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-   -   Armored cable shell as conduit? (http://www.diychatroom.com/f18/armored-cable-shell-conduit-160379/)

sixspeed 10-17-2012 01:38 PM

Armored cable shell as conduit?
 
Is it possible to use the existing armor of armored cable as a conduit to replace and run new conductors between two existing points?


Scenario: I have a single light switch controlling a ceiling light fixture in finished area. The light switch is wired as a switch loop, where line power enters at the light fixture junction box. There is one cable between the switch and light fixture: 14/2 armored (with bonding strip). The total linear distance between switch box and light fixture should be about 9 feet, with one 90 degree turn (the bend from wall run to ceiling run). I want to replace this switch with a dimmer which requires a neutral. Thus I need a neutral conductor at the switch box, or 14/3 cable between switch and fixture.

Dilemma: The existing armored 14/2 cable is fastened to each box inside with cable clamps. So I can easily detach the cable from each box. However, it seems the cable is fastened to structure behind the wall/ceiling. So I cannot pull out the old cable to feed in a new 14/3 cable. But the conductors within the armor seem to be free-floating with wiggle room. So would it be possible to pull out the 14/2 conductors from one end while pulling in 14/3 (stranded for more flexibility) conductors from the other end (using the outgoing conductors like a pull string; I'd figure it would be easier to swap the bundle of conductors rather than try to thread in a single neutral conductor)? Is the diameter of 14/2 armor cable the equivalent to 14/3 armor cable to make this physically possible as well as acceptable?

Thanks.

Jim Port 10-17-2012 01:49 PM

If you are dealing with a cable like MC or AC, commonly called BX, you will not be able to remove the conductors and pull in new ones. That is a cable assembly. However if you had flexible metal conduit, commonly called Greenfield, you can pull in new conductors.

The cable will be about 3/8" in diameter. The conduit is larger.

sixspeed 10-17-2012 02:08 PM

This is most likely BX cable (1970's vintage), although I can't confirm by reading any labeling or marking on the cable or the conductors.

What are some suggestions for replacing the entire cable segment with minimal destruction?

I do have access to the unfinished attic space above the ceiling, but the wall switch box is inside a finished wall. I suppose I can pry open the other cable knockout opening in the switch box, drill a new hole in the top plate of the wall, thread down a rod and see if I can pin the tail on the donkey...

jbfan 10-17-2012 02:37 PM

Sometimes it is quicker to remove the existing box, run the cable, then install a new box.

Jim Port 10-17-2012 02:43 PM

Another thing is the old cable should be stapled to the studs.

Your fished in cable will not need to be stapled.

Beepster 10-17-2012 02:59 PM

I would first be sure that you have BX and not Greenfield. I am not certain, but I believe that more of the older homes have Greenfield. My 1958 home in the Twin Cities has Greenfield and I am able to pull NM through. Only one NM 14-2 cable. I was told NO on 2 14-2 NM cables. And if the Greenfield does not terminate in a junction box, you must have a proper end with bushings to prevent the NM cable from getting nicked.

B

andrew79 10-17-2012 03:50 PM

If it was bx it wouldn't be free floating and there's no way you'd pull the wire out of it anyways over 9 feet plus 90.

sixspeed 10-17-2012 04:24 PM

Quote:

Originally Posted by jbfan (Post 1032710)
Sometimes it is quicker to remove the existing box, run the cable, then install a new box.

I've done this elsewhere, but only by planning to go larger (1-gang to 2-gang...) since it requires cutting some wall around the box for tool access to be able to undo the fasteners holding the mounting "wings" of the old metal box to the stud.

sixspeed 10-17-2012 04:43 PM

Quote:

Originally Posted by andrew79 (Post 1032772)
If it was bx it wouldn't be free floating and there's no way you'd pull the wire out of it anyways over 9 feet plus 90.

By free floating I mean the conductors inside the armor are not glued or in any way attached to the armor shell. Each insulated conductor also appears to be wrapped with a layer of brown paper. If you were to terminate this type of cable, you could cut the armor at say 1 feet from the end and easily pull that armor shell off. Whatever this armor cable I have, it is not a conventional conduit (smooth pipe).

rjniles 10-17-2012 07:21 PM

Quote:

Originally Posted by sixspeed (Post 1032786)
I've done this elsewhere, but only by planning to go larger (1-gang to 2-gang...) since it requires cutting some wall around the box for tool access to be able to undo the fasteners holding the mounting "wings" of the old metal box to the stud.

Not hard to remove a new work box without tearing out drywall. Ram a screwdriver between the stud and the box and pry open a saw blade worth of space. Use a saws-all with a metal blade to cut the box free. If you do a little drywall damage, caulk around the edge of the new box and install a mid-size plate.

Beepster 10-18-2012 08:38 AM

Quote:

Originally Posted by sixspeed (Post 1032810)
By free floating I mean the conductors inside the armor are not glued or in any way attached to the armor shell. Each insulated conductor also appears to be wrapped with a layer of brown paper. If you were to terminate this type of cable, you could cut the armor at say 1 feet from the end and easily pull that armor shell off. Whatever this armor cable I have, it is not a conventional conduit (smooth pipe).

It sounds like you have Greenfield. Pull all the wires out except one and use it to pull the 14-2 through. Not a problem here per the inspectors.

B

Jim Port 10-18-2012 08:49 AM

The conductors in Type AC cable were paper wrapped. The OP will not be able to pull them out.

sixspeed 10-18-2012 01:34 PM

Still a little bit confused about Greenfield vs. armored cable (BX), whether they are distinctly different. Depending on who you believe on the net, a mister Greenfield is credited as an inventor of the BX... :001_unsure:


Anyway, I'll figure a way to run a new cable. Thanks for everyone's insight.

Edit: I recently saw at the big box store an armored lighting whip (something like this: http://www.southwire.com/ProductCata...dcatsheetOEM35), where the conductors are loose enough inside the armor to yank out. Is this a greenfield type of armor cable?

mpoulton 10-18-2012 03:45 PM

Quote:

Originally Posted by sixspeed (Post 1033400)
Still a little bit confused about Greenfield vs. armored cable (BX), whether they are distinctly different. Depending on who you believe on the net, a mister Greenfield is credited as an inventor of the BX... :001_unsure:


Anyway, I'll figure a way to run a new cable. Thanks for everyone's insight.

Edit: I recently saw at the big box store an armored lighting whip (something like this: http://www.southwire.com/ProductCata...dcatsheetOEM35), where the conductors are loose enough inside the armor to yank out. Is this a greenfield type of armor cable?

Greenfield ("flexible metallic conduit" or FMC for code purposes) is a type of raceway, not a type of cable. It is intended to have conductors pulled into it, and has a diameter that is adequate for this and a relatively smooth interior. MC cable (aluminum spiral jacket) and AC cable (steel spiral jacket) are just that - cables. The cables are manufactured by spiral wrapping the jacket around the conductors, with some type of sheathing (paper or plastic) between the conductors and the jacket. The inner diameter is much smaller for the same conductor fill than FMC is, and it can sometimes be very difficult to pull the conductors out because they were never intended to be removed.

The bottom line is that it's not code compliant to use a cable jacket as a raceway, and there are some manufacturing differences between the two even though they look very similar.


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