What is an interlock kit?

An interlock kit is a mechanical safety device installed on an electrical service panel. The purpose of an interlock kit is to prevent the utility main breaker and the generator breaker from being turned on at the same time. This prevents dangerous back-feeding into the utility grid, protecting utility workers, electrical equipment, and your property. Interlock kits may have different appearances or configurations depending on the style of the utility panel. It's important to note that Interlock kits are not universally accepted in all jurisdictions.

Note: Generac does not manufacture, support, or provide installation instructions for interlock kits. This information is for reference purposes only.

Example of an interlock kit

Typical applications

• Portable generator backup: Used when homeowners want to power select circuits during an outage without installing a manual transfer switch.
• Low-cost solution: A cost-effective alternative to a manual transfer switch when powering limited electrical loads.
• Seasonal or occasional backup power: Ideal when backup power needs are infrequent or temporary.

How an interlock kit works

The interlock system mounts directly onto the breaker panel and uses a mechanical sliding plate that blocks either the main breaker or generator breaker from being switched on simultaneously. To energize the panel using a generator, the main breaker must first be turned OFF, allowing the generator breaker to be turned ON. The purpose is to help ensure utility and portable generator power sources are not active at the same time.

What is the difference between an interlock kit and a manual transfer switch?

	Interlock Kit	Manual Transfer Switch
Installation complexity	Installs on existing panel with minimal wiring; generally the lowest-cost option. Should only be installed by a licensed electrician.	Requires a dedicated enclosure and circuit rewiring; higher cost and more installation time.  Should only be installed by a licensed electrician or Generac authorized dealer.
Safety / Code compliance	Interlock kits are not universally accepted in all jurisdictions. Verify with your AHJ (authority having jurisdiction)  whether an interlock kit complies with local and/or national electrical codes.	Provides clear physical separation of circuits and is often preferred by inspectors and electricians.
Load flexibility	Dependent on existing panel layout; typically powers only essential loads.	Allows selective control over circuits and accommodates more complex load planning.
Cost	Lower overall material and labor cost.	Higher cost due to additional equipment and rewiring.
User operation	User must manually turn off the main breaker before activating the generator breaker.	User switches individual circuits from utility to generator power using the transfer switch.

Which option is right for my application?

While interlock kits are a simple, low cost way to prevent backfeeding and separate utility and generator power sources, they may not be code compliant in all jurisdictions. Using an interlock kit is also less flexible and more manually intensive than using a manual transfer switch.

When connecting a portable generator to a bulding's electrical system, we recommend installing a Generac manual transfer switch for a safe and code-compliant set-up.

 

Related articles

• Why you should never back-feed your electric panel with a generator
• What is the HomeLink manual transfer switch?