ATS Automatic Transfer Switch Guide

Q: Do I need an ATS for my generator?

If you need automatic backup power (no manual intervention), yes. An ATS is essential for: hospitals, data centers, telecom, fire pumps, security systems, and any application where a power gap is unacceptable. For occasional backup where someone can manually start the generator, a manual switch is sufficient and cheaper.

Q: How much does an ATS cost?

ATS prices range from $500 (100A, open transition, basic) to $15,000+ (1600A, closed transition, with monitoring). For a typical 200-500 kW generator, a 400-800A open transition ATS costs $1,500-5,000. HPOWER can include an ATS with your generator order, pre-wired and tested.

Q: Can I use one ATS with multiple generators?

Yes, with a multi-generator ATS or a paralleling switchgear system. Multiple generators can be paralleled for N+1 redundancy or to handle large loads. This is common in data centers and large industrial plants. The ATS/paralleling system manages which generators start and how they share the load.

Q: What is the difference between AMF and ATS?

AMF (Automatic Mains Failure) and ATS (Automatic Transfer Switch) are often used interchangeably. Strictly, AMF refers to the control module that detects grid failure and starts the generator, while ATS is the physical switch that transfers the load. In practice, most modern units combine both functions into one device called an AMF/ATS controller.

The ATS is the brain of your backup power system. Understand types, sizing, and operation to ensure seamless power transfer.

What is an Automatic Transfer Switch?

An ATS (Automatic Transfer Switch) is an electrical device that automatically switches a load between the utility (grid) power source and a backup generator. When the grid fails, the ATS signals the generator to start, waits for stable power, then transfers the load to the generator. When grid power returns, the ATS transfers back and shuts down the generator.

Without an ATS, someone must manually start the generator and flip the switch — unacceptable for hospitals, data centers, and critical facilities. The ATS makes backup power fully automatic.

ATS Operation Sequence

Here is the exact sequence when grid power fails:

1. ATS detects grid voltage drop below 80% of nominal (undervoltage)
2. ATS sends start signal to generator (after 1-3 second delay to avoid nuisance starts)
3. Generator engine cranks and starts (typically 3-10 seconds)
4. Generator stabilizes voltage and frequency (5-15 seconds)
5. ATS confirms generator power is stable (voltage ±5%, frequency ±0.5Hz)
6. ATS opens grid contactor, closes generator contactor (transfer takes 100-500ms)
7. Load is now powered by generator
8. Total transfer time: typically 10-30 seconds (configurable)
When grid power returns and is stable for 3-10 minutes, the ATS reverses the sequence and returns the load to grid power.

Open Transition vs Closed Transition

Open transition (break-before-make): The ATS disconnects from grid before connecting to generator. There is a brief power gap (100-500ms). This is standard for most applications and is less expensive.

Closed transition (make-before-break): The ATS briefly connects both grid and generator simultaneously (for <100ms), then disconnects the generator. No power interruption. Required for: data centers, hospitals with life-support equipment, semiconductor manufacturing, and any load that cannot tolerate even a 100ms gap.

Cost difference: Closed transition ATS costs 2-3x more than open transition. Most standby applications use open transition.

ATS Sizing

The ATS must be rated for the full load current of the system. Size the ATS based on the larger of:

- The generator full-load current, OR

- The maximum load current it will switch

Rule of thumb: ATS ampere rating ≥ generator rated current × 1.25 (25% safety margin).

Example: A 500 kW generator at 400V/3-phase produces ~903 amps. ATS should be rated ≥ 1130 amps (next standard size: 1200A or 1600A).

Common ATS sizes: 100A, 200A, 400A, 630A, 800A, 1000A, 1250A, 1600A, 2000A, 2500A, 3200A, 4000A.

Common ATS Faults

Failure to transfer: Usually caused by dirty contactors, failed voltage sensor, or loose control wiring. Annual inspection and cleaning prevents this.
Nuisance starts: ATS triggers generator on brief voltage dips. Fix: increase start delay to 3-5 seconds.
Failure to return to grid: Grid return timer too long, or return voltage threshold too high. Adjust settings.
Contactor welding: Caused by transferring under heavy inrush load (large motors). Use a time-delay bypass for motor loads.
Battery failure: The ATS start signal relies on generator battery. Dead battery = no transfer. Test battery monthly.

Frequently Asked Questions

Do I need an ATS for my generator?

If you need automatic backup power (no manual intervention), yes. An ATS is essential for: hospitals, data centers, telecom, fire pumps, security systems, and any application where a power gap is unacceptable. For occasional backup where someone can manually start the generator, a manual switch is sufficient and cheaper.

How much does an ATS cost?

ATS prices range from $500 (100A, open transition, basic) to $15,000+ (1600A, closed transition, with monitoring). For a typical 200-500 kW generator, a 400-800A open transition ATS costs $1,500-5,000. HPOWER can include an ATS with your generator order, pre-wired and tested.

Can I use one ATS with multiple generators?

Yes, with a multi-generator ATS or a paralleling switchgear system. Multiple generators can be paralleled for N+1 redundancy or to handle large loads. This is common in data centers and large industrial plants. The ATS/paralleling system manages which generators start and how they share the load.

What is the difference between AMF and ATS?

AMF (Automatic Mains Failure) and ATS (Automatic Transfer Switch) are often used interchangeably. Strictly, AMF refers to the control module that detects grid failure and starts the generator, while ATS is the physical switch that transfers the load. In practice, most modern units combine both functions into one device called an AMF/ATS controller.

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