Difference Between AC and BX

[big john]

This is just a curiosity, but I've heard a couple of people mention that there is in fact a difference between BX and AC.

Apparently BX is no longer used and everything now days is armored cable. What were the intrinsic differences between the two?

I always figured armored cable was called BX to avoid confusion. Simply because calling it AC might too easily be mistaken for MC.

-John

 

[Ed MacLaren]

Re: Difference Between AC and BX

You're right John, BX was actually one of the early manufacturer's trade-name for Armored Cable. Some of us old cats still call it BX.

Ed

 

[tonyi]

Re: Difference Between AC and BX

The earlier "BX" didn't have the internal bond wire needed to make the armor suitable for a EG. AC types always had the bond wire and armor suitable for EG.

I've found asking for AC in a lot of supply houses frequently brings a blank stare from the counter dudes. Ask for BX and they (usually) know what you mean<g>... asking for HCF might get you the blank stare again<g>

Everyone wants to push MC today it seems, but AC has some very distinct advantages in renovation work that are often overlooked by the Romex/plastic boxs and MC pushing crowd.

 

[Guest]

Re: Difference Between AC and BX

I found a pretty good history of BX. I'm asking the mikeholt.com webmaster if I can change my nickname here to Old-Cat per Ed

http://www.seatekco.com/bx.htm

Here is BX on the left, and AC on the right:

A modern cutter helps with making up "BX" (AC):

 

[big john]

Re: Difference Between AC and BX

I hate to admit it, but I'm more of a fan of MC than AC. Quicker installation with AC, and I know there are some different installation practices as per NEC, but I've always felt like the continuity of the ground in AC was a little too tenuous.

It's probably not based on anything other than the fact I'm just more comfortable having a EGC that I can wrap around a ground screw.

Once upon a time in a trade class in highschool, I would cut AC and MC with a hacksaw. I thought I'd died and gone to heaven when I joined the trade and they handed me a Roto-Split.

-John

 

[tonyi]

Re: Difference Between AC and BX

John, I generally avoid cheap connectors when doing AC, or anything needing a locknut that might work loose (like the old set screw connectors).

The AMC-50 Speedlock (Halex has an equivalent design too) is the hot ticket for AC work as far as I'm concerned. It snaps into the hole and an internal screw clamp/spreader expands and forces a good connection against the box at the same time its clamping down on the armor. You'd have to tear the cable apart before one of these came loose<g>. This type connector doesn't come in a 90 degree format though, which can be a PITA sometimes.

The grounding aspect almost as important for MC - the MC armor is supposed to be grounded even though its not the EG. Most of these snapin/pushin connectors with the little spring fingers retaining the MC and holding the connector to the box seem scary to me... They're listed, but I wonder about their long term performance...we all know how (un)well backstabs perform over the long run

 

[big john]

Re: Difference Between AC and BX

It never even occured to me that it was important to bond the armor of MC even though it wasn't used as an EGC... I suppose they've thought that into the design of any listed connector, but it makes you wonder about the design of the MC itself: If there was a fault in the middle of a run of MC that caused the armor to become energized, would it even pass enough current along to the nearest EG bond to trip the OCP or would it just hang out there, waiting for the next person who grabs hold of it?

Maybe they found that the incidence of that happening is low enough that it's not a concern...

-John

 

[iwire]

Re: Difference Between AC and BX

John, if we do our job right both ends of the MC armor are grounded to the box's so the fault current has two directions to go to clear the fault. Even though one of those directions may be a U turn in the box and back through the ground conductor in the MC that is faulting.

Unlike the an old BX feed that the fault may only have one direction to go.

Many guys forget that MC armor is still a metal raceway that must be grounded.

 

[tonyi]

Re: Difference Between AC and BX

John, for longer runs I've used HCF on some occasions rather than MC because of this. I'm not sure 100'+ of ordinary spiral MC armor could clear a fault without turning into a glowing fillamant ;-> With some of the specialty MC formats you won't have this option...AC/HCF cable configurations are more limited than MC, but in a lot of ordinary situations, they can often be interchanged (allowing for support and the few other minor installation differences)

A while ago I read somewhere the length of an average residential box-box run was in the range of 10'. I suspect the MC armor would be fine to clear a fault over that distance.

 

[roger]

Re: Difference Between AC and BX

The MC armor would never be needed to clear a fault unlike that of AC or HCFC

In MC installations we would bond the armor with an EGC and not call on it as an EGC.

We would have to think of this bonding the same as we would any device or equipment bonding.

With that being said, if we had a fault to the frame of any device at say 100' from the source, the EGC would have to be sufficient in it's properties to clear the fault on it's own.

Roger

 

[ryan_618]

Re: Difference Between AC and BX

If you apply the rules of art. 250.118 and justify the use of MC armor as an EGC, could you use regular 12-2 as an I.G. circuit with the green conductor as your "clean" ground?

Sorry if I'm hijacking the topic, but it seems to be heading that direction.

 

[earlydean]

Re: Difference Between AC and BX

Ryan,
I went to a seminar by a manufacturer of armored cable once. The rep. informed of us the differences between AC and MC. As I remember, the insulated ground is only part of the difference. He maintained that MC is not superior to AC, but only different, and has different uses. Every type cable is manufactured for a specific use or for several specific purposes. He also stated that the armor on MC could not be used as the ECG unless it is rated as an EGC, some are and some are not. They do make AC with an insulated grounding conductor for the application that you describe. Health Care Facility Cable is one of those speciality cables.(see 517.13(A))

Earl

 

[tonyi]

Re: Difference Between AC and BX

Roger, MC's armor could indeed be an (albeit unwilling, and possibly inadequate) participant in clearing a fault - just consider any number of possible damaged/crushed cable scenarios, or where some live thing comes into contact with the MC armor. It may not even be a fault to a breaker the piece of MC is hooked to if its an inadvertent contact kinda thing. Faults are not always related to loads being served.

 

[tonyi]

Re: Difference Between AC and BX

Ryan, I guess you're refering to 11(b) here.

The operative word in that section is "OR", as opposed to 11(a) where the operative word was "AND". So...IMO, the answer has to be yes, if you had some corrugated or tube type MC where the armor WAS listed for EG, then you could indeed hijack the green on say a 12-2 for an IG.

 

[iwire]

Re: Difference Between AC and BX

Originally posted by tonyi:
if its an inadvertent contact kinda thing. Faults are not always related to loads being served.

That is very true but is there any evidence to suggest that the MC armor would not clear this kind of fault?

Say a live wire from another circuit comes into contact with the MC armor in the middle of a 250' run of MC.

The fault current would have 2 paths back to a copper ground conductor, both paths would be less than 126'

 

[tonyi]

Re: Difference Between AC and BX

I'd say the same logic that disallows old BX armor as a gnd applies here. Lacking bond wire assist, spiral MC armor amounts to a glorified fillament if its long enough.

FWIW, I unwound about 8" of cutoff armor from a 10-2 MC and got a tape over 36" long.

 

[george t. everett]

Re: Difference Between AC and BX

I am from the old school AC or MC should be cut with a hack saw. Rotozip is for Johnnny homeowner. Cut plenty of AC & MC and never a problem. I guesss it is the sign of the times.