What it means
Also called a one-line diagram, it shows the structure of an electrical system at a glance: the utility service, transformers, switchgear, switchboards, panels, generators and transfer switches, and how they connect. A single line represents a circuit regardless of how many conductors it actually has, and standard symbols represent the equipment. Key ratings — such as voltages, transformer sizes and breaker ratings — are usually shown too.
Single-line diagrams are used to design systems, plan maintenance and shutdowns, and carry out system studies such as short-circuit, coordination and arc flash studies. Maintenance standards such as NFPA 70B expect them to be accurate and kept up to date.
An example: before a planned shutdown, a contractor uses the building’s single-line diagram to see which panels are fed from the switchboard section being worked on — and therefore which areas of the building will lose power.
Why it matters
- Safety and planning. It shows what’s upstream and downstream of any piece of equipment.
- Studies and maintenance. Arc flash and other system studies are only as good as the diagram they’re based on.
- Records drift. Changes to a building often aren’t reflected in its drawings, so updating them is common maintenance work.