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How Does a Generator Transfer Switch Work?

A Generator, be it portable or standby, acts as an alternative source of power or backup power in the event of a power outage. In some situations such as RV, Camping, or Worksites, generators become the main source of power. If your region has frequent blackouts or if the risk of storms or hurricanes is high, then Generators or Gensets become very useful as it takes some time to restore the grid. Speaking of the grid, we have to use the electricity from the generator only if the grid fails. A special device (or a piece of equipment) known as a Transfer Switch helps you switch between the utility power and the generator power. But how does a generator transfer switch work? What are the types of Transfer Switches?

If you bought a portable or standby generator (or planning to buy one), then you should also add a Transfer Switch to the cart. It will protect the residential infrastructure from any mishaps (the prominent one is back feeding to the grid) by safely switching between the two sources of power.

In this “Basics of Generators” guide, we will discuss an important part of the generator ecosystem which is the Generator Transfer Switch. We will see the need for transfer switches, how it works, different types, and many other important things.


The number of homeowners investing in backup power generators is increasing year after year. Whether it may be due to unreliable power supply from the utility (especially in rural areas) or due to unpredictable weather (or natural disasters), generators come in handy.

You can continue using essential electrical appliances (such as lighting, heaters, refrigerators, medical equipment, etc.) even when the grid fails. If you are a small business owner, then you can continue operating the business even during a prolonged power outage.

If you aren’t familiar, backup generators come in two basic forms; Portable Generators and Standby Generators. You can easily transport/move Portable Generators as they are relatively small and come with wheels (even smaller ones have a handle to carry them anywhere).

Standby Generators offer a permanent solution where we fix them in an ideal spot and wire the output of the generator to our home’s main breaker panel. Speaking of wiring, you can either permanently connect the generator to the existing residential electrical system.

Alternatively, you can run appliances by directly connecting them to the receptacles on the generator itself (this is usually possible on small and portable generators).

Regardless of the type of generator, if you are planning to hardwire the generator with the current wiring of your home, then don’t forget to include a Generator Transfer Switch in the process.

What is a Generator Transfer Switch?

A Generator Transfer Switch is a simple mechanical or electromechanical device that switches between the utility supply and the generator supply to the main circuit breaker panel.

Assume you are running all the electrical and electronic appliances in your home from the utility power supply. All of a sudden there is a power outage. You are not sure when will the utility restore the grid supply.

As you have a backup generator at your disposal, you want to fire it up and power your home with it. The load, which in this case is the entire home (technically, all the devices that are using electricity) must be connected to only one source at a time.

So, if you want to connect the supply from the generator, you have to first disconnect the utility supply at the main breaker panel and then connect the generator to the panel. This is where the Transfer Switch becomes important.

The Generator Transfer Switch physically disconnects the main breaker from the utility supply and connects it to the generator supply. Of course, this action is reversible.

When there is a power outage, the transfer switch disconnects the grid supply and connects the generator supply to the main circuit breaker. As the utility power comes back, it disconnects the generator supply and switches back to the grid supply.

In this way, the generator transfer switch ensures that the load is connected to only one source at any time.

If you are familiar with toggle or rocker switches, then the Generator Transfer Switch is analogous to a Single Pole Double Throw (SPDT) Switch.

What is the Need for a Transfer Switch?

Imagine the following scenario. You had a power outage in your area and the utility company is working hard to restore it as quickly as possible. In the meantime, you have a standby generator at your home and wanted to start it and run some important electrical devices.

If you haven’t installed any transfer switch and connected the generator directly to the existing electrical system, the supply from the generator can run back to the grid.

The worst is yet to come. We know that the street transformer or distribution transformer near our house steps down the 11kV supply from the substation to 120V or 240V. This is what you get from the utility.

An interesting about transformers is that if they step down the voltage in one direction (from utility to your house), they can step up in the other direction (from your house to the grid).

So, when you connect the generator directly to the main circuit breaker in your house during a power outage, this electricity is back fed into the grid. Once it reaches the street transformer, it steps up the voltage to thousands of volts.

If any utility technician is working on restoration at that time, the back feed from your generator could potentially kill him/her.

But if you have a Transfer Switch, it completely isolates the grid supply from the generator supply and makes sure that only one source supplies electricity to the load.

Additionally, the transfer switch protects the generator from the utility supply. If you don’t have a transfer switch and connected the generator directly to the main breaker, then as soon as the grid restores, the supply from the grid could completely damage the generator.

Types of Generator Transfer Switches

There are essentially two types of Transfer Switches. They are:

  • Manual Transfer Switches
  • Automatic Transfer Switches

The basic functionality of both these transfer switches is the same i.e., disconnect one source of power and then connect the other source to the load. The difference lies in how they implement this functionality. Let us now see some brief details about a Manual Transfer Switch and an Automatic Transfer Switch.

Manual Transfer Switch

Starting with the simplest of the two, a Manual Transfer Switch, as the name suggests, must be activated manually. When the power goes, the homeowner (or store owner) must go to the location of the transfer switch and turn a knob or pull a level (or something similar).

Manual Transfer Switches are almost completely mechanical devices. Internally, they have a couple of circuit breakers that switch between the utility supply and the generator supply.

An intelligent mechanical locking system in a Manual Transfer Switch ensures that the load connects to only one source. Also, this interlocking mechanism is important as it switches off one source completely before connecting the load to an alternative source.

The cost of a manual transfer switch is very low. Also, they are very easy to install. But the downside is, well, you have to manually switch the sources. As this is a time taking process, we generally do not use this type of switch in critical systems (such as medical applications).

For residential and small business applications, manual transfer switches are more than sufficient.

Automatic Transfer Switch

You might have guessed the operation of an Automatic Transfer Switch by now. As the name suggests, an Automatic Transfer Switch or ATS automatically disconnects the utility power in the event of a power outage and connects the load to the generator power.

The Automatic Transfer Switch will detect the power loss from the utility, starts the generator, waits till it stabilizes, disconnects the grid supply, and finally connects the generator supply. There is no need for any human intervention at all.

Internally, an Automatic Transfer Switch consists of several mechanical and electronic components. But the basic things are a locking mechanism, a motor (to move the locking mechanism), and a logic circuit to detect the grid supply and generator supply.

Automatic Transfer Switches are important (and sometimes necessary) in critical applications such as medical, emergency, food storage, and certain industries.

Speaking of industries, Automatic Transfer Switches are very common in manufacturing, assembling, and processing industries as it switches to backup power automatically and there won’t be any time delay in switching operation. This means no loss of work.

The downsides of Automatic Transfer Switches are they are extremely complicated devices with microprocessors (or another logic controller) and also very costly.

How Does a Generator Transfer Switch Work?

Now that we have seen the importance of a Generator Transfer Switch and its types, let us see how a transfer switch works.

From the discussion till now, it is clear that a generator transfer switch is an important part of the backup power supply system. But how does a generator transfer switch work?

Manual Transfer Switch

The manual transfer switch is typically installed near the main electrical panel and connected to both the utility power and generator power sources. Internally, the manual transfer switch has multiple switches that are used to manually select the power source.

For instance, if you want to switch from utility power to generator power, you would turn off the main breaker for the electrical panel, then use the switches on the manual transfer switch to select the generator power source.

When the manual transfer switch is set to the generator power source, the generator’s power will be directed to the electrical panel, providing power to your devices and appliances.

But when the generator is turned off or if the manual transfer switch is set to the utility power source, the power will come from the utility company. Some manual transfer switches may have built-in safety features like automatic transfer back to utility power when the generator is turned off or if there is a fault in the generator’s electrical system.

In short, a manual transfer switch provides a simple and effective way to switch between two sources of power, such as utility power and generator power. The manual transfer switch allows you to use the generator as a backup power source during power outages, or to switch to generator power when you need portable power for outdoor activities or other purposes.

Automatic Transfer Switch

An Automatic Transfer Switch operates in the following three steps.

Normal Operation

When utility power is present, the transfer switch connects the electrical load to the utility power, allowing the generator to remain idle (or even off). During this time, the switch continuously monitors the voltage of the incoming supply.

Power Outage

When a power outage occurs, the transfer switch detects the loss of utility power and automatically switches the electrical load to the generator power. But it doesn’t do that immediately.

First, as soon as it detects a power outage, it signals and starts the generator. Most generators take some time to get to speed and become stable. So, we shouldn’t connect any load as soon as we start the generator.

The Automatic Transfer Switch also gives some time for the generator to stabilize its engine speed and also output voltage. Once the output is stable, it disconnects the utility power first and then connects the load to the generator power.

Now, the generator starts supplying power to the electrical load. Even when the load is connected to the generator, the Automatic Transfer Switch still monitors the voltage of the utility supply.

Utility Power Restoration

When utility power is restored, the transfer switch detects the return of utility power and automatically switches the electrical load back to the utility power. It first disconnects the generator supply (with the generator still running) and then connects the load to the utility power.

After a certain observational period, the Automatic Transfer Switch signals the generator to turn off and the switch returns to normal operation.

Where is Generator Transfer Switch Useful?

A generator transfer switch is typically required in any location where a backup generator is used to provide power during a power outage or other emergencies. This includes:

  • Residential Applications: A generator transfer switch is often used in homes to provide backup power during power outages. It ensures that essential appliances, such as refrigerators, lights, and air conditioning systems, can continue to operate even when the main power source fails.
  • Commercial Buildings: Businesses often rely on a backup generator to keep essential operations running during power outages. A generator transfer switch ensures that computers, lighting, heating, and other electrical systems can continue to operate, minimizing downtime and financial losses.
  • Industries: Industrial facilities, such as factories and warehouses, often require a backup generator to ensure continuous operation during power outages. A generator transfer switch helps to ensure that essential electrical systems and equipment continue to function, minimizing downtime and production losses.
  • Emergency Response Centers: Hospitals, fire stations, and other emergency response centers require a consistent and reliable source of power to provide essential services during power outages or other emergencies. A generator transfer switch helps to ensure that these critical facilities have a reliable source of power during an emergency.
  • RVs: Recreational vehicles, such as RVs and boats, often require a generator to provide power while on the road or at sea. A generator transfer switch helps to ensure that essential appliances, such as lights and refrigerators, can continue to operate during power outages or other emergencies.

In each of these scenarios, a generator transfer switch helps to ensure a safe and seamless transition of power during an interruption of the main power source. It helps to prevent back feeding of power from the generator to the utility grid, which can be dangerous for utility workers, and ensures that essential appliances and systems continue to operate during a power outage.


A generator transfer switch is a device that connects a backup generator to the electrical load (e.g., appliances and lights) in a building. The switch automatically switches the electrical load from the utility power to the generator power and back, in the event of a power outage.

The generator transfer switch is an essential component of a backup generator system as it ensures a safe and seamless transfer of power between the generator and the utility power.

In this guide, we saw the basics of a generator transfer switch. We looked at the need for a transfer switch and also different types of transfer switches.

After that, we answered the question “How does a generator transfer switch work?” by explaining the operation of both the Manual Transfer Switch and Automatic Transfer Switch.

We hope that this guide on Generator Transfer Switches could help you understand all the essential things about a transfer switch. If you feel we missed something or want us to add anything, do let us know in the comments section below. It will not only help us but also other readers.

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