I am installing an electrical outlet above a gas fireplace for TV. I understand that electrical code requires securing the NM cable within 12" of outlet box. I am curious how one goes about achieving this in a finished wall being that there is really no room to get hand, hammer, drill or screwdriver inside the wall through that box opening. I am sure this is straightforward for experienced electricians but I have no idea how to go about this.
You're off the hook:
NEC 334.30(B)(B) Unsupported Cables. Nonmetallic-sheathed cable shall be permitted to be unsupported where the cable: (1) Is fished between access points through concealed spaces in finished buildings or structures and supporting is impracticable.