I believe that the 2020 requires that the conductors and terminal lugs supplying a Service Disconnecting Means breaker may not share a wiring compartment with any of the breakers it supplies. But that may only apply to exposed busbars and terminals The idea is that there has to be a way to deenergize the entire wiring compartment so that it never has to be worked hot. With a single enclosed double pole breaker that has the new non conductive terminal guards on the Service Entry Conductor termination lugs above your panel You can work in that enclosure with no exposed energized conductors, terminals nor busbars. When you open that breaker then everything in your small panel is deenergized and you don't have to work anything hot. Their is a strong push on from the workman's compensation insurance industry, OSHA, And the electrical industry as a whole to put an end to working hot on any kind of routine tasks.
230.71 Maximum Number of Disconnects. Each service
shall have only one disconnecting means unless the
requirements of 230.71(B) are met.
(B) Two to Six Service Disconnecting Means. 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 disconnecting means shall
be permitted to consist of a combination of any of the
(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) Switchboard(s) where there is only one service
disconnect 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
You could still run the service conductors into the terminal lugs of an appropriately sized double pole breaker and install it using a tie down kit so it could not be removed from the busbar accidentally or from ignorance while energized. I believe that the 2020 edition no longer permits exposed busbars, terminals, or other energized parts to be controlled by a breaker in the same cabinet as the portions of the panel it supplies unless it's supply terminals are in a separate compartment. With a single disconnecting means the inspector may except the terminal lugs of the main breaker as suitably guarded. Is the panel marked as "Suitable for use as service equipment?" If the inspector will except a back fed breaker with a tie down as the Service Disconnecting Means then you should be OK to run the energized service entry conductors into the terminal lugs of a tied down breaker. That will supply the busbars through the tied down breaker's busbar stab clamps and once it was opened it would deenergize everything else in the panel. If the panel is marked as suitable for use as service equipment that would do the job. You would then have to add the breakers for your individual loads.
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