Re: residential electrical panel main disconnect
408.16 Overcurrent Protection.
(A) Lighting and Appliance Branch-Circuit Panelboard Individually Protected. Each lighting and appliance branch-circuit panelboard shall be individually protected on the supply side by not more than two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard.
Exception No. 2: For existing installations, individual protection for lighting and appliance branch-circuit panelboards shall not be required where such panelboards are used as service equipment in supplying an individual residential occupancy.
Comment from the NEC Handbook:
The phrase ?for existing installations? in Exception No. 2 refers to the existing panelboard. It is not intended that a split-bus panelboard used in an individual residential occupancy be replaced if a circuit is added to the existing panelboard. It does mean, however, that for the installation of new panelboards in new or existing residential occupancies, a split-bus six-disconnect panelboard (with more than two circuit breakers or sets of fuses protecting the panelboard) is not permitted for the service equipment.
An individual residential occupancy could be a dwelling unit in a multifamily dwelling where the panelboard is used as service equipment. See the definition of dwelling unit in Article 100.
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The handbook has drawings as well.