Automatic Transfer Switch (ATS) Sizing & Selection Guide
A generator is useless without a mechanism to connect it to your building. The Automatic Transfer Switch (ATS) is the brain of your emergency power system. It constantly monitors the utility grid; when voltage drops, it signals the generator to start, mechanically disconnects your facility from the dead utility grid, and connects it to the generator. Selecting the wrong ATS can lead to electrocution of utility workers or catastrophic arc flashes.
Sizing the ATS (Amperage and Voltage)
An ATS is sized based on the maximum amperage it will carry, not the kW of the generator. The ATS must be rated for the full continuous current of the electrical panel it is feeding.
If the ATS is installed immediately after the utility meter to back up the entire facility, it must be sized to match the main utility breaker (e.g., 800 Amps, 1600 Amps). If it is only backing up a specific subpanel (like an IT server room), it is sized to match that specific subpanel's breaker (e.g., 200 Amps).
Types of ATS Transitions
The "transition" dictates how the switch moves the load between the utility grid and the generator.
1. Open Transition (Break-Before-Make)
The most common and cost-effective ATS. When power is restored, the switch completely disconnects the load from the generator (a momentary break in power) before snapping over to connect to the utility grid. Your lights will blink and computers will reboot unless they are on a UPS.
2. Delayed Transition
Similar to Open Transition, but it intentionally pauses in a "neutral" center position where the building is connected to nothing for 5-10 seconds. This allows large motors (like HVAC chillers) to coast down and lose their residual magnetic fields before being hit with new utility power, preventing out-of-phase mechanical shaft snapping.
3. Closed Transition (Make-Before-Break)
The most advanced and expensive option, favored by data centers and hospitals. When utility power returns, the ATS synchronizes the generator's AC sine wave perfectly with the utility grid's sine wave. It then connects to BOTH sources for 100 milliseconds, and disconnects the generator. The result? Zero interruption in power when transferring back to the grid. The lights do not blink.
Bypass Isolation Transfer Switches
In critical facilities like hospitals, you cannot shut off the power to perform maintenance on the ATS. A Bypass Isolation ATS is actually two switches built into one cabinet.
If the main ATS mechanism needs maintenance or fails, a technician can pull a massive lever to manually bypass the primary switch and route power through the secondary mechanical switch. The primary ATS cassette can then be physically pulled out of the cabinet like a drawer for safe repairs while the hospital remains fully powered.
Service Entrance Rated ATS
Normally, electrical code requires a main disconnect breaker immediately where the utility lines enter the building. A "Service Entrance Rated" ATS has this main utility circuit breaker built directly into the ATS cabinet. This saves immense space and installation labor, as you do not need to buy and mount a separate main breaker panel upstream of the ATS.
Frequently Asked Questions
Why does an ATS have to disconnect from the grid?
If a generator was connected to the building while the grid was still attached (without isolation), the generator would attempt to backfeed high-voltage power out into the city power lines. This would electrocute the utility linemen who are actively trying to repair the downed lines.
Can I use a manual transfer switch?
Yes, but it requires a human to realize the power is out, walk outside, start the generator, and throw a heavy mechanical lever. For industrial applications running 24/7, an automatic switch is almost always mandatory for operational continuity.