AEC definitions
A one-line diagram, also called a single-line diagram, shows an electrical power system as one line from the utility service through switchgear, transformers, and feeders to the panels. The single line stands for all phases. It is a schematic, not a conduit route.
Definition
Electricians and engineers use the one-line to see how power is distributed and how it is protected: breaker sizes, bus ratings, transformer kVA, and feeder designations. It does not show where conduit runs in the slab. The site plan, the power plans, and the panel schedules do the locating and the circuiting. Names on the one-line have to match those schedules exactly, or nobody can tell which panel is fed from which breaker.
In depth
A three-phase feeder is drawn as a single stroke, with the number of wires and the grounding conductor noted beside it. Breaking that convention and drawing each phase would make a wiring diagram, which is a different sheet used for controls and special systems.
Read the legend. A hash mark, a dot, or a note tells you wire count. Assuming three wires because the building is three-phase is how a neutral gets missed.
It establishes the path of power and the ratings of the equipment in that path. Arc-flash and coordination studies are built from it. Changing a breaker on the one-line without changing the panel schedule, the gear submittal, and the study produces three different buildings.
Utility requirements often govern the service side. The diagram should show the point of service and what the utility provides versus what the contract provides. NIC notes belong here as much as on a plan.
Panel LP-1, switchboard MSB, and transformer T-1 must be the same strings on the one-line, the riser, the schedules, and the labels the manufacturer engraves. Informal names in a meeting do not survive the shop drawing.
A reviewer should be able to pick a panel on a floor plan and walk backward to the breaker on the one-line without inventing a feeder. If that walk fails, the documents are not coordinated.
The one-line has no scale and no path through the building. Home runs on the lighting and power plans, and the specifications for conductors and raceways, are what the estimator measures. Using the one-line as a takeoff for conduit length will be wrong.
Large feeders may be shown schematically in a shaft on the plans. The one-line still governs the breaker they land on.
Adding a mechanical unit that needs a new feeder is a one-line revision, a panel or gear revision, and a mechanical coordination item. Cloud the diagram. A note in the mechanical schedule alone does not feed the unit.
Studies that were sealed against the old diagram are stale when the diagram changes. Say so in the revision, or the permit set and the study will disagree.
Examples
The one-line shows a 2500 A switchboard feeding distribution panel DP-1 through a 600 A breaker. The DP-1 schedule names that feeder as its source.
A 75 kVA transformer on the one-line steps 480 V down to 208 V for lighting panels. The panel schedule voltage matches.
An added rooftop unit is clouded on the one-line with a new 100 A feeder. The mechanical schedule and the power plan are revised in the same bulletin.
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