On Tue, 22 Jul 2014 01:14:05 +0000 (UTC), Doug Miller
wrote:

wrote in :

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!

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.

WRONG. The big question is whether the required cable connector is listed for use with 6-3
cable. If the connector is not listed for use with 6-3 cable, it's a Code violation to use it for
that, whether the cable fits or not.
Well, you show me a romex connector that will take a 6/3 cable that
isn't listed for use with 6/3 and you may have a point. The POINT is,
there is no romex connector that WILL accept a 6/3 - listed or not..