SmartSLDHow to Draw a Single-Line Diagram (Complete Guide)
A single-line diagram (SLD), also called a one-line diagram, is the most common way to describe an electrical power system on paper. Every transformer, breaker, bus, and feeder is shown as a single line — three-phase, two-phase, and single-phase are all drawn the same way unless explicitly noted. This makes the diagram fit on one page and stay readable at a glance.
This guide walks through what an SLD is, when you need one, the components you'll draw, and a step-by-step process for putting one together. At the end you'll find templates and a free browser editor you can use to draw your own.
What is a single-line diagram?
An SLD shows the topology of a power system: how power flows from source to load, the protective devices in the path, and the major equipment along the way. It's not a wiring diagram — it doesn't show every conductor, terminal, or pin. It's a schematic, simplified for human reading.
A typical SLD has these elements stacked vertically:
- Source — utility tie, generator, transformer secondary, or another upstream supply
- Service disconnect — main breaker, fused disconnect, or transfer switch
- Metering — utility meter, sub-meter, CT/PT instrumentation
- Bus / switchgear — the common conductor that distributes power
- Feeders / branch circuits — each with its own breaker, going to a load
- Loads — motors, panels, transformers, EV chargers, BESS, etc.
When do you need a single-line diagram?
You'll need an SLD in several situations:
- Electrical permit applications. Most jurisdictions require an SLD for any service upgrade, sub-panel addition, or substantial alteration.
- Utility interconnection. Connecting a solar array, battery system, or generator to the grid requires an interconnection SLD that the utility reviews.
- Bid and design submissions. Electrical contractors include SLDs in proposals to show their approach.
- Arc-flash and short-circuit studies. Any IEEE 1584 / NFPA 70E study starts from an SLD describing the power system.
- As-built documentation. After install, an updated SLD records what was actually built so the next person who touches the system can understand it.
Components you'll draw on a typical SLD
The symbols you place will depend on which standard you're following. North American work typically uses ANSI/IEEE symbols; European, Australian, and Asian work usually follows IEC 60617. The two are mostly interchangeable for common devices but differ on transformers, motors, and protective relays. See our SLD symbols cheat sheet for a side-by-side reference.
The most common symbols across both standards:
- Utility source — typically a circle with the system voltage labeled
- Disconnect / isolator — a knife switch or break in the line
- Circuit breaker — drawn as a switch with an arc or "X" through it
- Fuse — a small rectangle or zigzag interrupting the line
- Transformer — two interlocking circles (IEC) or two coils (ANSI)
- Bus — a thick horizontal or vertical line with feeders branching off
- Motor — a circle with "M" inside
- Battery / BESS — alternating long/short lines or a labeled box
- PV array — a square with diagonal lines or a labeled box "PV"
- Load — a labeled box or an arrow indicating power flow direction
Step-by-step: drawing your first SLD
1. Start with the source
Place the upstream supply at the top. For a residential service entrance, that's the utility connection — a circle labeled with the service voltage (e.g. "240V/120V split-phase") and amperage. For an industrial site, it might be a 13.8 kV bus from a utility substation.
If your system has multiple sources (utility + generator, utility + solar with backfeed), draw each at the top and trace down to where they tie together.
2. Add the main disconnect
Below the source, draw the main service disconnect — typically a main breaker for residential, or a fused disconnect / draw-out breaker for commercial. Label its rating in amps, and the AIC (interrupt capacity) if you're doing a fault study.
3. Add metering
If a utility meter or sub-metering is in the path, place it between the disconnect and the main bus. Show the CTs and PTs if they're separate from the meter itself. For most residential SLDs, a simple "M" in a circle is enough.
4. Draw the main bus
The bus is the horizontal line that distributes power to your branch circuits. Label the bus with its voltage and amperage rating (e.g. "200A · 120/240V"). Each branch feeder will tap off this bus.
5. Draw each feeder
For each circuit leaving the panel, draw a vertical line down from the bus. At the top of the line, place the branch breaker (label with amp rating). At the bottom, place the load (panel, motor, EV charger, sub-feed, etc.).
Group similar feeders. If you have six 20 A general-purpose breakers feeding lighting and outlets, you can show one representative branch and label it "qty 6" rather than drawing all six.
6. Add upstream protection details
For permit and engineering work, include device ratings: breaker frame size and trip setting, fuse rating and class, transformer kVA and impedance. The reviewer or anyone doing a short-circuit study will need these.
7. Label everything
Every component should have a unique ID (e.g. QF1, T1, M1) and a descriptive name. Use a consistent labeling convention — IEC standard prefixes (Q for switching, F for fuses, T for transformers, M for motors) are widely recognized.
Common mistakes
- Mixing standards. Pick IEC or ANSI for your project and stick with it. Reviewers spot mixed symbols immediately.
- Forgetting bus rating. Every bus needs an explicit amperage label. "200A" or "1200A" — don't make the reader guess.
- Drawing wires instead of conductors. An SLD line represents a complete circuit, not a single wire. Don't draw three lines for a three-phase feeder.
- Skipping the disconnect. Every load that needs to be locked out for service should show its disconnect on the SLD.
- No legend / title block. Include a title block with project name, drawing number, revision, and date. A legend explaining your symbol set is also helpful.
Tools you can use
You have several options, ranging from free generic diagram tools to dedicated electrical CAD software:
- AutoCAD Electrical — the industry standard. Comprehensive, but expensive (subscription > $1,500/year) and has a steep learning curve.
- draw.io / Lucidchart — free generic diagram tools. Easy to use but lack a real electrical component library and electrical-aware routing.
- SmartSLD — our free browser-based editor with a real electrical component library, automatic wire routing, and an AI agent that can build diagrams from a description or a photo. See our honest comparison of AutoCAD Electrical alternatives for context.
Templates to start from
If you want to skip the blank page, browse our worked examples:
Frequently asked questions
Is a one-line diagram the same as a single-line diagram?
Yes. "One-line" and "single-line" are used interchangeably. The IEEE prefers "one-line"; in practice both terms refer to the same drawing.
Do I need an SLD for residential work?
For most simple repairs or single-circuit additions, no. For a service upgrade, new sub-panel, solar interconnection, EV charger circuit, or generator transfer switch, your AHJ will almost always ask for one.
What scale should I use?
SLDs are not drawn to physical scale. Component spacing should be roughly proportional to readability, not distance in the field.
Can AI draw a single-line diagram for me?
Yes — SmartSLD includes an AI agent that can generate a complete SLD from a one-sentence description (e.g. "200 A residential service with a 12-circuit panel and a generator transfer switch"). Our guide to AI for electrical design covers how it works and where it falls short.
Drew this guide because we kept seeing the same questions in r/electricians. If anything's unclear or missing, email [email protected] and we'll update it.