Solar Single Line Diagrams (SLD): Complete Electrical Design Guide (2026)

Every solar installation in Europe requires a single line diagram before the grid operator will approve a connection. The SLD is not optional paperwork — it is the legal document that shows inspectors, engineers, and utilities exactly how the system is wired, what protection devices are fitted, and whether the design meets national safety standards. Get it wrong and permits are refused. Get it right and you shorten approval timelines by weeks.

This guide covers the complete electrical design process: what an SLD must show, how to size strings correctly, how to select cables and protection devices, and what IEC 60364-7-712 requires for European PV installations. The final section covers how solar design software can generate compliant SLDs automatically from your layout.

What Is a Solar Single Line Diagram?

A solar single line diagram is a simplified electrical schematic that represents a PV system's components and connections. It uses single lines to represent multi-conductor cables — so a three-core AC cable appears as one line, not three. The purpose is clarity: an SLD shows the system's electrical architecture at a glance without the complexity of a full wiring schematic.

Every SLD must show the path of current from PV modules to the grid: panels, string groupings, combiner boxes, DC protection, inverter, AC protection, metering, and grid connection point. Labels on each element give the inspector the information they need to verify compliance: cable cross-sections, voltages, currents, and equipment ratings.

SLD vs. wiring diagram vs. layout diagram

These three documents serve different purposes and are not interchangeable:

• Single line diagram (SLD): Electrical schematic. Shows components, connections, and ratings. Required for permits and grid applications.
• Wiring diagram (schematic): Full multi-conductor representation. Shows every individual wire, terminal, and connection point. Used during installation and commissioning.
• Layout diagram: Physical plan view. Shows where panels, inverters, and cables are located on the roof and in the building. Required alongside the SLD for building permits.

When SLDs are required

Building permit applications require an SLD in Germany (Baugenehmigung), France (déclaration préalable), and Italy (pratiche SUAP). Grid connection applications always require an SLD — the DNO or DSO uses it to verify anti-islanding compliance, protection device coordination, and that the AC output matches what was declared. In the UK, MCS certification for domestic systems requires a completed SLD. In Germany, registration in the Marktstammdatenregister (MaStR) requires documentation that includes electrical specifications captured in the SLD.

Solar SLD Components: What to Include

An SLD that passes inspection covers every major component from panel to grid. Here's what each section of the diagram must show.

DC side — array to inverter

• PV modules: Show module type, rated Voc and Isc at STC, number of modules per string, and number of strings. E.g., "12 × 400Wp, Voc 40.5V, Isc 9.8A".
• String combiner box: Required when more than two strings are connected in parallel. Show number of inputs, string fuse rating, and DC bus voltage.
• DC surge protection device (SPD): Show device type (Type 1 or Type 2), maximum continuous voltage (Uc), and discharge current (In). Installed between array and inverter.
• DC isolator/disconnect: Lockable switch rated for array Voc and 1.25 × Isc. Required between array and inverter input. Show voltage and current rating.
• DC fuse or circuit breaker: String-level protection. Show rated current, fuse type (gPV), and voltage rating.
• Cable labels: Show conductor cross-section (mm²) on every cable segment.

Inverter

Show the inverter as a box with labeled DC input terminals and AC output terminals. Include: model designation, rated DC input voltage range and MPPT window, rated AC output power (kW), and number of MPPT inputs. For multi-MPPT inverters, show each MPPT channel as a separate DC input with its string assignment.

AC side — inverter to grid

• AC isolator: Lockable disconnect on the AC output of the inverter. Rated for inverter output current and voltage.
• RCD: Show type (Type B for transformerless inverters), rated current (A), and sensitivity (mA). Installed downstream of the inverter on the AC side.
• MCB: Show rated current and trip curve (B or C). Sized for inverter rated AC output current.
• kWh meter: Bidirectional for net metering or feed-in measurement. Show meter class and whether it is import-only or import/export.
• Grid connection point: Labeled with system voltage and frequency (e.g., 230V AC / 50 Hz).

Battery storage (if applicable)

AC-coupled batteries connect on the AC bus between the inverter and the main distribution board. DC-coupled batteries connect via a separate charge controller or through a hybrid inverter's DC bus. The SLD must show: battery rated voltage (V) and capacity (kWh), battery management system (BMS), battery DC isolator, and whether the system includes backup/off-grid functionality (which triggers additional island protection requirements).

String Sizing: The Complete Calculation

String sizing determines how many modules can be wired in series. The string voltage must stay within the inverter's MPPT input range across all operating temperatures — too high in winter cold and the inverter shuts down to protect itself; too low in summer heat and the MPPT loses regulation.

Why temperature drives string voltage

Solar module Voc increases as cell temperature falls. On a cold winter morning, a module rated at Voc 40V at 25°C (STC) may actually produce 45V or more. Every module in the string adds to this, so a 15-module string could push 675V — well above many inverter input limits. The Voc temperature coefficient (typically -0.29% to -0.35%/°C for monocrystalline silicon) tells you exactly how much Voc changes per degree.

String sizing formulas

Use these two formulas to set the upper and lower boundaries:

Maximum modules per string (limited by inverter Vmax):

Max strings = floor( Vmax_inverter / (Voc × (1 + (Tmin − 25) × coeff)) )

Minimum modules per string (limited by MPPT Vmin):

Min strings = ceil( Vmin_MPPT / (Voc × (1 + (Tmax − 25) × coeff)) )

Where coeff is the Voc temperature coefficient expressed as a decimal per °C (e.g., -0.0029 for -0.29%/°C), Tmin is the lowest expected ambient temperature at the site, and Tmax is the highest.

Worked example: 400W TOPCon + SMA STP 10000TL

Panel specs: Voc 40.5V, Voc temperature coefficient -0.29%/°C (-0.0029/°C). Inverter: SMA STP 10000TL, MPPT range 200–800V, maximum input voltage 800V.

Germany (Tmin = -15°C, Tmax = 70°C cell temperature):

• Adjusted Voc at -15°C: 40.5 × (1 + (-15 − 25) × -0.0029) = 40.5 × 1.116 = 45.2V
• Max modules: floor(800 / 45.2) = floor(17.7) = 17 modules
• Adjusted Voc at 70°C cell temp: 40.5 × (1 + (70 − 25) × -0.0029) = 40.5 × 0.8695 = 35.2V
• Min modules: ceil(200 / 35.2) = ceil(5.68) = 6 modules

Spain (Tmin = 0°C, Tmax = 75°C cell temperature):

• Adjusted Voc at 0°C: 40.5 × (1 + (0 − 25) × -0.0029) = 40.5 × 1.0725 = 43.4V
• Max modules: floor(800 / 43.4) = floor(18.4) = 18 modules

Multi-MPPT string assignment

Modern inverters have two or more independent MPPT inputs. When panels face different orientations — for example, east- and west-facing roof planes — assign each orientation to a separate MPPT channel. This prevents the shaded or lower-irradiance string from dragging down the higher-performing string. Each MPPT input must be sized independently using the same temperature-based formulas above.

Cable Sizing for Solar DC Wiring

DC cable sizing determines conductor cross-section (mm²) based on maximum current, cable length, and allowable voltage drop. Undersized cables overheat; they also fail inspection because the SLD must show cable cross-sections and inspectors verify them against the current ratings.

Cable type requirements

PV DC cables must be double-insulated and rated for outdoor UV exposure. The two standard types in Europe are:

• PV1-F: Single-core, flexible (IEC 60228 Class 5), rated for 1000V DC and 70°C ambient. The most common type for string wiring.
• H1Z2Z2-K: Higher temperature rating (90°C), better UV and chemical resistance. Required in some installations with high ambient temperatures or conduit in direct sunlight.

Maximum DC string current

The cable from each string to the combiner box or inverter input must be rated for:

I_max = 1.25 × Isc × number_of_parallel_strings

The 1.25 factor accounts for irradiance above STC (which can happen briefly under certain cloud-edge conditions). For a single string with Isc 9.8A: I_max = 1.25 × 9.8 = 12.25A. This determines the minimum cable ampacity.

Cable sizing table

IEC 60364-7-712 recommends keeping DC-side voltage drop to 1% or less and AC-side voltage drop to 1% or less. The voltage drop formula for DC:

ΔV (%) = (2 × ρ × L × I) / (A × V_system) × 100

Where ρ is copper resistivity (0.0175 Ω·mm²/m), L is one-way cable length (m), I is current (A), A is cross-section (mm²), and V_system is the DC bus voltage (V).

Derating for bundled cables

When DC string cables run together in conduit or are bundled, the heat they generate is additive. Apply derating factors: two cables bundled = 0.85 × rated ampacity; three to five = 0.70; six to nine = 0.65. Cables in conduit exposed to direct sunlight require a further temperature derating of 0.82 (for 50°C ambient vs. the 30°C reference temperature used in standard ratings).

Protection Devices: Selection Guide

Every protection device in a solar SLD has a specific function. Selecting the wrong type — or omitting one — is both a code violation and a safety hazard.

DC surge protection device (SPD)

SPDs protect against transient overvoltages from lightning and switching events on the DC side. Selection rules:

• Type 1 SPD: Required when the building has a lightning protection system (LPS) installed. Rated to handle a partial lightning strike (10/350 µs waveform).
• Type 2 SPD: Standard for installations without an LPS. Protects against induced surges (8/20 µs waveform).
• Uc rating: Must be greater than 1.2 × Voc_max of the array. For an array with maximum Voc of 800V: Uc ≥ 960V.
• In rating: Minimum 5 kA for residential; 10–20 kA recommended for commercial systems in areas with high lightning density.

DC fuse (gPV type)

Conventional gG fuses are not suitable for solar DC circuits — they are designed for AC and may not safely interrupt DC fault currents. Use gPV-rated fuses specifically designed for PV applications. Rating: In ≥ 1.5 × Isc of the string; voltage rating must exceed array Voc at minimum temperature.

DC isolator

The DC isolator between the array and the inverter must be lockable (to prevent accidental re-energisation during maintenance) and rated per IEC 60947-3 for DC switching. Rating: Ue ≥ Voc_max, Ie ≥ 1.25 × Isc. In many European countries, the inverter's internal DC switch satisfies this requirement — but a separate external isolator is still required for maintenance access.

RCD on the AC side

Transformerless string inverters — virtually all modern residential and commercial string inverters — have no galvanic isolation between the DC and AC circuits. This means a DC ground fault can produce a smooth DC component in the fault current that a standard Type A RCD cannot detect (Type A only detects sinusoidal and pulsating DC fault currents). A Type B RCD detects all fault current types including smooth DC components and is mandatory under IEC 60364-7-712 for transformerless systems.

MCB (miniature circuit breaker)

The MCB on the AC output side protects the wiring and provides overcurrent protection. Select based on the inverter's rated AC output current. Trip curve: B-curve for residential (trips at 3–5× rated current), C-curve for commercial systems with higher inrush. The MCB rated current must not exceed the ampacity of the AC cable it protects.

Country-specific protection requirements

IEC 60364 Requirements for Solar PV in Europe

IEC 60364-7-712 is the international standard specifically addressing electrical requirements for photovoltaic power supply systems. Every European country has adopted it, either directly or through a national standard that references it. Understanding its key requirements prevents the most common compliance failures.

Segregation of DC wiring

PV DC cables must be kept separate from other electrical circuits where possible, or segregated by conduit if they must share a route. This prevents a DC fault from propagating into the building's general electrical installation. Where DC cables run inside the building, they must be installed in fire-resistant conduit or enclosed in fire-resistant cable trays — not clipped directly to joists or trunking shared with other circuits.

PV array marking requirements

The standard requires warning labels at the inverter, at the main AC distribution board, and at the array DC isolator. The label must warn that the system remains live during daylight even when the AC isolator is open. Labels must be in the local language: "WARNUNG: PV-Anlage unter Spannung" (German), "AVERTISSEMENT: Installation photovoltaïque sous tension" (French), "ATTENZIONE: Impianto fotovoltaico in tensione" (Italian).

Anti-islanding protection

All grid-connected inverters must disconnect from the grid within 0.2 seconds of detecting a grid fault (loss of mains). This prevents the inverter from continuing to energize the grid while utility workers assume it is de-energized. National implementations: VDE-AR-N 4105 (Germany), EN 50549-1 (European general), with specific frequency and voltage trip thresholds. Most modern inverters have anti-islanding built in and certified; the SLD must reference the inverter's anti-islanding certification.

Arc fault detection (AFCI)

Germany has required arc fault circuit interrupters (AFCIs) on DC circuits above 400V since 2022 under VDE-AR-N 4100. AFCIs detect the distinctive electrical signature of a DC arc — which does not self-extinguish like an AC arc — and shut down the inverter within 0.5 seconds. This requirement applies to new installations; retrofit on existing systems is not currently mandated but is strongly recommended. If an AFCI device is installed, it must be shown on the SLD with its rated voltage and detection threshold.

Equipment certification marks

The SLD does not need to list certifications explicitly, but every component must carry CE marking, and the inverter must be certified to IEC 62109-1 (safety) and IEC 62109-2 (grid connection). Modules must be certified to IEC 61730. These certifications are verified during the building permit and grid connection review — have the datasheets ready.

How to Draw a Solar SLD: Step-by-Step

Whether you're drawing by hand, in AutoCAD, or using a dedicated solar tool, the process follows the same sequence. Start from the DC side and work toward the grid.

1. List all components with specifications. Before drawing anything, compile a complete component list: module datasheet (Voc, Isc, Wp, temp coefficient), inverter datasheet (MPPT range, Vmax, rated current), cable types and cross-sections, all protection devices with ratings. This becomes the bill of materials and prevents errors in the diagram.
2. Draw the DC side. Start with the PV array symbol (panels in series forming a string), label the string voltage and current. Add the string combiner box if multiple strings are parallel. Add the DC SPD, DC fuse/breaker, and DC isolator in series between the array and inverter input. Label cable cross-sections on each segment.
3. Draw the inverter. Represent it as a rectangle with labeled DC input and AC output terminals. Note the model, rated power, and MPPT count. For hybrid inverters, add the battery port.
4. Draw the AC side. Starting from inverter AC output: AC isolator → RCD (Type B, rated current, sensitivity) → MCB → kWh meter → grid connection point. Label all cable cross-sections.
5. Add battery storage branch (if applicable). Show whether AC-coupled (connecting on AC bus) or DC-coupled (connecting to DC bus or hybrid inverter). Include battery isolator and BMS symbol.
6. Label all wire sizes, voltages, and currents. Every cable segment needs a label. Every major node needs a voltage label. Protection devices need rated current and type.
7. Add earthing and bonding connections. Show the PV array frame earthing conductor, the inverter chassis earth, and how these connect to the building's main earthing terminal. Earth conductors are typically shown as dashed lines.
8. Complete the title block. Bottom-right corner: project name, installation address, drawing date, engineer name and qualification, revision number (start at Rev. A), and the applicable standard (IEC 60364-7-712 and relevant national standard).

Solar Electrical Design Software

Drawing SLDs manually is time-consuming and error-prone. A missed label or an incorrect cable cross-section means a re-submission. Good solar design software eliminates this by generating the SLD automatically from the system design inputs.

When evaluating tools, look for these specific capabilities:

• Automatic string voltage checking: The software should flag strings that exceed inverter Vmax under minimum temperature conditions — not just at STC.
• Cable sizing integration: The tool should calculate required cross-sections based on current, length, and derating factors, and update the SLD labels automatically.
• IEC 60364-7-712 compliance checks: Built-in rules that verify protection device selection, RCD type, and SPD requirements against the standard.
• SLD export: PDF and DWG export for submission to utilities and building authorities. The SLD should include a title block populated from the project data.
• Component library: Up-to-date inverter and module databases so Voc, Isc, and temperature coefficients are correct without manual data entry.

SurgePV's solar design software auto-generates compliant SLDs from your panel layout. String sizing calculations use real temperature data for the project location — so the maximum string length for a site in Munich is different from one in Seville, and the tool handles that automatically. When you update the design, the SLD updates too. There's no separate drawing step.

For yield modeling that integrates with the electrical design, the generation and financial tool uses the same system parameters — cable losses, inverter efficiency, and string configuration — to produce accurate energy yield estimates alongside the SLD.

Common SLD Errors That Fail Inspections

These are the mistakes that appear most often in rejected SLD submissions. Each one either creates a safety hazard or demonstrates that the designer did not account for real operating conditions.

1. String voltage exceeding inverter Vmax. The most common error. Designers calculate string voltage at STC (25°C) but don't apply the low-temperature correction. In central European winters, a string that looks fine at STC may exceed the inverter's 800V limit by 50–80V. The fix: always calculate maximum string voltage at the site's minimum ambient temperature.
2. Wrong RCD type. Using a Type A or Type AC RCD with a transformerless inverter is a code violation under IEC 60364-7-712. Type A RCDs cannot detect the smooth DC fault currents that these inverters can produce. The fix is a direct swap — Type B RCDs are available at the same current ratings as Type A.
3. Missing DC isolator. Some designs omit the lockable DC isolator between array and inverter, relying solely on the inverter's internal switch. National standards require an accessible, lockable external isolator for maintenance isolation. The inverter's internal isolator does not satisfy this requirement in Germany, Italy, or France.
4. Cable sizes not shown. An SLD without cable cross-sections on every segment is incomplete. Inspectors cannot verify cable ampacity without knowing the size. Every cable segment — DC string, DC main feeder, AC output, and earthing conductor — must be labeled with its cross-section in mm².
5. No SPD on DC side in areas with lightning risk. SPDs are required by VDE-AR-N 4100 in Germany and CEI 0-21 in Italy regardless of whether a formal LPS is installed. Omitting the SPD from the SLD (even if it is physically installed) means it doesn't appear in the compliance documentation.
6. Earthing path not shown. The SLD must show how the PV array frame, inverter chassis, and all metallic enclosures connect to the building's main earth. Missing earthing connections are flagged in both the electrical inspection and the grid connection review.

Frequently Asked Questions

What is a solar single line diagram?

A solar single line diagram (SLD) is a simplified electrical schematic showing the major components of a PV system and the connections between them, using single lines to represent multi-conductor cables. It is required for building permits, grid connection applications, and certifications such as MCS (UK) and MaStR registration (Germany).

What must a solar SLD include?

A compliant solar SLD must show: PV modules (with Voc and Isc), string configuration and combiner box, DC surge protection device (SPD), DC isolator, fuses or breakers, inverter (DC and AC ports), AC isolator, Type B RCD, MCB, bidirectional kWh meter, grid connection point, cable cross-sections on every segment, and a title block with project address, engineer name, and revision date.

How do I size solar strings?

String sizing uses the inverter's MPPT voltage window and the module's Voc temperature coefficient. Calculate the maximum modules per string using the formula: floor(Vmax_inverter / (Voc × (1 + (Tmin − 25) × coeff))). Always use the site's minimum ambient temperature — not 25°C — for the maximum string calculation. For Germany, use -15°C; for Spain, 0°C is a common minimum.

What cable size is needed for solar DC wiring?

DC string cables must be PV-rated (PV1-F or H1Z2Z2-K). Size based on maximum current (1.25 × Isc × parallel strings) and cable length, keeping voltage drop below 1%. As a starting guide: 4 mm² for up to 30A over 10m, 6 mm² for up to 40A over 15m, 10 mm² for up to 50A over 25m. Apply derating factors for bundled cables or high-ambient-temperature installations.

Is a Type B RCD required for solar?

Yes, when using a transformerless inverter — which covers virtually all modern string inverters. Transformerless inverters can produce smooth DC fault currents that Type A RCDs cannot detect. IEC 60364-7-712 mandates Type B RCDs, and this is enforced by national standards in Germany, Italy, France, and the UK. Using a Type A RCD with a transformerless inverter is a code violation.

About the Contributors

Author

Rainer Neumann

Content Head · SurgePV

Rainer Neumann is Content Head at SurgePV and a solar PV engineer with 10+ years of experience designing commercial and utility-scale systems across Europe and MENA. He has delivered 500+ installations, tested 15+ solar design software platforms firsthand, and specialises in shading analysis, string sizing, and international electrical code compliance.

View all posts