On Mon, 21 Jul 2014 15:30:28 -0700 (PDT), wrote:

Hi,
I'm running a 6-3 Romex NM-B with ground for my new air conditioner system.
My main panel doesn't seem to have any spare (unoccupied) 3/4" size punch outs left; but there is a 1/2" punch out. It is possible for me to fit this 6-3 with ground in there, but does anyone know whether this is prohibited by the NEC?
All advice (and NEC references) appreciated!
Regards,
Theodore.
Can you get the 6-3 cable through the required cable connector, is
the big question. If you can fit the 6-3 into the connector that
threads into the 1/2" knockout, you are gold. If not you will need a
knockout punch to enlarge the hole to 3/4" size (Greenlee punch)