On Mon, 21 Jul 2014 17:23:30 -0600, Tony Hwang
wrote:

Doug Miller wrote:
wrote in
:

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!

I doubt very much that this would meet Code. First of all, you can't just pass the cable
through the knockout hole -- for the protection of the cable and the connections, Code
requires some sort of fitting on the hole (e.g. a cable clamp aka "Romex connector") which
secures the cable to the box. And I can't imagine that any fitting which would fit a 1/2"
knockout is listed for use with anything as large as 6-3; for example, the Romex connectors
used with 1/2" knockouts are listed for 14-2 through 10-3.

Does the panel have any vacant 1" knockouts? Or can you ream the 1/2" hole to 3/4"?

Hi.
The length of cable must be long, otherwise wouldn't 8-3 do? Enlarging
hole is easy using rimmer, cabinet is not really heavy gauge steel.
I assume you mean a reamer - which leaves all kinds of conductive
swarf all through the panel. The only RIGHT way to do it is with a
"knoclut punch" - aften referred to by the most common brand - a
"greenlee punch" Any electrician worth his salt will have one. All you
get that way is a metal ring.