Reusing Old Armored Cable as Conduit for New NM Wire

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Old 07-21-06, 06:54 AM
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Question Reusing Old Armored Cable as Conduit for New NM Wire

Hello,
I have never been given a straight forward answer regarding the practice of using empty (old) armored cable for a conduit (pulling new wire through). Is there a heat build up issue that could be encountered within the armored interior? I have several long runs of this armored sheating and large coils of NM that I'd like to put to use for interior work. I also desire the added protection of the armored cable especially to deter rodents in certain hidden areas. Am I OK to proceed as long as the new wire pulls through easily with no binding or possibility of nicks/abrasion. Additionally, bare ground or should the ground also be insulated?. I know, odd question but I have alot of material left that I'd like to put to use considering the huge rise in wire cost. Thanks in advance for your advise.
 
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Old 07-21-06, 08:08 AM
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You are allowed to run NM cable inside conduit when the conduit is for protection. This is often done in a garage or unfinished basement when running the cable down the wall to a receptacle or switch. The NM is protected by the conduit along the wall but left exposed in the ceiling.

I see nothing wrong with using some armored cable to protect NM, but you will have to place bushings on ends to protect the outer sheathing of the NM cable. And only run one piece of NM in the armored cable. As for the ground, I donít understand the question. The NM has it's own uninsulated ground wire. Just use that.
 
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Old 07-21-06, 08:11 AM
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Well you will probably get a lot of controversial answers to this question. Generally speaking it is considered bad practice to put NM-B cable in complete conduit systems.

But for academic reasons lets look at how you would approach doing this.

1st.. what size fmc conduit are we going to put the cable in...lets choose 1/2 inch FMC.

2nd..what size cable...lets look at putting one 14/2 g or one 12/2 g NM cable into the 1/2" FMC conduit but not both.

The conduit rules on the NEC require that we treat multiconductor cables like NM as one wire and we use the actual dimensions to calculate the cross sectional area of the cable. Also note that if only one wire (in your case one NM-B cable) is going into the conduit we are allowed to fill that conduit to 53% of its cross sectional area.

I measure my two cables and come up with 3/8" diameter for 14/2 g and 1/2" diameter for 12/2 g

The cross sectional area of 14/2 g is 0.110 in. squared

The cross sectional area of 12/2 g is 0.196 in. squared

The cross sectional area 1/2" FMC is 0.317 in. squared

53% of .317 in. squared is 0.168 in. squared

From this we see that we cannot have a NM-B cable larger than .0168 in. squared and be allowed to put it into 1/2" FMC and stay code compliant.

Our calculations show us that we could get one 14/2 g cable in the 1/2" FMC.

They also show us we would not be able to install a 12/2 g cable or we will exceed the 53% fill limit for one multiconductor cable or wire.

Again this is strictly looking at this academically but it is my opinion that you could stay code compliant using this approach.

My opinion also is dont run NM-B in conduit except as Bob explained.

The key questions are how big is the conduit and what size cable you are going to use and how many cables you need to run together in any given conduit size.

I'm sure this will create some other opinions


Roger
 
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Old 07-21-06, 10:59 AM
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I would agree except, like with NM, FMC, AC or MC are not permitted where subject to physical damage.


Originally Posted by Roger
My opinion also is dont run NM-B in conduit except as Bob explained.
Originally Posted by racraft
I see nothing wrong with using some armored cable to protect NM

Edited to remove the incorrect use of the term “raceways”. Thanks to those who pointed it out.
 

Last edited by Juhl; 07-21-06 at 10:00 PM.
  #5  
Old 07-21-06, 02:28 PM
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You might find that not to be the letter of the law in a lot of local juristictions, especially sleeves in residential applications for unfinished basements and garages. I do concede however that the Nec does not allow these conduits for physical protection.


Roger
 
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Old 07-21-06, 02:29 PM
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Amoured cable is a cable. You can not pull the wires out and use it as a conduit.

If you want conduit use conduit.
 
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Old 07-21-06, 02:51 PM
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I could understand the desire to do this if the old armored cable was already in concealed spaces and you needed to rewire. But to take empty armored cable (not just flex conduit which isn't the same thing anyway) and run NM thru it to install in new construction simply doesn't make sense to me.

In either case, the legalities have already been covered.

Is the supposed "empty armored cable" really old armored cable that has had the wires removed, or is this actually flexible conduit?
 
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Old 07-21-06, 04:31 PM
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Joe and Joe:.... my oversite.... you are correct AC and MC are cables not conduits. If what he has is one of the metal cable types with the wires stripped from them then they would be unuseable.

Roger
 
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Old 07-21-06, 04:50 PM
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UFORTUNATELY.... Sometimes it costs money to do it right.
Cheap is good...NOT... But then again my Family doesn't sleep there.
The time and effort you will spend trying to save a buck by doing this (depending on the actual scope of work, Wich has not been described completely), You could easly make up by just doing it right. Doing it twice and "fudging" it typicaly is harder and more time consuming.
If it is true flex from box to box use thh/wn. then your done with it.
 
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Old 07-21-06, 06:57 PM
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Just for my own education is greenfield still used or is this obsolete. Isn't "Seal-Tite" just greenfield with a protective rubber covering.
 
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Old 07-21-06, 07:20 PM
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If you are referring to greenfield as FMC (flexible metal conduit) it is alive and well and still being used.

Seal-tite or LFMC (liquidtight flexible metal conduit) is essentially FMC with a plastic cover but it also requires unique fittings for connectors.

FMC is cheaper than LFMC so it gets used where it can to save a buck. LFMC is more durable. I prefer it but the almighty buck makes many decisions for me so FMC gets used so the bill/bid is lower.
 
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Old 07-21-06, 08:23 PM
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OK.. I'm Not bidding.. Nor are any of us, So lets cut to the chase!!!!!!!
What exactley do you have? And...
what exactley are you trying to accomplish?
This could help us tremendously. and then we can stop the "Dating Game".And get to the question.
 
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Old 07-22-06, 04:10 AM
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Originally Posted by lectriclee
OK.. I'm Not bidding.. Nor are any of us, So lets cut to the chase!!!!!!!
What exactley do you have? And...
what exactley are you trying to accomplish?
This could help us tremendously. and then we can stop the "Dating Game".And get to the question.
I was merely answering the "added" question by the OP. (whoops, sorry, I thought I was answereing the OP. Now that I look back, I see it was a hijacker that just jumped in with his own question. Apologies to OP and all)

Other than that quick diversion, Lectriclee is correct. Explain your situation so you can get the correct answers and the best help.
 
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Old 07-22-06, 07:45 PM
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This Is The Situation That Began This Post

I have several long lenths of the older (heavy steel) armored cable. The kind you have to cut with a hacksaw blade. It is empty because its contents have been removed and sold for scrap. The original contents were 12 gauge (2wire) solid copper. Some of the insulation on this older wire is failing due to age (50 years) so I replace it with NM whenever a wall is exposed.

I plan in certain situations, where I desire the protection of this nearly indestructible armored cable, to pull 1 black, 1 white and 1 green wire from spools of 14 gauge that I have excess of. These spools contain the kind of wire you would use for exterior wiring pulled through PVC conduit. Something like Thnn or the like... I forget the exact letter designations.

My main reason for wanting to do this is protection from rodents when pulling new circuits through walls and ceilings in old construction. I don't like the idea of having NM cable laying loose between joists. If I'm just fishing wire and not doing a complete tear out there is no feasible way to safely staple the wire to joists. I also want to avoid removing walls unecessarily as the plaster is in excellent shape. But if I find a poor wire insulator situation, I need to correct that and I thought I could pull this new wire through this armored sheathing and end up with a safer upgraded wiring situation.

In short...I am trying to correct potential problems in wiring that is 50 years old without disturbing the original plaster walls. To date there have been no issues with tripped breakers due to insulation failure. I'm trying to keep it this way.

Is the wire that is used in NM cable different in composition than the stuff that comes in the bulk spools? Thinner insulation?
Can stranded be used the same as solid, or do they have precise applications. Is either rigidity or flexibility the only issue between the two? I'm trying to educate myself on areas not understood.
Hope this explains my thoughts better. Let me know.
 
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Old 07-22-06, 07:59 PM
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Even if this casing is acceptable to use as FMC, unless it is 1/2 inch trade size or large its uses a very limited and cannot be used for general use.per NEC 348.20.

This is actually kind of ironic since MC and AC cable is allowed for general use and is as small as 3/8 trade size.

Go figure.

The only possibility (which I don't believe is acceptable) is that if it is construed as you are remanufacturing the AC cable. Since AC and MC are factory made cables, I don;t believe this would be an acceptable practice. If this were to be allowed, you would need to use whatever insulation is used as original, I would think.
 
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Old 07-22-06, 08:27 PM
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Yes, It's at least 1/2" maybe more

I never measured it precisely. So FMC is the term of the blank armored stuff you can buy at supply houses? Not familiar with all the terms. I've only heard of MC lite.

I was wondering about the insulation as well. It was only a wound fiber based jacket that covered the rubber covered conductor wires. It pulls out when the wire is removed. I just thought you could use it like PVC conduit is used. It's just as strong if not more. Hammer blows bounce off the stuff. PVC would crack eventually. I've tried pulling 1black, 1white, 1 green already to test its feasibility. Easy pull.
 
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Old 07-22-06, 08:37 PM
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Sorry, guess I wasn't quite done

So code would rather have loose NM pulled into old construction that could, in some instances, lay above ceiling lath, where critters tend to hang out when they get in, and chew the hell out of everything? This sounds more dangerous than pulling new insulated wire off a spool and feeding it through a super tough armor. Not to mention, This armor will now gain a ground to add to its' safety. Originally only 2 wire.

Nope, ain't arguing with anyone here, just perplexed. Maybe NEC code is fearful that the one pulling new wire won't take the care to avoid nicks in the new wire insulation. Now why would someone do that.

Anyone know what the use of the blank 'Armored' (FMC) you find at supply houses is used for?
 
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Old 07-22-06, 08:41 PM
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Trade sizes are not the same as actual dimension. Trade size 1/2 inch actually measures about 7/8 of an inch. 1 inch trade size is about 1 3/8 inch. 3/8 inch trade size is about 1/2 inch or so.
Confusing isn't it.
FMC = flexible metal conduit. the spiral wound metal conduit. AKA greenfield

MC "lite" is a brand name of somebodies. MC cable is "metal clad" cable. It is a cable system from the factory.


What you are ending up with is essentially (probably) 3/8 inch FMC but since it was part of a factory made cable (MC or AC or BX) it is not listed ( I presume)for use as FMC and would therefor not be legal.

As I posted, nothing less than 1/2 inch trade size is allowed unless it is for certain listed applications. All in all what you are doing is probably not legal. An inspector may never catch it but that doesn;t make it legal or safe.
 
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Old 07-22-06, 08:54 PM
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Nec 348.20.

I just read the terminology for this code. It seems to me that the code exists, and limits use because of the grounding capability of FMC. The limit stated is 6 feet. It goes on later to state that a seperate conductor is need for longer runs. This is abviously due to the very real loss af grounding reliability in longer runs.

I guess I still don't see the 'danger' part if I use bushings, avoid use of the FMC to accomplish grounding of the circuit and ensure no nicking of the insualtion. Is it a heat issue buildup inside the cavity. It is the same principal as running through PVC. Once again, I am not arguing here (kind of feels like I am since I read no, and I keep asking why) Doesn't seem like this code applies as long as there is a grounding conductor fished with power and neutral.
 
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Old 07-22-06, 08:55 PM
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So code would rather have loose NM pulled into old construction that could, in some instances, lay above ceiling lath, where critters tend to hang out when they get in, and chew the hell out of everything? This sounds more dangerous than pulling new insulated wire off a spool and feeding it through a super tough armor. Not to mention, This armor will now gain a ground to add to its' safety. Originally only 2 wire.


Yup thats about it. I'm quite sure it's much deeper than that though. You are not required to secure old work installations, Except where exposed.The new insulation the critters don't realy carefore.
But anyway... Can you reach these locations from the basement,attic or closets. This would minimize any destruction of the plaster. Will the existing boxes accept the new cables or will they be too small.There is alot to consider. The old armored cable is somewhat grounded thru the armor, that was 1 of the original intentions.The code is the code. You could always ask the inspector. Who knows? stranger things happen.
 
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Old 07-22-06, 09:05 PM
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Originally Posted by earlybird
I just read the terminology for this code. It seems to me that the code exists, and limits use because of the grounding capability of FMC. The limit stated is 6 feet. It goes on later to state that a seperate conductor is need for longer runs. This is abviously due to the very real loss af grounding reliability in longer runs.

I guess I still don't see the 'danger' part if I use bushings, avoid use of the FMC to accomplish grounding of the circuit and ensure no nicking of the insualtion. Is it a heat issue buildup inside the cavity. It is the same principal as running through PVC. Once again, I am not arguing here (kind of feels like I am since I read no, and I keep asking why) Doesn't seem like this code applies as long as there is a grounding conductor fished with power and neutral.
You seem to be missing the point of the minimum acceptable size is 1/2 inch trade size. I have put in thousands upon thousands of feet of MC and AC and have not run across any that are 1/2 inch trade size (or larger) while using #12 wire.

But like I also stated, the irony of the fact that most common MC and AC cables are aproximately (if not exactly) 3/8 inch trade size.

Just as a side note, in Michigan, a seperate ground is required in any FMC construct, not just those longer than 6 feet. I do not know if you have a local or state code that supercedes the NEC.

As lee suggested, ask your local inspector if he will allow your proposed use. I have seen stranger things in this arena.

So it is not neccessarily a "danger" issue but one of legalities and liability or culpability were you to install it against code requirements.
 
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Old 07-23-06, 03:57 PM
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There is also a very significant technical issue here:

What you have is a _cable assembly_, not a _conduit_. A cable assembly is manufactured with the wires inside it, a conduit is one large enough to pull wires into it.

Now it turns out that very old cable assemblies use old types of insulation which are much thicker than modern cable assemblies. In particular I am thinking of the stuff with cloth wrapped rubber insulation with an over-all cloth wrapping. These cable assemblies are so large that it is very tempting to try to use them as conduits. But they are not conduit, and there are probably a good number of things that could simply go wrong.

When pulling wires into conduit, the NEC limits the conduit run to a maximum of 360 degrees of total bend between pull points. This means that you have to count the change in angle at any point where the conduit is not straight, add up the total of all the turns, and have less than 360 degrees for this to be a legal conduit run. Additionally, you are required to have a certain minimum radius on all bends, which changes with the size of the conduit. There are probably listing requirements for the inner surface of the conduit, and different requirements for the fittings that attach the conduit to the box. All of these requirements are there to ensure that conductors can be easily pulled into the conduit without damaging the conductors.

Cable assemblies are built up with the wires in place. There are no requirements for maintaining ease of pull. You can have tight bends, you can run the cable in circles, etc. The cable assembly is required to follow the supporting structure, even if this means many, many turns. Additionally, the inner surface may be manufactured differently; it needs to not damage the conductors that are in place when the cable assembly is moved around, fished, or stapled down, but it does not need to protect conductors being pulled through the armor.

It is extremely unlikely that any given armored cable run would follow a path suitable for use as a conduit, and also unlikely that the armored cable would be suitable as a conduit raceway.

-Jon
 
  #23  
Old 07-23-06, 05:22 PM
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The old casing is not listed as a conduit so you can't use it.
You don't know what the inside of it looks like. There could be sharp edges that would slice the insulation off as you pull wires though it.
 
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Old 07-23-06, 09:34 PM
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This isn't brain surgery. The NEC has specific rules that apply even if you are somehow unable to apply logic to them.

If the armor had wire in it before and it came from the factory that way, it is a cable assembly, as was explained . That means the armor isn't listed to have new wire installed inside nor to be used as conduit.

FMC is listed as conduit and doesn't have restrictions on it's use as such so long as the code conventions for it are followed.

If you want someone here to say to go ahead and use the stuff you have as you have indicated you want to it's probably not going to happen You can either do the job(s) to code or not. No amount of discussion here will change the facts of the situation, which are that what you propose isn't code compliant. It wouldn't matter if MC or AC cable came off the same machine that produced the FMC, if the armor isn't listed as conduit you can't use it for conduit.

Does it really take 23 posts to get the point across that this won't fly? Kind of a waste......
 
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Old 07-23-06, 10:07 PM
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If you want someone here to say to go ahead and use the stuff you have as you have indicated you want to it's probably not going to happen You can either do the job(s) to code or not. No amount of discussion here will change the facts of the situation, which are that what you propose isn't code compliant. It wouldn't matter if MC or AC cable came off the same machine that produced the FMC, if the armor isn't listed as conduit you can't use it for conduit.



VERY well put.
 
 

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