Generator Interlock vs Transfer Switch(What’s The Difference)

As soon as you hear about an approaching hurricane or a snowstorm, the first thing that comes to mind is the power outage that you might have to live through. In an event like this, having a backup generator is an absolute need. It allows your appliances to be up and running in no time as soon as it starts up. One of the most important things when opting for a generator is the decision between having an interlock kit or a transfer switch.

What is the difference between generator interlock and transfer switch?

An interlock is a small connector device that is installed on the front of the panel of your home breaker that allows you to use your generator to safely back feed your panel and power the circuits in your house. While transfer switch is a small switch that manages the electric load between two power sources and allows you to switch between the two easily.

Both of these devices help transfer the electricity supply from the main utility switch to the generator if the need arises. However, the choice depends on your power demands. Choosing the wrong device could cost you your appliances, safety of the building, and even your life.

Read on to know more about interlock kits and transfer switches and to find out what meets your needs in the best way.

The Difference Between Generator Interlock and Transfer Switch

Interlock – What is it?

An interlock is a small connector device that is installed on the front of the panel of your home breaker. It allows you to use your generator to safely back feed your panel and power the circuits in your house. The interlock manages the power source, making sure that your home panel is being supplied through only one source. It is designed to guarantee that the main breaker and the generator breaker cannot be turned on at the same time. 

How to Operate?

You will need an electrician to install the interlock kit in your home and wire your generator breaker. A fully installed system should have:

  • The interlock kit installed on your electrical panel
  • A generator breaker. This is from where the generator power will run through the panel
  • A waterproof inlet box on the outside of your house
  • A generator power extension cord that connects the generator and the inlet box

Operating an interlock is very simple as it is mostly automatic. We will explain the procedure in steps so it is clear:

  • Turn off the main breaker
  • Connect the generator power cord to the inlet box
  • Turn on the generator
  • Slide the interlock kit up
  • Turn on the breakers one by one according to your need or all if your generator is capable of supplying the whole load

And that is it.

How to turn off the generator:

When the power comes back on, you have to follow these steps:

  • Turn off all the breakers that are on
  • Slide the interlock kit down
  • Turn on the main breaker
  • Turn on the breakers one by one again


Interlocks have two types; some are automatic while some are manual. Manual interlock cannot detect that your home’s electrical panel has lost power so they need to be turned on manually in the way we have explained above. An automatic interlock senses that the circuits have lost or gained power and are triggered accordingly.

As soon as the power goes out, the interlock switch is slid up to turn on the generator breaker circuit. This effectively blocks the main power circuit breaker, turning it off. It ensures that the power is flowing in the electrical panel in only one way.

When power comes back on, the switch is moved down and blocks the generator back feed circuit breaker, allowing the main power source to flow freely.


  • Interlock kits are safer
  • They operate as many circuits at once as your generator’s capacity allows
  • They are inexpensive
  • They are more reliable


  • They do not carry UL Listing marking, so they cannot be used everywhere
  • It is permanent in some cases
  • You cannot know when the power comes back on unless you check manually
  • They are manual so you may need to step out of the house during a hurricane or a storm

Where is it used best?

Interlocks are best to be used in places where continuous power isn’t required like;

  • Residential homes
  • Small shops/business

Transfer Switch – What is it?

A transfer switch is also called a transfer panel. It comes pre-installed in some backup generators. It is a small switch that manages the electric load between two power sources and allows you to switch between the two easily. It has a switch dedicated to each circuit breaker and can operate 6 to 10 circuits at once. It is also useful if you want to power only one or two of your appliances; for example, a refrigerator or lights. A transfer switch also comes in two types; manual and automatic. 

How to operate?

Operating the transfer switch is extremely easy even if it is a manual transfer switch. They usually have three positions:

  • Utility: When the switch is on this option, the system is running on the primary utility power supply.
  • OFF: When the switch is in off position, the system is not running and needs to be turned on 
  • Generator:  Switch in this position indicates that the system is running on the power supply from the generator.

If you are using a manual switch, you have to follow the following steps to turn it on in case of lost power:

  • Make sure the transfer switch cord is plugged in the generator
  • Slide the switch to Generator mode
  • Turn on the generator

It is as simple as pressing a button.

How to turn off the generator:

When power comes back on:

  • Turn off the generator
  • Switch the button back to utility position


Automatic transfer switches are larger and have a push and pull relay system. It automatically detects when the power goes out and then pulls in or out and let the power come either in or out of the generator or the main power source. This way it energizes the circuit it is connected to in the circuit breaker panel.

Transfer switches usually have two modes of transmission; open transition and closed transition.

The open transition transfer switch breaks off one pathway of electricity before making another for it to flow. There is a short moment of interval in which the electric flow changes paths so it should not be used in places that cannot bear this interval like hospitals.

The closed transition transfer switch makes the other pathway for electricity to flow before it breaks off one. There is a very slight pause in the flow of electricity, almost 1/6th of a second. It can easily be used in places where the loads cannot be affected by even a slight loss of power. 


  • It offers extra flexibility, the circuits of choice can be provided power while keeping some appliances that do not need power 24/7 turned off
  • It can be used even when you are not home
  • It is very easy to operate
  • It is usually safer than the manual hookups used in interlocks especially in the cases of hurricanes or blizzards where stepping out of the house can be dangerous   


  • It is expensive
  • It can only feed a few circuits
  • Installation takes longer because the setup is custom to the place being installed to
  • The load connected to the generator has to be less than how much power can be supplied by the generator in case of the automatic transfer switch. This is because, with no one to operate and shed the load, appliances could get fried.

Where is it used best?

It is best used in places where the power must be brought back on immediately and automatically, like:

  • Factories
  • Hospitals
  • Food warehouses
  • Commercial freezers


Overall both are extremely good options for you, depending on your needs. But the automatic transfer switch does have a lot of benefits. It is quite inexpensive and gives the customer great value for money. Thereby, if you are a little low on the budget then it will be a great choice for you. Not only this, but it is also very safe and secure if you have kids at home, so it stands out as a great choice.


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