"Josh" wrote in message
...
I'm adding a couple of steel outlet boxes and switch boxes in my
residence, fed by 12-2 w/g Romex. I've noticed in Lowes some of the
boxes have clamps built inside to secure the Romex, and some don't.
Are clamps required by NEC?

The other thing I'm wondering about is the knockout holes on the
sides
and back of the boxes. Are rubber grommets required for the holes I
run the Romex through?

Whether they are required or not, I always use the clamps if they come
with the box. Otherwise I use rubber grommets. Metal rubbing on
plastic usually winds up with the metal winning and with electricity,
the results are often not pretty.