What is an ATS (Automatic Transfer Switch) and why your diesel generator is useless without a properly sized one

The component that decides whether your diesel generator works or not

An ATS (Automatic Transfer Switch) is the electrical device that detects the loss of the normal power source (CFE) and automatically commands the start of the emergency generator, transferring the load to the generator when it reaches the correct voltage and frequency. Without an ATS properly sized, the diesel generator is just an expensive piece of equipment that does not know when to start, does not know how to take the load, and does not know when to return to CFE when normal service is restored.

The ATS is the brain of the emergency power system. It is not a passive component: it actively monitors the two sources (CFE and generator), decides when to transfer the load, and synchronizes the generator startup sequence with the load transfer. A generator without an ATS, or with a poorly sized ATS, does not provide the reliability the data center needs.

What an ATS actually does

The ATS performs four functions in the emergency power system:

1. Detection of CFE failure: continuously monitors the voltage and frequency of the CFE source. When the voltage drops below the configurable threshold (typically 80-85% of nominal) for a configurable time (typically 0.5-3 seconds), the ATS declares a failure of the normal source.
2. Generator start command: sends a start signal to the diesel generator. The generator takes 5-30 seconds to start, stabilize voltage and frequency, and reach the ready state.
3. Load transfer to the generator: when the generator is ready, the ATS opens the CFE contactor and closes the generator contactor, transferring the load to the emergency source. The transfer time is 1-10 seconds depending on the type of ATS.
4. Return to CFE when normal service is restored: once CFE returns, the ATS waits for a configurable cool-down time (typically 5-30 minutes), then transfers the load back to CFE and commands the generator to stop.

Additionally, the ATS typically includes a weekly test cycle that starts the generator, transfers the load, runs for a configurable time (typically 30 minutes), and returns to CFE, all without operator intervention. This test is critical to keep the generator in operational condition and to detect failures before a real emergency.

The types of transition

There are three types of transition in an ATS, with different applications:

• Open transition (break-before-make): the ATS disconnects from one source before connecting to the other. There is a brief interruption of power to the load (typically 1-5 seconds). This is the most common type and the least expensive. Used for loads that can tolerate a brief interruption (lighting, HVAC, mechanical systems, IT loads with UPS downstream).
• Closed transition (make-before-break): the ATS connects to the new source before disconnecting from the old source, momentarily paralleling the two sources. This requires synchronization between the sources and is more expensive. Used when the load cannot tolerate any interruption, or when the transfer must be seamless to the user.
• Soft loading transition: the ATS gradually transfers the load from one source to the other over a few seconds, without a complete disconnection. Used for very large loads where a sudden transfer would cause disturbances in the power system.

For data center applications, the open transition with UPS downstream is the most common. The UPS covers the 1-5 second interruption during the transfer, and the data center IT load does not register the event.

How the ATS is sized

ATS sizing depends on:

• Total load current: the sum of the currents of all the loads that the ATS will feed. Typically calculated at 80% of the nominal capacity of the ATS for continuous operation.
• Voltage and phases: typically 220 V, 480 V, or 13.8 kV in three-phase for data center applications in Mexico.
• Type of transition: open, closed, or soft loading. The type affects the cost and complexity of the device.
• Short-circuit capacity: the ATS must withstand the short-circuit current of both sources (CFE and generator) without damage. The generator typically has lower short-circuit capacity than CFE, so the ATS must be sized for the lower of the two.
• Required accessories: bypass isolation, metering, communications (Modbus, SNMP), exerciser clock, etc.

An ATS of 800 A in three-phase 480 V can feed approximately 665 kW of load (800 A × 480 V × √3 × 0.80 = 531 kVA, ~665 kW at PF 0.8). For a 1 MW data center, two ATS of 800 A in parallel are needed, or one ATS of 1600 A.

The most common sizing errors

The most common sizing errors in ATS for data centers:

• Sizing based on the generator power without considering the actual load: the generator may be sized for the future load, but the ATS is sized for the current load. This causes overload of the ATS when the load grows.
• Not considering the inrush currents: motors, transformers, and UPS have high inrush currents at startup (3-6 times the nominal current for 1-10 cycles). The ATS must withstand these inrushes without tripping.
• Not considering the harmonic distortion: UPS and variable frequency drives generate harmonic distortion that increases the RMS current. The ATS must be sized for the real RMS current, not just the fundamental.
• Choosing open transition when the load requires closed: some critical loads cannot tolerate the 1-5 second interruption of the open transition. Choosing the wrong type of transition can cause load drops.
• Not including the bypass isolation: the bypass isolation allows the ATS to be taken out of service for maintenance without cutting power to the load. Without bypass, the maintenance of the ATS requires shutting down the load.
• Sizing for the available fault current of CFE without considering the generator: the generator has lower short-circuit capacity than CFE (typically 3-5 times the nominal current, vs 10-25 times for CFE). The ATS sized for CFE may not coordinate with the protections of the generator.

The standards that apply

ATS for data center applications must meet several standards:

• UL 1008: transfer switch equipment, for the safety and performance of the device. The most cited standard in North America.
• IEC 60947-6-1: low-voltage switchgear and controlgear, part 6-1: multiple function equipment, transfer switching equipment. The international equivalent.
• IEEE 446 (Orange Book): recommended practice for emergency and standby power systems. Covers the ATS as part of the redundant architecture.
• NFPA 110: standard for emergency and standby power systems. Defines the classification of the emergency power system and the requirements for the transfer switch.
• NEC (National Electrical Code): for installation in the United States. In Mexico, NOM-001-SEDE applies based on the NEC.

Manufacturers of ATS for data center applications include ABB, Eaton, Schneider Electric (ASCO), Vertiv (Liebert), Generac, and ASCO Power Technologies. The choice depends on the size, type of transition, certifications, and integration with the existing power architecture.

Coordination with the generator

The ATS does not work in isolation: it must coordinate with the diesel generator for the system to work properly. Critical coordination points:

• Generator start delay: the ATS must wait the appropriate time for the generator to start and stabilize before transferring the load. Typical delay: 5-30 seconds, configurable in the ATS.
• Generator cool-down: when CFE returns, the ATS keeps the generator running for a cool-down time (typically 5-30 minutes) before stopping it. This protects the generator from thermal shock.
• Weekly exercise: the ATS programs a weekly test that starts the generator, transfers the load, runs for a configurable time, and returns. The exercise keeps the generator in operational condition.
• Load shedding: in systems with several generators, the ATS can perform load shedding during the generator startup to reduce the inrush and avoid overload.
• Generator protections: the ATS and the generator controls must coordinate so that the protections of the generator (overload, overheating, low oil pressure) do not interfere with the transfer.

Without proper coordination, the data center can experience failures during a CFE outage: the generator starts but the ATS does not transfer the load, the transfer is made before the generator is ready, or the generator stops prematurely. Each of these failures can cause an unplanned interruption of the data center.

What your next electrical RFP should ask

If the data center needs an ATS for the emergency power system, the questions that must appear in the RFP:

• What is the type of transition of the ATS? Is it open, closed, or soft loading? Which is needed for the critical load?
• What is the nominal current of the ATS? Is it sized for the current load with 25% margin for growth? Does it consider the inrush currents of the motors and the UPS?
• What is the short-circuit capacity (Withstand and Closing Rating, WCR)? Is it coordinated with the protections of the CFE and the generator?
• Does the ATS have bypass isolation? Is the bypass manual or automatic? Is it make-before-break or break-before-make?
• What certifications does the ATS have? UL 1008, IEC 60947-6-1, IEEE 446, NFPA 110?
• What is the transfer time? Is it less than 10 seconds for the critical load?
• What communication protocols does it support? Modbus, SNMP, BACnet? Does it integrate with the data center’s BMS?
• What is the O&M cost? Are there preventive maintenance kits? What is their frequency?
• What is the warranty? What is the expected useful life?
• Does the manufacturer have local technical support in Mexico? What is the response time in case of failure?

The answers to these questions determine whether the ATS is properly sized for the data center, and whether it will provide the reliability the operation requires.

Sources

1. NFPA (2022). NFPA 110 Standard for Emergency and Standby Power Systems. https://www.nfpa.org/codes-and-standards/nfpa-110-standard-development/110
2. IEEE (2012). IEEE 446 Recommended Practice for Emergency and Standby Power Systems. https://standards.ieee.org/ieee/446/5676
3. ASCO Power Technologies (2024). Automatic Transfer Switches — Technical Reference. https://www.emerson.com/en-us/asco-power-technologies
4. Eaton (2024). Automatic Transfer Switch Product Guide. https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/transfer-switches/eaton-transfer-switch-product-guide.pdf
5. Schneider Electric (2024). TransferPacT Automatic Transfer Switches — Data Center Application Guide. https://www.se.com/ww/en/product-range/64101-transferpact-automatic-transfer-switches/