Fundamentals of Single-Line Diagrams in the PE Power Exam

If you’re an aspiring professional engineer looking to conquer the complexities of electrical systems, understanding single-line diagrams in the PE Power exam is a crucial skill you can’t afford to overlook.

In this blog, we’ll unravel the importance of single-line diagrams in the PE Power exam as per NCEES® guidelines and provide a clear course roadmap according to NCEES® PE Power exam criteria and course descriptions to master this essential aspect of your exam preparation.

According to NCEES® PE Power exam course guidelines, single-line diagrams in the PE Power exam are an integral part of the exam, serving as visual representations of electrical systems that engineers must interpret and analyze. They are like blueprints that guide professionals in designing, operating, and maintaining complex industrial power systems.

This is part 1 of our in-depth study guide series of single-line diagrams in the PE Power exam. Let’s start with the fundamentals of single-line diagrams, explore their depths in the PE Power exam, and pave the way to your engineering success.

Disclaimer: The mentions of external drawings are purely for illustrative purposes and do not refer to any factual documents within this context or elsewhere in the article.

Fundamentals of Single-Line Diagrams

The Single-Line Diagram (SLD) is a fundamental representation of an electrical system, providing a simplified view of its components, interconnections, and electrical flow paths. It is a one-line drawing that depicts the power distribution and control circuits clearly and concisely.

Before moving further, have a quick look at the following sample single-line diagrams to learn how detail-oriented, technically deep, and insightful single-line diagrams can be:

*The information in these SLDs is based on hypothetical scenarios. For real-world applications, it is essential to comply with relevant industry standards, such as those by the National Electrical Code (NEC), International Electrotechnical Commission (IEC), and other regional regulations governing electrical installations and safety standards.

Critical Components of a Single-Line Diagram

The single-line diagrams represent a complex circuit with the respective symbols or notations of the following critical components.

  • Power Sources – The SLD showcases the power sources, such as generators or utility supplies, indicating their voltage levels and connection points to the electrical system.
  • Electrical Equipment – Various electrical components like transformers, circuit breakers, switches, motors, and loads are represented by standardized symbols. These symbols provide valuable information about the type and ratings of the equipment.
  • Bus Arrangement – The diagram illustrates the bus arrangement, which includes bus bars for power distribution at different voltage levels. It shows how power is routed from one location to another within the electrical network.
  • Protective Devices – Protective devices like fuses, circuit breakers, and relays are depicted to highlight their role in safeguarding the system against overcurrents, short circuits, and other electrical faults.
  • Metering and Instrumentation – The SLD may include metering devices (like an ammeter, voltmeter, ohmmeter, wattmeter, etc.), instruments, and measuring points for monitoring and control purposes.

The table below showcases individual components of single-line diagrams with their purpose and use cases in electrical and power circuits.

TransformerA device that transfers electrical energy between two or more circuits through electromagnetic induction, typically used to step-up or step-down voltage.
Removable or drawout circuit breakerA circuit breaker that can be withdrawn from its housing for maintenance or replacement purposes without disturbing the electrical connections.
Future removable or drawout circuit breaker positionA designated space on the single-line diagram intended for a potential drawout circuit breaker that may be added in the future.
Non-drawout circuit breakerA circuit breaker that is fixed in its housing and cannot be withdrawn for maintenance; it can only be replaced as a whole unit.
Disconnect switchA switch used to isolate electrical equipment from the power source for maintenance or safety purposes, providing visible means of isolation.
FuseA protective device that interrupts the flow of current in a circuit when it exceeds a certain level, protecting electrical equipment from damage.
Bus ductA metal enclosure used to house bus bars, providing a means of distributing power to various electrical components in a building or facility.
Current transformerA device that converts high currents to measurable and manageable values for use in protective relays or metering applications.
Potential or voltage transformerA device that steps down voltage levels for measurement or control purposes, providing lower voltage outputs proportional to the input voltage.
Ground (earth)A connection to the earth’s surface used to prevent electrical shock and provide a safe path for fault currents.
BatteryA storage device that provides a source of electrical power in the event of a power outage or for backup purposes.
MotorAn electrical machine that converts electrical energy into mechanical energy to perform mechanical work.
Normally open (NO) contactA contact that is open (non-conductive) in its normal state and closes (becomes conductive) when activated.
Normally closed (NC) contactA contact that is closed (conductive) in its normal state and opens (becomes non-conductive) when activated.
Indicating lightA visual signaling device, such as an LED or lamp, used to indicate the status of a particular electrical circuit or equipment.
Overload relayA protective device that detects excessive currents and trips the circuit to prevent damage to electrical equipment from overheating.
CapacitorA passive electronic component that stores and releases electrical energy, often used for power factor correction or energy storage purposes.
AmmeterA measuring instrument used to determine the current flowing through a circuit.
Instantaneous overcurrent protective relayA protective relay that operates instantly when the current exceeds a preset threshold, providing rapid circuit protection.
Emergency generatorA generator that provides backup power in emergencies or during power outages.
Fused disconnect switchA disconnect switch equipped with fuses, which protect the circuit from excessive currents and faults.
Low voltage motor controlControl equipment used for starting, stopping, and protecting low voltage motors.
Medium voltage motor starterControl equipment used for starting, stopping, and protecting medium voltage motors.
Meter centerA distribution panel or enclosure that houses electricity meters for multiple circuits or tenants.
Load center or panelboardAn electrical distribution board that receives power from the main source and distributes it to various branch circuits within a building.
Transfer switchA switch that allows the transfer of electrical power between two sources, commonly used to switch between main power and backup power.
Current transformer with connected ammeterA current transformer connected to an ammeter for measuring and monitoring electrical current.
Protective relays connected to current transformerProtective relays that use the output of a current transformer to detect faults and protect electrical systems.

*Remember the components listed above are commonly found in electrical systems and single-line diagrams, but the specific layout and components may vary depending on the application and complexity of the electrical system.

Purpose of Single-Line Devices in Power Engineering

The purpose of studying single-line devices in the PE Power exam preparation is their utmost importance in power engineering. Apart from sharing a significant portion of the PE Power exam, they impact the following aspects of the power engineer’s role.

  • Design and Planning – Single-Line Diagrams are essential during electrical systems’ initial design and planning stages. They help engineers visualize the network’s layout, identify potential design issues, and optimize the system for efficiency and reliability. Let’s have a quick look at some key areas that single-line diagrams impact significantly:
  • Documentation – SLDs are critical documentation for electrical installations, providing a comprehensive record of the system’s configuration and components. They are vital for future maintenance, expansions, and modifications.
  • Safety and Troubleshooting – The diagram aids in safety protocols and troubleshooting procedures. Engineers and electricians can quickly locate components and understand how the system is interconnected, allowing for faster identification and resolution of faults.
  • Regulatory Compliance – Single-Line Diagrams are often required by regulatory authorities as part of electrical system approvals and compliance assessments. They ensure that installations meet safety standards and code requirements.
  • Communication – SLDs facilitate effective communication between various stakeholders involved in the electrical system, including engineers, operators, maintenance personnel, and management. It presents complex information in a simple, standardized format.
  • System Optimization – By analyzing the SLD, engineers can identify potential improvements in power distribution, load balancing, and overall system efficiency. This leads to better resource utilization and energy conservation.

Single-Line Diagrams in Power Engineering – Layout and Organization

Single-line diagrams are essential for visualizing electrical systems in a simplified and easy-to-understand manner. They represent the flow of electrical power and the connection of various components within a system. That’s why power engineers commonly use these diagrams for circuit illustration and documentation for internal use or support and external distribution. Below are some general layout guidelines for creating single-line diagrams:

  • One-Line Representation – Single-line diagrams use a single line to represent all the electrical components and connections. This helps in reducing complexity and providing a clear overview of the system.
  • Unidirectional Flow – Electrical power is typically shown to flow from the top of the diagram to the bottom, following a unidirectional flow from the source to the loads.
  • Logical Arrangement – Components are arranged logically, starting with the power source at the top and then proceeding to the loads at the bottom.
  • Labels and Symbols – Standardized symbols represent each component uniquely, and labels are used to identify the type and ratings of the equipment.
  • Breakers and Disconnects –  Circuit breakers and disconnect switches are strategically placed to indicate their protective roles and isolation points within the system.

Bus Arrangement and Voltage Levels in Single-Line Diagrams

Bus arrangement and voltage levels are crucial in determining how electrical power is distributed within a system. The buses’ arrangement and voltage levels define the different sections and levels of the electrical network. Look at a glance to discover key arrangements and voltage levels of different components of single-line diagrams.

  • Bus Duct – Bus ducts carry high currents between different electrical components or sections of the system. They ensure efficient power distribution and reduce voltage drop over longer distances.
  • Transformer – Transformers are used to step up or down voltage levels. They are placed strategically to convert high-voltage power from the utility grid to lower voltages suitable for distribution or to increase the voltage for long-distance transmission. For instance, A 132 kV to 11 kV transformer steps down the voltage from the transmission level (132 kV) to a distribution level (11 kV) to supply power to local distribution networks.
  • Voltage Transformers (VT) and Current Transformers (CT) – VTs and CTs are used to measure voltage and current levels for metering, protection, and control purposes. For instance, A 11 kV/110 V voltage transformer steps down the voltage for metering or protection purposes, allowing accurate measurements at a lower voltage level.

Equipment and Device Representation in Single-Line Diagrams

The symbols used to represent equipment and devices in single-line diagrams are standardized for clarity and consistency. Understanding these symbols is essential for interpreting the diagram. Here are some key components and their representation:

  • Motor – A motor is represented by a circle with the letter “M” inside, indicating its location and connection point in the system.
  • Battery – A battery is shown as a collection of cells or a rectangle with a plus and minus sign to represent the positive and negative terminals
  • Fuse – A fuse is represented by a zigzag line, acting as a protective device against excessive currents. It interrupts the circuit when the current exceeds a specific value, protecting equipment from damage. For instance, a fuse can be placed in a circuit supplying power to a motor. If the motor experiences a short circuit or overload, the fuse will blow, preventing damage to the motor.
  • Capacitor – A capacitor is represented by two parallel lines, illustrating its ability to store and release electrical energy. For instance, capacitors are often used in power factor correction systems to improve the overall power efficiency of electrical networks.

Circuit Breakers and Protective Devices in Single-Line Diagrams

circuit breakers and protective devices

Circuit breakers and protective devices are crucial for ensuring the safety and reliability of electrical systems. They protect equipment and personnel from potential faults and overloads. Here’s their role in single-line diagrams:

  • Removable or Drawout Circuit Breaker – These circuit breakers can be withdrawn from their housing without disconnecting the electrical connections. They facilitate easy maintenance and replacement of faulty breakers. For instance, a draw-out circuit breaker might protect a critical process in an industrial plant. In case of a fault, the draw-out feature allows quick replacement, minimizing downtime.
  • Non-Drawout Circuit Breaker – Non-drawout circuit breakers are fixed in their housings and cannot be withdrawn. They are commonly used for less critical circuits or where regular maintenance is not required.
  • Disconnect Switch – Disconnect switches provide a visible means of isolation and are used to de-energize equipment for maintenance or emergencies. For instance, a disconnect switch might be placed ahead of a motor to isolate it from the power supply during maintenance or repair work.
  • Protective Relays – Protective relays are connected to current transformers and other sensors to detect abnormal conditions such as overcurrent, undercurrent, or faults. They trigger circuit breakers to open and protect equipment. For instance, a protective relay connected to a current transformer monitors the current flowing to a motor. The relay trips the associated circuit breaker to prevent motor damage if the current exceeds a preset threshold due to a fault.

By following the standard organization and understanding the role of each component, power engineers and students can create accurate and informative single-line diagrams that effectively represent the electrical systems they are working with. These diagrams become invaluable tools for planning, maintaining, and troubleshooting complex electrical networks.

*Ensuring technical accuracy and safety is vital while creating or interpreting single-line diagrams. In the second last section, we will discuss this in more detail with a range of insightful and detailed examples as per the industry regulations and guidelines.

Safety Considerations and Documentation in Single-Line Diagrams

Safety considerations, clear documentation, compliance with regulatory standards, and appropriate safety labels and warnings are critical when creating and using SLDs. This documentation details crucial circuit elements to enhance safety, emergency shutdown procedures, documentation, and regulatory compliance in single-line diagrams.

Safety Symbols and Nomenclature

The North American market, in collaboration with the Institute of Electrical & Electronics Engineers (IEEE), has standardized drawing symbols and nomenclature for electrical drawings, including SLDs. These symbols and labels help represent common devices, such as circuit protective devices, transformers, current transformers, etc. (IEEE 315-1975/ANSI Y32.9).


These voltage-stepping devices come in various winding configurations and are essential components of power systems. Properly indicating the type of transformer, its kVA ratings, cooling methods, and any surge or lightning protection devices on the SLD is crucial for safety and documentation purposes.


In SLDs, transformers are represented by standardized symbols, with detailed information provided for each type. For instance:

  • Transformer Winding Configurations – Transformers come in various winding configurations, such as delta-wye (Δ-Y), delta-delta (Δ-Δ), and wye-wye (Y-Y). Each configuration has specific applications based on the system’s voltage requirements and grounding schemes.
  • Transformer Ratings – Transformers are rated in kilovolt-amperes (kVA) to indicate their power-carrying capacity. The rating is crucial for determining the transformer’s capability and suitability for the application.
  • Cooling Methods – Transformers can be cooled using different techniques, including natural air convection (KNAN) and forced air cooling (KNAF). The cooling method affects the transformer’s thermal performance and is essential for proper sizing and operation.
  • Surge and Lightning Protection – Surge arresters (metal oxide varistors) are often used to safeguard transformers from voltage surges and lightning strikes. These devices are represented on the SLD and connected to the transformer’s incoming terminations.

Circuit Protective Devices

circuit protective devices

Circuit protective devices protect power systems from overcurrents and faults. Commonly represented circuit protective devices on SLDs include:

  • Circuit Breakers – Circuit breakers are automatic switches that interrupt current flow in the event of a fault. They are represented using standardized symbols and can be of various types, such as molded-case circuit breakers (MCCBs) or air circuit breakers (ACBs).
  • Fuses – Fuses are overcurrent protection devices that consist of a metal filament or conductor that melts when subjected to excessive current. They are represented on the SLD and are available in different ratings for specific applications.

Current Transformers (CTs)

Current transformers are used for sensing and measuring current in power systems. They are represented in SLDs using specific symbols and can be of different types:

  • Bar-Type CTs – These CTs encircle the conductor whose current is to be measured. They are commonly used in medium-voltage applications.
  • Donut-Style CTs – Donut-style CTs are bolted in series with the load conductors. They find applications in both low-voltage and medium-voltage systems.
  • CT Polarity – The polarity marks on the conductor face toward the current source, while the polarity mark on the CT winding represents its relationship to the H1 medium-voltage terminal for bar-type CTs or its input orientation for donut-style CTs.

Voltage Transformers

As mentioned several times, they are used to step down the high voltages to be consumed safely by different devices on the consumer end. They are also called potential transformers.

MV Cable Sizing and Termination Considerations

Medium-voltage (MV) cables are crucial components in power systems, and their sizing and termination must be carefully considered in SLDs:

  • MV Cable Sizing – The sizing of MV cables should adhere to the NEC regulations, considering factors such as load current, cable length, ambient temperature, and derating based on the type of raceway or duct bank used.
  • Termination Types – Proper termination types must be specified for higher transformer secondary currents. Stress cone cable and porcelain terminators are options with higher short-circuit ratings, ensuring safe and reliable connections.

Incorporating these technical insights into the single-line diagram documentation ensures a comprehensive representation of power system components and enhances safety, efficiency, and compliance with industry standards.

Emergency Shutdown Procedures in Single-Line Diagrams

  • Short-Circuit Ratings – The impedance percentage (%Z) of transformers is a critical parameter that must be shown on the SLD. This information is necessary to calculate the required ratings of downstream equipment, ensuring they can withstand fault currents. See the example below:
short-circuit ratings
  • Secondary Short-Circuit Considerations – If a transformer requires an absolute minimum impedance to prevent secondary short-circuit levels from exceeding critical values, it must be noted on the SLD and in project specifications.
  • Termination Considerations – Depending on the available short-circuit ratings, medium-voltage impedance with rubber dead-front terminations may not be suitable. Proper selection and application of terminations, such as stress cone cables or porcelain terminators, should be specified.

Documentation and Record Keeping for Single-Line Diagrams

  • Detailed Device Representations – The SLD must provide detailed device representations, including symbols, ratings, and nomenclature. This documentation aids maintenance personnel in understanding the equipment and its capabilities.
  • Busway and Connection Specifications – For higher transformer secondary currents, busway flanges and non-segregated busways should be documented. For proper illustration, the SLD should specify the rated short-time and short-circuit withstand current values.

Compliance with Regulatory Standards in Single-Line Diagrams

  • ANSI and IEEE Standards – The SLD should adhere to the ANSI Y32.9 and IEEE 315-1975 standards for symbol language and nomenclature in North America. Additionally, incorporating International Electrotechnical Commission (IEC) symbols recognized worldwide enhances international trade.
  • NEC Regulations – Compliance with the National Electrical Code (NEC) for MV cable sizing, derating factors, and other safety considerations must be ensured.
  • NCEES® – National Council of Examiners for Engineering and Surveying
  • IEEE 315-1975 (Reaffirmed 1993)/ANSI Y32.9 – Standard for North American electrical drawing symbols and nomenclature.
  • National Electrical Code (NEC) – Regulations for electrical installations and safety standards in the United States.
  • International Electrotechnical Commission (IEC) – Global standards organization for electrical, electronic, and related technologies.
  • Electrical Engineering Portal
  • Iowa State University Press Books


And there you have it—the ultimate guide to single-line diagrams in the PE Power exam! We’ve uncovered the importance of mastering these diagrams per the NCEES® PE Power exam guidelines. Remember, single-line diagrams are not just a box to check on your journey to becoming a licensed professional engineer; they are the roadmap to navigating complex industrial electrical systems.

Ready to take your preparation to the next level? Study for FE is your go-to platform for comprehensive PE Power exam preparation. With expert resources, practice tests, and personalized study plans, Study for FE will guide you throughout your preparation. Don’t wait any longer; start studying with Study for FE today.


Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.