Single Line Diagram
A one-page map of how power flows through a building's electrical system.
Quick Answer
A single line diagram, also called a one-line diagram, is a simplified drawing that represents a three-phase electrical system with a single line per connection. It shows the major components, such as the utility service, switchgear, transformers, generators, breakers, and panelboards, and how power flows between them, along with ratings and sizes.
The Full Picture
Electrical systems are three-phase, which would make a full wiring drawing cluttered. A single line diagram simplifies that: one line stands in for all the phase conductors, and standard symbols represent equipment. The result is an overview of the power system on a sheet or two, which engineers, contractors, and inspectors all use.
A typical one-line starts at the utility service and flows downward or across through the main disconnect, switchboard or switchgear, feeders, transformers, distribution panels, and branch panels. Each element is labeled with ratings: voltage, ampacity, breaker or fuse sizes, transformer kVA, feeder conductor and conduit sizes, and often short-circuit current ratings. Generators and automatic transfer switches appear where emergency or standby power is provided.
The diagram is a design and coordination tool. Engineers use it to verify capacity, protection, and fault current studies. Contractors use it to build the system and to see what the gear submittals need to match. Owners and facility staff rely on it later for safe operation and maintenance, and utilities often require one for service approvals.
Because it shows ratings in one place, the one-line is where mismatches show up. If a panel schedule, equipment schedule, and one-line disagree about a breaker or feeder size, someone has to resolve it through an RFI. Cross-checking those drawings is a common part of preconstruction document review.
Real Examples
Common Misconceptions
People assume: A one-line shows the actual wiring layout.
Actually: It is a schematic of how components connect and their ratings. It does not show physical locations, routing, or every conductor, which appear on plans and riser details.
People assume: One-line diagrams are only for large industrial plants.
Actually: Nearly every commercial project has one, and utilities, inspectors, and arc-flash studies often depend on it, even for modest buildings.
Frequently Asked Questions
What does a single line diagram show?
It shows major electrical equipment and how it is connected, from the utility service through switchgear, transformers, and panels, with ratings and feeder sizes.
Why is it called a single line diagram?
Because one line represents all the conductors of a three-phase circuit, which keeps the drawing simple and readable.
Who prepares the one-line diagram?
The electrical engineer of record prepares the design one-line. Contractors and manufacturers may produce updated or as-built versions.
How is a one-line different from a panel schedule?
The one-line shows the system's overall structure. A panel schedule lists the circuits, breakers, and loads inside one panelboard.