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  #1   IP: 12.2.142.7
Old July 8th, 2004, 10:27 AM
Dan Dan is offline
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Question BX Cable Grounding

I am doing some rework in a system that has a combination on conduit and bx cable. The bx drops to light fixtures from junction boxes. In looking at new bx cable, I find that two types are available. Some has a light, bare aluminum (?) wire as perhaps some type of ground. For 12-2 bx, there are 2 insulated conductors and this little wire bundled in the package. Other 12-2 bx cable has a fully insulated, full size ground bundled with it. Most of what I see in the home center electrical section is like the latter. The other has a special, lower price and is being discontinued. Did Code requirements change? The existing structure bx (or conduit) wiring has no ground wire and obviously relied on the bx shield as the ground. Which wire should I buy? When putting into the old system, should the ground wire be terminated in the box (connected to conduit) with grounding screw, and then ground the box like is done with Romex? What are the requirements for rework on the existing system.

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Dan
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  #2   IP: 66.59.126.127
Old July 8th, 2004, 02:49 PM
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mdshunk mdshunk is offline
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You're calling two different types of cable by the name "BX".

The type with two insulated conductors and a thin bare strip is rightly called by the NEC type AC cable, for "armored cable". This is the type of cable that we call by the slang name "BX" cable, after it's original name "Type B, Experimental". In this type of cable system, you wrap the strip back over the cable's armor at each termination, and wrap it into the corrugations for several inches. To connect this type of cable to the box, you need an AC cable connector. It has a set screw, and looks sort of like an EMT connector.

The other type of cable with the fully insulated, full sized grounding conductor is type MC cable, for "metal clad". It's terminated in a squeeze connector, that looks sort of like a Romex connector. You deal with the ground the same way you would when using Romex in metal boxes. Nothing special is done with the armor of this cable.

It's important to point out that with both types of cable, you need to use the red anti-short bushings at each termination ("Red Heads"). Note that type AC cable normally has steel armour, and it quite heavy in roll or reel form. Type MC cable normally has aluminium armour, and is much lighter and easier to work with. Type MC cable is what's currently being used, and type AC cable is almost unheard of nowadays.
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  #3   IP: 12.2.142.7
Old July 9th, 2004, 06:25 AM
Dan Dan is offline
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Default Thanks - MC vs AC cable

Thanks for the MC vs AC cable description. I've also seen the end terminations that screw into the armor. Do these work with either type of cable? Do you use the anti-short bushing with these?

Thanks again,
Dan
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  #4   IP: 38.164.3.131
Old July 9th, 2004, 07:30 AM
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Ron Ron is offline
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As a rule of thumb, it is good to use the red heads for both AC and MC cable. With that said, it is only required for AC cable.
See the NEMA link below.
http://www.nema.org/DocUploads/8B117...EF3/BULL90.pdf
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  #5   IP: 66.59.126.169
Old July 9th, 2004, 03:16 PM
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mdshunk mdshunk is offline
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Quote:
Originally Posted by Dan
Thanks for the MC vs AC cable description. I've also seen the end terminations that screw into the armor. Do these work with either type of cable? Do you use the anti-short bushing with these?

Thanks again,
Dan
I think that it's technically impossible to use a red-head with the screw in connectors. I have never used them. You'd have to check the package to see if they say they are for AC or MC cable. I have a feeling they are only for metal clad and not armoured. I don't see how they could be used with AC and still give the metal strip a place to wrap back around.


RON.... thanks for that link. I never knew that red-heads were optional with type MC. I always get cranked up when I get a roll of MC that doesn't have the bushings attached. Now I know why. Thanks...
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  #6   IP: 148.78.243.122
Old July 19th, 2004, 05:53 PM
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Wgoodrich Wgoodrich is offline
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The metal ribbon found in type AC cable is not required to attach to anything. The ribbon only is intended to augment the connection of the metal tape of the AC cable. It is not required to wrap to or attach to anything. It does its job by runnning the full length of the flex sheathing to ensure a fault path between any loop of the wraps of the flex sheathing.

You are required to use listed connectors approved for the use with this cable that is all. Read the manufacturers installation instructions. You will find this connection design there.

Hope this helps

Wg
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