Generator + ATS backup power

Scenario

A facility engineer draws this one-line during the design review of a data-center UPS bypass or hospital emergency power system. The single-line diagram (SLD) is the first document a utility inspector or commissioning engineer asks for — it must show every source, switching device, bus, and load path in a single horizontal view without wiring details.

Annotation key

• utility — mains supply; drawn as the IEEE 315 utility symbol (three-line source)
• generator — diesel or gas genset; drawn as rotating-machine circle with winding symbol
• ats — Automatic Transfer Switch; drawn as the NEMA/IEEE transfer-switch symbol
• bus — horizontal bus bar; all connected devices share the same voltage rail
• breaker — molded-case or air circuit breaker; drawn as the IEEE 315 breaker symbol
• load — end-consumer device or feeder
• UTIL -> ATS1 — directed line representing the power path from source to device

How to read

Two sources (utility and generator) feed into the ATS. The ATS selects the live source and connects it to the 480 V critical load bus. From the bus, two independently-fused circuit breakers (CB1, CB2) feed their respective critical loads. If utility power fails, the ATS senses the loss, the generator starts, and the ATS transfers within seconds — all without interrupting the bus downstream.