What's the difference between BX and Greenfield

[Nate Nagel]

Just got one of those ideas that seems to simple to possibly work. please tell me where the flaw in my thinking is. I have a circuit that is fed directly from the breaker panel to a receptacle two stories above. It's 14/2 BX without ground. Tester says that there is a ground through the armor but I've heard several places that older BX may provide a very high impedance ground due to the armor rusting between spirals. So here's my idea... why not pull out the old cloth covered wires and pull new THHN through the now-empty armor, kind of like if I were using greenfield for new construction. this way I could pull three conductors, assuming the ID of the old BX armor would allow me to do so.

What do you think? It seems like a pretty easy way of getting the ground where I need it, if it'll work. Certainly easier than fishing a separate conductor (although I'll have to do that on the other side of the house for the bathroom, but that's another issue for another time.)

nate

[Berkshire Bill]

The old conductors are not that easy to remove from the BX and the THNN will likely get nicked when you pull the cable.

Bill

[Edwin Pawlowski]

It is.

Try pulling the wire and you will see the flaw in seconds. May be OK for a few feet, but not for a long run.

I think your crazy, but hey, that just my opinion. Well, a few of your neighbors do also. Your mother still loves you though.

[L. M. Rappaport]

On Sun, 24 Dec 2006 09:11:06 -0500, Nate Nagel wrote (with possible editing):

I agree with Bill. I further think that there's no way you're going to pull those old, cloth covered conductors through the armor.

In addition, there's a logical mistake in the above. While the armor might rust (usually doesn't happen, btw) the fact that it is wound in a spiral means that rust doesn't matter. The stuff is usually galvanized and the coating (far more conductive than steel) is what provides a continuous ground. Still not as good as copper, though.

FWIW, assuming that it isn't stapled, I'd use the old BX to pull 14/2 w/g Romex. Taper the tape carefully.

[RBM]

With some types of armor cable it's tough enough to cut and pull off six inches of the armor, there's no way you'd be able to remove the conductors from the armor, also you'd need to use 1/2 inch greenfield to pull new conductors through, and if it's over six feet, you need a separate ground conductor

[Nate Nagel]

It is stapled, and furthermore, is pulled up inside a wall with no access. this is why I'm asking these questions. otherwise I either a) let it go or b) pull a wire across the house to a plumbing chase, up that into the attic, and then back down again to the 2nd floor.

I don't know how difficult it would be to pull through it... it's pretty much a straight shot and probably only 20 feet or less.

nate

[gfretwell]

If this is the type AC with the thin metal tape in it you can be pretty confident of the grounding assuming the electrician made up the connectors right. I did a survey of a building full of WWII EMT and BX, checking ground paths with an Ecos tester. They were all sound with less than 1 ohm total impedance on all of them. This was old style craftsmanship but it proved to me the wiring method was sound when properly installed.

[Nate Nagel]

It's not. house was built in '48 or '49, I forget which.

I guess that will have to be my next check, just for peace of mind. If it ohms out OK I will let it be, since the majority opinion seems to be that pulling new wire through old AC is probably difficult at best.

nate

[mm]

I don't think the idea is to conduct from one layer of spiral to the other. The idea is to go spiralling to the end.

It is only pi times as long, which still isn't much. (no, that doesn't make sense. It's more than that. It's maybe 3 inches, one circumference, for every half inch (one width of the spiral), so it's

6 times as long. But remember how big the cross section of the bx is compared to a wire. So it's still not much.

So I don't know wht these several places have in mind.

[gfretwell]

The problem with that is the "choke" effect. That is why Greenfield has a 6 foot length limitation when used as the EGC and why Type AC has that little bonding strip. It won't carry full fault current but it will knock off the leading edge of the fault. By that time the delayed current through the choke will catch up.

BTW the new style MC will use a "one size up" bare aluminum bonding wire under the armor instead of the green wire, or in addition to it in health care cable. You cut this off flush with the armor and use a new style bonding connector at the box.

[Tom The Great]

This is why BX (now AC) usually has a bonding wire installed to short out the spirals, do you see such a bonding wire? If so, then it appears your BX is up to today's common AC.

This would be considered a violation of the manufactures instructions, a NEC violation, unless you can prove it was safe. Also, unwarrented. If there is a grounding path, and you are still concerned, you can install a GFCI for extra protection, even for the downstream receptacles.

Now the information I offered was to contridict your information. What is a real fact, if you are concerned about your electrical system, get a qualified electrician to check it out. If work needs to be done depending on the amount, I would get three estimates.

Merry Christmas,

tom @