There have been some questions on the site lately, that reference using interlock kits with portable generators.

Interlock in service panel

This has inspired me to read through article 702 Optional Standby Systems of the NEC (which is short so it wasn't much effort), where I found the following items that seem relevant.

NEC 2008
702.5 Capacity and Rating.
(B) System Capacity.
(1) Manual Transfer Equipment. Where manual transfer equipment is used, an optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system.

So in the case of an interlock, would the generator have to be rated to supply the whole house, or only the items the user wants to run (which could change at any time)?

702.6 Transfer Equipment. Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment.

A properly installed interlock seems to meet this requirement, so there's nothing here to prevent one. However, there is an exception to this section which may disallow the use of interlocks in residential systems.

Exception: Temporary connection of a portable generator without transfer equipment shall be permitted where conditions of maintenance and supervision ensure that only qualified persons service the installation and where the normal supply is physically isolated by a lockable disconnecting means or by disconnection of the normal supply conductors.

In my mind, this sentence "where conditions of maintenance and supervision ensure that only qualified persons service the installation". Would mean an interlock cannot be installed in a residential panel, because there is no way to prevent the homeowner from servicing the installation.

Are interlocks a cheap and easy solution for homeowners, or can they not be used in residential situations?


I found this interesting response in this thread, on the Electrical Contractor Network Forum. Which appears to be from a representative from a company who makes these types of devices.

Forum Entry

Which basically says the device is NEC compliant because it's the user that is the transfer equipment, not the device.

I also found this semi-related, not relevant to the question at hand, section in the NEC.

408.36 Overcurrent Protection.
(D) Back-Fed Devices. Plug-in-type overcurrent protection devices or plug-in type main lug assemblies that are backfed and used to terminate field-installed ungrounded supply conductors shall be secured in place by an additional fastener that requires other than a pull to release the device from the mounting means on the panel.

Which means if you're backfeeding through a breaker, you'll need to strap the breaker in place somehow.

  • IMO I think the key words are, Temporary connection of a portable generator meaning that transfer equipment may not even be used here. Portable generators are well... portable. If I install proper transfer equipment then in my opinion that is not a temporary installation but a mostly permanent installation of a generator sitting on standby. However what it sounds like is not allowed is for an unqualified DIY'er manually bypassing the main service line to an existing box without transfer equipment. Commented Jan 7, 2013 at 14:54
  • @maple_shaft I think the purpose of the exception, is to prevent an unqualified person from turning the main service on while the generator is connected. Which could easily be done if the cover was removed from the panel, during service to the installation (interlocks are disabled by removing the panel cover).
    – Tester101
    Commented Jan 7, 2013 at 15:36
  • "The user is the transferer of power." OMG! If that logic was any more twisted, it would self implode into a sub atomic black hole! The only possible transferer of power here is the unfortunate utility worker. Transfer equipment is clearly intended to be current carrying switch gear, not flimsy easily defeatable bits of sheet metal. Somewhat clever, I'll grant that, but I'm fairly sure no electric service provider any where would give anyone permission to use this.
    – bcworkz
    Commented Jan 8, 2013 at 0:50
  • I've only ever seen manual transfer switches as subpanels (with 2 to 12 circuits on them), with mechanical switches that flip one way for generator power, and the other way for utility power (and the switch design itself prevents both). This also limits the generator to powering only the sub-panel, which is typically wired for fridge, freezer, furnace fan, sump/well pumps (if applicable), and maybe a couple lights. I agree with @bcworkz, this flimsy metal cover looks sketchy -- not to mention it seems to rely on manually turning off unnecessary circuits to prevent overloading the generator.
    – gregmac
    Commented Jan 8, 2013 at 5:32
  • 3
    I have personally seen an interlock in a panel, it's far from being cheap . there made of stainless steal and are very sturdy. as far as operation, you have to shut of main to even slide up the interlock before you can flip on generator breaker. then, turn on your cercuits that are tagged for emergency power only. the only thing that would get damaged, if a home owner did not properly operate is his 1000.00 generator
    – user15712
    Commented Oct 21, 2013 at 19:40

3 Answers 3


It seems the decision is ultimately in the hands of the Authority Having Jurisdiction (AHJ), as long as "equivalent objectives can be achieved" using the device.

National Electrical Code 2008

ARTICLE 100 Definitions

Approved. Acceptable to the authority having jurisdiction.

Authority Having Jurisdiction (AHJ). An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure.

ARTICLE 110 Requirements for Electrical Installations

110.2 Approval. The conductors and equipment required or permitted by this Code shall be acceptable only if approved.

ARTICLE 90 Introduction

90.4 Enforcement. This Code is intended to be suitable for mandatory application by governmental bodies that exercise legal jurisdiction over electrical installations, including signaling and communications systems, and for use by insurance inspectors. The authority having jurisdiction for enforcement of the Code has the responsibility for making interpretations of the rules, for deciding on the approval of equipment and materials, and for granting the special permission contemplated in a number of the rules.

By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.

This Code may require new products, constructions, or materials that may not yet be available at the time the Code is adopted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code adopted by the jurisdiction.


While these devices may or may not be approved by the AHJ, they are not the best solution available. They do not prevent the user from overloading the generator, since the load selection is made by the user dynamically during operation. The better, safer solution, is to install manual or automatic transfer switches. This allows you to pre-select the loads making it a more user friendly solution.

  • 1
    I have helped install several generators, always with a manual or automatic transfer switch. Never something that lets you choose or change the loads during operation. I didn't even think that was a possibility but sounds like trouble :O Commented Feb 21, 2013 at 12:51
  • +1 - questions like this can only be answered by talking with your local AHJ. Commented Jun 13, 2013 at 3:37

As a Master Electrician, interlock kits are a great option and if they have been tested by UL or other groups like CSA they are generally accepted by AHJs. I would not buy a cheap, un-tested one but you can purchase them from OEM's and get the test report with the kit.


Based on the quote:

Temporary connection of a portable generator without transfer equipment shall be permitted where conditions of maintenance and supervision ensure that only qualified persons service the installation and where the normal supply is physically isolated by a lockable disconnecting means or by disconnection of the normal supply conductors.


I would say that any home owner who isn't a licensed electrician may NOT install an interlock. The only safe way to install a temporary interlock for a portable generator - say in the case of a power outage - is to disconnect the main power grid from the house (even if that just means shutting off the main).

Joe Average Homeowner will not have the training necessary to keep that safety fact in mind.

In addition, as stated in the comments, Joe Average Homeowner can easily remove the panel cover, which would disable the interlock by removing it, allowing a scenario where the generator switch and main circuit can be engaged simultaneously.

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