How to Create a Single-Line Diagram for a PV System Plan Set

Creating a compliant pv system single-line diagram (SLD) is a critical skill for any electrician working in solar. This diagram is the electrical roadmap of your photovoltaic installation, providing a clear, concise overview of the entire system for the Authority Having Jurisdiction (AHJ) submission. A successful SLD includes all major components, from the PV modules to the point of interconnection, detailing conductor sizes, overcurrent protection devices, and disconnecting means. Adhering to the latest NEC Article 690 standards is non-negotiable for approval. The diagram must clearly show the path of power, grounding details, and all safety mechanisms like rapid shutdown requirements. Errors or omissions in the SLD are a leading cause for permit rejection, leading to costly project delays. Mastering this document is essential for efficient and profitable solar installations.

What is a PV System Single-Line Diagram?

A single-line diagram, often abbreviated as SLD, is a simplified schematic that uses single lines and standardized symbols to represent the electrical distribution path of a PV system. Unlike a complex wiring or schematic diagram that shows every conductor, the SLD focuses on the big picture: how major components are connected and protected. Think of it as the system’s electrical blueprint.

For any journeyman electrician or solar installer, the primary purpose of the SLD is to clearly communicate the design to the plan reviewer at the local building department, also known as the Authority Having Jurisdiction (AHJ). A well-executed diagram demonstrates that the system design is safe, efficient, and compliant with the nec code book. It is fundamentally different from other drawings, like riser diagrams, which focus more on the physical location and routing of conduits through a building. If you need a refresher on those, you can learn more about how to read electrical riser diagrams.

Core Components of a Compliant PV Single-Line Diagram

To ensure your AHJ submission is approved without corrections, your SLD must meticulously detail every key part of the system. Omitting any of this information is a common reason for rejection. AHJs are increasingly using digital plan reviews, which means clarity and completeness are more critical than ever to avoid delays.

PV Array and Source Circuits

This is the heart of your power generation. You must specify the total number of PV modules, the manufacturer, and the model number. Clearly indicate their electrical characteristics (Voc, Isc, Vmp, Imp). Show how the modules are wired, whether in a series vs parallel circuit configuration, and detail the number of modules per string. The term for this, according to the NEC 2023, is the PV source circuit. If you are using module-level power electronics (MLPE) like optimizers or DC-to-DC converters, they must be shown for each module. This is also a good place to remember the fundamentals of wiring, such as correctly installing MC4 connectors.

Overcurrent Protection and Disconnecting Means

Safety is paramount, and your SLD must prove it. Clearly label all overcurrent protection devices (OCPDs), including fuses and circuit breakers, with their amperage ratings. This includes specialized breakers like the arc-fault circuit interrupter (AFCI), now a standard requirement for PV systems on or in buildings. The PV system disconnecting means must be clearly identified, showing their location and rating. This section must also illustrate compliance with rapid shutdown requirements, a critical aspect of PV hazard control. The diagram should show the rapid shutdown initiator and which circuits are controlled.

Inverter and Balance of System (BOS) Components

The inverter is the brain of the operation. Your diagram needs to show the make and model of the inverter and specify its continuous power output rating. The inverter input/output circuit details are crucial, including the maximum input voltage and current from the array and the output voltage and current to the load side. Other Balance of System (BOS) components like junction boxes, combiners, and monitoring systems should also be represented with appropriate symbols and labels.

Grounding and Conductor Details

A compliant SLD provides a complete picture of the system’s grounding and bonding. You must show the path of the equipment grounding conductor (EGC) from the PV array racking all the way back to the main service panel. If a supplementary grounding rod is used for the PV system, its location and connection point must be drawn. Furthermore, every single conductor on the diagram must be labeled with its size (AWG) and type (e.g., THWN-2, PV Wire). These labels are not optional; they are essential for verifying your calculations.

Point of Interconnection (POI)

The final step on the diagram is showing how the PV system connects to the existing electrical infrastructure. This is the point of interconnection. Typically, this is a dedicated breaker in the main service breaker panel. Your diagram must specify the busbar rating of the panel and the position and rating of the new PV back-fed breaker. For larger commercial projects, the POI might involve more complex equipment, sometimes even requiring knowledge of 3-phase transformer configurations.

Step-by-Step: Key Calculations for Your Diagram

Your SLD is a visual representation of several critical electrical calculations. The AHJ will check the numbers on your diagram to ensure they are accurate and code-compliant. Modern solar design software can automate many of these, reducing errors and design time significantly.

1. Conductor Sizing Calculations: Based on the current from your PV string circuits and inverter output, you must perform conductor sizing calculations to select the appropriate wire gauge. This is determined by the wire’s ampacity after adjusting for conditions of use (e.g., temperature, conduit fill). Always reference a standard wire ampacity chart from the current NEC edition.
2. Voltage Drop Calculations: Power loss is a critical performance factor. You must perform voltage drop calculations for both the DC and AC sides of the system to ensure it remains within acceptable limits (typically 2-3%). Using an online voltage drop calculator can streamline this process, and showing the results (e.g., percentage drop) on the diagram is a recommended best practice for proving efficiency and streamlining AHJ review.
3. Conduit Fill Calculations: The NEC places strict limits on the number of conductors allowed within a conduit of a given size to prevent overheating. Use a conduit fill calculator to verify that your planned conduit runs are compliant. Remember that local jurisdictions may have specific rules, so it’s always wise to check for local code amendments, similar to the published Alaska NEC amendments that modify the national code.

Common Mistakes to Avoid for a Flawless AHJ Submission

Soft costs, including permitting and design, can represent a huge portion of a residential PV system’s total price, and rejected plans are a major contributor to these costs. Errors in electrical diagrams are a leading cause of permit resubmission. Here are some key mistakes to avoid:

• Incomplete Component Information: Failing to list make, model, and electrical ratings for all modules, inverters, and OCPDs.
• Ignoring NEC Updates: Not designing according to the nec code book edition currently enforced by your AHJ, especially regarding rapid shutdown requirements or AFCI protection.
• Calculation Errors: Submitting a diagram with incorrect conductor sizing calculations (a direct code violation) or voltage drop values that are too high, which can cause performance issues and may be questioned by the AHJ.
• Missing Grounding Details: Showing an incomplete or ambiguous path for the equipment grounding conductor or neglecting to specify the grounding electrode system connection.
• Illegible or Unclear Drawings: Submitting a hand-drawn diagram that is messy, hard to read, or uses non-standard symbols can lead to an immediate rejection.

Investing in your technical skills is the best way to avoid these costly errors. To produce professional plan sets and master technical drawing, consider enrolling in our PV-focused online electrical courses to ensure your projects pass inspection the first time.

Primary Sources

The information presented in this article regarding electrical codes and standards is based on the National Fire Protection Association’s (NFPA) National Electrical Code (NEC). For the most accurate and up-to-date requirements, always refer to the official publication from the NFPA.

Frequently Asked Questions

What is the most important information on a pv system single-line diagram?

The most critical information includes all major components with their ratings, conductor sizes and types, the complete grounding system, all overcurrent protection devices, and all required disconnecting means. The diagram must provide a complete electrical picture from the modules to the point of interconnection.

Which NEC article governs pv system single-line diagrams?

NEC Article 690, “Solar Photovoltaic (PV) Systems,” is the primary article that governs the design and installation of PV systems, and therefore dictates most of the requirements that must be shown on the single-line diagram.

How do rapid shutdown requirements affect the single-line diagram?

The SLD must visually represent the rapid shutdown system. This means showing the location of the initiator, identifying the controlled conductors, and indicating which components (like an inverter or specialized shutdown device) are used to achieve the shutdown function, a key part of PV hazard control.

Can I draw a single-line diagram by hand for my journeyman electrician project?

While some smaller jurisdictions may still accept legible hand-drawn diagrams, it is highly discouraged. Professional-looking diagrams created with design software are far less likely to be rejected. Today, the growing use of cloud-based solar design software makes it easier than ever to produce clean, accurate, and compliant diagrams.