On Thu, 29 Jul 2010 11:56:14 -0700 (PDT), Josh
wrote:

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?

Probably. I'm not an electrician and I've never had a copy of the NEC,
so I can't say yes, but you should use clamps regardless.

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?

You're supposed to use add-on clamps for those knockouts. They not
only hold the romex in place, they fill the rest of the hole, so that
sparking and fire will stay in the box and not spread to the wall.

I've never heard of the use of grommets here and I woudln't consider
using them. They don't fill the hole.

Felix has a point. IF you want to do this yourself, at least read a
couple books on the subject. They have DIY books that deal with
electricity. Some have long chapters. If they're long, maybe 2 or 3
of them would be enough. they have them at the libary and the book
department of Home Depot etc. (If they only have in stock, "How to
illuminate your patio", that will probalby go too fast past the basic
electrical wiring.)