Is this BX or MC?

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mark32

mark32

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I just came across 4 250' rolls of armored cable (12/3) at the HD I work, the armor is that of MC but there is no seperate ground. We don't sell it but someone slapped a 12/3 mc tag on it and returned it as such, no surprise. When would you use this cable, I was under the impression that the armor on MC is not to be considered or used as the equipment ground, am I incorrect?
 
If there is a suspicion about the authenticity of the label slapped on it, strip out a foot or so of the end and look for internal identifiers, you know, like paper or plastic filler (AC - paper, MC - plastic); or a running ribbon with printing on it. Depending upon the amount of text you might find, you might have to expose more than a foot. Let us know what you find.
 
Okay, the conductors are wrapped in paper and there is a plastic ribbon that states something like "Set screw type connectors are not to be used with this armor".
 
mark32 said:
Okay, the conductors are wrapped in paper and there is a plastic ribbon that states something like "Set screw type connectors are not to be used with this armor".
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OK. It does sound like this is actually AC.

For the coup de gras, is there a little light guage aluminum wire, the "drain" wire, between the paper and against the armor? If so, the cable is absolutely AC, not MC.
 
al hildenbrand said:
For the coup de gras, is there a little light guage aluminum wire, the "drain" wire, between the paper and against the armor? If so, the cable is absolutely AC, not MC.
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Al, I don't mean to get off subject and I'm not trying to get knit-picky here, I just want to get my facts and education straight. I was always under the impression that the bare aluminum conductor inside AC cable was considered a shunt and not a drain. My understanding of a drain is something (like the nylon string inside CAT-5 cable) that aids in stripping the outer jacket off that cable. The shunt provides a means of continuous contact between the curls of the AC cable, thereby making the AC jacket part of the grounding system of that cable.

Please correct me if I'm wrong.
 
goldstar said:
Al, I don't mean to get off subject and I'm not trying to get knit-picky here, I just want to get my facts and education straight. I was always under the impression that the bare aluminum conductor inside AC cable was considered a shunt and not a drain. My understanding of a drain is something (like the nylon string inside CAT-5 cable) that aids in stripping the outer jacket off that cable. The shunt provides a means of continuous contact between the curls of the AC cable, thereby making the AC jacket part of the grounding system of that cable.

Please correct me if I'm wrong.
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like everything in our business, it probably is called by several names.
In the product description Southwire simply calls it what it is, a BOND wire
 
goldstar said:
The shunt provides a means of continuous contact between the curls of the AC cable, thereby making the AC jacket part of the grounding system of that cable.

Please correct me if I'm wrong.
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That is correct, it actually shunts or shorts each of the spirals in the jacket together to allow the fault current to travel in a straight line.
 
Thank you all for the input, and indeed Al, there is a light gauge strip contained within the sheathing. Now for my last question, if this product is available (Aluminum sheathed AC) why make MC? I've never seen this product before and actually thought it was a manufacturer's defect. While we're at it, does steel sheathed MC exist?
 
goldstar said:
I was always under the impression that the bare aluminum conductor inside AC cable was considered a shunt and not a drain.
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I love a good turn of phrase. "Shunt" or "Bond" works great for me.
goldstar said:
My understanding of a drain is something (like the nylon string inside CAT-5 cable) that aids in stripping the outer jacket off that cable.
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When I think of a drain, I'm imagining the complex impedance that exists for time varying current and/or voltage that is seeing all the paths that exist along classic spiral wound armour of Type AC cable. The inductive portion of that impedance is the problematic characteristic of the spiral (think transformer coil without a core - that is, a choke) and it is minimized by providing an alternative path to the spiral path, a path through the bond or shunt wire.

In my mind the little aluminum wire "drains" current away from the inductive impedance path of the spiral.

The faster the current is changing, the higher the inductive impedance, the more current drains across adjacent wraps of spiral armour that are in poor edge to edge contact.
 
Al, the way you described the spirals of armor being a choke, without the bond/drain wire, makes me think about the 6' limit on FMC when the armor is used as the equipment ground. Would you agree to this that the limit is because of the choke effect?
 
mark32 said:
I've never seen this product before and actually thought it was a manufacturer's defect.
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Type AC cable is one of the granddaddies of the wiring methods in Chapter 3 of the NEC. It goes back to the early 1900s, and was, early on, given the trade jargon name of BX. Back then a steel armor was ubiquitous.

BX was very popular as a means of extending existing Knob & Tube branch circuits.

By the 1950s, the inductive impedance choking problem shown by fire statistics, resulted in the required upgrade to the modern "bond wire" assembly.

I work in a Metro Area that has a long history of "all metal code" being enforced for multiple decades. As a result, chunks of my work area have a lot of AC installed, as that was the economic wiring method for the construction type and the local code constraints.

I can go to other areas of this Metro, and find whole subdivisions that were not bound by "all metal code" and they are wired in NM.
 
Al,

Thanks for the response. I'm not sure I'm clear on this but at least we know what's going on in your mind. I was basically looking for a term that's an acceptable standard for our industry (BTW, I still walk into the supply house and ask for MCM and not kcmils - they know what I'm talking about). In any case, whether it's considered a shunt or a drain it serves the purpose of completing the electrical path across the curls of armored cable. However, I would not consider it a bonding wire.

Regards,

Phil
 
al hildenbrand said:
By the 1950s, the inductive impedance choking problem shown by fire statistics, resulted in the required upgrade to the modern "bond wire" assembly.
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Al,

I've heard this too but never found an real empirical evidence to support it. Do you have any?
 
mark32 said:
Al, the way you described the spirals of armor being a choke, without the bond/drain wire, makes me think about the 6' limit on FMC when the armor is used as the equipment ground. Would you agree to this that the limit is because of the choke effect?
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I suspect that was part of the substantiation for the NEC change, but I don't recall.
 
One could say that the strip is a bond between wraps, but the wrap is the EGC.

A drain is a wire added to foil-shielded cable to make a ground connection easier.
 
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