic diagram showing how incoming power is distributed to equipment. Below is the CSA Z462 single line diagram definition:

A4.1.1 Single-Line (One-Line) Diagram: A diagram which shows, by means of single lines and graphic symbols, the course of an electric circuit or system of circuits and the component devices or parts used therein.

Having a ‘single-line’ allows the diagram to stay readable despite communicating a lot of information about an electrical system.

This electrical one line diagram is the primary reference for maintenance and operations for lockout/tagout procedures, as well as for any engineering power system studies.

In this post, I will show why you need an SLD and how to make it.

Why do I need a single-line diagram?

There are two main reasons you need it:

For everyday operations and maintenance, as well as engineering power system studies.

Both require that the electrical single line diagram be kept up-to-date and available.

Operations and maintenance

To plan your lockout/tagout procedures, you need up-to-date primary sources of information.

“4.2.2.2 Lockout procedure

A lockout procedure shall be developed on the basis of the existing electrical equipment and system and shall use suitable documentation, including up-to-date drawings and diagrams.” — CSA Z462

SLDs help verify electrical circuit interlocks will not result in the re-energization of the circuit being worked on.

“4.2.2.4 Electrical circuit interlocks

Suitable documentation, including up-to-date drawings and diagrams, shall be consulted to ensure that no electrical circuit interlock operation can result in re-energizing the circuit being worked on.” — CSA Z462

Use up-to-date diagrams in establishing an electrically safe work condition.

“4.2.5 Process for establishing and verifying an electrically safe work condition

a) Determine all possible sources of electrical supply to the specific equipment. Check applicable up-to-date drawings, diagrams, and identification tags.” — CSA Z462

Having an up-to-date SLD can help avoid longer downtimes and keep everyone safe.

Power system studies

Having an up-to-date SLD is required to complete a power system study.

“6.12.3 Power system studies and single line diagram

Power system studies and one-line drawings are critical to the safe and reliable operation of electrical power systems. The studies and drawings shall be readily available and maintained on a consistent basis.

A main program shall include the continual upkeep and review of the following power system studies and drawings:

  1. One-line diagrams;
  2. Short-circuit studies;
  3. Coordination study;
  4. Arc flash incident energy study; and
  5. Load flow study.”

—CSA Z463

The information on the SLD can be used for different types of power system studies for your site.

  • Short circuit study to ensure equipment can withstand a fault.
  • Coordination study to ensure the right devices are tripped in time.
  • Incident energy study to know the arc flash hazard levels on equipment.
  • Load flow study to know the continuous current through the system.

SLD Updates

CSA Z463 - Maintenance of Electrical Systems, recommends a single-line diagram be reviewed after 5 years, or when there has been a significant change.

A significant change could be:

  • A new installation or system modification
  • Change in utility or source
  • Change in system impedance, configuration, or loading
  • Change in protection devices or settings

How to draw electrical single line diagrams

What Is a Single Line Diagram & How to Draw a Circuit Diagram (2)

Ideally, you should not have to draw your own single line diagram.

There would have been a drawing made for the design of the site or one made for a new project.

But maybe you cannot find it or there have been so many untracked changes it isn’t any good.

If you are working on a new SLD, the equipment itself is the best source for data.

Getting the connections right between equipment is the most important part of the diagram.

Between looking at your equipment tags and nameplates you should be able to make all the necessary updates.

Wiring diagram symbols

To start you should know what symbols to use to represent your equipment.

The source for standardized electrical diagram symbols comes from the document IEEE Std 315, ANSI Y32.9, CSA Z99.

Here are the most used electrical schematic symbols you will need to start drawing your system.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (3)

Next, I will go through each symbol and look at symbol variations and data to include in your electrical schematic.

EquipmentSymbolsData
The utility or AC
current source
symbol is used to
show where power
is coming from.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (4)

1. Incoming voltage

2. Fault level and impedance (optional)

These symbols could
all represent
an AC generator.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (5)

1. Watts

2. KVA

3. Voltage

4. # of Phases

5. Frequency

Two winding
transformers can be
represented with
either of these symbols
What Is a Single Line Diagram & How to Draw a Circuit Diagram (6)

1. connection type (Δ, Y)

2. KVA

3. Voltages

4. %Z Impedance

These are the Delta
and Wye connection
configurations, which
can be added in.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (7)N/A
These switches
symbols can be used
to represent a disconnect
or transfer switch.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (8)

1. Rated

Amperage

Fuses can be
represented with
either graphic symbol.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (9)

1. Rated

Amperage

2. Model

Low voltage circuit
breaker symbols,
with the second
indicating a draw-out
type.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (10)

1. Rated amperage

2. Model

3. Trip settings (optional)

Medium voltage
circuit breaker
symbols, with the
second indicating
a draw-out type.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (11)

1. Rated amperage

2. Model

This is the motor
symbol, one with a M
and one with a delta
connection symbol.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (12)1.Power (HP)
These symbols show
a current transformer (CT)
above and a potential
transformer (PT)
on the bottom.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (13)1.Turns ratio
This is the relay symbol
attached to a CT
What Is a Single Line Diagram & How to Draw a Circuit Diagram (14)

1. Function number

2. Instrumentation connection

Electrical one line diagram design

There are a few things that make a single-line diagram special and help to keep it readable.

  • Remember that you are using a single line to represent multiple conductors.
  • Diagrams start at the top of the page with the incoming source of a system’s power.
  • Electrical symbols are typically fed from the top and feed from the bottom.
  • There is no physical location or size represented of the electrical equipment.

Apart from lots of symbols, the standard also makes note of some drafting practices:

Orientation: Alterations of the orientation
of the symbol do not alter its meaning.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (15)
Line Width: The line width does not change
the meaning but may be used for emphasis.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (16)
Enlargement or Reduction: There is no meaning
associated with different symbol sizes.
What Is a Single Line Diagram & How to Draw a Circuit Diagram (17)

Terminal symbol (o): This symbol can be
added to attachment points of connecting lines
to a graphical symbol.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (18)

You might also wonder how much of the site to include on the diagram.

A common level of detail to stop at is once you have included all the distribution equipment.

This means once you have all your panelboards and MCCs you can transition to using equipment schedules in combination with a single line diagram.

It is also possible to build supporting documents to include more detailed information on equipment and keep the electrical diagram itself readable.

Connect the schematic symbols

To start, connect your electrical symbols using one line.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (19)

You can use a horizontal line to indicate a piece of distribution equipment such as switchgear, MCC, splitter, or panel.

You will notice that this diagram is missing the cables, these can be added in without a symbol using some arrows and annotation.

Indicate equipment separation

You can also group symbols using a dash-dotted box to indicate they are part of one piece of equipment.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (20)

Here notation was added for the cables using a loop with a line to point to cable data, and dash-dotted boxes for equipment enclosed together.

This shows there are three main parts: an incoming fused disconnect, a transformer, and the main switchgear.

This info is easy to indicate and can be very useful in determining how something should be de-energized.

Add equipment data

Once you have all the symbols and connections figured out you can start adding the equipment info.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (21)

Here I have added in the equipment data that is typically found on a wiring diagram.

The amount of information added can vary depending on what is being used for.

Electrical equipment information

The single-line drawing provides the blueprint for communicating different types of information about a power system.

The most important information to include is:

  1. Incoming service voltage
  2. Equipment rated current
  3. Identification names of equipment
  4. Bus voltage, frequency, phases, and short circuit current withstand ratings
  5. Cable sizes, #of cables, and lengths
  6. Transformer connection type, kVA, voltages, and impedance
  7. Generator voltage and kW
  8. Motor HP
  9. Current and Voltage ratios of instrument transformers
  10. Relay device numbers

Looking at our diagram again we can separate the info into voltage, amperage, and impedance to make sense of what is included for each piece of equipment.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (22)

Most of this can be found on the equipment nameplates.

Voltage

The incoming voltage is 12.47 kV and is down to the transformer primary.

The transformer steps the voltage down to 600 V.

From the transformer secondary to the switchboard is 600 V.

The disconnect, fuse, cables, and circuit breaker do not have voltages shown but should be rated for the associated voltages.

Amperage

Current rating:

The current rating is the maximum amount of continuous current a piece of equipment can pass without deterioration.

First, let us look for equipment current ratings:

  • Disconnect, 150A
  • Fuse, 140 A
  • Circuit breaker, 2000 A

The transformer does not directly list its rated current, but the info is bundled into the 1500kVA rating.

  • 1500kVA / 12.47 kV / √ 3 = 69.5 A
  • 1500kVA / 0.6 kV / √ 3 = 1443.5 A

The cables do not list their rated currents, but general info can be looked up in cable ampacity tables from a given size.

Short circuit current rating:

The short circuit current rating is the maximum amount of current a piece of equipment can withstand temporarily without sustaining damage.

At the incoming, the available short circuit fault data for a three-phase fault and a line to ground fault can be added if received from the utility.

This current is then used to calculate the maximum short circuit at any location in the system and compared to the equipment withstand ratings.

On this diagram, only the switchboard short circuit current rating is shown at 86 kA, but each piece of equipment has a limit.

Interrupting rating:

The maximum current that a device can interrupt safely is called the interrupt rating.

On this diagram, none of the interrupt ratings are shown, but the fused disconnect and circuit breaker would both have an interrupt rating.

Impedance

Impedance affects the amount of current dissipated and is used to determine load flows and short circuit fault levels.

The only equipment that lists the impedance here is the transformer at 5.83 %.

The cable size and length can also be used to approximate impedance.

The other pieces of equipment would have negligible impedance.

Relays

In larger systems, relays may be used in combination with circuit breakers.

There are many different relay functions and numbers associated with each type.

Below are a few of the commonly used.

  • 50 - Instantaneous Overcurrent Relay
  • 51 - AC Time Overcurrent Relay
  • 86 - Lock-Out Relay, Master Trip Relay
  • 87 - Differential Protective Relay

For complete reference see IEEE Std. C37.2 Standard Electrical Power System Device Function Numbers.

Including the relays and current transformers is important to understand what protections are in place.

Some sites may create a separate document to indicate the relay control signals, but it is also possible to put these signals directly on the SLD.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (24)

In the diagram, you can see the use of a draw-out high voltage circuit breaker at 13.8kV marked with double arrows.

Coming into the draw-out breaker is a control signal shown as a red dotted line, which comes from the relays.

The relays show the current transformers (CT) they are connected to, and the CTs show their CT ratio.

Device numbers may be combined if the device has multiple functions (50/51).

There are also suffix letters that may be used with the device number to specify Neutral or Ground protection (50N/50G).

You can also see the differential relay 87 is connected to the relays above and below it. This shows that it is using the same CT data.

Title block

The title block helps to manage documentation by tracking changes and dates on a drawing.

It is usually in the lower right corner, but also includes the border around the entire diagram.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (25)

One of the first things to check before you begin looking at a single-line drawing is the revisions.

This is a list of the changes that have been made to the document with the date.

It is also worth noting that just because the revision date is recent does not always mean the entire drawing is up-to-date.

In this example, you can see the drawing is being used for tender, for construction, as-built, additions, and removals.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (26)

If making revisions a method of communicating exactly what has changed on the drawing is to circle the change with a ‘revision cloud’.

What Is a Single Line Diagram & How to Draw a Circuit Diagram (27)

Here the main incoming fuse size has changed to 100A from 80A.

This change would be associated with a revision letter, which could also be placed directly next to the cloud on the diagram.

The next section is to list the reference drawings, allowing you to trace the info to other documents.

There should also be a section that lists the people who drew the drawing, managed the project, the dates, and their company.

The client company, name of the drawing, drawing number, and revision version are also listed in the bottom corner.

Conclusion

I hope this article has helped to better explain how to make a single line diagram.

Do you know anyone else that would benefit from this blog? Use the share buttons below to share the content.

If you have any questions, you can always reach out to me at jon.dobbelsteyn@leafelectricalsafety.com.

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What Is a Single Line Diagram & How to Draw a Circuit Diagram (2024)

FAQs

What Is a Single Line Diagram & How to Draw a Circuit Diagram? ›

The single-line diagram is a graphical representation of an electrical installation. It differs from other diagrams because, in the case of the Single Line diagram, each circuit is represented by a single line, in which all the conductors of the corresponding section are represented.

What is a single-line diagram? ›

The definition of a single-line diagram or SLD is an electrical diagram or drawing that represents the components of an electrical installation system represented by symbols, and describes how the components are related.

What is a single line drawing? ›

A one line drawing, also known as a single line drawing, is a drawing made with just one line.

What is a circuit diagram short answer? ›

What is a Circuit Diagram? A circuit diagram, also known as an electrical diagram, elementary diagram, or electronic schematic, is a graphical representation that simplifies an electrical circuit. It serves as a visual tool for the design, construction, and maintenance of electrical and electronic equipment.

What is single circuit line? ›

A single circuit transmission line has three sets of conductors, while a double circuit transmission line is two independent circuits on the same structure with each circuit made up of three sets of conductors.

How to draw a single-line diagram in Word? ›

Draw a line in Word or Outlook
  1. On the ribbon, select Insert > Shapes. ...
  2. From the Lines gallery, select the kind of line you want to draw.
  3. On the canvas, click and drag your mouse to draw the line. ...
  4. With the line still selected, on the ribbon, on the Shape Format tab, select Shape Outline.

What is the meaning of single line? ›

(ˈwʌnˌlaɪn ) adjective. consisting of one line. The Department issued a one-line statement identifying the dead man. Finally he replied to her letters with a one-line message: 'The package is here.

What is a single line chart? ›

A simple line graph is the most basic type of line graph. In this graph, only one dependent variable is tracked, so there is only a single line connecting all data points on the graph. All points on the graph relate to the same item, and the only purpose of the graph is to track the changes of that variable over time.

What is a single circuit? ›

A modular single-circuit electrical system is defined as a system that consists of one circuit in the same components or enclosures that are interconnected together by an installer. This system is typically connected to the building's Branch Circuit by a corded power entry—no electrician required!

Why do we draw circuit diagrams? ›

Circuit diagrams are used for the design (circuit design), construction (such as PCB layout), and maintenance of electrical and electronic equipment. In computer science, circuit diagrams are useful when visualizing expressions using Boolean algebra.

What is a line circuit diagram? ›

∎ 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.

What is a circuit diagram kids? ›

Circuit diagrams give an outline of the circuit in clear, precise terms that are easy to understand, and circuit models allow students to have tactile interaction with circuits, build some for themselves, and understand the significance of what a circuit is.

What is the difference between single line and multiline diagram? ›

In contrast to multi-line representation, which shows all component connection points and the total number of connections of cable connections, single-line representation shows greatly simplified components with one, or a maximum of two, connections and cable connections with a single line.

What is the difference between a single-line diagram and a schematic? ›

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).

How do you read a single-line diagram? ›

A one line typically starts at the top of the page and works its way down. It will start with the utility or other means of incoming power and its disconnecting device. It will then flow down to the distribution equipment like a switchboard or MCC and then finally it will end with the loads, like a motor or panelboard.

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