Publish Time: 2026-10-15 Origin: Site
An ATS for a diesel generator, or Automatic Transfer Switch, is a device that automatically transfers electrical loads from the main utility supply to a backup generator when a power failure occurs.
For commercial buildings, industrial plants, hospitals, data centers, and other facilities that depend on reliable electricity, an ATS plays an important role in maintaining backup power availability.
However, not every diesel generator requires an automatic transfer switch. The decision depends on the application, electrical load, operating requirements, and acceptable power interruption time.
In this guide, we explain how an automatic transfer switch works with a diesel generator, when an ATS is necessary, the differences between automatic and manual transfer switches, and how to select the correct ATS for your backup power system.
Table of Contents
An Automatic Transfer Switch (ATS) is an electrical switching device that monitors the normal power supply and transfers a connected load to an alternative power source when required.
In a typical diesel generator backup system, the ATS connects three major components:
Utility Power Supply: The primary source of electricity during normal operation.
Diesel Generator Set: The standby power source that operates during a utility outage.
Electrical Load: The building equipment, industrial machinery, or essential circuits requiring electricity.
When utility power becomes unavailable or falls outside acceptable operating limits, the ATS detects the condition and initiates the backup power sequence.
If the diesel generator supports automatic starting, the ATS can send a start signal to the generator controller. Once the generator reaches acceptable operating conditions, the ATS transfers the designated electrical load to generator power.
When the normal utility supply returns and stabilizes, the ATS transfers the load back to utility power and initiates the generator shutdown sequence.
It is important to understand that an ATS does not produce electricity or increase generator capacity. Its primary function is to coordinate automatic power-source switching.
To understand the power generation equipment behind the system, read our guide on What Is a Diesel Generator and How Does It Work?.
An ATS operates through a coordinated sequence involving power monitoring, generator starting, electrical switching, and utility power restoration.
A typical diesel generator ATS system follows six steps.
During normal operation, the ATS continuously monitors the incoming utility power.
Depending on the controller configuration, it may monitor voltage, frequency, and phase conditions to determine whether the normal source is suitable for supplying the connected load.
When utility power fails or moves outside configured operating limits, the ATS detects the abnormal condition.
A programmed time delay may be used to prevent unnecessary generator starts caused by momentary power disturbances.
The ATS sends a remote-start signal to the diesel generator controller.
The generator starts, and its alternator begins producing electrical power.
Before allowing load transfer, the control system verifies that the generator has reached acceptable voltage and frequency.
Once generator power is available, the ATS disconnects the load from the utility source and connects it to the backup generator.
In a conventional open-transition ATS, the switching process follows a break-before-make sequence.
This prevents the normal and generator sources from being directly interconnected during transfer.
When utility power returns, the ATS monitors the restored supply.
The controller generally applies a retransfer delay to confirm that the normal source is stable before switching the load back.
The ATS transfers the connected load back to utility power.
The diesel generator typically continues operating without load during a programmed cool-down period before shutting down.
The system then returns to standby monitoring mode.
Important: A conventional diesel generator ATS system does not necessarily provide uninterrupted electricity. A short power interruption normally occurs while the generator starts, stabilizes, and accepts the load. Facilities requiring uninterrupted electricity may also need an Uninterruptible Power Supply (UPS).
For more detailed technical information, refer to the Cummins Transfer Switch Application Manual.
An ATS is particularly useful when electrical loads must automatically switch to backup power without requiring manual intervention.
The following applications commonly benefit from automatic transfer systems.
Commercial buildings may rely on backup generators for emergency lighting, security equipment, communication systems, elevators, and selected building services.
An ATS allows designated circuits to receive generator power automatically when the utility supply fails.
This can reduce the need for immediate operator intervention during unexpected outages.
Hospitals and healthcare facilities often require highly reliable emergency power systems.
Critical medical equipment, emergency lighting, and essential electrical circuits may need backup electricity within specified restoration times.
An appropriately designed ATS system can help coordinate the transition from utility power to generator power.
However, the complete emergency power system must meet the applicable healthcare regulations, electrical codes, and performance requirements.
Data centers depend on reliable electricity to support servers, networking equipment, cooling systems, and communication infrastructure.
An ATS can automatically connect designated loads to standby generator power when the normal source fails.
For equipment that cannot tolerate an interruption, the ATS is commonly integrated with UPS equipment and a properly engineered power distribution system.
Unexpected power failures can interrupt manufacturing processes, stop production lines, and affect industrial equipment.
An ATS can help restore power automatically to designated production equipment, control systems, and essential facility loads.
Engineers must also evaluate motor starting currents, equipment restart sequences, and generator load capacity when designing these systems.
Remote pumping stations, telecommunications towers, and unmanned industrial facilities may not have operators available during an outage.
An automatic transfer system allows a compatible standby generator to respond without requiring personnel to arrive on site.
An ATS may not be required in situations where:
A generator is used only for occasional or temporary operation.
Manual power restoration is acceptable.
Qualified personnel are available to operate approved manual transfer equipment.
The generator supplies dedicated equipment independently of the utility source.
Another suitable automatic power transfer arrangement is already installed.
The choice between automatic and manual transfer should always be based on the application's operational needs and applicable electrical requirements.
Both an Automatic Transfer Switch and a Manual Transfer Switch allow loads to be supplied from an alternative power source.
The primary difference is whether the switching process occurs automatically or requires operator intervention.
Feature |
Automatic Transfer Switch (ATS) |
Manual Transfer Switch (MTS) |
|---|---|---|
Power failure detection |
Automatic |
Operator-dependent |
Generator starting |
Automatic with compatible controls |
Usually manual |
Load transfer |
Automatic |
Manual |
Operator required |
Normally no |
Yes |
Initial equipment cost |
Generally higher |
Generally lower |
Control complexity |
More advanced |
Simpler |
Utility power restoration |
Automatic retransfer |
Manual retransfer |
Typical applications |
Commercial, industrial, critical backup power |
Occasional and noncritical backup power |
For facilities that require automatic backup power restoration, an ATS is generally the more suitable option.
For occasional backup applications where manual intervention is acceptable, a manual transfer switch may provide a simpler solution.
Both devices must be correctly rated and installed to prevent unsafe power-source connections.
For another comparison of backup power switching arrangements, read Automatic Transfer Switch vs. Interlock: Which Is Best?.
Different ATS designs are available to meet specific electrical and operational requirements.
Understanding these configurations helps engineers select appropriate switching equipment.
An open-transition ATS disconnects the electrical load from one power source before connecting it to the alternative source.
This operating method is commonly described as break-before-make switching.
Open-transition switches are widely used in conventional standby diesel generator installations because they provide source separation during transfer.
A closed-transition ATS can briefly connect two acceptable, synchronized power sources during a controlled transfer.
This arrangement can reduce transfer interruptions under suitable operating conditions.
However, closed-transition switching requires appropriate synchronization, electrical protection, and utility-interconnection approval where applicable.
It should only be selected when the complete electrical system supports this operating method.
A delayed-transition ATS introduces a programmed pause between disconnecting one electrical source and connecting another.
This arrangement can be useful in selected applications involving motor loads or equipment requiring a controlled transition interval.
The correct ATS type depends on the electrical load characteristics and the overall power system design.
Choosing an appropriate ATS involves more than matching the transfer switch to the generator's rated power.
The ATS must also be compatible with the utility supply, electrical load, control system, installation conditions, and available fault current.
Diesel generator capacity is generally expressed in kilowatts (kW) or kilovolt-amperes (kVA).
ATS capacity is normally specified by its rated current in amperes (A).
For a balanced three-phase system, the approximate line current can be calculated using:
Current (A) = kVA × 1,000 ÷ (√3 × Voltage)
For example, consider a 100 kVA diesel generator operating at 400 V, three-phase.
Current = 100 × 1,000 ÷ (1.732 × 400)
Current ≈ 144 A
This calculation provides the approximate rated generator current.
However, it does not automatically determine the required ATS current rating.
The final ATS size must consider the maximum load current, utility supply configuration, equipment ratings, applicable electrical codes, and the design of the distribution system.
For additional guidance on choosing the generating equipment, read How to Choose the Right Generator Set.
The ATS must be compatible with the electrical system's rated voltage and operating frequency.
Common configurations include:
230/400 V, 50 Hz
120/208 V, 60 Hz
277/480 V, 60 Hz
Actual configurations vary depending on regional standards and installation requirements.
The ATS controller must also support the appropriate voltage and frequency monitoring functions.
Transfer switches must match the electrical distribution system.
Single-phase and three-phase installations have different conductor and switching requirements.
For three-phase applications, engineers may need to select between 3-pole and 4-pole switching arrangements.
The correct configuration depends on neutral switching requirements, grounding, and applicable electrical regulations.
Not every diesel generator supports automatic starting.
Before choosing an ATS, confirm that the generator controller has the necessary automatic-start functions and remote-control connections.
Possible interface requirements include:
Remote start and stop signals
Generator running status
Voltage and frequency monitoring
Alarm and fault signals
Load transfer readiness
A compatible control interface is essential for reliable automatic operation.
The type of electrical load affects the overall backup power system design.
Examples include:
Industrial motors
Pumps and compressors
HVAC equipment
Manufacturing machinery
Lighting systems
Data center equipment
Large motor starting currents, simultaneous load restoration, and sensitive electronic loads may require additional engineering consideration.
An ATS must be suitable for the available electrical fault conditions.
The selection process should consider short-circuit withstand and closing ratings, upstream protective devices, grounding arrangements, and neutral switching requirements.
For additional information, read How to Earth a Generator Set Properly.
A useful external technical reference is Schneider Electric's Backup Generator Grounding Guide.
The ATS enclosure must be appropriate for the intended installation location.
Important considerations include:
Indoor or outdoor installation
Ambient temperature
Humidity and condensation
Dust and corrosion exposure
Enclosure protection rating
Maintenance accessibility
Electrical engineers should select equipment that meets the applicable installation and environmental requirements.
Consider a commercial facility installing a 200 kVA diesel generator for standby power.
The generator operates at 400 V, three-phase.
The approximate rated current is:
200,000 ÷ (1.732 × 400) ≈ 289 A
This provides an initial reference for system planning.
However, choosing the final ATS rating requires additional information.
The engineer must evaluate:
Maximum connected electrical load.
Utility-side current requirements.
Generator output capacity.
Available short-circuit current.
Electrical protection coordination.
Neutral switching and grounding requirements.
Applicable codes and installation standards.
For example, a facility may have a 400 A utility service but a standby generator rated at only 200 kVA.
If the ATS supplies the entire electrical service, it must be appropriately rated for the utility-side load current.
The generator may also require load shedding or a dedicated essential-load distribution system to prevent overload during emergency operation.
Key consideration: Generator kVA and ATS ampere ratings must be coordinated, but they represent different equipment specifications.
Although automatic transfer switches are designed for reliable operation, periodic inspection and functional testing are necessary.
Possible causes include:
Low generator starting battery voltage
Incorrect controller operating mode
Damaged remote-start wiring
Generator controller faults
ATS control signal problems
Technicians should review generator alarms and verify the ATS control interface.
Possible reasons include:
Generator voltage or frequency outside acceptable limits
Active transfer time delay
Control circuit problems
Mechanical switching faults
Incorrect ATS settings
Qualified technicians should inspect the equipment using approved electrical safety procedures.
An ATS may delay retransfer when utility voltage or frequency is unstable.
Other causes can include incorrect time-delay settings, sensing problems, or control faults.
Overheating can result from loose connections, excessive current, unsuitable conductor sizing, or deteriorated electrical contacts.
Routine inspection and appropriate electrical testing help identify potential problems.
Regular maintenance should include:
Visual inspection of the ATS enclosure
Review of controller status and alarm records
Verification of automatic operating settings
Scheduled functional transfer tests
Electrical connection inspection
Documentation of maintenance activities
Maintenance intervals should follow the manufacturer's recommendations and applicable regulatory requirements.
For emergency and standby power installations, refer to the relevant provisions of NFPA 110 and applicable local standards.
An automatic transfer switch is only one component of a reliable standby power system.
The diesel generator, electrical distribution equipment, control panel, protective devices, and ATS must operate together.
When choosing a diesel generator for automatic standby operation, project planners should evaluate:
Required standby power capacity
Generator voltage and frequency
Automatic-start controller compatibility
Expected electrical load
Motor starting and load restoration requirements
Installation conditions
Electrical protection requirements
Future system expansion
At DIYPOWER, we provide diesel generators and power equipment for commercial and industrial applications.
Our generator product range covers different power capacities and operating requirements, allowing customers to evaluate equipment based on their project needs.
When planning a diesel generator and ATS installation, it is useful to prepare the following information before requesting a quotation:
Project Information |
Details Required |
|---|---|
Generator capacity |
Required kW or kVA |
Electrical system |
Voltage, frequency, and number of phases |
Application |
Commercial, industrial, emergency standby, or other |
ATS requirement |
Automatic or manual transfer |
Connected loads |
Load types, maximum demand, and starting requirements |
Installation |
Indoor or outdoor |
Control functions |
Remote start, monitoring, and load management |
Project location |
Country and applicable electrical standards |
Providing complete project information helps the technical team evaluate equipment compatibility and identify the appropriate generator configuration.
No. An ATS is generally required when automatic transfer between utility electricity and standby generator power is necessary. Some applications can use properly installed manual transfer equipment instead. The final requirement depends on the installation, operating conditions, and applicable electrical codes.
Yes. A diesel generator can operate independently or supply electrical loads through an approved manual transfer system. However, without suitable automatic switching equipment, transferring power between utility and generator sources may require manual intervention.
The total power restoration time depends on outage detection, controller time delays, generator starting time, voltage and frequency stabilization, and the ATS switching mechanism. A conventional standby generator system typically requires several seconds to restore generator power. The complete system must be verified when a specific restoration time is required.
A generator control panel monitors and manages generator operation, including starting, stopping, alarms, and electrical parameters. An ATS monitors power-source availability and transfers the electrical load between sources. Both devices work together in an automatic standby power system.
A 3-pole ATS typically switches the three phase conductors, while a 4-pole ATS also switches the neutral conductor. The correct choice depends on grounding arrangements, neutral requirements, electrical system design, and applicable regulations. A qualified electrical engineer should determine the appropriate configuration.
Yes, an ATS may form part of a backup power system supplied by multiple generator sets. However, such installations may require additional paralleling equipment, generator controls, protective devices, and load management systems.
An ATS primarily manages power-source transfer. Although some systems include load-management functions, generator overload protection depends on correctly designed electrical protection and generator control equipment.
An Automatic Transfer Switch for a diesel generator is an important component of standby power systems that require automatic electrical load transfer during utility outages.
It is particularly valuable in commercial buildings, industrial facilities, healthcare environments, data centers, and other applications where reliable backup power is important.
However, choosing the right ATS requires careful evaluation of generator capacity, electrical current ratings, voltage, control compatibility, grounding arrangements, and load characteristics.
At DIYPOWER, we help customers evaluate diesel generator solutions for different commercial and industrial power requirements.
Explore our diesel generator product range or contact DIYPOWER to discuss your project's generator capacity, electrical configuration, and ATS integration requirements.
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