A single-line diagram is the map of a building's electrical system. It shows every major piece of equipment from the utility service down to the panels, how each is fed and what it is rated for, drawn with one line standing in for the several conductors of each circuit. If you own or manage a building of any size, it is the most useful electrical document you can have, as long as it matches what is actually installed.
Electricians, engineers, utility staff and emergency crews all use the single-line diagram, often abbreviated SLD or called a one-line. It answers the first questions in almost every electrical task: where does this panel get its power, what else is fed from the same place, and what do I have to switch off to work on it safely?
Why one line?
Most building circuits are three-phase, with three phase conductors, often a neutral and a bonding conductor. Drawing all of them for every circuit would turn a building's system into an unreadable tangle. The single-line diagram draws each circuit as one line and puts the details, such as the number and size of conductors, in a note beside it. The result is a diagram you can read top to bottom like a family tree, from the source to the smallest panel on it.
Reading the symbols
Symbols vary a little between designers and drawing standards, and every good drawing set has a legend. The common ones are simple. Start at the source at the top of the sheet and follow the lines down.
| Symbol | What it means | What to look for beside it |
|---|---|---|
| Two overlapping circles | Transformer | kVA rating, primary and secondary voltage, impedance |
| Small square or curved switch | Breaker or switch | Frame and trip rating in amperes, interrupting rating |
| Fuse symbol on the line | Fused switch | Fuse rating and class |
| Thick horizontal bar | Bus in a switchboard or panel | Bus rating, voltage, short-circuit rating |
| Circle with M or a meter label | Revenue or sub-meter | Who owns it, what it measures |
| Circle with G | Generator | kW rating, voltage, fuel |
| Box labelled ATS | Automatic transfer switch | Ampere rating, which loads it serves |
What a good single-line diagram tells you
- Sources. The utility service, generators, solar PV and any other sources that can energize the system.
- Equipment names. Each switchboard, panel, transformer and motor control centre with a unique name that matches the label on the equipment.
- Ratings. Voltage, ampere rating and short-circuit or interrupting rating for each piece of equipment.
- Protective devices. The breaker or fuse protecting each feeder, its rating and, for adjustable breakers, a reference to the settings.
- Conductors. The size, number and type of conductors in each feeder, and often its approximate length.
- Location. The room or floor where each piece of equipment sits.
- Grounding. Where the system is grounded and bonded at the service and at separately derived systems such as transformers.
Equipment names that make sense
The names on the diagram are what everyone uses on site, so they need to be consistent and printed on the equipment itself. Many buildings use a short code that says what the equipment is and where it is, for example a letter for the type (main distribution, panel, transformer, motor control centre), the floor or area, and a number. A panel name that also hints at its voltage or whether it is on emergency power helps a worker see at a glance what they are dealing with.
Whatever system you choose, apply it everywhere: the diagram, the panel schedules, the equipment labels, the arc flash labels and the maintenance software. When a renovation adds a panel, give it a name in the same pattern and add it to every one of those records at the same time.
Why keeping it current matters
A single-line diagram that is out of date is worse than an inconvenience. Lockout procedures rely on knowing every source that feeds a piece of equipment. Short-circuit and arc flash studies are built on it, so a wrong diagram gives wrong labels. Emergency responders and utility crews may use it to isolate power during a fire or flood. And every new project starts by asking how much capacity is left, which nobody can answer from a diagram that does not match the building.
CSA Z462 expects employers to keep diagrams accurate where they are used as part of the electrical safety program. In practice, the diagram should be updated every time equipment is added, removed or replaced, and checked against the building during periodic maintenance.
Signs your diagram has drifted
- Panel names on the diagram do not match the labels on the equipment.
- Panels exist in the building that do not appear on the diagram at all.
- The diagram is dated before the last major renovation or tenant fit-out.
- A generator, solar system or EV charging panel has been added but is not shown.
- Breaker sizes on the diagram do not match the breakers in the switchboard.
Bringing an old diagram up to date
Take a typical case: a property manager takes over a 1990s commercial building with a diagram from the original construction. Since then there have been several tenant renovations, a new rooftop unit and a set of EV chargers. The update starts at the service and works downstream. A qualified person records the nameplate data of each switchboard, panel and transformer, notes the breaker sizes, and traces where each feeder goes. Equipment that is not on the diagram is added; equipment that has been removed is deleted. Where a feeder cannot be traced without opening energized equipment, the work is planned with the electrical contractor, sometimes during a short shutdown.
The result is a diagram that matches the labels on the equipment, with spare breakers and capacity identified. It becomes the base for any short-circuit or arc flash study and the first document handed to the next designer or contractor.
Do not trace circuits in live equipment yourself. Opening energized panels and switchboards exposes people to shock and arc flash hazards. Field verification is done by qualified electrical workers following your electrical safety procedures.
How ehZee can help
We prepare single-line diagrams for new systems and bring existing ones up to date through a site survey, as part of our power distribution design work. An accurate diagram is also the starting point for short-circuit and coordination studies. For the bigger picture, see how power gets from the street to the outlet.
Common questions
Is a single-line diagram the same as a riser diagram?
Not quite. A riser diagram shows equipment by floor and how feeders run vertically through the building. A single-line diagram focuses on the electrical connections and ratings. Many drawing sets include both.
Where should the diagram be kept?
A current copy is commonly posted in or near the main electrical room, with the master kept with your building records in a format that can be edited. Everyone who updates the system should know where the master is.
Do houses have single-line diagrams?
Rarely, because a house usually has one service and one panel. They become useful as soon as a building has more than a few panels, transformers or sources.