How to Size an Automatic Transfer Switch for Dual Power Supply Systems

An automatic transfer switch, often called an ATS, is a key device in dual power supply systems. It automatically transfers electrical loads from the main power source to a backup source when the main supply fails or becomes unstable. When the normal power supply is restored, the ATS can transfer the load back safely.

ATS devices are widely used in commercial buildings, hospitals, factories, data centers, residential backup systems, and distribution panels where power continuity is important. However, choosing the right ATS is not only about selecting a current rating. A reliable ATS must match the system voltage, load type, number of poles, short-circuit capacity, installation environment, and backup power design.

This guide explains how to size an automatic transfer switch for dual power supply systems and what factors should be checked before selection.

What Is an Automatic Transfer Switch?

An automatic transfer switch is an electrical switching device that connects a load to one of two power sources. In most systems, these sources are:

• Utility power and generator power

• Main utility and backup utility

• Utility power and UPS system

• Generator and emergency power source

The ATS continuously monitors the normal power source. If the voltage drops, phase loss occurs, or the main supply fails, the ATS disconnects the load from the normal source and connects it to the standby source. This helps maintain power supply for critical equipment.

In a dual power supply system, the ATS helps improve reliability, reduce downtime, and protect electrical equipment from unstable switching.

Why Correct ATS Sizing Matters

Incorrect ATS sizing can cause serious problems in an electrical system. If the ATS is undersized, it may overheat, fail during transfer, or suffer contact damage. If the ATS does not match the fault level of the system, it may not withstand short-circuit conditions safely.

Correct ATS sizing helps to:

• Ensure reliable power transfer during outages

• Prevent overheating and equipment failure

• Protect motors, lighting, HVAC, and control systems

• Reduce nuisance tripping

• Support future load expansion

• Improve system safety and compliance

For critical applications, such as hospitals or data centers, ATS reliability is especially important because even a short power interruption can cause operational risks.

Key Factors When Sizing an ATS

1. Rated Current

Rated current is one of the first factors to check. The ATS current rating must be equal to or higher than the maximum load current of the system.

For example, if the calculated load current is 180A, the selected ATS should not be 180A exactly. It is usually better to select the next standard size, such as 200A or 250A, depending on project requirements and future expansion.

Common ATS current ratings include 63A, 100A, 160A, 250A, 400A, 630A, 800A, and 1000A.

2. System Voltage

The ATS voltage rating must match the electrical system. Common voltage systems include:

• 220V or 230V single-phase systems

• 380V, 400V, or 415V three-phase systems

• 480V industrial systems

Using an ATS with the wrong voltage rating can lead to unsafe operation, control failure, or insulation problems.

3. Number of Poles

The number of ATS poles should match the system wiring and grounding design.

A 2-pole ATS is commonly used for single-phase systems. A 3-pole ATS is used for three-phase systems where the neutral is not switched. A 4-pole ATS is used when the neutral conductor also needs to be switched.

Whether to choose a 3P or 4P ATS depends on the earthing system, neutral arrangement, generator design, and local electrical standards.

4. Load Type

Different loads behave differently during startup and operation. This affects ATS selection.

Resistive loads, such as heaters, are relatively simple. Motor loads, pumps, compressors, HVAC systems, and transformers may create high inrush current during startup. IT equipment and UPS systems may also have specific transfer requirements.

If the system includes many motors or inductive loads, the ATS should be selected with enough capacity and proper coordination with upstream protection devices.

5. Short-Circuit Withstand Capacity

The ATS must be able to withstand the available short-circuit current at the installation point. This is often expressed through ratings such as short-time withstand current or making capacity.

The ATS should also be coordinated with upstream circuit breakers or fuses. If a short circuit occurs, the protection device should clear the fault safely while the ATS remains protected.

Ignoring short-circuit withstand capacity is one of the most common and dangerous ATS selection mistakes.

6. Transfer Type

Different ATS systems use different transfer methods.

Open transition transfer is the most common type. It disconnects the load from one source before connecting it to the other source. This creates a short interruption but is suitable for many commercial and industrial applications.

Delayed transition transfer adds a short delay between disconnection and reconnection. It is useful for loads that need time to discharge or reset.

Closed transition transfer transfers power with almost no interruption, but it requires source synchronization and is used in more demanding applications.

For most standard dual power supply systems, open transition ATS is commonly selected.

7. Installation Environment

The installation environment also affects ATS selection. Consider:

• Indoor or outdoor installation

• Ambient temperature

• Humidity

• Dust and pollution level

• Ventilation

• Altitude

• Vibration

• Enclosure protection rating

For outdoor or harsh environments, the ATS should be installed in a suitable enclosure with proper protection against dust, moisture, and corrosion.

Basic ATS Sizing Formula

To estimate load current, use the appropriate formula based on the system type.

For three-phase systems:

 Current (A) = Power (W) / (√3 × Voltage × Power Factor)

For single-phase systems:

 Current (A) = Power (W) / (Voltage × Power Factor)

After calculating the current, add a safety margin. A common margin is 20% to 25%, depending on the project and future expansion needs.

Then select the next standard ATS current rating.

ATS Sizing Example

Suppose a facility has a 100 kW three-phase load, operating at 400V with a power factor of 0.8.

 Current = 100,000 / (1.732 × 400 × 0.8)
Current ≈ 180A

Add a 25% margin:

 180A × 1.25 = 225A

The recommended ATS rating would be the next standard size above 225A.

 Recommended ATS size: 250A

However, this is only the current rating. Before final selection, the engineer should also check voltage, poles, short-circuit withstand capacity, transfer mode, load type, and enclosure requirements.

Common ATS Sizing Mistakes

When selecting an ATS, avoid these common mistakes:

• Choosing the ATS only based on generator capacity

• Ignoring actual load current

• Forgetting motor starting current

• Selecting the wrong number of poles

• Not checking short-circuit withstand capacity

• Ignoring neutral switching requirements

• Leaving no margin for future expansion

• Using an indoor ATS in outdoor or harsh environments

• Failing to coordinate the ATS with circuit breakers and fuses

A correctly sized ATS should be selected based on the complete electrical system, not a single parameter.

ATS Selection Checklist

Before choosing an automatic transfer switch, check the following items:

• Rated current

• Rated voltage

• Single-phase or three-phase system

• 2P, 3P, or 4P configuration

• Load type and starting current

• Main and backup power source type

• Short-circuit withstand capacity

• Transfer mode

• Control voltage

• Manual and automatic operation requirements

• Enclosure protection level

• Installation environment

• Monitoring or communication needs

• Applicable electrical standards

This checklist can help engineers, contractors, and project buyers avoid under-sizing or selecting the wrong ATS model.

DELIXI Automatic Transfer Switch Solutions

DELIXI automatic transfer switch products are designed for dual power supply systems in commercial buildings, industrial facilities, distribution panels, and backup power applications. They help ensure stable switching between normal and standby power sources, improving power continuity and system reliability.

For projects with different current ratings, voltage systems, installation environments, and backup power designs, DELIXI can provide suitable ATS options for power distribution and electrical control systems.

When selecting an ATS, it is recommended to review the full system design or consult technical support to ensure proper sizing and safe operation.

Conclusion

Sizing an automatic transfer switch is more than choosing an ampere rating. A reliable ATS must match the system current, voltage, number of poles, load characteristics, short-circuit capacity, transfer mode, and installation environment.

For a dual power supply system, the ATS plays a critical role in maintaining power continuity and protecting connected equipment. By calculating the load current, adding proper margin, checking system conditions, and selecting the correct ATS type, you can build a safer and more reliable backup power system.