What is an Automatic Transfer Switch?
An automatic transfer switch (ATS) is the critical link between your electrical panel, the utility power grid, and your standby generator. It performs three essential functions: detecting when utility power fails, signaling the generator to start, and transferring your electrical load from utility to generator power — all automatically, without human intervention.
When utility power is restored, the ATS monitors the incoming power for stability (typically for a few minutes), then transfers back to utility power and signals the generator to shut down. This entire process happens automatically every time power is lost and restored.
Manual Transfer Switch vs Automatic Transfer Switch
A manual transfer switch (MTS) requires you to physically flip a switch or turn a handle to transfer from utility to generator power. This is appropriate for portable generators and situations where someone will always be present during an outage. Manual transfer switches cost $300–$800 and are simpler to install.
An automatic transfer switch (ATS) handles the entire process without intervention. This is required for standby generators and any application where power must be restored quickly and reliably regardless of whether anyone is present. ATS units cost $1,500–$12,000 depending on amperage and features.
Open Transition vs Closed Transition
Open transition (break-before-make): The ATS disconnects from one power source before connecting to the other. There is a brief interruption (typically 100–500 milliseconds) during the transfer. This is the standard type for most residential and commercial applications. Open transition is simpler, more reliable, and less expensive.
Closed transition (make-before-break): The ATS briefly parallels both power sources during transfer, resulting in zero interruption. This is used in critical applications like hospitals, data centers, and industrial processes where even a 100ms interruption is unacceptable. Closed transition ATS units are significantly more expensive and require utility company approval because they momentarily connect the generator to the utility grid.
How to Size Your Transfer Switch
The transfer switch must be rated for at least the amperage of your main electrical panel breaker. Common residential service entrance ratings:
- 100A service: 100A transfer switch — typically for smaller homes with gas heat and hot water
- 200A service: 200A transfer switch — the standard for most modern homes
- 400A service: 400A transfer switch — larger homes with multiple HVAC systems, pools, or workshops
Commercial and industrial applications may require 600A, 800A, or higher-rated transfer switches, often in three-phase configurations.
Single-Phase vs Three-Phase
Single-phase power (120V/240V) is standard for residential and small commercial applications. Most homes and small businesses have single-phase service.
Three-phase power (208V, 240V, 277V, or 480V) is used in medium to large commercial and industrial facilities. Three-phase motors are more efficient and powerful than single-phase equivalents. If your facility has three-phase service, you need a three-phase generator and transfer switch.
NEMA Enclosure Ratings
- NEMA 1 (indoor): Designed for indoor installation in clean, dry, controlled environments. Lower cost but no weather protection.
- NEMA 3R (outdoor): Weather-resistant enclosure suitable for outdoor installation. Protects against rain, sleet, and external ice formation. Required for any outdoor or semi-exposed installation.
Most residential ATS units are NEMA 3R since they're typically installed on an exterior wall near the electrical panel. Indoor installations in utility rooms can use NEMA 1 enclosures.
Load Shedding and Load Management
Load shedding allows a smaller generator to serve a larger electrical panel by selectively disconnecting non-critical circuits during peak demand. For example, a 200A service entrance with a 22 kW generator might use load shedding to temporarily disconnect the electric dryer and water heater when the central AC compressor starts — then reconnect them once the compressor reaches running speed.
This is an effective way to avoid oversizing your generator while still maintaining whole-home coverage.
Compatible Brands at GeneratorProDirect
GeneratorProDirect carries transfer switches from multiple manufacturers:
- Cummins: OTEC series automatic transfer switches — 100A to 400A, single and three-phase. Known for reliability in commercial applications.
- Kohler: RXT and RDT series — 100A to 600A with advanced load management features. Integrates seamlessly with Kohler generators.
- Generac: RXSW and RTSW series — widely available, competitively priced, excellent residential options.
Browse Transfer Switches
View all 43 transfer switch options at GeneratorProDirect.com — all shipping tax-free in 45+ states with free freight over $499. Browse Transfer Switches
