The single-line diagram (SLD) is the document AHJ reviewers spend the most time on during a solar permit review. It is the electrical map of the entire system. If anything is wrong or missing on the SLD, the permit comes back for corrections, regardless of how solid the rest of the submittal is.
Most installers understand that an SLD is required. Fewer know exactly what a correct SLD includes, what reviewers look for, and where the most common errors occur. This post covers all of it. For a broader overview of what goes into a complete plan set, see our post on solar plan sets explained.
What a Single-Line Diagram Is and Why It Matters
A single-line diagram is a simplified schematic that shows the electrical components of a PV system and how they connect, using single lines to represent conductors rather than showing each wire individually. The SLD gives the AHJ reviewer a complete picture of the system at a glance.
Reviewers use the SLD to verify that the electrical design is safe, that the components are correctly rated, that the wiring follows code requirements, and that the system will operate correctly when connected to the grid. It is also the document inspectors reference in the field during final inspection.
A well-drawn SLD reduces back-and-forth with the AHJ, shortens review times, and gives field crews a reliable reference document during installation. A poorly drawn SLD creates corrections, delays, and confusion on the job site.
Required Elements on a Solar SLD
PV Source Circuits
The SLD must show each PV source circuit: the string configuration (how many modules in series), the conductor size, conduit type, and the maximum values for current and voltage. For each string, the SLD should show the calculated open-circuit voltage (Voc) and short-circuit current (Isc) at worst-case temperature conditions.
Temperature-corrected Voc is not optional. Reviewers check that the maximum system voltage as designed stays within the inverter input voltage range and does not exceed the allowable voltage for the wiring methods used. In many jurisdictions, PV source circuit wiring must use conductors rated for direct exposure to sunlight and listed for outdoor use.
Combiner Boxes and DC Disconnects
When multiple strings combine before reaching the inverter, the SLD must show the combiner box location, the overcurrent protection device (OCPD) for each string, and the combined current value at the combiner output. The OCPD ratings must be consistent with NEC 690.9, which sets requirements for PV source circuit and output circuit overcurrent protection.
DC disconnects must be shown with their ampere rating and voltage rating, along with a label confirming they are rated for DC service. AC-rated disconnects are not acceptable for DC applications, and reviewers look for this specifically.
Inverter
The inverter is the centerpiece of the SLD. The diagram must show: the model number, maximum DC input voltage and current, AC output voltage and current, and confirmation that the unit is UL-listed. For microinverter systems, show the number of units, the per-unit AC output, and the aggregate AC output current.
The inverter section of the SLD should also indicate whether the inverter includes built-in ground fault protection, arc fault protection, and rapid shutdown control. AHJ reviewers increasingly look for these confirmations as separate line items on the SLD rather than implied by the model number.
AC Disconnect and Service Panel
The SLD must show the AC disconnect (if separate from the inverter), the wire size and conduit from the inverter to the service panel, and the interconnection breaker at the service panel. The interconnection breaker must comply with the 120 percent rule from NEC 705.12(B) or the alternative load-side connection rules. Show the main breaker size, the busbar rating, and the interconnection breaker size to demonstrate compliance.
If the solar system requires an AC combiner panel, meter socket, or utility revenue-grade meter, show those components explicitly on the SLD with their ratings.
Rapid Shutdown System
Rapid shutdown must be shown on the SLD with enough detail for the reviewer to confirm compliance with the applicable version of NEC 690.12. The SLD must show: the rapid shutdown initiator, the module-level power electronics or rapid shutdown devices if required, and the initiating circuit from the service entrance or disconnect to the array boundary.
For systems using microinverters, the rapid shutdown compliance method is built into the product design. Show the microinverter model and confirm that the listed rapid shutdown response is consistent with the applicable code edition. For string inverter systems using MLPE (module-level power electronics like DC optimizers), show the MLPE model and confirm the communication method.
Grounding and Bonding
The grounding and bonding path must be visible on the SLD. This includes the equipment grounding conductor (EGC) size and type, the grounding electrode conductor, the grounding electrode system, and confirmation that module frames and racking are bonded. Some AHJs require a separate grounding and bonding schedule in addition to what is shown on the SLD.
For transformerless inverters, the SLD must document that the inverter includes ground fault detection and interruption (GFDI) in compliance with NEC 690.5.
Labeling Requirements on the SLD
NEC requires specific labels on PV systems, and the SLD should indicate where these labels are placed and what they say. Key labeling requirements under NEC 690 include: rapid shutdown labels at the service entrance and on the inverter, photovoltaic system labels at the service panel interconnection point, and maximum system voltage labels at each PV output circuit combiner or junction.
NEC 690.56 (in 2023 NEC) introduces additional requirements around system labeling and equipment marking. Jurisdictions that have adopted NEC 2023 may require updated label text that differs from prior editions. Confirm which code edition the AHJ is using before finalizing your SLD. Our permit design team tracks adopted code editions by jurisdiction and applies the correct labeling requirements to every plan set.
Common SLD Errors That Lead to Permit Corrections
Missing temperature-corrected Voc calculations. This is the most common correction on SLDs for string inverter systems. The SLD must show the calculated maximum voltage, not just the module Voc from the spec sheet.
Conductor size shown without ampacity calculations. Reviewers want to see that the conductor was sized for the actual load with temperature and conduit fill corrections applied. Showing a wire gauge without the supporting calculation is insufficient.
Inverter model mismatch. When the SLD shows a different inverter model than the spec sheet, reviewers flag it as an error. This happens most often when plan sets are templated and the inverter is updated without revising the SLD.
Incomplete grounding path. A grounding schedule that does not trace from the module frame through the racking, conduit, EGC, and grounding electrode will not satisfy the reviewer. Every link in the chain must be documented.
Rapid shutdown documentation that does not match the installed system. If the SLD shows an MLPE-based rapid shutdown system but the spec sheets show a string inverter with no MLPE, the reviewer will send it back. These sections must be internally consistent. See our post on permit drawings vs. engineering stamps for more on how reviewers evaluate plan set consistency.
What a PE-Stamped SLD Adds
In jurisdictions that require a PE stamp on the electrical plan set, the SLD carries more weight than a non-stamped version. A licensed PE has reviewed the calculations behind the diagram and accepts responsibility for the design.
PE-stamped SLDs typically see faster review times in jurisdictions that are familiar with them. Reviewers know that a licensed engineer has already checked the calculations, which shifts some of the burden of review. Our post on PE stamps for solar explains the stamp requirement by jurisdiction and how the stamping process works.
How Right Angle Engineering Handles SLDs
Every plan set Right Angle Engineering produces includes a complete, jurisdiction-specific SLD. Our engineers draw each SLD based on the actual equipment, site conditions, and AHJ requirements for the project. We do not use generic templates that require the installer to fill in component details after the fact.
We verify inverter compatibility, calculate temperature-corrected Voc, confirm rapid shutdown compliance, and check the grounding path before the SLD leaves our team. For projects that require a PE stamp, a licensed electrical engineer reviews and stamps the drawing. Turnaround is 12 hours for most residential projects. Contact us at Right Angle Engineering to get started.
Frequently Asked Questions
What is the difference between a single-line diagram and a three-line diagram?
A single-line diagram uses one line to represent all conductors in a circuit, showing components and connections in a simplified format. A three-line diagram shows each conductor individually and is more common in commercial and utility-scale work where phase relationships matter. Residential and most commercial solar permits use SLDs.
Can I use a generic SLD template for multiple projects?
Generic templates are a leading cause of permit corrections. An SLD must reflect the actual equipment, string configuration, conductor sizing, and AHJ requirements for each specific project. Using a template without updating all fields, especially inverter model, string count, and voltage calculations, will result in corrections. See our overview of AHJ solar requirements to understand how requirements vary by jurisdiction.
Does the SLD need to show the utility meter?
Most AHJs expect the SLD to show the utility meter, the interconnection point, and the point of common coupling with the utility. Some also require notation of whether the meter is a standard or bidirectional/net metering meter. Confirm with the local AHJ what level of utility interconnection detail they require.
What happens if the SLD does not match the spec sheets?
A mismatch between the SLD and the equipment spec sheets will result in a correction notice from the AHJ. The correction requires revising the SLD to match the actual equipment, resubmitting, and waiting for re-review. This adds time to every project it affects. Internal consistency checks before submittal are the most effective way to prevent this.
How quickly can Right Angle Engineering produce a PE-stamped SLD?
For most residential projects, Right Angle Engineering delivers a complete, PE-stamped electrical plan set including the SLD within 12 hours. Commercial projects may take longer depending on system complexity. We work with installers on expedited timelines when projects have critical permit deadlines.
References
National Fire Protection Association. NFPA 70: National Electrical Code, Article 690.
Right Angle Engineering. Solar Plan Sets Explained.
Right Angle Engineering. PE Stamps for Solar.
Right Angle Engineering. Permit Drawings vs. Engineering Stamps.