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.
Clamps are required. The knockout holes require separate clamps. The reason those holes are used is to allow connection of conduit as well as Romex and BX type cables. (you simply use a different clamp/ connector for each type of wiring.) If you buy boxes with integral clamps make sure you get the ones for Romex not BX (they look similar but are subtly different.)
For Romex, you can also use the little black Arlington buttons instead of clamps
Romex clamp:
Thanks. The Arlington buttons look pretty nifty, serving as both a grommet and clamp. Can they be split and added to existing steel boxes too.....without pulling the Romex back out and gaining access to the back or side of the outlet box?
No. They're supposed to be pushed through the hole from the outside, and I don't see how you would accomplish that without pulling out the Romex, split or no. There's a flap inside the button similar to the ones used on plastic old work boxes, to grab the cable - trying to push the cable through the button in the reverse direction from what is intended seems doomed to failure.
I don't think you need to worry about having a "grommet" as a properly installed cable clamp will not allow the cable to move relative to the box at all, so no possibility of chafing exists. The Arlington button and the traditional Romex clamp accomplish the same thing but in different ways. The main advantage of the Arlington button is reduced installation time.
nate
If the question is indicative of your your electrical knowledge, please call an electrician.
Depending upon the box type and composition, clamps or connectors may or may not be required. For metal boxes, I'm a big fan of the Arlington button. They take up very little space and are easy to use. Be sure to use boxes of adequate size for the size and amount of cables you plan to use in each box.
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.
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.)
*Depending on the manufacturer, some button type Romex connectors come from the factory with a slit in them. I have installed a few of these after the wire was installed. I used Channellocks to squeeze the button over itself a little and pushed it into the hole at the same time while also trying not to damage the cable. A third hand could be useful, but not required. One trick I learned in the process is to install the button over the individual conductors and not the outer sheath. Once the button is in place you can pull the cable with the outer sheath through.
Thanks for the info. What about the little plastic (often blue in color) outlet boxes? They have rectangular knockouts on the back end; is there a clamping fixture for them, or does the wire just hang freely?
Jon
You are not supposed to completely remove those flaps (for lack of a better word), as they close back against the cable and hold it. I absolutely hate plastic boxes.
Some plastic boxes have built in "clamps", but others have plastic squares that get completely knocked out.
Aye, I absolutely hate them as well, but they are (to my knowledge) the only ones available with the flip-out "wings" to install them into a pre-existing wall (without cutting out a hole larger than the box itself).
Jon
Interesting, thanks for that.
Jon
There are a number of box supports available for metal switch boxes that don't require opening the wall larger than the box. I'm kind of partial to the old fashioned "madison bars", but clip on wings and other stuff are also available.
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I am curious...., why don't plastic boxes come with round knock-outs like metal boxes have?
Instead of having to deal with the blue plastic bendable flaps, I would much rather have a plastic box with round knock-out holes, and then use Arlington buttons to secure the incoming wires.
As far as I know, the clamps are mandatory, and I've never seen grommets used in interior domestic wiring.
Lots of steel "rework" boxed also have capability of installing from the front, through the hole, with expanding "jaws" to hold to the drywall
You can get larger plastic boxes, like 4" square with knockouts, but I've never seen them on single gang boxes. I'm not real sure that a button will hold tight in a plastic box. It may be slightly thicker than steel
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