A single-line diagram (also called a one-line diagram) is a simplified drawing of a power distribution system in which a single line represents all three phases of each circuit. It shows how power flows from the source — the utility service, a generator, or both — through transformers, breakers, disconnects, and buses, down to the equipment that uses it. It is the map of the electrical system: not every wire, but every path. If you can read one, you can answer the two questions that matter most before touching any electrical equipment — where does this get its power, and what turns it off?
A full wiring diagram of even a modest facility would be unreadable: three phase conductors, a neutral, grounding conductors, and control wiring for every circuit, times hundreds of circuits. The single-line diagram exists to throw that detail away on purpose. It keeps exactly one thing: the topology of the power system — what feeds what, through which devices, in what order.
On a well-drawn one-line you can see, on a single page, the utility service entering the building, the transformers that step voltage down, the main breaker, the buses that distribute power, the feeder breakers that protect each branch, and the major loads at the bottom — motors, panels, equipment. Alongside the symbols, the drawing carries the data that describes each device: voltage levels, transformer sizes and connections, breaker ratings and settings, conductor sizes, and equipment names that match the labels on the gear itself.
That last part is what makes the drawing useful in the field: the box labeled “MDP” on the drawing is the switchboard labeled “MDP” on the wall. When the names match, anyone can move between the drawing and the equipment without guessing.
Most commercial and industrial power is three-phase: three conductors carrying alternating current offset in time from one another. In a balanced system, each phase sees the same equipment — the same breaker, the same transformer winding, the same cable run — and carries roughly the same current, so drawing all three phases would triple the ink without adding information.
So the convention collapses them: one line stands for the whole circuit, however many conductors it physically contains. Where the number of phases or conductors matters, the drawing says so with notation — short slash marks across the line, or a label like “3Φ, 4W” (three-phase, four-wire). A single-phase branch, a neutral, or a grounding connection is called out where it is relevant rather than drawn everywhere.
Every line on a one-line is a real set of conductors, and every symbol is a real device in a real enclosure somewhere in the facility. Keep that translation running — this line is a conduit, this square is a breaker I could put my hands on — and the drawing stops being an abstract chart.
Symbol styles vary a little between drawing packages and eras, but the common North American conventions are consistent enough that a handful of shapes covers most of what you will see. Here are the ones that do most of the work, with the standard rendering and what each device actually does.
A circle (often with a wave inside) at the top of the drawing: the incoming service from the power company. Everything below it depends on it.
Two overlapping or stacked circles, one per winding. Changes voltage between the line entering and the line leaving — the drawing usually notes both voltages and the size.
A square or rectangle in the line (larger drawout power breakers are often drawn as an open box). Opens automatically on overload or fault, and can be opened by hand.
A hinged blade drawn swung open off the line. A manual isolation point — no automatic protection by itself, but a place where the circuit can be visibly opened.
A narrow rectangle in the line. Melts open on excess current and must be replaced afterward. Often drawn paired with a disconnect as a fused switch.
A heavy horizontal line. The common conductor bar inside switchgear or a panelboard where one supply splits into many feeders. Everything hanging from a bus shares its source.
A circle labeled for what it measures (kWh, A, V). Utility revenue meters sit near the service entrance; instrument symbols appear wherever the system is monitored.
A circle with an M, usually at the bottom of a branch — a load, the end of a power path. The drawing typically notes horsepower or current rating beside it.
A circle with a G — a source, like the utility symbol but on site. Standby generators usually connect through a transfer switch that selects between utility and generator power.
Beyond these, you will meet variations: a transfer switch (two sources feeding one selectable output), current and potential transformers feeding meters and relays, capacitor banks, surge arresters, and grounding symbols. All of them follow the same logic — a shape on the line, a label beside it, a real device behind it.
One-lines are conventionally drawn with the source at the top and loads at the bottom, so power “flows down the page.” Here is a deliberately simple facility:
A minimal one-line: utility service through a transformer and main breaker to a distribution bus, feeding a motor, a lighting panel, and a rooftop unit through its local fused disconnect.
Read it the way power flows. The utility feeds transformer T1, which steps the voltage down. The main breaker is the first protective device on the secondary side and the switch that kills the entire bus below it. Three feeder breakers hang from the bus, each protecting its own branch: one ends at a motor, one at a lighting panel (detailed on its own panel schedule elsewhere), and one passes through a local disconnect and fuse before reaching a rooftop unit.
Now read it the other way. Stand at the rooftop unit and walk up the page: its fused disconnect, its feeder breaker at the bus, the main breaker, the transformer, the utility. That upward walk lists every point where this equipment can be isolated — the single most valuable read a one-line offers.
When something fails — a shorted motor winding, a damaged cable — fault current pours from the source toward the failure through every device in between. On the one-line, that path is simply the line from the source down to the fault. In a well-coordinated system the protective device closest to the fault opens first, so the smallest possible part of the system goes dark.
This gives you two practical reads:
For working electricians and safety professionals, the one-line earns its keep in a handful of recurring jobs:
An accurate one-line is the foundation of planning safe electrical work. Every step of a sound isolation sequence — identify all sources, interrupt the load, open the disconnecting means, verify absence of voltage — leans on knowing the system's actual topology, and the one-line is where that knowledge lives. The most dangerous electrical assumptions are topological ones: that a panel has one feed when it has two, that opening a disconnect killed the whole enclosure when a separately-fed control circuit is still live, that the breaker labeled for a load actually feeds it.
The drawing is also the shared reference that lets a job briefing work. When the crew, the supervisor, and the facility owner are all pointing at the same current drawing, “we are locking out here, which kills this” is a verifiable statement instead of tribal knowledge.
A one-line is a snapshot, and facilities change: feeders are added, transformers replaced, breaker settings adjusted, solar and battery systems interconnected. Each change that is not drawn turns the diagram from an asset into a hazard, because people extend trust to a document that no longer describes reality.
Good practice is straightforward:
Nothing — they are two names for the same drawing, used interchangeably across the industry. Both describe a simplified power-system drawing in which one line represents all three phases of a circuit.
The one-line shows the power path with one line per circuit, so you can see the whole system at once. A schematic or wiring diagram shows every conductor and control connection inside a specific circuit, wire by wire. You use the one-line to understand the system and plan isolation; you use the schematic to troubleshoot inside the equipment.
Engineers, for design and power-system studies. Electricians and maintenance technicians, for finding upstream disconnects and planning de-energization and lockout. Safety professionals, for reviewing work plans and briefings. Utilities, inspectors, and emergency responders may also use them to understand how a facility is fed.
Whenever the system changes, as part of closing out the change — plus a periodic field verification even without known changes, because undocumented modifications accumulate. A one-line that no longer matches the installed system is worse than none, because people trust it.
Because in a balanced three-phase system all three phases pass through the same equipment, drawing them separately adds clutter without information. The convention collapses them to one line, with slash marks or labels indicating phase and conductor count where it matters.
This guide is written and reviewed by a Certified Safety Professional (CSP) with 35+ years teaching electrical safety nationwide to electricians, EHS teams, and Fortune 500 operators.
Reading the one-line is step one; working safely on what it describes is a trained skill. Live, instructor-led electrical safety classes run every week. See the schedule.