I prefer using the metal connectors with metal boxes because a lot of the single cable plastic box connectors are very hard to use, or at least I do not have flubber high strength fingers for doing them. The 2 cable hitlocks are very easy to use.

But, Like plastic boxes that have internal PLASTIC clamps. The brown plastic boxes that have metal internal clamps strikle me as the kind that would cut into cable insulation.

Also, some electricians were taught to stick romex into a box first and then strip it. This is bad practice as it actually takes longer to strip the cable, you are more likely to nick the insulation, and they are probably causing too much stress at the cable connector or clamp. Easier way is to mark the cable with a felt pen, then cut and strip. The only times that I put the cable in first and then strip it is when it is going through a protective conduit or it is a communications or fire alarm cable that has a rip cord.

It seems that some people do not know their own strength. I have a Siemens soft start that somebody gave me because somebody destroyed a box lug by overtightening it. I eventually talked Ilsco into giving me some of these specialized lugs from the quality control bin and refurbished this soft start and would really like to sell it.

I have been an electrical contractor since 1982. I had never had problems with romex connectors or staples until the newer NM-B wire came into use. The problem did not start immedately but would occur after the romex jobs had been completed anywhere from a week to six months. Close inspection of the wire involved revealed that the NM-B plastic was much softer than the older NM and after a period of time the romex connector or staple would squeeze completely thru the NM-B plastic. I discussed this with the local inspector and we deceided that we would use only plastic clamps or bushings and romex staples that had an insulating cushion that actually touched the wire instead of the metal staple. This worked well and we had no further problems except for the service calls to repair short circuits where romex connectors had been used.

Eddie Hodges MST

The older cable had three to four paper strands inside the cable assembly making the cable larger . In Mass I have not seen 12/3 used in a long time.I will pass this along to the electricians that I have jurisdiction over.

Mike, in regards to this information about romex connectors, I'd like to refer to an incident I encountered on a service call a while back. I recieved a call that people were getting shocked on the gas meter of the house, and also that a dog was shocked that was tied to the gas meter. I went to the house and searched all the circuits might be connected to the gas line in someway. I checked the ground wires etc. To make a long story short, I found that the gas furnace was connected by a piece of 12/2 romex cable, secured by a romex connector. The connector was secured too tight and penetrated the conductors, thus allowing 120 volts to energize the frame of the furnace, which in turn also energized the gas pipe out to the gas meter. Several people recieved a shock, including a small boy. Thankfully no one was severely hurt, though the potential was certainly there. The ground system was not in tact as it should have been, thereby creating a dangerous situation. Just thought I would relay this in hopes it will help us all to think a little bit more of the small things we may overlook that may save someone's life.

Mike, Have also experienced the same with MC cable using the two screw external clamps rated for MC and RX. The faults occur with aluminum sheathed cables easier than the steel sheathed. Most people seem to a cordless dril without properly setting the torque on the drill. Thanks. Steve Green

I have only experienced this problem once or twice in my career of thirty years, much of which was spent in residential wiring. I have sen this occur when romex connectors are over tightened and when romex staples are over hammered. I see this as bad craftsmanship as opposed to a product defect. Just like you can ring the head off of a 1/4-20 bolt you can over tighten a connector but it doesn't mean the bolt or connector are defective.

Do not use power screw drivers or drills for tightening the clamps. These clamps have been iin use for years with out any problems.

This is one reason why I had switched to plastis, pushin romex connectors.

My firm has also had some experience with this condition on two or three different projects. Because we use plastic romex connectors 95% of the time this has not been a major issue but there have been two cases where an electrician stapled a 12/3 home run too tight and caused a short. These were not visable afterwards either, it was only when staples were pulled out because we assumed we had a bad piece of cable that needed to be replaced that we found out what was going on!

I wonder if a combination of metallic and non-metallic parts (metallic body/ non metallic clamp) would work, where the plastic clamp would destort when over-tightened?

I've noticed that the NM cable produced over the past decade has less paper filler surrounding the conductors and thinner overall jacketing. A competent electrician shouldn't have trouble knowing how much to tighten the clamps, though. I've seen a short like the one described only once in the past 20 years, and that was due to an installer using a cordless screwdriver to tighten the clamp screws.

I have been in the business for over 25 years, and after fixing several problemss like this, I ONLY use plastic connectors. The current metal connectors oughta be outlawed!!!

It Seems to me that only the very inexperienced / novice / homeowner would be the only individuals who would have these types of problems! Similar to the need to color coding romex!! The novice / inexperienced!! ARE WE GOING TO TRY TO IDIOT PROOF SOCIETY, or are we just going to use common sense in the trades! When will this insanity end!

Dear Mike, Although I do see a problem with tightening the clamps on a metal box to tight , I really see more of a problem in the way the romex is manufactured. Now I am sure you remember when romex was larger years ago do to the different insulation on the interior wire,well there also was a more thicker jacket on the wire. I have been physically doing residential and commercial work for over 30 years and the first thing I tell my employees is to never over tighten clamps just snug them. After all where is the wire going once it is stapled properly. When doing renovation work I always try to bring my wires in on top for this reason. Mike I am not pinning medals on myself but have yet to have a short on ( Romex ) do to overtightening the installer just needs to use more caution. Thanks Rich

This problem is not new and is not limited to 12-3 nm. I have been in the trade for 35 years. This problem has been around all of those years. There has never been a true method of testing when too tight is too tight. I have always been taught that you don't use the clamp to secure the wire to the box, it is used to close the open space in the connector to reduce arcing outside the box or fixture in case of a short circuit and flash within the junction space. Using a metallic pressure clamp and tightening to the point of testing for movement of the conductors inside the box when the wires are moved exterior to the box is totally asking for a future problem.

We, as professionals in the trade, have to use good common sense and learn from experience (especially of others experience) and apply our skills logically and inform the new talent in our trade as to our personal experience with items such as this to keep everyone safe.

I have done it -once-.

Since my first mistake overtightening a xx-3 cable clamp, I have viewed the romex clamp and the nearest staple as a system that sufficiently secures the cable without damage. The reduce clamping force allowed by this system viewpoint has been my good workmanship standard ever since.

I once toyed with distorting the sheetmetal clamp by pre-tightening over a metal pipe, but I have since adopted good workmanship as the standard response to this silly hazard.

I would much prefer delisting these clamps for XX-3 and selling a very nice substiute that had a clamp profile for round cable. I am not so fond of plastic cable clamps that I am ready to give up on metal clamps.

Cheers, ken

I've seen this many, many times. It is always with 12/3 NM-B cable and the clamp mentioned in the article. I've also seen NM cable of all types shorting to theses clamps when the cable is pulled while preparing for removal of the sheath. These clamps tend to "dig" into the cable(s). It is an identifiable issue that needs corrected.

Why is this news? Any electrician with more then a year's experience knows this problem and has learned to tighten the nm connectors screws just enough to cause enough resistance to stop the wire from pulling.

This has been an issue for over 40 years, why bring it up now? Is it truely a mystery to the Manufacturers or the NEC?

I have had many issues with 12/3 nm-b and 1/2 inch romex connectors.I have found that if cable is snug it is too tight. The insulation jacket has become too soft.

I have seen this problem in the past & what I have read is correct what I would like the code to adress is the circular mill area of the 12-3 nmb vs the i.d. of the connector. I believe if you do the math you will figure it has the same end result as over fill. Something to think about. Also leaves no room for expansion

It seems to be a problem only with 12/3 nmb, so perhaps this style of connector could be delisted for that aplication. However, I know few is any professional electricians who still use this type of connector. Most electricians who work consistently with NMB now use a plastic type push-in connector, which does not require any set screws. Using these would eliminate this problem, and the only people I know who use the old fashioned connectors are homeowners - and we all know about them...

That cable clamp also damages underground feeder cable also. I had a recent experience with such.

We got away from metallic romex connectors years ago because of this. It is easy to damage the insulation as you are installing the cables in them and then once you tighten the clamp its a toss up as to whether it will short out or not. All we use is plastic, and they are quicker to install.

I have been a electrical high school teacher for a number of years and this is a common problem that the students run into all the time with 14-3, 12-3, 14-4, and 12-4 romex. They always tighten the screws too much and it arcs out the connector severely enough to start a fire. There is much merit to this issue and it should be addressed.

The problem of having NM connectors cut into NM cable as to create a fault condition is nothing new and is has been present on both old and new NM cable. It is so common we don't report it. At one time it was considered a rite of passage for a new electrician's helper to do so. When a new cable kicks a breaker, the fault is normally due to an NM connector or a metal staple.

I just did this two days ago with 8/3 NM with a grounding wire. It was a one inch, two screw connector.. Upon screwing the clamp down and then energizing the circuit, I created an immediate short circuit and ensuing (embarrassing) explosion in front of my customer.

The problem described is common among do-it-yourselfers.

Real electricians don't tighten connectors down until the wires short nor do real electricians drive staples so hard that wires are damaged.

While we're on the subject, I've noticed that when light bulbs are over tightened the lamp breaks every time. Also it's a little known fact that you can strip the threads on device screws by over tightening those too.

I was told 50 years ago when I was a 14 year old summer helper not to drive the staples "home" or to over tighten the Romex clamps. My most recent experience was this past summer when I over tightened a wall case clamp. I agree that thinner plastic jacket and the use of THHN has made the problem worse. With the older, thicker plastic or braided fabric jacket and TW insulation the cable wasn't subject to damage so easily.

I have found this to be very correct. My solution that has worked very well is to pay attention to how much the clamp is tightened and to wrap a few rounds of good electrical tape where the NM passses through the connector.

It appears as though economics and convenience have taken precedence over safety. Also, couldn't a non-metallic connector cause problems between conductors?

I had a similar problem with 10-3 with ground MC cable recently. It's hard to get those 4 # 10 wires through the hole in the 3/8 inch connectors that work with that size MC cable. I know from personal experience, that if you're not real careful the wire will short to the connector.

I teach electrical classes at Albany Technical College in Albany, Ga. This problem is not new and few electricians with experience overtighten these clamps. However, I think that the problem is worse than realized. Over time a clamp with pressure against the cable can eventually cut through the jacket and the insulation even when initially it seems to be good. I believe that the metal romex connector needs to be redesigned and until then we need to use plastic connectors. Even better use plastic boxes. They are less expensive and have more room for conductors and are less apt to start fires.

I teach students to not overtighten any cable connector or cable clamp because of this. Someone always manages to do this in my classroom and it is a good lesson learned. I also teach them to look at most cable connectors and clamps because they have indentations in them that don't help when they are overtightened. They should be secure according to code, but a definiton of what too secure is, maybe should be defined. This also happens in other wire not just NM B. If they are too tight it may short out, but also cause the circuit to just not work. They loosen the connector and everything flows.

It may be that a feedback-loop user interfaced threaded fastener driver apparatus is called for.