DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Electrical (
-   -   romex clamped against bx knockout in old box (

kbm774 01-01-2010 04:16 PM

romex clamped against bx knockout in old box
My house was built in 1930 and still has some bx runs.

In one of my rooms, I want to replace the bx with new romex. The old metal boxes are perfect. I want to pull new romex into the old boxes where the BX used to be.

Problem is that the knockouts for the bx are inset into the receptacle box by roughly 1/8 inch. If I clamp the romex against the back of the box, this 1/8 metal lip will cut into the romex.

I have previously dealt with this years ago by putting a metal dowel pin on its side between the romex and the back of the old box, so that the romex clamps against a smooth surface instead of the 1/8 inch lip. This worked fine, but there must be a better way.

I don't want to open the can of worms of replacing the boxes and destroying my walls.

Any suggestions are much appreciated.

joed 01-01-2010 05:51 PM

BX clamps are not suitable for Romex. Replace them with proper clamps.

kbm774 01-01-2010 07:22 PM

I know about the clamps. I replace the existing bx clamp with the kind that comes with a new metal romex box, which presses the romex against the back wall of the box.

The problem is with the old boxes themselves - they have a round knockout where the bx enters and the o.d. of the knockout is 1/8" from the wall of the box. If I bring the new romex through this opening and use the new type of clamp, that's where I have a problem. This is in contrast to the new boxes, which have the knockout hole open right up against the wall of the box so there's no metal lip.

joed 01-01-2010 08:09 PM

Not sure what type of clamp you are using but it sounds like the wrong type. All the clamps I have seen for round knock outs don't care if the hole is near the bottom. They fill the entire hole and clamp the cable.
You should be using something that looks like this

kbm774 01-01-2010 08:51 PM

thank you - maybe I'm thinking of this wrong. Here is what it looks like now (with clamp removed and my messed-up wall from where I started to dig out the existing box)...

It's hard to tell from this picture, but the hole that the bx passes through is not flush with the back wall of the box.

I planned to use a clamp from a new metal box, like this one:

Would the clamp which you suggested work on the inside of my old bx box (I have no access to the outside of the box)?

frenchelectrican 01-01-2010 10:22 PM

As you show us the photo of new metal box with interal BX clamp that is listed for both BX et Romex cables but some new metal box will NOT take BX at all so just watchout on that one.

Keep the antishort bushing aka redhead or reddevil or other tradenames will show up that is very important part with BX but not need in MC cable.


kbm774 01-01-2010 10:29 PM

thank you. just to clarify - I want to keep the old box and use NM instead of the old bx. I need an internal clamp/connector to hold the NM. Do they make a connector designed to go in a knockout and hold NM from INSIDE the box? Seems like the only connectors that would do this are the push-in connectors, but these are designed for use outside the box.

An internal clamp won't work for me because the knockout isn't flush with the wall of the box, so if I clamp NM against it, the lip of metal will cut into the romex. Sounds like maybe the approach I used last time I did this might be the best way - to put a metal spacer in between the romex and the back wall of the box, and then use a standard clamp.

frenchelectrican 01-01-2010 10:39 PM


Any chance did you save the old clamps from the old box ? that look like tee shaped or double holes clamp one of the two.

If so save them you will need them however do not overtighten them otherwise you will pinch the insluating materals.

That box I was looking at the photo I am sure it is listed to work with BX, MC , AC, Romex and old style cloth/tar coated Romex cable.

and make sure you ground the box if you can't find the screws they typically take either 10-24 or 10-32 threads depending on which screw hole you are using.


kbm774 01-03-2010 08:48 AM

still looking for help on this one, thx

WontonSoup 03-02-2010 04:15 PM

Looks like the "Tom Two Way" is for you. Came across it while looking for AC connectors.

Cool user name 08-16-2010 01:51 PM

I have the exact same situation. I am running Romex into a metal box of the same age. The KOs are much smaller than the current standard 1/2" or 3/8 and will not fit the common external Romex clamp. I still have the internal clamp, but I am not sure if it is appropriate to use due to space/safety, etc.

Does anyone know where I can find a smaller size external clamp or fitting to secure Romex? I can replace/update the box if absolutely necessary, but I'd like to mimimize extra work patching the wall. Thanks in advance.

Jim Port 08-16-2010 01:53 PM

The internal clamps from a new metal box can be used. Just look for a metal box with a NM clamp.

All times are GMT -5. The time now is 04:46 AM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1