Automatic transfer switch for generator

Do you have a backup generator?

If something happened and you lost power, would you be prepared? With an automatic transfer switch for generator, you can rest assured that you will always have power when you need it.

An automatic transfer switch is the safest and easiest way to connect your backup generator to your home. It’s also the smartest way to protect your appliances from damage caused by power surges.

Read through the complete article to learn more about automatic transfer switches for generators and find the best one for your needs.

What is an automatic transfer switch for generator?

An automatic transfer switch (ATS) ensures continuous power from one of the two power sources to the electrical loads connected. It is an intelligent power switching device of self-acting type, controlled by the dedicated microprocessor-based control logic.

This controller monitors the electrical parameters like voltage and frequency of both primary and alternate power sources. If the primary power source fails, the controller will automatically switch to the alternate power source if it is in healthy condition.

Its first priority is always the primary source, and it will transfer the load to the alternate source only when the primary source fails or under a manual operator command to do so. It returns to the primary power as soon as it is healthy.

In the case of home standby generators, they act as the alternate source, and ATS carries out the additional function of generating start and stop commands to the standby generator on failure and restoration of the utility service.

How does an ATS work?

A typical transfer sequence from the utility electrical power to the emergency power from the generator is described below for easy understanding. The actual sequence may be more elaborate depending on the complexity of the selected ATS.

  1. It Continuously monitors the health of the primary power source. Let us say that the primary voltage drops below a certain preset percentage, V1 (let us say 80% of the nominal voltage), or the frequency drops below F1 (97% of the nominal) and stays at this reduced level for more than a certain preset time, T1.
  2. ATS will isolate either the whole house wiring or the designated emergency circuits from the grid power, depending on how the internal wiring in your house is arranged.
  3. It will start the DG Set through the electric start system.
  4. Monitor the DG Set voltage and frequency, and switch loads to the generator power when the voltage and frequency reach a certain preset level V2/F2 and stay above this value for more than the preset time T2.
  5. It restores the system back in reverse order when the grid power returns and the voltage exceeds the preset V3 level for T3 time.
  6. It shuts the generator down after a certain pre-programmed time.

ATS switches are available for two and three power sources.

  • Utility – Utility
  • Utility – Generator
  • Generator – Generator
  • Utility—Generator—Generator
  • Utility—Utility—Generator

Modes of operation of automatic transfer switches

Automatic transfer switches can operate in three basic modes, as explained below.

  1. Automatic mode – In this mode, ATS carries out the entire switching, including sensing, initiation, and operation.
  2. Non-automatic mode – Initiation is manual, either through a local button or remote, but the entire operation is carried out by the controller.
  3. Manual mode – Both initiation and operation are done locally, mostly through a handle.

Automatic Transfer Switch Diagram

Schematic and detailed automatic transfer switch circuit diagrams are presented below to give you an insight into the operation of ATS and guide you to connect them to your existing house wiring.

Can you use an ATS with a portable generator?

It is possible to use ATS with large-sized portable generators. However, to use them effectively, there must be a provision of a continuous supply of fuel from the lines that your house uses for heating purposes like natural gas, diesel, or propane.

Are automatic transfer switches safe?

In addition to the convenience in operation, the transfer switches provide protection against back feeding, which can be a serious safety hazard that may cause electrocution to utility personnel and members of your family, fire, and damage to your house and equipment.

What kind of ATS do I need?

Before purchasing the ATS, you must understand that most automatic transfer switches come in various types and configurations, comply with different codes and standards, have different switching mechanisms, and have different types of transitions. Let us briefly familiarize you with these technicalities so that you can select the correct house transfer switch for your application.

Main Types of Automatic Transfer Switches

The first distinction is based on how the transition between one source to the other is made. Selecting the correct transition time is very crucial for your safety.

Types By Transition

  1. Open transition – these are also known as the “break before make” types of transitions, where the load has to be disconnected from the old source before connecting to the new one. These are further classified into “open delayed” or “open in-phase” types. Open -delayed are used for large inductive loads like motors. A delay is introduced between the old source’s disconnection and the new source’s reconnection. An open in-phase transition is used where the delays are undesirable. The controller monitors the voltage and phase of both the sources and makes the transition when both of them match. Since the break is involved, the open type is less desired for critical loads.
  2. Close transition – These are used where no interruption in power is acceptable. As expected, these are “make before break” types where connection to the new power source is made before disconnection from the old. It also works by ensuring that the voltage and phase between the two sources match each other before initiating the transfer. If the old source fails completely, it works like an open mode. The transition path to the old source is a close transition.

Types By Application

  1. Service Entrance-rated Transition Switch – It may be cost-effective to install a service entrance-rated ATS between the utility meter and the main panel for new building installations. Such an ATS performs the function of the main breaker and the whole house transfer switch. Since it is before the main panel, it can be located outside, minimizing the cable entries to the house and the costs.
  2. Switched Neutral Type – You may require a switched neutral type ATS if you have a floating neutral in your portable generators or your standby generator system is separately derived.
  3. Other types like Drawout, Bypass Isolation, and Maintenance Bypass type are normally required for commercial applications.

Types By Switching Mechanisms

Depending on your application, ATS might be called upon to exhibit different operating characteristics. In general, they should have the ability to

  • Close even in presence of high inrush currents,
  • Carry the full rated current continuously,
  • Interrupt the service currents,
  • Withstand the fault currents.

The selected switching mechanism must be able to carry the required functionalities out of those listed above. The different types based on the switching mechanism are:

  1. Contactor Type – This is the most common and economical type. The contactors used are constructed as a double throw, electrically controlled switch. A single operator opens a set of contacts and closes the other set. These contacts are not the ones used for motor starting or lighting applications. They are circuit breaker-type contacts with arc chutes and arcing horns.
  2. Molded Case Type – Also known as the circuit breaker type switches, these can be operated by mechanical means through an over-center toggle or by electrical means through motors. A molded case switch resembles a molded case breaker except for the magnetic and thermal trip elements. They find use where a compact and high-capacity disconnect switch is required.
  3. Power Frame Type – These are larger, faster, and rated for a few thousand amps.

Other Important Features

Other useful features that many automatic transfer switch makes provide are:

  1. Automatic transfer switches are available for both indoor and outdoor applications. For indoor applications, you can use NEMA – 1; General purpose ATS, while NEMA 3R – weather-resistant type ATS is required for outdoor applications.
  2. Many ATS also provide such protection for the electrical equipment in your entire home. This is particularly helpful for your sensitive electronic devices.
  3. Some ATS have truly active power management and energy monitoring systems, where the ATS is capable of actively balancing the electrical loads at your home. The system will continuously monitor the power usage and ensure that a hundred percent of the generator capacity is utilized. They will also shed some of your loads if they find the generator cannot meet those loads’ demands, like big central air conditioners.
  4. They are also available in the generator-ready load center configuration, where the load centers in the form of a single kit carry a factory-installed ATS. They have multiple circuits for essential and non-essential loads and hence are one step solution to provide automatic backup power. These load centers can replace your entire main incoming electrical panel and are capable of providing electricity only to essential circuits during an outage.
  5. Three-Way Transfer Switch – ATS is also available in a three-way configuration in case you want to connect the power sources as discussed above.
  6. ATS must conform to international standards like UL 1008, NEMA standard ICS10, or IEEE446 (commercial and Industrial installations).

Sizing Your Automatic Transfer Switch

You must decide the loads that will receive power from the ATS, like in the case of manual transfer switches. Does it include a sump pump, water heater, or only the essential loads? Based on this decision, you can select either

  • Whole house transfer switches, or
  • Backup Generator Sub Panel Automatic Transfer Switches

For whole house applications, the simplest way to select the amperage of your ATS is to match it with the rating of your main incoming breaker for receiving the utility power. If the main breaker is 100 A, select an ATS of 100A. Check this against the maximum amperage from your standby generator.

However, if you are using a load center type panel and backing up only some critical loads, then the ATS can match the amperage of your generator or that of the selected loads.

Can I install my own ATS?

It is best to utilize the services of a qualified electrician as you require a permit from the authorities to install and operate an ATS for your standby generator. The authorities will not issue the permit without certification from a qualified electrician.

Some insurance companies offer a discount on your homeowner’s insurance if you have an ATS installed. These companies also require certification from the qualified person that ATS has been installed correctly.


Automatic transfer switches are an important piece of equipment for your generator. They allow the generator to seamlessly switch to utility power during a power outage and back to the generator when the power is restored. There are many different types of automatic transfer switches available, so it’s important to select the right one for your needs.

We’ve outlined some of the most common types of automatic transfer switches and what you need to consider when selecting one. Please let us know in the comments section whether you found the article helpful or if you have any questions about which type of switch would be best for you.

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