wrote in message
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My circuit breaker panel cabinet has 2 existing, already-installed
breakers with nothing attached. I would like to use one as a
dedicated line for a spare refrigerator in the basement. All of the
existing bx-cables exit the cabinet from the top, and _every_ existing
panel knock-out on the top of the cabinet is occupied. There are
knock-outs on the side of the cabinet, but I’m wondering, is there
some rule or code that prohibits exiting from the side?


No. If there is room go ahead and use a side knockout.