BX Cable & Ground

[FMC1959]

My understanding is that armored cable, also referred to as BX cable, sometimes yes, but usually does not have a ground; the armored shell carries the ground. So if it is coming from a junction or receptacle box, which is grounded, the BX clamped into the box with one of the screwed down clamps, grounds the cable which would then continue down the line and ground the box at the other end. This is considered safe and to code? What if the cable is not going to a box but to a light, motor or anything else that has a screw for ground, are there metal clamps that clamp to the BX and have a ground wire coming off this clamp for when the need arises?

 

[wyliesdiesels]

yes BX (not made anymore; replaced w/ AC) has a bonding strip in the jacket so it is code permissible to use it as an EGC.

If terminating to a light fixture and you need an EGC wire to go to the fixture, you could connect the EGC from the fixture to the metal box ground screw. no need for a special clamp as the connector for the BX would carry the continuity to the metal junction box.

 

[FMC1959]

"(not made anymore; replaced w/ AC)", what is the difference between the two?

 

[alfredeneuman]

The 1972 NEC had an Article 250 grounding rule that ended BX.
(It required that grounding receptacles be used,for instance).
The workaround is a thin copper or aluminum strip outside of the paper in intimate contact with the armor. The modified type is called Type AC.

 

[KenC]

"(not made anymore; replaced w/ AC)", what is the difference between the two?

Waitin' for the real answer, but IME the old BX encountered in vintage buildings was always steel while AC is aluminum. but that may be brand dependent. That and the bonding strip.

 

[dscheidt]

"(not made anymore; replaced w/ AC)", what is the difference between the two?

Whether it was sold by the owner of the BX trademark or not. BX is/was (not sure it's still a live mark, and I'm not going to check) a trademark for AC cable, it was originally owned by the inventors of the cable type. They were acquired by GE, and later Southwire ended up with the trademarks. What you got when you bought BX in 1903 was not what you got in 1940 which was not what you got in 1960, which wasn't what you got after 1972, because the code requirements for AC cable changed. But it's all BX, because the trademark owner says so. There's lots of confustion about this, with many people calling colder types BX, and newer varieties something else, but this isn't consistent from place to place, and if it matters, clarification is needed.

This is exactly analogous to Romex and type NM cable. Romex was a type of cable invented by the Rome wire company, added to the code as NM. NM made by Rome and their successors (now Southwire) is Romex. Doesn't matter if it's cloth jacket over type R insulated wires, cloth over TW, PVC over TW, or PVC over THHN (as in NM-B) it was all still Romex. NM or NM-B made by anyone else isn't Romex, even if it's exactly the same construction, and even if you got it by asking the supply house for Romex.

 

[mikedodge]

Whatever you're going to call it the most common type of BX/armored cable you see usually DOES have a ground wire in it.

 

[rlitman]

Whatever you're going to call it the most common type of BX/armored cable you see usually DOES have a ground wire in it.

MC has an insulated ground wire in it. The interwebs tells me it MAY also include a bare metal bonding wire that allows the the armor to be used as an EGC (I suppose this would be handy for an isolated ground situation or hospital grade).

AC has no separate insulated ground. Just the bare metal bonding wire under the armor (and outside of any paper or plastic protecting the wires).

 

[mikedodge]

MC has an insulated ground wire in it. The interwebs tells me it MAY also include a bare metal bonding wire that allows the the armor to be used as an EGC (I suppose this would be handy for an isolated ground situation or hospital grade).

AC has no separate insulated ground. Just the bare metal bonding wire under the armor (and outside of any paper or plastic protecting the wires).

Was that an AI generated answer from somewhere?

As the name implies that's still the bonding wire, it's not solely relying on the outer armored cover as the ground.

AC can come with the ground wire inside the paper insulation and MC can come without it being insulated and outside of the paper. It all depends on model of cable. Southwire's website shows the different variations.

 

[rlitman]

Was that an AI generated answer from somewhere?

As the name implies that's still the bonding wire, it's not solely relying on the outer armored cover as the ground.

AC can come with the ground wire inside the paper insulation and MC can come without it being insulated and outside of the paper. It all depends on model of cable. Southwire's website shows the different variations.

Nothing in anything I post ever has any element of AI (unless I specifically call it out as AI). So clearly you're not understanding what I'm writing. Let me try again. Perhaps you missed the pertinent details or don't understand the difference between AC and MC.

MC has an insulated ground wire in it. The interwebs tells me it MAY also include a bare metal bonding wire that allows the the armor to be used as an EGC (I suppose this would be handy for an isolated ground situation or hospital grade).

AC has no separate insulated ground. Just the bare metal bonding wire under the armor (and outside of any paper or plastic protecting the wires).

In MC, you will find an actual ground wire with green insulation. This is the EGC (equipment grounding conductor). If it includes a bonding wire (more on this in a moment), then the armor can be used as a code compliant EGC as well.

In AC, there is no insulated ground wire. Instead, the ARMOR itself is the EGC, and a bonding wire is provided underneath and in intimate contact with the armor, that ensures the armor is a safe EGC. The bare wire is NOT an EGC, it is not a ground wire, and it is not mean to be connected to anything. As a matter of principle, I usually bend the bonding wire around the end of the armor and wrap it up a few turns so it gets hit by the clamp screw, but it's not a requirement. Anyway, the bonding wire is NOT considered a conductor.

The Southwire website diagrams are great. Starting here: https://www.southwire.com/wire-cable/metal-clad-cable/c/c-mcmain
Filter for type "AC".
All four examples show an aluminum bonding wire OUTSIDE of the paper that wraps the conductors.
Then filter for "AC90", and the copper wire type shows a copper bonding wire in contact with the aluminum armor.

If you turn off the filter and browse through all 76 variants, you'll notice that only the AC types have paper surrounding the conductors, a bonding wire OUTSIDE of the paper, and no green wire. All of the MC types have a plastic wrapper and include a green insulated ground wire with the rest of the conductors. The green armored MC types also include a bare bonding wire between the plastic wrapping and the armor.

Actual BX predates the bonding wire. Without the bonding wire under the armor, the armor isn't considered a safe and reliable EGC.

 

[reader2580]

I expect the majority of GJers wanting to use a metal wrapped cable for a new installation are likely to be using MC cable, not AC. The AC and BX discusssion is helpful for those that are working with old installations that have existing AC and/or BX cable.

 

[mikedodge]

Nothing in anything I post ever has any element of AI (unless I specifically call it out as AI). So clearly you're not understanding what I'm writing. Let me try again. Perhaps you missed the pertinent details or don't understand the difference between AC and MC.


In MC, you will find an actual ground wire with green insulation. This is the EGC (equipment grounding conductor). If it includes a bonding wire (more on this in a moment), then the armor can be used as a code compliant EGC as well.

In AC, there is no insulated ground wire. Instead, the ARMOR itself is the EGC, and a bonding wire is provided underneath and in intimate contact with the armor, that ensures the armor is a safe EGC. The bare wire is NOT an EGC, it is not a ground wire, and it is not mean to be connected to anything. As a matter of principle, I usually bend the bonding wire around the end of the armor and wrap it up a few turns so it gets hit by the clamp screw, but it's not a requirement. Anyway, the bonding wire is NOT considered a conductor.

The Southwire website diagrams are great. Starting here: https://www.southwire.com/wire-cable/metal-clad-cable/c/c-mcmain
Filter for type "AC".
All four examples show an aluminum bonding wire OUTSIDE of the paper that wraps the conductors.
Then filter for "AC90", and the copper wire type shows a copper bonding wire in contact with the aluminum armor.

If you turn off the filter and browse through all 76 variants, you'll notice that only the AC types have paper surrounding the conductors, a bonding wire OUTSIDE of the paper, and no green wire. All of the MC types have a plastic wrapper and include a green insulated ground wire with the rest of the conductors. The green armored MC types also include a bare bonding wire between the plastic wrapping and the armor.

Actual BX predates the bonding wire. Without the bonding wire under the armor, the armor isn't considered a safe and reliable EGC.

I was only asking because of this comment. It's hard to tell these days where people get their information.

"The interwebs tells me it MAY also include a bare metal bonding wire that allows the the armor to be used as an EGC (I suppose this would be handy for an isolated ground situation or hospital grade)."

Obviously I have looked at Southwest's website because I mentioned it and as I said they make MC and AC in multiple formats. Yes their images and notes are great.

Either way the original post in this thread wasn't entirely correct, BX in modern form does have a grounding wire in it in some form and doesn't rely on the metal exterior armored cover as it's sole grounding source. The OP also didn't mention what exactly he was up to. If it's reworking something existing that old style BX might become an issue. If it's new work just get the armored cable with the proper ground wire. Any supplier would have that in stock even regular stores like home depot.

 

[rlitman]

...Either way the original post in this thread wasn't entirely correct, BX in modern form...

Well, that's because BX hasn't been made for over 50 years. The term has stuck around, but most people don't actually encounter it.
My house was wired in 1929 with BX (which was a huge step up from knob and tube), though even in my house, it's use is pretty limited by now.

 

[dave*99]

Well, that's because BX hasn't been made for over 50 years. The term has stuck around, but most people don't actually encounter it.
My house was wired in 1929 with BX (which was a huge step up from knob and tube), though even in my house, it's use is pretty limited by now.

My son's old house was built in the same time frame. BX stretched tight to minimize expense I assume. The basement lights looked like an octopus. BX running diagonally under floor joists that were a full 2" thick. No grounds on anything. No 3 prong receptacles. The conductors in the BX looked like they had cotton insulation. Joints were soldered and taped with friction tape.

If has all since been replaced with NM-B.

 

[rlitman]

My son's old house was built in the same time frame. BX stretched tight to minimize expense I assume. The basement lights looked like an octopus. BX running diagonally under floor joists that were a full 2" thick. No grounds on anything. No 3 prong receptacles. The conductors in the BX looked like they had cotton insulation. Joints were soldered and taped with friction tape.

If has all since been replaced with NM-B.

Wow. Yeah, good workmanship practice was still being learned... Mine has twisted and soldered splices with ceramic wire nuts that then got friction taped. I've replaced quite a bit.

 

[wyliesdiesels]

MC has an insulated ground wire in it. The interwebs tells me it MAY also include a bare metal bonding wire that allows the the armor to be used as an EGC (I suppose this would be handy for an isolated ground situation or hospital grade).

AC has no separate insulated ground. Just the bare metal bonding wire under the armor (and outside of any paper or plastic protecting the wires).

nope it does not.... i have never seen a bonding wire in any MC i have installed.... and i have used a few brands... furthermore, code does not allow the use of the MC jacket for the EGC....

 

[wyliesdiesels]

AC can come with the ground wire inside the paper insulation and MC can come without it being insulated and outside of the paper. It all depends on model of cable. Southwire's website shows the different variations.

paper insulation? there is no paper insulation in MC cable.... there is a plastic liner though...

 

[dscheidt]

nope it does not.... i have never seen a bonding wire in any MC i have installed.... and i have used a few brands... furthermore, code does not allow the use of the MC jacket for the EGC....

You're not installing the right stuff, then. southwire make two MC product lines with a bare bonding wire. They call them MCAP and HCF-MCAP for 'metal clad all-purpose' and 'healthcare facility....'. MCAP is MC with a bare aluminum bonding strip, in contact with the armor, and is available in a bunch of sizes and conductor counts. HCF -MCAP is the same thing, but with a green insulated conductor as well, to meet the requirements for use in a health care facility; most (maybe all) is also got a green painted armor jacket. other companies make similar products, they'll all be marked 'armor is equipment ground' or something like that. Southwire pushed it for the labor savings, as with a suitable connector, all you have to do is attach it to the box, and the box is properly bonded.

 

[FMC1959]

My house was built in the year 1900. I am sure all kinds of changes have been made over the years.

I have some old 8ft neon's that are old, and super heavy, meaning when I took them down, I never expected to feel the weight these things have. I am going to put led's. The wire coming out of the wall is a BX cable with 2 wire, no ground. At the switch is a 14/2 Romex type wire. so somewhere in the wall there is a junction box (plenty of those in this house). I am not digging into the walls, I just wanted to aware of what the grounding through the BX would be.

 

[mm08822]

My house was built in the year 1900. I am sure all kinds of changes have been made over the years.

I have some old 8ft neon's that are old, and super heavy, meaning when I took them down, I never expected to feel the weight these things have. I am going to put led's. The wire coming out of the wall is a BX cable with 2 wire, no ground. At the switch is a 14/2 Romex type wire. so somewhere in the wall there is a junction box (plenty of those in this house). I am not digging into the walls, I just wanted to aware of what the grounding through the BX would be.

A quick check to understand if any connection (hopefully grounding) is provided by the metal jacket is to turn on the switch and read the voltage between the 2 conductors expecting it to be 120v.

Then check 120v exists between the hot and the jacket.

There should also be continuity between neutral and jacket. Even measure resistance between neutral and jacket expecting 0.x ohms.

You can also check the jacket continuity to ground using the known ground from a confirmed properly wired receptacle. Use an extension cord to bring that ground into close proximity to the bx jacket.

 

[rlitman]

A quick check to understand if any connection (hopefully grounding) is provided by the metal jacket is to turn on the switch and read the voltage between the 2 conductors expecting it to be 120v.

Then check 120v exists between the hot and the jacket.

There should also be continuity between neutral and jacket. Even measure resistance between neutral and jacket expecting 0.x ohms.

You can also check the jacket continuity to ground using the known ground from a confirmed properly wired receptacle. Use an extension cord to bring that ground into close proximity to the bx jacket.

I wouldn't suggest this as safe or even good advice. Measuring resistance near a live circuit is not something to be done by untrained hands and most certainly without an appropriate Category rated meter with a safe fuse (most multimeters are not safe for this sort of test).

Additionally, a measurement of resistance between neutral and armor of BX (real BX) may give a false impression of safety. To be truly safe with BX, just use a GFCI breaker and don't worry about the grounding.

 

[mm08822]

I wouldn't suggest this as safe or even good advice. Measuring resistance near a live circuit is not something to be done by untrained hands and most certainly without an appropriate Category rated meter with a safe fuse (most multimeters are not safe for this sort of test).

Additionally, a measurement of resistance between neutral and armor of BX (real BX) may give a false impression of safety. To be truly safe with BX, just use a GFCI breaker and don't worry about the grounding.

Fair enough - I omitted a step of checking for both dc and ac voltages present between the cord ground and the jacket in the method I suggested, that would be done before switching to the resistance check. ( I would anyway.)

I would want to know if the jacket of that old bx was grounded, floating or even hot due to a nicked hot wire somewhere being as the OP is not sure how that transition occurs. Especially when installing another metallic fixture back up onto non-conductive surfaces. That could be a booby trap waiting for someone given the right conditions.

The gfci cb may work, but being it is a house from 1900, I have serious concerns about it holding due to the various wiring changes and age of insulation having possible leakage for a slew of reasons. Worth a try, but don't be surprised if it trips.

 

[dscheidt]

The gfci cb may work, but being it is a house from 1900, I have serious concerns about it holding due to the various wiring changes and age of insulation having possible leakage for a slew of reasons. Worth a try, but don't be surprised if it trips.

If it trips, something is wrong that could result in a fire.

 

[rlitman]

If it trips, something is wrong that could result in a fire.

Agreed. In my parent's house, I had one leg of BX that I connected downstream of a GFCI outlet I installed by a sink, and that outlet wouldn't hold with the BX on the load side. That was enough to convince me to go through the effort to replace it.

Fair enough - I omitted a step of checking for both dc and ac voltages present between the cord ground and the jacket in the method I suggested, that would be done before switching to the resistance check. ( I would anyway.)

I would want to know if the jacket of that old bx was grounded, floating or even hot due to a nicked hot wire somewhere being as the OP is not sure how that transition occurs. Especially when installing another metallic fixture back up onto non-conductive surfaces. That could be a booby trap waiting for someone given the right conditions...

I also agree that you would want to be aware if a BX armor is floating or hot. It's possible, but unlikely, and if GFCI protected, gladly impossible.

Anyway, I always worry when I see advice for people to get measure ohms in-circuit. It's the most likely test where the inexperienced have meters blow up in their faces.

 

[mm08822]

Agreed. In my parent's house, I had one leg of BX that I connected downstream of a GFCI outlet I installed by a sink, and that outlet wouldn't hold with the BX on the load side. That was enough to convince me to go through the effort to replace it.


I also agree that you would want to be aware if a BX armor is floating or hot. It's possible, but unlikely, and if GFCI protected, gladly impossible.

Anyway, I always worry when I see advice for people to get measure ohms in-circuit. It's the most likely test where the inexperienced have meters blow up in their faces.

A piece of bx where the jacket is not terminated properly at its supply end could easily become energized due to chafing, etc. and remain energized if it were not in contact with any path to egc or even the neutral wire. You don't know what is in the walls for certain.

While replacing any mystery wiring is the best assurance, we can't be certain that the OP will consider changing to a gfci cb or if it would even hold.

 

[dscheidt]

A piece of bx where the jacket is not terminated properly at its supply end could easily become energized due to chafing, etc. and remain energized if it were not in contact with any path to egc or even the neutral wire. You don't know what is in the walls for certain.

When I was in college, some friends rented a house that had mostly knob and tube wiring, with a smattering of BX in the basement. One of them went downstairs and found a short section of bx glowing. She mentioned that to one of her housemates, who said, "oh yeah, I've seen that." some discussion led to a chat with the guys at the fire station around the corner, which led to the utility pulling the meter, and then some hurried rewiring by the landlord.

 

[wyliesdiesels]

You're not installing the right stuff, then. southwire make two MC product lines with a bare bonding wire. They call them MCAP and HCF-MCAP for 'metal clad all-purpose' and 'healthcare facility....'. MCAP is MC with a bare aluminum bonding strip, in contact with the armor, and is available in a bunch of sizes and conductor counts. HCF -MCAP is the same thing, but with a green insulated conductor as well, to meet the requirements for use in a health care facility; most (maybe all) is also got a green painted armor jacket. other companies make similar products, they'll all be marked 'armor is equipment ground' or something like that. Southwire pushed it for the labor savings, as with a suitable connector, all you have to do is attach it to the box, and the box is properly bonded.

i dont do work in healthcare facilities or hospitals

 

[u2slow]

If this is a Canadian installation of the last 50 years, AC90 has a proper copper bond wire.



Pigtail your bonds together just like with NM cable. Use the red anti-short bushings and AC90 rated connectors. A rated connector bonds the armour to the device box or metallic device enclosure.

 

[mm08822]

When I was in college, some friends rented a house that had mostly knob and tube wiring, with a smattering of BX in the basement. One of them went downstairs and found a short section of bx glowing. She mentioned that to one of her housemates, who said, "oh yeah, I've seen that." some discussion led to a chat with the guys at the fire station around the corner, which led to the utility pulling the meter, and then some hurried rewiring by the landlord.

What this indicates is the jacket was carrying either hot wire or neutral wire current, either part of it or all. It depends on whether the conductor was broken and the jacket completed the circuit or the conductor was just just chafed in a few places causing the current to split between two paths for some distance.

• In either of these cases, without a ground path diverting current, a gfci device would not be tripping.
• It would also not cause a standard cb to trip.
• Even if there an afci cb, it might have tripped at the instance of damage but if reset and the resulting damaged connections remained secure enough, it might never see another cause to trip.

 

[rlitman]

...In either of these cases, without a ground path diverting current, a gfci device would not be tripping...

All a GFCI needs to trigger is 5ma. It'll find that with contact of the armor with the structure. Every single staple into wood may be good enough for this.

 

[dscheidt]

What this indicates is the jacket was carrying either hot wire or neutral wire current, either part of it or all. It depends on whether the conductor was broken and the jacket completed the circuit or the conductor was just just chafed in a few places causing the current to split between two paths for some distance.

• In either of these cases, without a ground path diverting current, a gfci device would not be tripping.
• It would also not cause a standard cb to trip.
• Even if there an afci cb, it might have tripped at the instance of damage but if reset and the resulting damaged connections remained secure enough, it might never see another cause to trip.

In those cases, a GFCI would trip when a ground path, like a human, was introduced, preventing an electric shock. I'm pretty sure that's not what was happening in this case, though.

 

[mm08822]

All a GFCI needs to trigger is 5ma. It'll find that with contact of the armor with the structure. Every single staple into wood may be good enough for this.

And they may not. I've seen energized conductors touching new lumber (copper making contact) protected by gfci's that didn't trip.