Generator Interlock Vs. Transfer Switch (Here Are The Differences)

Sean Jarvis
by Sean Jarvis

During certain moments it may become necessary to use a standby or backup generator. This is especially true during emergency situations. These can restore the lights, ventilation, computers, devices, and other critical systems.

When using a portable generator, you will need either a generator interlock kit or a transfer switch. Which to use is dependent on a variety of factors.

The difference between a generator interlock and a transfer switch is that transfer switches are safer to use, and they are best used in situations where the power must come on immediately. A generator interlock involved training and understanding on how to operate, as it is manual and not automatic.

Portable generators are often used during natural disasters such as hurricanes, and in locations with well-known bad weather patterns. This will supply back up power when the electrical grid goes down and keep sump pumps working to protect your home.

Portable generators come in many different sizes and it’s important to know how many watts you will need to run your home. Whether you are looking to only power critical appliances, like the sump pump and fridge, or run your entire home like you normally would, there are generators that can handle nearly any load.

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What is a Generator Interlock Kit?

A generator interlock kit is a device that allows for the safe powering of a home by a portable generator during a power outage.

An interlock kit manages the power source to the electrical panel. It ensures that the generator circuit breaker and the main circuit breaker cannot be turned on at the same time. This makes it safe for anyone working on the electrical lines and for preventing dangerous feedback.

A generator interlock kit gives the same end result as a transfer switch, by adding an external interlock onto an existing breaker panel that allows the main breaker to be turned on. It can also designate one load breaker to be turned on, though not both at the same time.

How Does a Generator Interlock Kit Work?

Manually operated, the interlock switch is moved from the main power breaker to the generator breaker when the utility power goes out.

A generator interlock kit is installed on the front cover of a breaker panel. There are two sliding steel or plastic plates that are held together by bolts. When moved down, the plate block the generator back-feed circuit breaker and allow the main breaker to stay on.

When it is moved up, the generator back-feed circuit breaker may be turned on, but the main circuit breaker is blocked and will stay off.

The generator back-feed circuit breaker is connected to a generator inlet installed on the outside of the structure. A short cord is what connects the generator to the house inlet, through a twist lock plug and socket. Once in generator mode, the back-feed breaker is on and accepting power from the generator and to the panel. The main breaker is off which isolates the mains.

The home is fed by back-feeding the panel, but the panel cannot back-feed the external mains. If the external mains is back-fed, this can potentially electrocute line-workers, start fires, and overload or damage the generator.

Upsides to a Generator Interlock

An interlock kits lets the generator energize the panel and all of its circuits. It can be installed directly on the breaker panel and does not require a separate panel to operate. It also removes the need for extension cords to bring power to different appliances.

Downsides to a Generator Interlock

Generator Interlocks do not carry the UL listing mark, though many are tested by an independent lab to a UL standard. Without this UL listing, some electrical inspectors will not allow the use of interlocking kits. If you remove the electrical panel cover, it could allow the generator back-feed breaker to be turned on while the main is on, possibly energizing the power lines or overloading the generator, if connected.

Because the main breaker needs to be shut off, you will have no way of knowing when the utility power is restored. The only way you will be able to know this is by manually turning the generators break off and the main breaker back on.

If you are not a licensed electrician, it is best to not install these yourself. The only safe way to install a temporary interlock for a portable generator is to disconnect the main power grid from the house.

What is a Transfer Switch?

A transfer switch, also referred to as a transfer panel, switches between power sources. A manual transfer switch requires someone to manually flip the switch when the power goes out.

An automatic transfer switch starts the generator without any human interaction, when the power goes out or becomes unstable, and then switches back when the power is restored.

Many commercial buildings require an alternate power source to comply with the National Electric Code (NED) and other building codes. Refrigerated food cases in grocery stores and lifesaving equipment in hospitals, must be connected to an automatic switch.

How a Transfer Switch Works

If the utility power source fails, an automatic transfer switch (ATS), will switch on and start the backup generator. It commands the back-up generator to start based on the voltage monitored on the primary supply.

In a home that is equipped with a backup generator and an automatic transfer switch, the ATS will tell the backup generator when to start. Once the generator is ready to provide power, the transfer switch breaks the connection to the electric utility and connects the generator to the home’s electrical panel.

The generator then supplies the home with power and is not connected to the electric utility lines. This is to protect the generator from overload in powering loads in the house and for safety, as line workers will expect the electrical lines to be dead.

A transfer switch can be used to provide power to entire electric subpanels of the home, or specifically critical circuits. There are also transfer switches that are able to reduce peak load demand from a utility by smoothly transferring from the utility to the synchronized generators.

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Related Questions

Is a Transfer Switch Necessary?

It is not only necessary but legally mandated. You must install a transfer switch in order to operate a portable generator for your home.

How Far Can a Generator Be from the Transfer Switch?

Air-cooled generators should be located as close as possible to the transfer switch and fuel supply. There must be 5 feet of clearance around the gas meter as well.

How Big of a Generator Do I Need to Run My House?

Most critical household equipment can be run with a generator rated at 5,000 to 7,500 watts. These include a well pump, fridge and freezer, and all lighting circuits. The more watts that your generator is capable of, the more likely that you can run multiple appliances at once.

Sean Jarvis
Sean Jarvis

Sean Jarvis is an interior decorator, writer, and expert handyman. Well versed in everything home improvement, he is a savant at manipulating words and spaces and upgrading everything around him. Sean specializes in writing concise guides about appliance repair and installation, home and lifestyle, and other residential projects.

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