The Short Answer
A one-line diagram (also called a single-line diagram) is a schematic that represents a building's electrical power distribution using a single line to show the flow of power from the utility service through the main equipment to panels and major loads. It documents service size, overcurrent devices, conductors, grounding, and equipment ratings in one clear drawing. The one-line is a core sheet in any commercial electrical set and is typically required for permit.
What a One-Line Diagram Shows
The one-line traces power from its source to distribution. Starting at the utility service or transformer, it shows the main service equipment, main breaker or fused disconnect, metering, and then the feeders running to distribution panels, switchboards, and major equipment. For each device it notes the rating — service ampacity, breaker sizes, panel ratings, and conductor sizes. Grounding and bonding are also represented. It's called a "one-line" because a single line represents what may physically be three-phase conductors plus neutral and ground.
Why It's a Single Line
Drawing every conductor of a three-phase system would clutter the diagram and obscure the overall distribution scheme. The single-line convention lets engineers, contractors, and inspectors quickly understand the power architecture — how the service is arranged, how it's protected, and how it feeds the building — without tracing individual wires. Detailed conductor and connection information lives in panel schedules and plans that accompany the one-line.
How It Supports the Rest of the Set
The one-line ties the electrical set together. It reflects the load calculation that sizes the service, connects to the panel schedules that detail each panel's circuits, and coordinates with the power plans that show physical routing. Overcurrent device coordination and equipment ratings shown on the one-line demonstrate a safe, code-compliant distribution system. Reviewers rely on it to confirm the design at a glance.
When Do You Need This?
A one-line diagram is required for essentially all commercial electrical permits involving new or modified distribution — new construction, service upgrades, tenant improvements adding panels or significant load, and generator or standby power installations. If your project changes the electrical distribution, expect the one-line to be part of the submitted, stamped set.
Frequently Asked Questions
Is a one-line the same as a riser diagram?
They're related but different. A one-line focuses on power distribution and device ratings; a riser diagram often emphasizes the vertical arrangement of equipment across floors. Some sets include both depending on project complexity.
Do small projects need a one-line?
Many do, even limited-scope projects that modify distribution. Whether it's required depends on the scope and the jurisdiction's submittal checklist, which we can review for you.
Who prepares and stamps the one-line?
A licensed electrical engineer prepares, reviews, and seals the one-line as part of the electrical design and accepts responsibility for it. We don't stamp work we haven't engineered or reviewed.