SLD (Single Line Diagram)

Quick Facts

What is an SLD?

Formal definition

A Single Line Diagram is a one-line schematic that depicts the major components and conductors of an electrical system using single symbolic lines to represent multi-conductor paths. For solar PV, the SLD covers the entire DC and AC path from modules to the point of interconnection.

Engineering definition

The SLD captures all design decisions in a single drawing: module string configuration, inverter input topology, all OCPDs and disconnects with ratings, conductor types and ampacities, grounding electrode system, and the interconnection method to the utility. It is the foundation drawing referenced by every other electrical drawing in the permit set.

Industry definition

Designers, EPCs, and AHJs treat the SLD as the project’s “electrical fingerprint.” Reviewers verify code compliance, calculate worst-case currents, validate breaker coordination, and confirm rapid-shutdown placement directly from the SLD.

Permitting definition

The SLD is the single most-reviewed document in any solar permit submission. Plan reviewers check it against NEC code citations, the equipment cut sheets, and the structural drawings. Most permit rejections cite SLD errors — missing labels, mismatched conductor sizes, or unsafe interconnection methods.

SLD Explained Simply

For installers: Think of the SLD as the wiring blueprint — every wire, breaker, and disconnect the inspector will look for must be on it. If it’s not on the SLD, it can’t be in the field.

For homeowners: The SLD is the technical drawing that proves your solar system is wired safely and meets the electrical code. Your city’s permit office uses it to approve the project before you turn the system on.

For junior designers: The SLD condenses complex three-phase, multi-circuit wiring into a single readable line. Master the symbols (PV array, inverter, disconnect, OCPD, transformer, grounding electrode) and you have read 90% of any solar permit set.

For new engineers: The SLD is your design audit trail. Every NEC calculation — string Voc, conductor ampacity, OCPD sizing, voltage drop, ground fault detection — terminates in an annotation on the SLD.

Analogy: A road map showing highways without the lane markings. The SLD strips away three-phase complexity to show the topology cleanly, while the detail drawings (three-line, conduit/conductor schedule, panel schedule) fill in the missing detail.

Why an SLD Matters

Safety. First responders read the SLD label on the service panel to locate the PV disconnect and rapid-shutdown initiator before entering a structure. NEC 690.13 and 690.56 label requirements exist specifically because firefighters need to interpret the SLD in seconds.

Code compliance. NEC 690.4(B), 690.8, 690.9, 690.13, 690.56, 705.12, 706 — every relevant code citation gets evaluated against the SLD. Plan reviewers literally check off code line-by-line.

Engineering impact. Every wire size, breaker rating, and disconnect specification flows from the SLD. Get the SLD wrong and you’ve designed an unsafe — or simply unbuildable — system.

Project cost. SLD errors caught after permit approval cost more in change orders. SLD errors caught after construction can cost a system commissioning delay of weeks.

Permitting impact. A clean, well-labeled SLD speeds AHJ approval from weeks to days. SolarApp+ automated review depends entirely on a structured SLD machine-readable through field-mapped templates.

How an SLD Is Built

1. Capture site data. Service voltage, main breaker rating, available bus, existing loads, utility tariff.
2. Lock the module and inverter. Datasheet values feed every downstream calculation.
3. Run string sizing. Determine modules-per-string at ASHRAE extreme temperatures (see MPPT for worked example).
4. Place DC components. Combiner boxes, optimizers/microinverters if used, DC disconnects.
5. Place the inverter. Indoor/outdoor location, distance from main service.
6. Place AC components. Inverter output OCPD, AC disconnect, production meter (if required), interconnection method.
7. Apply protection rules. NEC 705.12 120% rule, line-side tap, supply-side connection, dedicated breaker — pick the legal method.
8. Add code labels. Per NEC 690.13/690.56 plus AHJ-specific labels.
9. Size conductors and OCPDs. Apply NEC 690.8(B), temperature derating, conduit-fill derating.
10. Add the title block, revision history, PE stamp area. Cross-reference cut sheets and structural drawings.

Engineering Deep Dive

Required SLD symbols

• PV array: module count, string count, modules/string with Voc and Vmp at extreme temperatures.
• Inverter: make/model, AC and DC ratings, MPPT count, UL 1741 listing.
• DC disconnect: ampacity, voltage rating, fused/unfused.
• AC disconnect: lockable, visible-open, ampacity, voltage rating.
• OCPDs: circuit breakers and fuses with AIC rating, trip rating, type (HACR, etc.).
• Conductors: size (AWG/kcmil), type (PV Wire, USE-2, THHN, XHHW-2), insulation rating, conductor count, ground/neutral identifier.
• Conduit: type (EMT, RMC, PVC, ENT), trade size, fill calculation reference.
• Grounding: EGC size per NEC 250.122, GEC connections, equipment bonding jumpers.
• Rapid shutdown initiator: location, signal type (PLC, wireless), compliant with NEC 690.12.
• Point of interconnection: main service panel, line-side tap, dedicated meter pedestal.

Worked example — string Voc on the SLD label

Module Voc = 41.6 V, β_Voc = −0.27 %/°C, site ASHRAE 99.6% min = −8 °C, 22 modules in series.

Voc(T) = 41.6 × [1 + (−0.0027)(−8 − 25)]
       = 41.6 × [1 + 0.0891]
       = 41.6 × 1.0891
       = 45.31 V per module
String Voc = 45.31 × 22 = 996.8 V

The SLD must annotate Vmax string = 996.8 V at −8 °C, confirming NEC 690.7 compliance for a 1,000 V system.

NEC 705.12 interconnection methods

Each method appears differently on the SLD — picking the right one early avoids re-drafts.

Required SLD labels (NEC 2023)

• 690.13(B) — DC disconnect labeling
• 690.56(B) — “PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN” warning at service entrance
• 690.56(C) — Map of rapid shutdown initiator location
• 705.10 — “WARNING: DUAL POWER SOURCE” labels at all disconnects
• 705.12(B)(3)(2) — Backfed breaker label
• 710.10 — Stand-alone PV labels (where applicable)

Each citation must appear on the SLD or in a labels schedule sheet that cross-references back to the SLD.

Design Considerations

• Scale. Residential SLDs fit on a single 11×17 sheet. Commercial SLDs span 4–8 sheets with separate DC and AC SLDs plus a system-level SLD.
• AHJ template. Many AHJs (LADWP, SCE, PG&E, Phoenix) publish SLD templates. Use them — reviewers compare line-by-line.
• As-builts. Always plan for an as-built SLD revision after construction.
• Cross-references. Pin numbers, panel schedules, equipment schedules must all key back to the SLD.
• Vector vs. raster. AHJs increasingly require vector PDFs for SolarApp+ machine review. Don’t submit flattened raster scans.

Permitting Implications

AHJ requirements vary. California IOUs (PG&E, SCE, SDG&E) require an SLD that matches their interconnection template. NYC DOB requires structural and electrical SLDs on separate sheets. Phoenix and Tucson use SolarApp+ for automated review with strict template adherence.

Common rejection reasons.

1. Missing rapid shutdown initiator on the SLD.
2. Conductor sizes don’t match the NEC 690.8(B) ampacity calculation.
3. 120% rule violated for backfed breaker.
4. Missing or incorrect compliance labels.
5. Inverter make/model not on the AHJ’s approved equipment list.
6. Three-phase systems missing neutral conductor on the SLD (use a 3LD for these).
7. EGC undersized per NEC 250.122.

Inspection. The field inspector physically checks installed equipment against the SLD. Any deviation requires an as-built revision before final approval.

Utility Interconnection Impact

For systems > 11 kW (residential) or any commercial project, the utility interconnection application typically requires the SLD plus a 3LD. Rule 21 (California) and IEEE 1547 compliance items are verified directly from the SLD — anti-islanding test report, UL 1741-SB listing for the inverter, MPPT count and capacity.

For systems > 1 MW, the utility’s protection engineer reviews the SLD as part of the system impact study. Protective relaying, transformer specifications, and switchgear configurations are all keyed to the SLD.

US Code Requirements

• NEC 690.4 — Source circuit installation requirements.
• NEC 690.7 — Maximum system voltage (string Voc at coldest expected temperature).
• NEC 690.8 — Circuit current and conductor ampacity.
• NEC 690.9 — Overcurrent protection.
• NEC 690.11 — Arc fault detection.
• NEC 690.12 — Rapid shutdown.
• NEC 690.13 — DC disconnect requirements.
• NEC 690.41–47 — Grounding.
• NEC 690.56 — Required labels.
• NEC 705.10–12 — Source interconnection.
• NEC 706 — Energy storage (where ESS is integrated).
• NEC 710 — Stand-alone systems.

India Regulatory Context

In India, the SLD is required for CEIG (Chief Electrical Inspector General) approval before energization. CEIG-approved SLDs typically include:

• Earthing and lightning protection scheme per IS 3043
• Surge protection per IEC 62305 / IS 16942
• Compliance with the CEA Connectivity Regulations for grid connection
• DISCOM-specific labels and net-metering interconnection
• Reference to MNRE-approved inverters and ALMM-listed modules

Indian SLDs follow IS 12032 conventions and are typically drawn in metric units with IEC standard symbols.

Real-World Examples

Residential — 7.2 kW, San Jose CA

18 modules in 2 strings of 9 on a SolarEdge SE7600H-USA with HD-Wave optimizers, load-side breaker connection (120% rule satisfied), 30 A backfed breaker, all NEC 690 labels visible on the SLD. Permit approved within 4 days via SolarApp+.

Commercial — 487 kW carport, Phoenix AZ

1,218 modules across two parking canopies, 4 × Sungrow SG110CX inverters (9 MPPTs each), one combiner box per inverter, dedicated AC sub-panel, line-side tap at the existing service. SLD spans 3 sheets with separate DC SLD, AC SLD, and labels schedule.

Utility-scale — 50 MW tracker plant, Texas

5 × 4 MW central inverters, 1,500 V DC system, two recombiner boxes per inverter, 4.16 kV medium-voltage collection, step-up to 138 kV at the substation. SLD is part of an 18-sheet set including SLD, 3LD, protective device coordination, grounding, lightning protection, and SCADA.

Common Mistakes

1. Mismatched conductor sizes between SLD and conduit schedule. Always cross-check.
2. Missing rapid shutdown initiator. NEC 690.12 violation; instant rejection.
3. Backfed breaker at busbar end without 120% calc reference. Plan reviewers want to see the calculation on the SLD.
4. Wrong inverter listing. UL 1741 vs. UL 1741-SA vs. UL 1741-SB matters for state grid codes.
5. Misplaced disconnect — outside the line-of-sight from the inverter. NEC 690.13(A) requires readily accessible disconnect.
6. Missing AHJ-required labels (e.g., LA County’s “PV CIRCUIT” labels, Hawaii’s “PV SYSTEM POWER SOURCE”).
7. Incorrect EGC sizing. Apply NEC 250.122 based on OCPD rating.
8. Ignoring temperature correction for conductor ampacity. NEC 310.15(B) ambient correction is mandatory.
9. Using a residential SLD template for commercial. Three-phase needs a 3LD or explicit per-phase SLD.
10. Missing rapid shutdown placard. NEC 690.56(C) map of initiator locations.

Best Practices

Design.

• Use the AHJ’s published SLD template where one exists.
• Annotate every NEC calculation directly on the SLD so reviewers don’t need a separate calc sheet.
• Include the production meter, monitoring system, and any storage interconnection.

Drafting.

• Use a consistent symbol library (IEEE 315 / IEC 60617).
• Keep the SLD on a separate sheet from the site plan and structural drawings.
• Maintain a visible revision history.

Permitting.

• Submit vector PDFs, not flattened scans.
• Include a one-page labels schedule.
• Stamp clearly with PE seal, signature, date.

Installation.

• Field-print the SLD and post at the inverter location.
• Update for as-builts before commissioning.

Comparison: SLD vs. Three-Line Diagram

Standards & Certifications

• IEEE 315 — Standard symbols for electrical and electronics diagrams
• IEC 60617 — International graphical symbols for diagrams
• NEC 690, 705, 706, 710, 712 — PV and energy storage articles
• UL 1741, UL 1741-SB — Inverter listings cited on SLDs
• IEEE 1547 — Distributed energy resource interconnection
• IS 12032 (India) — Graphical symbols for electrical diagrams
• IS 3043 (India) — Earthing code referenced on SLDs

Key Takeaways

• The SLD is the master one-line schematic of a solar PV system and the single most-reviewed document in any permit.
• It captures every NEC 690 and 705 design decision: string Voc, conductor sizes, OCPD ratings, disconnects, rapid shutdown, and interconnection method.
• Residential SLDs are concise (1–2 sheets); commercial SLDs span multiple sheets with separate DC, AC, and labels schedules.
• Use AHJ-published templates when available and submit vector PDFs for SolarApp+ machine review.
• A clean, well-labeled SLD speeds permit approval from weeks to days; errors are the #1 cause of solar permit rejection.