What Is Electrical SLD (Single Line Diagram)? Definition & Guide

Key Takeaways

A single line diagram (SLD) is a simplified electrical schematic that uses one line to represent all conductors between components — it is the most commonly required electrical drawing in solar permit packages
Every SLD must show every component from the PV source (modules) to the point of interconnection (POI) with the utility, including all disconnects, overcurrent protective devices (OCPDs), inverters, and the revenue meter
NEC Article 690 governs the electrical requirements that an SLD must reflect, including conductor sizing, OCPD ratings, grounding, and rapid shutdown compliance
Incomplete or incorrect SLDs are the single most common reason AHJs reject residential and commercial solar permit applications — missing wire sizes and OCPD ratings account for the majority of rejections
Residential, commercial, battery storage, and utility interconnection SLDs each have distinct requirements for the components shown and the level of detail expected
Modern solar design software generates SLDs automatically from the electrical design, eliminating manual drafting errors and cutting permit preparation time from hours to minutes

What Is an Electrical SLD (Single Line Diagram)?

An electrical SLD (single line diagram) is a simplified schematic drawing that represents the major electrical components and connections of a solar PV system. Instead of drawing every individual conductor (which a three-line diagram does), the SLD uses a single line to represent the entire circuit path between components. This makes the diagram readable at a glance while still conveying all the information an AHJ plan reviewer needs to verify code compliance.

The SLD is the backbone of any solar permit package. It tells the reviewer exactly how the system is wired: what modules feed into what inverter, what size wire connects each component, what overcurrent protection exists at each point, where the disconnects are located, and how the system ties into the existing electrical service. Without a complete and accurate SLD, the permit application goes nowhere.

The SLD is not a wiring diagram. It does not show physical wire routing, conduit paths, or terminal connections. Its purpose is to represent the electrical relationship between components in a way that a plan reviewer can verify against NEC requirements in minutes rather than hours. Think of it as a road map, not turn-by-turn directions. Every component gets a standardized symbol, every conductor gets a size label, and every protective device gets a rating. If any of those are missing, the permit gets kicked back.

Types of Solar SLDs

Residential

Residential SLD

Covers grid-tied residential systems up to 25 kW. Shows PV modules, string configuration, inverter (string or micro), AC disconnect, production meter (if required), main service panel, and utility meter. Most AHJs accept a single-page SLD for residential permits. Must include conductor sizes, OCPD ratings, grounding electrode conductor size, and rapid shutdown compliance notation per NEC 690.12.

Commercial

Commercial SLD

Covers systems above 25 kW on commercial buildings or ground mounts. Requires more detail than residential: multiple string combiner boxes, fused or unfused combiners, DC and AC disconnects, switchgear, transformer connections (if applicable), and main distribution panel or switchboard. Often spans multiple pages and must show short-circuit current ratings (SCCR) at each point in the system.

Storage

Battery Storage SLD

Adds energy storage components to the base SLD: battery bank, battery inverter or hybrid inverter, charge controller (if separate), transfer switch or automatic transfer switch (ATS), critical load panel, and battery disconnect. Must show NEC Article 706 compliance including battery OCPD, DC disconnect, and ventilation notation. AC-coupled and DC-coupled configurations have different SLD layouts.

Utility

Utility Interconnection SLD

Focused specifically on the point of interconnection with the utility grid. Required by most utilities as part of the interconnection application, separate from the AHJ permit. Shows the revenue meter, utility disconnect (if required), service entrance equipment, main breaker rating, bus bar rating, and the 120% rule calculation for supply-side or load-side connections per NEC 705.12. May require utility-specific formatting.

SLD Component Reference Table

This table lists the standard components shown on a solar SLD, their schematic symbols, the information required for each, and the applicable NEC reference. Every component from the PV source to the point of interconnection must appear on the diagram.

SLD Component	Symbol	Required Information	NEC Reference
PV Module/Array	Rectangle with + and - terminals	Module wattage, Voc, Isc, quantity, string configuration	NEC 690.7, 690.8
String Fuse/OCPD	Rectangle with curved break	Ampere rating, voltage rating, interrupt rating	NEC 690.9
Combiner Box	Rectangle with multiple inputs	Number of inputs, fuse ratings, output conductor size	NEC 690.9(A)
DC Disconnect	Switch symbol (open/close)	Ampere rating, voltage rating, lockable notation	NEC 690.15
Inverter	Triangle or rectangle with ~ symbol	Model, kW rating, input/output voltage, phase	NEC 690.4
AC Disconnect	Switch symbol (open/close)	Ampere rating, voltage rating, lockable, visible/accessible	NEC 690.15
AC OCPD (Breaker)	Rectangle with curved break	Ampere rating, number of poles, AIC rating	NEC 690.9, 705.12
Main Service Panel	Rectangle with bus	Bus rating, main breaker rating, available spaces	NEC 705.12
Revenue/Production Meter	Circle with M	Meter type, CT ratio (if applicable)	Utility requirement
Grounding Electrode	Three horizontal lines (decreasing)	GEC size, grounding electrode type, bonding jumper size	NEC 690.47, 250.166
Rapid Shutdown	Dashed boundary box	Initiation device, controlled conductors, array boundary	NEC 690.12
Surge Protection (SPD)	Diamond or MOV symbol	Type (1, 2, or 3), voltage rating, location	NEC 690.35, 242.24

SLD Completeness Check

All components from PV source to POI must be shown with: equipment ratings + conductor sizes (AWG/kcmil) + OCPD ratings (A) + grounding conductor sizes + disconnect locations

A complete SLD traces an unbroken path from the PV modules to the utility point of interconnection. At every component along that path, the diagram must label:

Equipment ratings. Inverter kW, panel bus ampacity, disconnect ampere rating, module Voc and Isc. These let the plan reviewer verify that each device is rated for the voltages and currents it will see.
Conductor sizes. Every line on the SLD must be labeled with the wire gauge (e.g., #10 AWG Cu THWN-2) and the number of conductors. This includes DC source circuits, DC output circuits, inverter output circuits, and feeder circuits.
OCPD ratings. Every overcurrent protective device — fuses, breakers, supplementary protectors — must show its ampere rating. The reviewer checks these against NEC 690.9 and the conductor ampacity to verify proper protection.
Grounding and bonding. The equipment grounding conductor (EGC) size, grounding electrode conductor (GEC) size, and bonding jumper sizes must appear on the SLD. Ground fault protection method (fuse or electronic) should be noted.
Disconnect locations. Every required disconnect must be shown with its rating and a note about accessibility — visible, lockable, and within sight of the equipment it serves.

If any of these five categories is missing from any component on the SLD, expect a plan review correction notice.

Why AHJs Reject SLDs

Incomplete or incorrect SLDs are the leading cause of solar permit rejections across U.S. jurisdictions. A 2022 NREL study on solar permitting timelines found that electrical plan deficiencies — primarily missing conductor sizes, unlabeled OCPD ratings, and absent grounding details on SLDs — accounted for the majority of first-round correction notices. Each rejection adds 1-3 weeks to the permitting timeline. The fix is straightforward: every line on the SLD needs a wire size label, every protective device needs an ampere rating, and every disconnect needs a voltage and ampere rating. No exceptions. Using solar design software that auto-generates SLDs with all required labels eliminates these rejections entirely.

Practical Guidance

Label every conductor on the SLD. Every single line between components must show the wire gauge, insulation type, number of conductors, and conduit type. Write it as “#10 AWG Cu THWN-2 (2) in 3/4” EMT” — not just “#10.” Plan reviewers reject diagrams that omit conductor count or insulation type.
Include the 120% rule calculation on the SLD. For load-side connections per NEC 705.12(B)(2), show the math directly on the diagram: main breaker rating + solar breaker rating must not exceed 120% of the bus bar rating. For example: 200A bus × 120% = 240A; 200A main + 40A solar = 240A. This saves the reviewer time and prevents back-and-forth.
Show rapid shutdown compliance clearly. NEC 690.12 requires controlled conductors within the array boundary to be reduced to 80V and 5A within 30 seconds of shutdown initiation. Note the compliance method on the SLD — module-level shutdown (MLPEs), inverter-level shutdown, or listed rapid shutdown system. Mark the array boundary with a dashed line.
Use standardized IEEE/ANSI symbols. Plan reviewers expect to see standard electrical symbols per IEEE Std 315 and ANSI Y32.2. Non-standard or hand-drawn symbols slow down the review and can lead to misinterpretation. Solar design software uses the correct symbol set automatically, so every SLD follows the conventions that reviewers recognize.

Keep a copy of the approved SLD on site during installation. The SLD is your wiring reference. If the field conditions require a deviation from the approved SLD — different conduit routing, additional disconnect, wire size change — document the change and submit a revised SLD to the AHJ before the final inspection.
Verify equipment nameplate data against the SLD. Before wiring anything, confirm that the inverter model, breaker ratings, disconnect ratings, and module specifications on the SLD match the actual equipment delivered to the site. Substitutions happen, and an inspector will compare the installed equipment to the approved plans.
Follow the SLD for grounding and bonding exactly. Grounding is where most inspection failures happen after the SLD itself is approved. The SLD specifies the GEC size, bonding jumper sizes, and grounding electrode type. Install exactly what the diagram shows — do not substitute a smaller grounding conductor because it is easier to pull.
Match disconnect locations to the SLD. The SLD shows where each disconnect is located relative to the equipment it serves. NEC 690.15 requires disconnects to be “within sight” of the inverter and readily accessible. If you move a disconnect to a different location than what the SLD shows, the inspector may fail the installation even if the new location technically complies with code.

Explain that the SLD is included in every permit package. Homeowners sometimes ask what is in the “engineering documents” they are paying for. The SLD is the centerpiece — it shows exactly how their system connects to their home’s electrical service. A professionally generated SLD signals that the company takes engineering seriously.
Use SLD quality as a competitive differentiator. Many smaller installers still produce SLDs manually or use generic templates that frequently get rejected. If your company uses software that generates project-specific, code-compliant SLDs automatically, that is a real advantage: fewer permit rejections, faster approval, and faster installation start dates.
Know the cost of a permit rejection. Every SLD-related permit rejection adds 1-3 weeks to the project timeline. That means delayed interconnection, delayed customer savings, and a crew sitting idle waiting for resubmission approval. On a 20-project-per-month operation, even a 10% rejection rate costs tens of thousands in delayed revenue and rework hours annually.
Do not promise customers a specific permit timeline. SLD approval is one piece of the permit process. AHJ review times vary from 1 day to 6 weeks depending on the jurisdiction. What you can promise is that the SLD will be complete and code-compliant on first submission, which eliminates the most common cause of delays.

Generate Permit-Ready SLDs Automatically

SurgePV creates project-specific single line diagrams from your electrical design — with all conductor sizes, OCPD ratings, disconnect locations, and NEC references filled in automatically. No manual drafting. No missed labels. No permit rejections.

Book a Demo

No commitment required · 20 minutes · Live project walkthrough

Sources & References

NFPA 70 (NEC) Article 690 — Solar Photovoltaic Systems. Covers all electrical requirements for PV system design including conductor sizing (690.8), overcurrent protection (690.9), disconnecting means (690.15), and rapid shutdown (690.12) — all of which must be reflected on the SLD.
IEEE Std 315 / ANSI Y32.2 — Graphic Symbols for Electrical and Electronics Diagrams. Defines the standardized symbols used on SLDs including switches, fuses, breakers, transformers, meters, and grounding electrodes.
NREL Solar Permitting and Inspection Studies — Research on solar soft costs, permitting timelines, and common plan review deficiencies across U.S. jurisdictions, including the impact of incomplete electrical drawings on permit approval rates.

Frequently Asked Questions

What is the difference between a single line diagram and a three line diagram?

A single line diagram (SLD) represents all conductors in a circuit as one line, simplifying the schematic so that the overall system architecture is clear at a glance. A three line diagram shows each individual phase conductor (L1, L2, L3 or L1, L2, N) as a separate line, providing more detail about phase connections, neutral routing, and individual conductor protection. Most residential solar permits require only an SLD. Commercial systems and three-phase installations may require a three line diagram in addition to the SLD, depending on the AHJ. The SLD shows what is connected and how; the three line diagram shows the exact conductor-level wiring between those connections.

What information must be included on a solar SLD for permit approval?

A permit-ready solar SLD must include: PV module specifications (wattage, Voc, Isc, quantity, and string configuration), inverter model and ratings (kW, input voltage range, output voltage), all conductor sizes with insulation type and material (e.g., #10 AWG Cu THWN-2), every overcurrent protective device with its ampere rating and interrupt capacity, all disconnect switches with ampere and voltage ratings plus “lockable” notation, grounding electrode conductor and equipment grounding conductor sizes, rapid shutdown compliance method per NEC 690.12, the main service panel bus rating and main breaker size, the 120% rule calculation for load-side connections, and the utility meter and point of interconnection. Some AHJs also require surge protection device (SPD) notation, arc-fault circuit interrupter (AFCI) notation, and the listing/certification marks for major components.

Can I create a solar SLD manually or do I need software?

You can create an SLD manually using CAD software, Visio, or even hand-drawn on engineering paper — there is no NEC requirement that it be software-generated. However, manual SLDs are the primary source of permit rejections because designers frequently omit required labels, use non-standard symbols, or miscalculate conductor sizes and OCPD ratings. For companies processing more than a few permits per month, dedicated solar design software is significantly faster and more reliable. Tools like SurgePV generate the SLD directly from the system’s electrical design, so every component, wire size, and protective device rating is populated automatically and matches the rest of the permit package. The time savings alone — typically 30-60 minutes per project — justify the switch from manual drafting.