What Is a Single Line Diagram?

A 480V switchgear lineup with six breakers, three transformers, a generator, and 200 meters of cable. Try explaining that system in a paragraph. Now try it with one drawing. That's what a single line diagram does.

A single line diagram (SLD) — also called a one-line diagram — represents an entire electrical power system using a single line per circuit instead of drawing every conductor. Three-phase feeders, neutral conductors, and grounding paths all collapse into one line with standardized symbols. The result is a readable map of your system's power flow, from the utility service entrance down to individual loads.

Why It's Called "Single Line"

A three-phase, four-wire feeder has four physical conductors. Drawing all four for every circuit in a facility creates an unreadable mess. The SLD convention replaces those parallel conductors with one line and relies on notation to convey the details — voltage level, number of phases, conductor size, and conduit type.

This isn't a wiring diagram. It doesn't show physical routing or terminal connections. It shows the electrical architecture: what connects to what, at what voltage, through what protective devices.

What an SLD Shows

A typical SLD includes:

• Utility service entrance (voltage, available fault current)
• Main switchgear and distribution switchboards
• Transformers (kVA rating, impedance, voltage ratio)
• Generators and automatic transfer switches
• Buses and bus ties
• Circuit breakers and fuses (trip ratings, interrupting capacity)
• Cables and feeders (size, length, type)
• Motors, panels, and other loads
• Grounding scheme

Each component gets a standardized symbol defined by IEC 60617 (the actively maintained international standard for graphical symbols). Breakers look like small boxes on the line, transformers are paired circles, buses are thick horizontal bars, and so on.

The best SLDs also annotate each element with its key ratings — a transformer marked "1500 kVA, 13.8kV-480/277V, 5.75% Z" tells you everything you need for a quick engineering assessment.

When You Need One

You need an SLD for essentially any non-trivial electrical system. Specifically:

• Engineering studies. Short circuit analysis, arc flash studies, coordination studies, and load flow analysis all start from the SLD. It defines the system model. Without one, you're building a model from scratch before any analysis can begin.
• Construction and commissioning. Electricians and commissioning agents reference the SLD to understand system topology before they open a single panel.
• Operations and maintenance. Lockout/tagout procedures rely on the SLD to identify upstream protective devices and isolation points. NFPA 70E Section 205.3 requires that single-line diagrams be maintained where provided, and Section 120.5(A)(1) references them as the primary means for identifying energy sources.
• Code compliance. NEC 408.4(A) requires switchboards and panelboards to have a circuit directory. The SLD serves as the system-level equivalent.
• Insurance and facility audits. Many insurance carriers and AHJs (authorities having jurisdiction) require a current SLD as part of facility documentation.

In practice, any facility with multiple voltage levels, medium-voltage equipment, or complex distribution benefits from having an SLD. Many AHJs, insurers, and engineering standards effectively require one for these systems, even where no single code section mandates it outright.

What Makes a Good SLD

A good SLD balances completeness with readability. Some guidelines:

Include the right level of detail

Show every protective device, every transformer, and every bus. Annotate voltage levels at each bus. Include equipment ratings that matter for engineering analysis — breaker trip settings, transformer impedances, cable lengths and sizes.

Don't include wiring-level detail like terminal numbers, conduit routing, or junction boxes. That belongs on wiring diagrams and panel schedules.

Follow convention

Power flows top to bottom. The utility source sits at the top. Voltage decreases as you move down the page. Main switchgear comes first, then distribution, then branch circuits. Left-to-right layout within a voltage level is acceptable for wide systems.

Keep it current

An outdated SLD is worse than no SLD. If someone added a 500 kVA transformer and a new MCC last year but the SLD still shows the original design, every study based on that drawing is wrong. Every lockout/tagout procedure is suspect.

This is where traditional CAD-based SLDs fall short. Updating a Revit or AutoCAD drawing requires opening the file, finding the right sheet, making changes, and re-issuing. Many facilities let their SLDs drift out of date because the update process is too cumbersome.

SLD vs. Other Electrical Drawings

The SLD is one of several drawings in a complete electrical documentation set:

The SLD is the starting point. It defines the system. The other drawings provide implementation detail.

Building SLDs with Software

Traditional SLD creation means AutoCAD or Visio — draw symbols, connect lines, add text annotations manually. It works, but the drawing is static. Change a breaker rating and you have to find every reference and update it by hand.

Modern single line diagram software like ekx takes a different approach. The SLD is a live model of your electrical system, not a static drawing. Each component has real electrical properties — impedance, ratings, connections — that feed directly into short circuit calculations, voltage drop analysis, and auto equipment sizing. Update a transformer rating on the diagram and the calculations update automatically.

This model-based approach eliminates the gap between "the drawing" and "the analysis." They're the same thing.

Key Takeaways

• A single line diagram represents an entire electrical system using one line per circuit with standardized symbols
• SLDs are required for engineering studies (short circuit, arc flash, coordination), safe maintenance, and code compliance
• A good SLD shows equipment ratings, voltage levels, and protective devices — but not wiring-level detail
• Power flows top to bottom, utility at the top, with decreasing voltage levels as you move down
• Keep your SLD current — an outdated diagram undermines every study and safety procedure that depends on it