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Automatic transfer switch sizing is an important selection step
Automatic transfer switch sizing is an important selection step
Resource: https://www.youtube.com/watch?v=U-7vj6Hlwok

There’s no one-size-fits-all when it comes to ATS switches, and anyone looking to buy automatic transfer switch equipment should understand the things to consider. That’s what we put together for you in this guide. Read on to learn all that you need to know about ATS selection.

How to Buy Automatic Transfer Switch

An automatic transfer switch swaps your electricity supply to the appropriate source based on the electric power quality and availability. But the switch must be appropriate for your type of application or it could cause more problems than it solves.

In order to help you determine what type of automatic transfer switch is best for your needs, we’ve put together a list of the things to consider. These will answer your common questions about ATS selection and help you make the right decision. The things to take into account include:

Automatic Transfer Switch Type

In order to buy automatic transfer switch equipment that will fit your power system, you need to determine which type of switch will be appropriate. These are usually classified based on their transition methods. Typically, you can choose from these ATS types:

  • Closed transition transfer switch
  • Open transition transfer switch
  • Soft loading transfer switch
  • Bypass isolation transfer switch

The open transition automatic transfer switch is the most common. As its name indicates, it first breaks the connection with the original power source before making a new one. Use this switch if your power system is not sensitive to outages or periods of power loss, such as if powering lights.

Closed transition automatic transfer switches, on the other hand, keep the two power sources active during the switchover. This ensures a more seamless experience for your devices that are using power. This type of switch is recommended for mission critical applications.

The soft loading transfer switch is somewhere in between these two, slowly ramping up power from the new source while simultaneously ramping down power from the original one. Use this ATS type if powering inductive loads such as industrial motors.

In a bypass isolation transfer switch, two switches are used to allow the main service to be serviced without interrupting the power to the loads. This type of switch is best for an electrical system cannot remain unpowered for any length of time without power during switch servicing.

Automatic Transfer Switch Application

Depending on your type of alternate power source and transfer switch application, you may want to choose between a load center and service disconnect switch. A load center transfer switch will selectively serve individual circuits, typically up to 16 different circuits, and fit specific backup power sources.

For example, many emergency generators or backup UPS systems cannot power entire electrical systems. If using these, you want the switch to power preferred loads. This is usually wired to the specific circuits in the panel.

On the other hand, a large generator may be able to supply power to the entire system (or it may need to) in such cases, a service disconnect transfer switch can be installed. This type of switch is mounted between your utility meter and the main breaker panel.

Switch Transfer Time

Another thing to consider when it comes to the process to buy automatic transfer switch equipment is the transfer time. This is the amount of time it takes for the switch to transition from one power source to another. When powering non-inductive loads, faster is better.

For inductive loads, you want the transition to take longer and allow residual current to decay. This can take up to 20 seconds. There are transfer switch types that come with this transitioning function built-in. We recommend using these to avoid system or equipment damage.

Automatic Transfer Switch Ratings

When you want to buy automatic transfer switch equipment, you need to consider the voltage and current ratings of the device. The voltage rating is the maximum voltage that the device can handle. The current rating is the maximum current that the device can handle.

As a general rule, the voltage rating should be equal to or greater than the nominal voltage of your system. The current rating should be equal to or greater than the maximum current of your system. If these are exceeded, the device could be damaged.

Typical automating transfer switch ratings for voltage include 120V, 208V, 240V, 480V, and 600V. For current, ratings typically range from 50A to 4000A, while frequency is usually either 60Hz or 50Hz.

Transfer Switch Enclosure Type

Another thing to consider during ATS selection is the type of enclosure you need. The enclosure protects the device from the environment and can be either indoor or outdoor. Enclosures are usually NEMA-rated and come in several different types. The most common types are:

Indoor Transfer Switch

NEMA 1 – offers protection against contact with live parts as well as dust.

NEMA 12 – To protect against dust, oil, and water.

Outdoor Transfer Switch

NEMA 3R – used outdoors and protects against rain, sleet, and snow.

NEMA 4 – protects against water, dust, and other corrosive materials.

The type of enclosure you need will depend on your application. For example, if you’re using the switch outdoors in wet climate, you’ll need a NEMA 3R enclosure

Other Automatic Transfer Switch Specifications

In addition to the considerations above, there are other features you may want to look for in your automatic transfer switch equipment. The most important of these automatic transfer switch specifications include the following:

  • Modular design – This feature allows you to easily expand or modify your system as needed.
  • Generator compatibility – A diesel generator automatic switch, for example, has to deal with the unique characteristics of diesel generators, such as high inrush currents and must be able to cope.
  • Load shedding – This feature allows you to selectively disconnect non-essential loads to reduce the demand on your system.
  • Fault current protection Fault currents can cause thermal and magnetic stress damage, and your ATS switch should be able to protect against them.

Automatic Transfer Switch Sizing

Automatic transfer switch sizing refers to the process of choosing a switch that will suit the electrical system it is intended to protect. This is usually done by considering the system’s voltage and current ratings, or power ratings.

The rule is to ensure that the amperage of your switch is perfect or matches that of your electrical system. Done right, automatic transfer switch sizing can help ensure the following:

  • That your electrical system is adequately protected against power outages and other disruptions.
  • That the switch can provide safe and efficient power to your system when it is needed.

As you can see, there are a lot of things to consider when selecting automatic transfer switch equipment. But, if you keep the considerations above in mind, you’ll be able to find the right ATS switch for your application.

In most cases, though, the best way to select an automatic transfer switch is to consult with an expert. An experienced engineer can help you understand your options and choose the right switch for your needs.

Alternatively, you can contact a trusted automatic transfer switch company for advice on how to size or select your switch. Many automatic transfer switch manufacturers also offer free consultations, and will guide you through the selection process.


The process to buy an automatic transfer switch can be daunting, but it doesn’t have to be. Just remember to keep the considerations covered in this article in mind, and you’ll be well on your way to finding the right ATS switch for your needs. You may also want to talk to an expert to get the best advice possible, or consult a manufacturer to get started on the right foot.

IGOYE Automatic Transfer Switch Catalog