Interesting article from U/L on armored cable

In the late 1800s and early 1900s, one of the earliest wire raceways was called “moulding” (sic).
It consisted of two pieces of wood, a backing and a capping. The backing had two or more individual grooves, one for each wire, and a capping to cover the grooves. The grooves separated the wires by 1/2 inch, and provided mechanical protection.

Although wood moulding was not intended for use in concealed spaces or damp locations, it was required by the National Electrical Code® (NEC®) to have two coats of waterproof paint, or be impregnated with moisture repellant. This product continued to be permitted by the NEC until the 1935 edition. However, by the mid-1910s, some large cities had banned its use, and wood moulding was being phased out as metal moulding and armored cable were becoming widely available.

Armored cable was first Listed by Underwriters Laboratories Inc. in 1899 for Sprague Electric Co. of New York, and was originally called “Greenfield Flexible Steel-Armored Conductors,” after one of its inventors, Harry Greenfield. Armored cable first appeared in the 1903 NEC.

There were originally two initial versions of this product, one called “AX” and the other “BX,” with the “X” standing for “experimental.” The “BX” version became the one that eventually was produced, and hence the name “BX” stuck. This was the registered trade name of armored
cable for General Electric (GE), who later acquired Sprague Electric. Armored cable made after 1959 requires an aluminum bonding strip under the armor to help improve the conductivity of the armor. This continues to be a popular wiring method today, and is described in Article 320 of the NEC.

Published in the U/L “Electrical Connections” column


The problem with BX today is people are thinking it is listed to use the outer case as a low resistance path EGC and it is not intended for the purpose…so many HI’s run into a “FLIP” inspection where the person has replaced the 2 prong with 3 prong plugs and now think it has a good LOW resistance path…yeah it has a path…but not very LOW in terms of aiding in clearing a fault if you get Objectionable currents running on it it simply can act as a heat strip…seen it a few times in a basement where it was RED as fire…

Don’t you hate when companies “experiment” on ya…love it…AX and BX…an experimental project for the consumers…nice.

Are you still whining about that tracking chip the CIA implanted in your head? Get back on your meds, and you’ll forget all about that. :-;;

Gee all this time I thought that thin aluminum strip was to hold the fish paper ( anti Short ) in place .

Roy Cooke

The armor af type AC is listed for and perfectly capable of opening the OCD and that is the intent of the EGC.

AHh…greg I was talking about the old BX…it does NOT have this strip and is not capable of opening a OCPD…been their…seen it with my own eyes.

marc…I wish I was on the MEDS…been the longest 4 days seminar trip yet…I am still sitting in the airport and no the tracking chip came out years ago…at least thats what the people with the white jackets and padded walls told me.

I would still question that if this is a 15 or 20a circuit and the armor is properly connected. That is particularly true if there is already some load current on the circuit. It would take an awful lot of BX to generate 5 ohms of impedance from the inductive effect alone. The fact that this lasted in the market without the strip for 60 years is an indication. Bear in mind there will always be some continuity between the wraps that shunts out the choke effect so it is not one long single inductor.

When folks start screwing in the “greenies” all bets are off.

Lasting 60 years is one thing Greg…converting it to use the outer case as a EGC is another thing. The older BX was simply never listed for this use at all.

That’s true, mainly due to the fact that “listings”, as we know them, didn’t exist during that era. :roll:

NO problem…so I imagine the " GLOWING " BX I see in basements…hey if you guys like BX…too all his own…you know my opinion on it and it stands…:slight_smile:

Pictures! Pictures! Or, alternately, calculations!

I’m in several different basements almost every waking day of my life, and I’ve yet to see such an instance.

If you feel like you’re being ganged up on, you are. This inductor theory is a bunch of crapola on a stick. The armor of style BX is no where near a textbook inductor. The bond strip was only added to give 100% assurance that it can never be.

Resist the groupthink, Paul. It’s rather unbecoming.

marc, I agree that 99.9% of the time the old BX will open the ocpd, but that still leaves that .1% of the instances where it won’t. When I was taking code classes, the instructor on the VHS told of a house fire that he investigated and found the cause to be a very long BX run to a bathroom light that had been shorted for a long time. Over time the drilled holes in the framing became charcoal (there is a term for this I can’t remember) and the house caught fire. (I think the instructor was David Shapiro.)

Another electrician reported a dull red glowing BX cable in an attic. I think it was on Mike Holt’s site.

There is enough of a chance to where I will not intentionally ever use it for grounding.

Obviously it would be wasted to have YOU explain objectionable currents and how it could happen with BX…but since you seem to be a FAN of it…believe what you will…oh why not ask Mr. Holt about it…see his opinion on BX being used as a EGC since you seem to be a fan of it…

gang up on me all you wish service man…:)…it’s all good I dont want to question an electrician in a state with no licensing…we will just say Mr. Shunk is for using older BX as a EGC and leave it be at that ( he is entitled to his opinion…)…while other factors can play into it…I saw it…sorry no pictures as I was not doing websites and teaching when I viewed it…

It is futile Brian, we is on a mission tonight it seems…I guess I am gonna have to run over to Charlottesville and see that MYSTERY CRAPOLA that did not exist…since Marc seems to be all knowing tonight.

Oh wait…you better go over to Mike Holts and inform Bob he is wrong…

Member # 47 posteddocument.writeln(timestamp(new Date(2003,9,31,18,52,0), dfrm, tfrm, 0, 0, 0, 0)); October 31, 2003 06:52 PM The old looking cable is BX and it has no ground conductor.

That must be 12/3 or 14/3 White, Black and Red

In your house if it is truly conduit (pipe) it can be and still is used as a grounding conductor.

The old style BX as Wayne has said will read on your meter like a grounding conductor, however under the wrong circumstances it can heat up and glow like a toaster element instead of operating the over current device.

Posts: 9203 | Registered: Feb 2003 | IP: Logged |

Where ya at Shunkie…seems like some VERY big names over on the holt site believe BX is also not something that should be used for an EGC…so are you saying to the HI’s who have extreme liability that you feel if they choose to convert 2 prong plugs on an old BX system over to 3 prong plugs and simply BOND it …that they are fine…hmmmmm…have not been working with HI’s very long have you Marc…

I try to KEEP them out of legal issues…NOT set them UP for them.

hmmm…seems I am not alone…

big john](;u=00005871)
Member # 5871 posteddocument.writeln(timestamp(new Date(2004,4,24,20,9,0), dfrm, tfrm, 0, 0, 0, 0)); May 24, 2004 08:09 PM Wdalrymple,

Not only is your boss violating electrical codes, but if the receptacles are fed by BX (metal sheathed cable lacking any equipment ground) then he is also creating an additional hazard: A ground fault can heat the metal armor of the cable just like a light-bulb filament.

What was only a shock hazard is now a shock hazard and a fire risk.

-John Posts: 168 | Registered: Jun 2003 | IP: Logged | posteddocument.writeln(timestamp(new Date(2005,5,11,9,5,0), dfrm, tfrm, 0, 0, 0, 0)); June 11, 2005 09:05 AM
Using the term “BX” for armored cable that is currently manufactured is a mistake, but is widely accepted, hence the slang “BX”.

What is really important to understand about the difference between the two types of cable is the grounding path.

**The cable sheath of BX is not permitted to be used for grounding purposes, as it is very unreliable as a grounding path. It may work, or it may not


I believe that AC cable was manufactured sometime in the '50’s.

BX cable is generally larger in diameter and sometimes even has a double outer metallic sheath (the worst stuff to deal with when renovating old work


When replacing older 2-prong receptacles look for the bonding strip that is manufactured in the cable with AC Cable. If you see the bonding stip, there is a very good chance the box is grounded(bonded).
Pull out your tester, and check from hot conductor to grounded conductor, then from hot conductor to the box, this will give you an indication of whether there is a grounding path. Just remember that you may get a ground path reading off of the old style BX… again, check for the bonding strip.

Sometimes you will even see the bonding strip of AC Cable is wrapped around a screw in the enclosure - that is not necessary, as it is a “bonding strip”, not an equipment grounding conductor.

Pierre Belarge
Instructor & Industry Advocate, NY

So marc…when I speak I do so for the SAFETY of the HI industry…so why dont you ask Pierre if he likes the idea of the old BX ( without a bonding strip ) as a EGC…and then lets have HI’s ignore this potential issue and they can just defer to you on this topic…sound good?


I have worked for (2) years training these HI’s and helping them and I have their BEST interest at heart. There are MANY cased of the old armoured cable causing fires because of its poor qualities…couple that with the OLD breakers they find and the situations they are within and you simply are doing the HI harm by making the statement you have made…

It is FAR from CRAPOLA and I would begin to DEFY you on this…these guys have way more liability that you or I have as they are targets many times and they need to be safe and aware…how about you READ this article and understand the very first case it reflects…

How can you assist these guys in one breath and put them at risk with the other…all in your attempt to pull punches with me…you dont have to like me as I knew your intent when YOU KNOW WHO asked you to come here…but do not put these guys at risk making those statements.

Seen it glow on a test bench about 35 years ago in Electric 101 class. Forget the test parameters, but it does glow!

Were they testing Federal Pacific breakers? :wink:

Considering the number of FPE that are out on the market…probably a good chance greg is was…the known resistance per 100ft on the old BX is about 2.068 per 100ft…

It is over 2 X the resistance of todays AC cable and even higher in a normal conductor.

I simply do not believe a person should put their faith in old BX of pre-1960 and feel secure knowing the system itself is old…the breakers in the system are also probably as old…and I am not alone in this concern.

Thats all…increased resistance( impedance ) is never a good thing when trying to clear a fault or if you have objectionable current issues in older homes because of DIYers doing something like trying to retro an older home with pre 1960 BX and make it appear to be a effective and low impedance system for the fault current…as always we can agree to disagree but atleast you keep it straight and to the point…

The largest concern probably with OBJ is improperly done remote distribution panels and the incorrect bonding of which causes a big problem with a casing that is not designed for this intent. The advent of the bonding strip and better metals used helps reduce this risk…thus why we still have MC and AC cable…completly relavant issues.

Thanks for keepin it real…

No was simply was a high school teacher showing what can happen when Primative Pete shows up on the job.

Some things in life are grey, electricity is black and white, when grey shows up, many times it turns red (attributing to loss of life or limb and hopefully before it reaches that point it gets a RED TAG).