Electrical – Are generator interlock kits permitted in residential applications

code-complianceelectricalgenerator

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?

Update

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.

Best Answer

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.

WARNING

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.

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