Hi Jason, thank you for the kind words, glad to hear you enjoy the CQD. Section 300.21 is made up of two parts/sentences, with the first sentence establishing the intent:
“Electrical installations in hollow spaces, vertical shafts, and ventilation or air-handling ducts shall be made so that the possible spread of fire or products of combustion will not be substantially increased”.
The second part/sentence establishes the rule:
“Openings around electrical penetrations into or through fire-resistant-rated walls, partitions, floors, or ceilings shall be fire-stopped using approved methods to maintain the fire resistance rating.”
This would indicate that only those penetrations through fire-resistant-rated walls, ceilings, etc. need to be fire-stopped to maintain the fire-resistance-rating.