Installing a Service Disconnect

Many changes have occurred to the National Electrical Code (NEC) since the first NEC was published on Aug. 31, 1897. Even though the electrical industry was in its infancy, this “first” NEC was remarkably insightful and has withstood the test of time. Despite the many changes since the first NEC was established, some of the original requirements are still found in the 2002 NEC.

Electrical technology has advanced greatly in the 100-plus years since the adoption of the first NEC, but the laws of physics have stayed constant. What is truly amazing, however, is the level of safety that the early electrical pioneers recognized and placed in this first Code. Some of these safety issues are still addressed in the present Code. The language may have been modified, but the original concept is still there.

The intent of Section 230.70 in the 2002 NEC for the location of the service disconnecting means at a building or structure has remained relatively unchanged since 1897. In 1897, the electrical experts of the time decided “there must be a main disconnecting switch installed at every building to disconnect all service wires, either underground or overhead, and that it be located in a readily accessible place as near as possible to the point where the wires enter the building. This disconnecting means must be arranged to cut off the entire current.” These early safety experts also felt that electrical power should be turned off when the last person left the building for the evening, so the disconnect was required to be reasonably easy to access as that person exited the building.

The actual text in Section 230.70(A)(1) is as follows: “The service disconnecting means shall be installed at a readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors.” By closely studying the actual text in this section, it becomes obvious that the disconnect switch can be located anywhere outside the building or structure with no maximum or minimum distance from the building. However, as soon as the service conductors enter into the building, the service disconnect must be installed at the location where these unprotected conductors enter. This location will vary depending upon the construction of the building or structure and the accessibility of the disconnecting means inside the building.

Section 230.6 provides some relief where, due to the construction of the building, it is more desirable to install the service disconnecting means at a location further inside the building. The service conductors can be installed under not less than two inches of concrete beneath the building or structure or installed within the building or structure in a raceway that is encased in two inches of concrete or brick. Where the floor of the building is not concrete, the service conductors may be installed in conduit under not less than 18 inches of earth. The service conductors can also be installed within a three-hour rated vault that meets the construction requirements of Part III of Article 450 for transformers.

Where any of these four requirements are satisfied, the service disconnecting means would be required to be installed where the service conductors emerge into the building. This would permit the service disconnecting means to be located on the roof of the building or somewhere in the interior of the building. It is always best to limit the distance that service conductors can travel within a building or structure, thus limiting potential danger to the building and occupants.

A recent addition to Section 230.70 is permission to use shunt trip devices at the service disconnecting means located within the building but locating the shunt trip remote operating devices on the outside of the building. Locating these remote operating devices outside the building permits the fire department or other emergency response teams to disconnect power to the building without having to enter the building first. These remote devices are not listed for use as service disconnecting means, as required by 230.66. It should be noted, however, that shunt trip devices do not guarantee that operating the remote device will actually disconnect power to the building. Without power to these remote devices, the shunt trip devices would fail to operate. Always verify that power is turned off before working on the electrical equipment and make sure the power has been locked off.

Many of the tools that we now use to provide the data necessary to design and install electrical installations were just simply not available to the electrical pioneers that first provided us with a National Electrical Code back in 1897. Yet, we still can recognize the basic concepts of these first safety requirements imbedded in the present National Electrical Code. EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at



About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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