Sometime between the 1925 and 1933 National Electrical Codes , the maximum number of service disconnects changed from one or two disconnects to a maximum of six. I cannot verify the exact edition where the change occurred. I have original copies of the 1925 and 1933 NEC books and the 1933 NEC handbook written by Arthur L. Abbott, so I can verify the one or two disconnect rule in the 1925 NEC and the six disconnect rule in the 1933 NEC .
First, I will explain the reasoning behind the one or two service disconnecting means. There were two reasons for the one or two main service disconnect rule. The two service disconnecting rule permitted a service disconnecting means for electrical supply power to a store or business on the bottom floor of a building with the second service disconnecting means for a residence on the top floor of the same building. A common building configuration at that time was a business occupying the bottom floor and the owner living on the top floor with his family.
Most single-family residences not involving a business were required to have a single main, but two main service disconnecting means could be installed with one disconnect for receptacles, lighting and other small loads, as well as one large disconnecting means for a larger load, such as an appliance, requiring a larger branch circuit.
The 1933 NEC, however, specifically stated that the single main could be omitted for a single-family residence and up to six branch circuits could be installed at the service with additional requirements. The six branch circuit breakers were required to be “grouped in a single readily accessible cabinet at the point of the service entrance, provided the approved-type circuit breakers may be operated and reset without opening the cabinet, that approved means are available within the cabinet for opening the grounded conductor and that the ungrounded service conductors are not exposed to contact when the door of the cabinet is open or its cover is removed.”
The reason for the expansion to six branch circuits in a residence—as explained by the chairman of the NEC Correlating Committee and the Code-Making Panel, who had jurisdiction of Article 405 covering service installations between 1925 and 1933—was to permit adding branch circuits for appliances without having to change the entire service. During the meeting for this NEC change, the panel members discussed the number of disconnecting means for residences, and the number six was decided on as a reasonable maximum number of disconnecting means. In the 1937 NEC , a change to Section 2351a permitted the six disconnect rule for all electrical services, with the added capability of having the six disconnects in separate enclosures or mounted within the same enclosure.
The six disconnect rule has been the requirement since the 1933 NEC for residences and the 1937 NEC for all occupancies. The 2020 NEC made a dramatic change to 230.71 by stating in (A) that each service shall have only one disconnecting means unless the requirements in 230.71(B) are met.
Section 230.71(B) now states the following: “Two to six service disconnects shall be permitted for each service permitted by 230.2 or for each set of service-entrance conductors permitted by 230.40, Exception No. 1, 3, 4, or 5. The two to six service discontenting means shall be permitted to consist of a combination of any of the following: (1) Separate enclosures with a main service disconnecting means in each enclosure; (2) Panelboards with a main service disconnecting means in each panelboard enclosure; (3) Switchboards(s) where there is only one service disconnecting means in each separate vertical section where there are barriers separating each vertical section; (4) Service disconnects in switchgear or metering centers where each disconnect is located in a separate compartment.”
This 2020 NEC revision will have major impacts and ramifications for new installations and additions to existing electrical installations. Requiring a separate enclosure with a main for each enclosure or a main for each panelboard will affect single- and two- and multifamily dwellings and commercial and industrial buildings.
In a future article, I will explain why the change was made to the 1933 NEC for residences and the 1937 NEC for all other occupancies. I will also address examples and issues that will be affected by the changes in the 2020 NEC.