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Armor Clad Cable (old BX) still allowed as ground


Brandon Whitmore
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AFAIK the AC needs the bond strip in order to be used as a grounding method.

I have seen old stuff without the bond strip glowing red from a fault to the casing without tripping the breaker.

Hi Jim,

If the casing isn't grounded, and you have a short-to-ground/casing as you say, what is it that is drawing the current - so much so - that would cause enough heat to be generated to make the casing you saw glow? (Sounds like you found a bad breaker, not a ground problem/lack of bond strip problem like you seem to be saying.)

Also, if in your opinion old armoured cable isn't *adequate* to provide reliable grounding, do you think we should recommend re-wiring an entire house to address such a risk?

Thanks for your thoughts.

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I'm just curious as to whether 1920's AC/ BX cable is still considered to be acceptable as a grounding path when people upgrade to 3 prong receptacles? From what I know, newer AC has a bonding strip installed while the old AC/ BX did not.

No. If you have older BX that lacks a bonding strip, you shouldn't see the armor being used as an equipment grounding conductor. The armor was never constructed or intended for that use. Without the bonding strip, the outer steel coil is simply too poor and unreliable a conductor to provide a reliable ground path.

I've heard stories of the armor glowing red, as Jim Port describes, but I've never seen it. It doesn't surprise me though. Steel isn't a particularly good conductor compared to copper or aluminum and without the bonding strip, it could be a loooooong way from a fault to the panel. I imagine that ribbon of steel could warm up quite a bit if a fault were travelling on it.

- Jim Katen, Oregon

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Also, if in your opinion old armoured cable isn't *adequate* to provide reliable grounding, do you think we should recommend re-wiring an entire house to address such a risk?

I would not write it up as a defect as long as they are all 2 prong receptacles. They can always go the GFCI route and mark the converted 3 prong receptacles, GFCI Protected-Ungrounded.

I would however recommend an upgrade for safety enhancement

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I would not write it up as a defect as long as they are all 2 prong receptacles

They upgraded to 3 prong receptacles. Seeing as how it's not allowed to individually ground receptacles to the nearest water pipe, it becomes pretty difficult to provide proper grounds. I guess GFCI protection would be the best option in this case. If it were my house, I'd still ground to the nearest water pipe, and then add GFCI protection as well.

Thanks all.

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I would not write it up as a defect as long as they are all 2 prong receptacles

They upgraded to 3 prong receptacles. Seeing as how it's not allowed to individually ground receptacles to the nearest water pipe, it becomes pretty difficult to provide proper grounds. I guess GFCI protection would be the best option in this case. If it were my house, I'd still ground to the nearest water pipe, and then add GFCI protection as well.

Thanks all.

Hi Brandon,

Grounding the house electrical system to a water pipe (or ground stake/stakes - depending on local requirements/type water piping), and grounding individual receptacles are two completely different things. Just going by your post, you might want to check up on the difference.

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Grounding the house electrical system to a water pipe (or ground stake/stakes - depending on local requirements/type water piping), and grounding individual receptacles are two completely different things. Just going by your post, you might want to check up on the difference.

Thanks Jerry,

I know the difference. I read that since 1996, all grounds are supposed to be within 5' of the pipe entrance to the home-- that goes for GEC's and EGC's. I'm still trying to find out for sure as I just read that last night (fishing)

Here's a little discussion on this: http://ths.gardenweb.com/forums/load/wi ... 15848.html

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  • 2 weeks later...

I'm just curious as to whether 1920's AC/ BX cable is still considered to be acceptable as a grounding path when people upgrade to 3 prong receptacles? From what I know, newer AC has a bonding strip installed while the old AC/ BX did not.

No. If you have older BX that lacks a bonding strip, you shouldn't see the armor being used as an equipment grounding conductor. The armor was never constructed or intended for that use. Without the bonding strip, the outer steel coil is simply too poor and unreliable a conductor to provide a reliable ground path.

. . .

- Jim Katen, Oregon

I'm afraid that, in light of newly acquired knowledge, I must disagree with my own self.

While in Powells yesterday, I picked up, "Wiring Materials for All Purposes" published by General Electric in 1936. It has this to say about BX cable:

". . . Over the entire assembly is an interlocked, galvanized steel strip, giving excellent mechanical protection and also affording a means of grounding systems wired with this class of material."

From this, I suppose that the old BX that lacked a bonding strip was, indeed, intended for use as a grounding conductor -- when needed.

- Jim Katen, Oregon

Download Attachment: icon_adobe.gif bx002.pdf

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I'm glad to hear that.......

Reason being, I see miles of AC all the time (no NMC in Chicago), and it's accepted knowledge that the jacket is the grounding path.

Then, I get the NEC guru's saying it isn't, and I got all scared that I've been a dolt for the last 2 decades.........

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Here I am thinking I need to change a report that has already been sent out.................

Question: The new AC requires the bonding strip. The old BX lacks the grounding strip, and was not being used as a ground. When installing 3 prong receptacles, would that old ground (no longer allowed since a bonding strip is installed) be allowed to be used as the ground ?

I may just leave my report the way it was written, and see what the electrician says. It's not like there weren't any other issues.

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Here I am thinking I need to change a report that has already been sent out.................

Question: The new AC requires the bonding strip. The old BX lacks the grounding strip, and was not being used as a ground. When installing 3 prong receptacles, would that old ground (no longer allowed since a bonding strip is installed) be allowed to be used as the ground ?

In 1935 -- as far as I can tell -- it was ok to use the spiral steel jacket (with no bonding strip) as an equipment grounding conductor. Today, if there's an ungrounded BX circuit still in use, you aren't supposed to change the circuit so that the jacket is used as a grounding conductor.

They added the bonding strip for a very good reason: without it, the spiral steel jacket sucks as a grounding conductor.

I only posted the information from that book because I found it historically interesting. Old BX without a bond strip is an obsolete product. I wouldn't push my luck with it.

I may just leave my report the way it was written, and see what the electrician says. It's not like there weren't any other issues.

I wouldn't advise changing it. The electrician might agree with you.

- Jim Katen, Oregon

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They added the bonding strip for a very good reason: without it, the spiral steel jacket sucks as a grounding conductor.

That alone, tells me it should be upgraded, or at least GFCI protected. Will surge protectors will work when the BX is used as the EGC?

I'm kinda thinking that they can have the best of both worlds. A pseudo- grounded surge protected receptacle, with GFCI protection for safety.

Do you think at some point they will start to require GFCI protection throughout a house? Seems like a good idea to me.

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I guess I don't understand why the metal jacket is such a lousy ground path.

It's metal conductor, one can look at the connectors to see if the clamping screws are tight, check the connection to the boxes, etc. Why wouldn't it function as a ground path, or be any less reliable than NMC grounds, where one can't see or check the connections without taking it apart?

Why does the metal jacket suck, but having that little strip of aluminum in there (that invariably breaks and isn't reliable) not suck?

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If you think about the true length of the spiral sheath vs the length of the cable you will see that the spiral is many times longer than the cable itself. Add in the fact that you are depending on the intergrity of the spiral between coils and it should not be hard to see why it would be a poor grounding path.

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I guess I don't understand why the metal jacket is such a lousy ground path.

It's metal conductor, one can look at the connectors to see if the clamping screws are tight, check the connection to the boxes, etc. Why wouldn't it function as a ground path, or be any less reliable than NMC grounds, where one can't see or check the connections without taking it apart?

Why does the metal jacket suck, but having that little strip of aluminum in there (that invariably breaks and isn't reliable) not suck?

I think that the biggest problem with it is what happens when the spiral jacket comes apart. Without the bonding strip in there, you have a choke coil and some question about whether or not the jacket would actually clear a fault.

-Jim Katen, Oregon

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If you think about the true length of the spiral sheath vs the length of the cable you will see that the spiral is many times longer than the cable itself. Add in the fact that you are depending on the intergrity of the spiral between coils and it should not be hard to see why it would be a poor grounding path.

I'm assuming you're suggesting that resistance is a problem, i.e., the length of the spiral relative to the length of the wiring run....(?).... Given the lengths in use in any home, I just can't see that as a problem.

The "integrity" of the spiral between the coils? What's that mean? It's a single piece of metal band run through some form of machinery that creates a spiral coil out of that single piece of banding. It's an unbroken conductor, bonded with a clamp and screw. It's not a bad way to make a connection. Personally, I like EMT with a redundant grounding wire in it, but I realize that's only in my world.

What about the integrity of NMC? Overdriven staples, pulls, crappy bonding in the plastic boxes, and any number of other possible installation errors makes me think NMC is kinda crappy compared to AC.

Anyone that's ever worked with AC cable, and if one lives in Chicago, they work with it constantly, one knows that the damn stuff is almost impossible to cut or dismantle. It's one of the reasons I hate it as much as I do. Sure, you can yank on it and make it "despiral", but when it gets to the amount of force necessary to make it do that, my money is on AC over NMC regarding damage, i.e., the AC would hold up better.

Baseline, if the NEC says no, that's what I'm saying, but the idea that the spiral sheath is inappropriate for a ground path kinda leaves me wondering why. The fact that they think that tiny strip of aluminum does something makes me wonder even more.

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I think that the biggest problem with it is what happens when the spiral jacket comes apart. Without the bonding strip in there, you have a choke coil and some question about whether or not the jacket would actually clear a fault.

-Jim Katen, Oregon

I had to look up choke coil..........

What does happen when the spiral jacket comes apart? It's still a single piece of metal banding with continuity....(?)....

Why wouldn't that clear a fault? How or why would that little piece of approx. #18 awg clear something that a heavy piece of metal banding wouldn't?

The yes man is your enemy. Only your friend will argue with you.

---------old Russian proverb

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. . . What does happen when the spiral jacket comes apart? It's still a single piece of metal banding with continuity....(?)....

Why wouldn't that clear a fault? How or why would that little piece of approx. #18 awg clear something that a heavy piece of metal banding wouldn't?. . .

As I understand it -- and be warned that my understanding might be faulty -- when you pass current though a conductor it creates electric and magnetic fields around the conductor. When you arrange this conductor in a coil, these fields can induce or resist current flow in nearby conductors. This action -- for reasons I don't understand -- causes the resistance of the coil itself to go up. If the coil is made from a magnetic material, such as steel, the effect seems to be amplified.

Bottom line: if the jackets coils become separated, the resistance of the long piece of steel tape goes way up -- much higher than it would be if it were just a straight piece of steel. Any current that flows on that steel could heat it up considerably.

The little aluminum bonding strip in later versions of AC cable is there to short circuit that coil effect. The current can just jump from one spiral to the next without having to go around the spiral. I suspect that the aluminum strip would work even if it were cut in several pieces.

If there are any electrical engineers out there who can explain this better, please do. Matt Steger?

- Jim Katen, Oregon

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  • 2 weeks later...

Not sure when the bonding strip was added to AC. Some report as late as 1959. My place was built in '58 and has it. My understanding was that it was actually added in the 40's sometime.

The bonding strip was first *required* in AC cable in the 1959 NEC. The requirement isn't there in the '56 edition. Back in those days, the NEC requirements generally followed innovations in manufacturing. So it makes sense that the bonding strip would have appeared before the NEC requirement came out.

In much the same way, grounded receptacles were first *required* in 1962, but manufacturers had been making them for some time before that.

In contrast, the requiements for AFCIs came out before we had working stock on the shelves. Go figure.

- Jim Katen, Oregon

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Courtesy NFPA NECH

Article 320 III. Construction Specifications

320.100 Construction.

Type AC cable shall have an armor of flexible metal tape and shall have an internal bonding strip of copper or aluminum in intimate contact with the armor for its entire length.

The armor of Type AC cable is recognized as an equipment grounding conductor by 250.118. The required internal bonding strip can be simply cut off at the termination of the armored cable, or it can be bent back on the armor. It is not necessary to connect it to an equipment grounding terminal. It reduces the inductive reactance of the spiral armor and increases the armor's effectiveness as an equipment ground. Many installers use this strip to help prevent the insulating (anti-short) bushing required by 320.40 (the “red headâ€

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  • 2 months later...

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