Generator Accessories Generator Care

Are Generator Interlock Kits Legal & Safe for You?

A generator interlock kit is a perfect alternative to a generator transfer switch. It will ensure that you meet the necessary electrical codes and ensure your generator backup line safely powers your home during a power outage. Most people go for it as an inexpensive option, but a question bothers most customers: are generator interlock kits legal?

Yes, generator interlock kits are legal, just like a transfer switch. However, it would be best if you had it installed by a qualified electrician. The installation has to meet the strict National Electrical Code requirements and National Fire Protection Code. And with most kits listed under UL 67 by independent labs, you can legally use them in the USA.

Unfortunately, some are not legal in other countries, such as Canada. It’s also crucially important that the unit be tested, serviced, and maintained by qualified personnel. Everything you need to know about generator kits, codes, installation, and more is in this post.

What are the Regulations and Requirements to be Met When Installing a Generator Interlock Kit

For an interlock to be legal in the USA, its installation must meet a set of ordinances and requirements

Guidelines for Operating a Portable Generator Using a Manual Transfer Equipment:

  • Equipment installed permanently for a portable generator connection must comply with 2008 NEC (National Electrical Code) section 702.6.
  • According to this requirement, the manual transfer equipment must have the interlocking device designed for the purpose at hand.
  • The interlock device must prevent inadvertent interconnection between the main power supply and the optional (generator) supply.
  • It must also be a standard 231 UL Listed Power Outlet. Lastly, the manufacturer’s installation info must be on-site for inspection and the device installed per the manufacturer’s instructions.

Guidelines for Operating Permanent Generators Using a Manual Transfer Switch:

  • When using a permanently installed generator (standby generator) with an interlock kit (permanently installed manual transfer device), the whole setup must comply with the provisions provided for portable generators or use a panelboard or a listed transfer switch.
  • • The interlock device must be installed to allow you to select the loads during manual transfer. It must also be installed as per the instructions provided by the manufacturer and the manufacturer’s installation information available on-site for inspections.

NFPA 70 – National Electrical Code®

  • Under UL 1008 standard, transfer equipment must prevent removing panel covers or doors from making the necessary interlocking mechanism useless.
  • The standard installation of a UL 67 approved generator interlock kit needs to be done on the panel board cover. If the cover is removed, the interlocking device won’t work.

National Electrical Code ANSI/NFPA 70 Article 70

  • The ordinance states that your generator interlock MUST only allow one of the two breakers to be on the ‘ON’ position at a time.

About Generator Interlock Kits

An interlock generator kit is designed to safely make the generator hook up to the mains breaker box. It’s designed to lock out the home panel main breaker and allow the backup generator (portable) to power specific breakers by back-feeding through the generator breaker.

It’s an inexpensive alternative to the famous, dedicated transfer switch, both installation, and purchasing. It works the same way as a transfer switch and is repurposed as the main “back feed” breaker.

How does the Interlock kit work?

The mains breaker stays on during the expected condition, allowing the panel’s utility electricity to power. The interlock device keeps the back feed breaker off, thus isolating the generator power.

That way, the utility power energizes the main panel but cannot back feed (transfer charges to) the generator. That prevents electrocution and risks damaging your generator.

When there is a power outage, you’re required to set the generator mode on and the mains power breaker off. That ensures the power panel receives power from the generator while preventing beck feeding the utility power.

Doing so is not only unsafe and dangerous but also illegal. It can potentially cause electrocution to the line-workers, overload the line, start a fire or even damage your properties.

The early versions of the interlock kit were used freely, but today, you need to buy a UL-listed model to use it in a UL-listed power panel.

And now, with a single connection, you can power everything in your home using a backup generator since all your circuits are activated through the breaker panel.

It’s also a perfect way to eliminate running those unsightly and potentially risky extension cords to your home from the generator and plugging and unplugging in appliances and lights.

Once the interlock kit is installed, it is easy to transfer power from the utility line to the generator power during an emergency. And the same applies when the power is restored. Here are the few steps involved:

  • Turn your main panel breaker to its “OFF” position
  • Switch OFF all the branch circuits
  • Slide the plate to the backup/generator mode (whichever point allows the main power to stay at the “OFF” position)
  • Hook up your generator to the outlet and start the engine
  • Now turn the main generator breaker ON
  • With the panel energized, turn on your essential circuits, one at a time. Remember to allow the appliance to start before you can engage the next branch breaker.


Some appliances (the motor-driven types) such as microwaves, dryers, the water heater might not run simultaneously. It’d be best to engage the circuits one by one and wait for such appliances to start before engaging the following circuit. Ensure you understand the generator’s amount of power and list things you can run with the generator to single out their circuits in the breaker.

Where Can You Use Generator Interlock Kits

Generator interlock kits are primarily used in residential and small commercial applications and less frequent power outages.

Yes, they are straightforward to operate. However, you might need additional power management skills to know what circuits to turn on and turn off to prevent exceeding the generator capacity.

At times, the owner might need proper training or on-site operation experience to understand operating the device.

In summary, generator interlock kits work best with:

  • Residential and smaller commercial applications
  • Applications requiring less continuous power during an outage
  • Applications where the homeowner or on-site personnel can operate it manually

Related Questions

How much does a generator interlock kit cost?

A generator interlock device is cheap; it costs anywhere between $25 and $150, depending on the complexity and construction quality. However, its installation might cost you between $100 and $350, depending on the installation complexity and the labor rates in your area.

How can you avoid generator overloading when using a generator interlock kit?

When using a generator interlock kit, you have to manually turn off all the breakers one by one and then turn them back on one by one. Every though this works, there is still the risk of overloading your generator. And overloading your generator can result in damaging your appliances or the generator itself. And to avoid such problems, it’d be best to install meters on the generator feed to the breaker panel. It can help you know the amount of load carried at any time and give you the amount of power spared for additional loads. That way, you can determine which circuits to turn on and which to turn off.

About the author

Sharif Gen

Leave a Comment