How to Connect Grounded NM Wire to Ungrounded BX Wire

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  #1  
Old 05-22-06, 02:15 PM
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How to Connect Grounded NM Wire to Ungrounded BX Wire

Hi

I have to move a light switch to an adjoining wall. The existing wire is ungrounded BX. My question pertains to connecting grounded NM wire to the ungrounded BX wire.

I can simply connect WHITE to WHITE and BLACK to BLACK with wire nuts and electrical tape. The question is what to do with the NM grounding wire. Is there a special connector that I have to use or can I just wrap it around the BX casing and secure/cover with electrical tape?

I'd appreciate any help with this!

Thanks.

Jim
 
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Old 05-22-06, 03:09 PM
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Without aproper ground you cannot legally extend this circuit.
 
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Old 05-22-06, 05:08 PM
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Originally Posted by jdegeorge
I have to move a light switch to an adjoining wall. The existing wire is ungrounded BX.
I never heard of groundless AC.


> The question is what to do with the NM grounding wire.
It attaches to the box with a machine screw.

> Is there a special connector that I have to use?
A green machine screw is common (not special).


> can I just wrap it around the BX casing and secure/cover with electrical tape?

No, no, and no!

You must continue to use the existing metal box for the splice!
 
  #4  
Old 05-22-06, 07:13 PM
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Originally Posted by bolide
I never heard of groundless AC.

> The question is what to do with the NM grounding wire.
It attaches to the box with a machine screw.

> Is there a special connector that I have to use?
A green machine screw is common (not special).

You must continue to use the existing metal box for the splice!
I also have groundless AC at my house.

I think what Racraft is saying--you can not legally extend an ungrounded armored cable circuit. --with NM or otherwise.
(I don't believe the metal jacket will suffice as a legal ground--even if it electrically reads as ground.)
As such you can't legally put NM into box with armored cable and extend the ground by screwing it to the box.

This is the latest info I received on this.

--Curious what others have to say.
 
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Old 05-22-06, 07:24 PM
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It sounds like what you folks have is not mc or ac but BX.

It is no longer a cable assembly listed for use by the NEC.
and now you know one reason why.
 
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Old 05-22-06, 07:28 PM
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OK--they don't make BX anymore--wasn't it just a brand name former maker of armored cable?
 
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Old 05-22-06, 07:43 PM
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Originally Posted by Capt
OK--they don't make BX anymore--wasn't it just a brand name former maker of armored cable?
No BX was a listed cable at one time.

As well AC (armored cable) and MC (metal clad) are not the same thing either. AC has a solid copper or aluminum bare wire that runs inside the metal wrap where MC doesn't. They are listed in the NEC and described there.

I have not has the opportunity to work with BX when it was being used and do not know the correct way to handle it. I would think the wrap was accepted as a ground but correct connectors would assuredly be needed to provide legally accepted continuity. But that is only a guess.
 
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Old 05-22-06, 08:24 PM
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Ac cable is the correct name for what is commonly known as BX.
 
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Old 05-22-06, 09:05 PM
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Originally Posted by pcboss
Ac cable is the correct name for what is commonly known as BX.

no it is most definately not. There used to be BX cable. It is no longer an acceptable cable. Many people do call either MC or AC by the name BX but they are each a seperate type of cable.

BX is actually closer to MC than AC but even there , there are differences in the insulation requirements.

AC , as I posted earlier, has a solid wire inside the metal sheathing that was not included in BX or MC.
 

Last edited by nap; 05-22-06 at 09:20 PM.
  #10  
Old 05-23-06, 05:20 AM
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My understanding is that the code _always_ called the stuff type AC cable, and that BX was a brand name.

Under the earlier code the armor was used as the equipment grounding conductor. However it was determined that without the 'bonding strip' the armor was not always sufficient to act as the EGC. Thus the requirements for AC cable were changed to include the 'bonding strip'. I am working from memory here, and don't have previous code editions on hand to confirm the above.

This bonding strip may actually be present in the AC cable that the original poster was asking about. This bonding strip is never brought into the junction box, but is instead wrapped back over the anti-short bushing and folded under the cable clamp. If this bonding strip is present, then the combination of armor and bonding strip is considered a sufficient EGC, and this circuit could be extended.

If the AC cable has a proper ground, and is in a proper cable clamp to a metal box, then the box is properly grounded. The ground wire from the romex could then be connected to the box.

-Jon
 
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Old 05-23-06, 05:56 AM
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BX was not a trade named item --
it was a separate cable type used thru 1950s --

extend the circuit by setting a nonmetallic junction box, connecting the grounded NM cable hot & neutral, leave the grounding wire unused, terminate the new cable in a nonmetallic box for the switch. -- providing the outside metal of the BX is terminated with a waxed heavy cardboard or plastic insert to keep the metal from digging into the inner wires.

BX was discontinued because the metal sheath was not suitable to handle fault currents due to poor contact from one helix to another, and the industry needed to provide safer installations by interconnecting all metal enclosures and utilitization devices | appliance metal chassis against shocks, electrocutions, fire ignition.

to stop fault currents, they must get back to the source quickly, normally protected by a fuse or breaker.
 
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Old 05-23-06, 09:17 AM
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From the www.codecheck.com website

AC(BX) Armor clad cable
BX is the common trade name for AC. BX was the trademark of cable made by G.E.'s Sprague Electric division.

189?- Gus Johnson and Harry Greenfield patent AC

1910- AC receiving acceptance.

1920s or the early 1930s widespread adoption.

1932 NEC- Armored cable was officially called Type AC

1952- Aluminum clad AC introduced.

1959 NEC- Aluminum bonding wire required under metal sheathing.
 
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Old 05-23-06, 11:04 AM
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BX designation of GE type AC cable

BX was the General Electric Code for the Bronx plant were they manufactured type AC cable. neither the NEC or UL ever used it as a cable designation. Much like Kleenex and Xerox it has become the common name for the original construction of type AC cable. As another poster has pointed out the requirement for a bonding strip came many years after type AC was in common use.
 
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Old 05-23-06, 05:15 PM
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I am getting full of crow, guys. This is two servings today for me.

Does anybody even use AC?

Now do note, there is another cable listed in the NEC that is similar. MC (metal clad) it is made in two different styles but the common one in use around my area is extremely similar to AC but it does not have the redundant ground wire included.

The only time AC cable is used or required in my experience is in a medical facility or similar area. Other than that MC is generally used. Both are available with an EGC as part of the system.

BTW: winnie, I had an inspector that required a minimum of 4 backwraps the laid in the valley of the spiral to be acceptable to him. I have also had an inspector that DID require the redundant ground to be brough into the box and joined with the insulated EGC. How do you win? It seems every inspector has his interpretation as to what is right even when they are wrong.
 
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Old 05-23-06, 07:17 PM
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I think you'll find that Article 330 identifies MC cable as having a sheath that cannot be used for grounding, and a conductor provided within the cable for grounding.
Article 320 requires a bonding strip within the cable.
I agree that BX was an old type of AC cable that is not compliant with current requirements.
 
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Old 05-23-06, 08:16 PM
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Originally Posted by HandyRon
I think you'll find that Article 330 identifies MC cable as having a sheath that cannot be used for grounding, and a conductor provided within the cable for grounding.
Article 320 requires a bonding strip within the cable.
I agree that BX was an old type of AC cable that is not compliant with current requirements.
That is why type MC cable can be used as a pool or spa circuit. They don't require an redundant ground but they do require an insulated ground.
 
  #17  
Old 05-26-06, 01:55 PM
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Still Don't Know What To Do

I forgot to mark this post so I would get a response!

Anywho, thanks to all of you for responding so quickly. Interesting how my question sparked a lengthy discussion about the history of AC!

I said BX because that's what I was told it was, but yes, it's ARMORED CABLE with the twisted metal covering that looks like a screw.

It has only 2 wires (White and Black) unlike today's AC that also has a grounding wire. The house was built in the 20's.

Anywho, there is definitely no grounding wire in this box. However, this past summer we had an electrician upgrade the service and service panel. The new panel is grounded with #6 twisted copper cable, attached from the panel's neutral bar to an 8' copper ground rod which is embedded in the ground.

The existing house wiring was left alone, but a few new lines were added with NM.

I thought this sounded like the system was properly grounded. Isn't it?

As for my current situation, it sounds like that BOLIDE says:

> The question is what to do with the NM grounding wire.
It attaches to the box with a machine screw.
> Is there a special connector that I have to use?
A green machine screw is common (not special).
> can I just wrap it around the BX casing and secure/cover with electrical tape?
No, no, and no! You must continue to use the existing metal box for the splice!
So, if I make the splice in the metal box, connect WHITE to WHITE and BLACK to BLACK with wire nuts and electrical tape and attach the NM wire's ground wire to the metal box with a green grounding screw I'll be okay?

Have a great holiday weekend everyone, and thanks!
 
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Old 05-26-06, 03:22 PM
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No, if your existing cabling method is not currently grounded neither will the new part even though it would contain a grounding conductor. There would be no way for a fault current to get back to the panel.

AC cable does not contain a grounding conductor. It depends on a thin bond wire under the metallic sheath and the sheath to provide a ground path.
 
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Old 05-26-06, 07:56 PM
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I now understand that there's no way for this puppy to be grounded, but what do I have to do to splice the NM into the non-grounded BX, falling short of running an NM homerun from the panel?

Would it be unsafe to make the splice in the metal box, connect WHITE to WHITE and BLACK to BLACK with wire nuts and electrical tape and attach the NM wire's ground wire to the metal box with a green grounding screw?

I can't see it being any less safe than if I continued the run with more non-grounded BX, or as if I didn't move the light switch at all.

Please let me know. Thanks!
 
  #20  
Old 05-27-06, 08:15 AM
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jdegeorge,

You are not permitted to _extend_ an ungrounded circuit. You are permitted to keep using it unchanged, you are permitted to repair it, but you are not permitted to extend it.

An ungrounded circuit, properly installed, is reasonably safe, though not as safe as a properly grounded circuit. You are permitted to let the reduced safety remain ('grandfather clause'), but you are not permitted to _extend_ this situation elsewhere.

nap,

As I understand the proper use of the bonding strip in AC cable, the inspector that required extra wraps of the bonding strip was being a stickler for (perhaps excessive) workmanship, but not asking you to do something that was harmful or wrong. The inspector that required you to join the strip like an ordinary wire was asking you to do something potentially (though very unlikely to be) harmful.

-Jon
 
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Old 05-27-06, 08:57 AM
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Originally Posted by winnie
nap,

As I understand the proper use of the bonding strip in AC cable, the inspector that required extra wraps of the bonding strip was being a stickler for (perhaps excessive) workmanship, but not asking you to do something that was harmful or wrong. The inspector that required you to join the strip like an ordinary wire was asking you to do something potentially (though very unlikely to be) harmful.

-Jon

The anecdotes were more of a comment on the inspectors being anal than anything. I realize the wrapping served no true purpose, just the level of insanity (read: power mad) that inspectors can show.

A question on the other part though. How would joining the ground strip actually cause any problem? It actually would seem to provide some benefit. It removes the reliance on the ac connector to be the sole bonding connection between the ac metal wrap and the junction box. Like in all installations, when everything is perfect, all is well. When something comes apart or is installed less than perfect, backup is a good thing. What would be the functional difference between this and using a bond bushing on a conduit system that includes a egc?
 
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Old 05-27-06, 09:34 AM
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This is a little side point not relevant to the original poster's question, but an interesting discussion:
Originally Posted by nap
How would joining the ground strip actually cause any problem? It actually would seem to provide some benefit. It removes the reliance on the ac connector to be the sole bonding connection between the ac metal wrap and the junction box. Like in all installations, when everything is perfect, all is well. When something comes apart or is installed less than perfect, backup is a good thing. What would be the functional difference between this and using a bond bushing on a conduit system that includes a egc?
As I said above, this would be _very_ unlikely to actually cause a problem, and frankly I would need to come up with a pretty convoluted scenario. But that bonding strip is usually quite thin, often the equivalent of a 16 or 18ga aluminium wire. The bonding strip is _not_ intended to be an equipment grounding conductor, but is instead intended to function as _part_ of the equipment grounding conductor. I could conceive of a situation where, if you directly connected to this little wire, most or all of the fault current would flow through this wire, causing excessive heating. Far more likely is that this little wire would be part of a redundant parallel path of equipment grounding conductors, and thus not a problem.

You asked what the difference would be between using this bonding strip and an EGC connected to a bonding bushing on conduit; imagine that you install conduit with a bonding bushing, and then attach 16ga aluminium wire to the bonding bushing. For the most part the metal enclosure would act as the EGC, and the wire attached to the bonding bushing wouldn't matter. But in the situation where the redundancy matters (the conduit somehow breaking loose) the undersized bonding conductor might be worse than nothing at all.

-Jon
 
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