What Is an Automatic Transfer Switch?
An Automatic Transfer Switch is a power-transfer device that moves selected electrical loads between a utility source and a standby generator. The switch monitors the normal source, signals the generator when needed, and connects the generator after stable power is available. It also returns the load to utility power when normal service is restored.
Quick Facts About Automatic Transfer Switch
Category
Generator transfer equipment
Used for
Moving selected loads to standby power
Common confusion
Not the same as a generator or interlock
Also called
ATS
Often discussed with
Generator Installation
Key Takeaways About Automatic Transfer Switch
- An Automatic Transfer Switch manages source changes automatically.
- The switch serves only circuits connected to its protected load side.
- A generator and an Automatic Transfer Switch perform different jobs.
How Automatic Transfer Switch Works
An Automatic Transfer Switch monitors incoming utility voltage through control circuitry. If the normal source fails, the switch sends a start signal to the generator. After generator power reaches an acceptable condition, the switch opens the utility connection and closes the generator connection for the circuits assigned to the standby system.
Related glossary terms: Generator Inlet, Generator Interlock, Load Shedding.
When utility power returns, the switch transfers the selected loads back to the normal source and signals the generator to stop after its shutdown sequence. The equipment must match the generator, service arrangement, neutral configuration, and connected load. A Generator Interlock uses a different operating method because a person moves the breaker position manually. An Automatic Transfer Switch supports automatic operation for loads wired through its transfer equipment.
From the GMB Electric team
When we install or inspect an Automatic Transfer Switch, we trace the utility and generator conductors, verify the protected load connections, and review the control wiring. We also test source transfer and return operation so the switch responds as the system design requires.
Automatic Transfer Switch in Practice: A Real-World Example
A home with a standby generator may route selected heating, refrigeration, sump pump, and lighting circuits through an Automatic Transfer Switch. During a utility outage, the switch starts the generator and transfers those connected circuits to generator power.
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Load Shedding
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Wiring Diagram
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