We have some BX runs laid out in a "star" (or spoke-and-hub) configuration in our basement.
The electrician suggested replacing the old BX with Romex since he said that the BX relies on the cladding for grounding which can rust leaving the circuits ungrounded.
Is this a real concern with BX? Is Romex better than old BX or does the metal cladding give you a better quality cable?
imho:
If it aint broke dont fix. Just keep an eye on it. If it rusts, you have another problem, tackle the humidity issue asap.
Personally I like the dedicated equipment grounding conductor, but then I have to remind myself, BX(AC) has a dedicated equipment grounding conductor, and unlike romex(NM) it also shields the conductors from accidental abuse.
What is the condition of the cladding now?
OH, btw, don't let anyone unnessarly touch the cables. I bet electrician is handling them alot. The oils and salts on your hands will attract moisture and discolor(and possibly corrode) the cladding. Make it look like it needs replaceing.
BTW, I''ve been in houses that were in crappy conditions structurally, but the BX (AC) was going strong.
later,
tom @
You can bang a nail thru Romex a whole lot easier than you can BX .
BX isn't AC. BX doesn't have the grounding conductor, which means it acts like an inductor when the cladding is used as a conductor. This is why the bare grounding conductor was added, and the name changed to AC.
This has been discussed here many times.
The modern AC is repeatedly referred to as BX by electricians and electrical supply stores. I was in HD a couple of days ago and they had a special on 250ft 12/3 ***BX***. DIY books seem to use the BX designation more frequently than AC. Like Romex and Kleenex and Xerox, BX was a trade name used by the inventors, GE, and referred to the location of the plant where it was invented: the Bronx, i.e. BX. Similarly Romex was invented (!) in Rome NY, hence the name.
To which discussions you obviously didn't pay attention. The discussion of induced current in the spiral sheathing resulted in the conclusion that at 60hz any induced current is infinitesimal. In present day AC the small gauge follower wire is not the ECG and is electrically not connected to anything except the outside sheathing. The outside sheathing IS the ECG.
Although it's difficult to find out the exact history the consensus seems to be that BX was introduced not primarily to prevent damage to the conductors from nails and the like but to stop rodents gnawing on them. The ability to use the outside sheathing as a ground was simply serendipitous.
As to where the "induction" idea came from I heard an interesting comment from an old guy who was certainly around at the time of the introduction of the follower wire (early sixties). He said that the problem with the old no-follower-wire BX was that it caused interference with the TV sets which were becoming popular at the time. The solution was the follower wire which stopped the spiral from becoming an antenna. This may of course be a crock-of-shit. If anyone has any firm documented information from the era, I'd be interested in hearing about it.
Not picking on you, or proclaiming an expert, but if the bonding wire prevented the clading from being sometype of interferring antena, wouldn't MC have the same type bonding wire?
Just tossing that out for thought.
later,
tom @
This is because the conductors are seperated. If they are together, they kinda cancel out their magentic fluxes, reducint inductive heating. I think that's why it's not much of a concern in bundled conductors, and even code requires same circuit conductors to be organized together.
Same here, the code is very indepte, and sometimes not knowing the history behind a decision, or why a group think, leaves the reader confused.
Just learning too.....
later,
tom @
I made the point of the difference, since people here working on older houses can encounter the older BX, and we can not see what is really happening at their end. I'm not expecting to change the lingo of every electrician in the world.
Thanks for the post with the details.
According to HorneTD :
I was involved in that discussion, and as you may recall, I agreed _completely_ with you over considering cable armor alone as "adequate grounding".
The only point where I disagreed with you is the suggestion that the "coil reactive impedance" is likely to play a significant factor between tripping the OCPD and not tripping the OCPD. Corrosion is going to be the main factor in virtually every real world case if you're dealing with a mere 60Hz.
At 400Mhz, on the other hand, a couple turns of wire is essentially an open circuit.
Remember that inductive heating isn't the same thing at all as reactive impedance. So one doesn't imply the other and v-v.
I'd love to see this test report. What current level/voltage was that at?
In order for this to occur at anything remotely resembling the power/frequency levels that occur in people's homes, there'd have to be some pretty subtle metalurgical changes going on in the metal box.
There ain't no way that 15A at 60hz going through separate holes in, say, 16ga steel is going to induce enough current flow to generate appreciable heat in said steel. No matter how long you waited. Unless you had the whole assembly in a dewar flask and didn't mind waiting the months for micro-degree daily increments to get to dangerous levels.
Indeed, inductive heating _still_ occurs _even if_ the whole circuit goes through one hole. Thus, the same thing would happen (albeit take longer) even if the whole cable went through one hole.
Unbalanced circuits (eg: hot plus neutral) radiate, period. Thus they induce current in surrounding plate penetration, one or two hole. Thus they generate heat.
But, the frequency is VERY important.
The amount of inductive heating at 60hz on simple plate penetration is small. Not non-existant, but _extremely_ small. So small to be dwarfed by conduction/convection in any real-world situation we're likely to see. Now, if we're talking million amp plus flows, that's different.
I'm not one of those posters. I consider you a highly skilled and knowledgeable professional, with very valuable real experience. Just don't know RF very well - s'alright, you know a _lot_ more about code than I do.
Like I said this might be a crock-of-shit; it just seemed to be an interesting hypothesis given the apparent problems with the induced resistance mantra. I can only guess (and the physics are way beyond my level) that perhaps the interference only occurs when the EGC (outside sheathing) is carrying current (i.e. there is some sort of fault). With MC the armor never carries current even if a fault occurs. This would also explain why there's no rush to replace the installed no-follower-wire AC: the armor has to be used as an EGC AND a fault has to occur.
"Always question." "Always be wary of asserted authority."
In none of the above do you make reference to anything we can check or to anything which explains the apparent anomaly of a perfectly good steel strip connecting the source of the fault with the building ground not working properly if wound in the form of a spiral but being OK if a ridiculously small gauge wire follows along the spiral. The proffered explanation of inductive resistance seems to be definitively crushed by Chris Lewis who in the earlier discussions was supported by other posters who had presumably done the same calculations. In any event no one pointed to errors in his reasoning.
You (or UL) seem to think that the EGC depends on the contact between each of the spirals. Why? AC is a continuous length of steel that just happens to be in a spiral form. If one were to pull apart the spirals thereby extending the steel to its full length and properly connect both ends wouldn't that form an adequate EGC? Corrosion between the spirals is really immaterial. The only questions would seem to be: Is the steel armor of sufficient gauge to carry the current and are the ends properly connected? In the latter case you can badly connect the NM ground too and connections can work loose over time so how does that differ?
And please spare us the drama. "Carrying out the dead" indeed!
Snip
I have been a volunteer in Fire and Rescue since 1971. In those thirty three years I have helped to carry out the remains of fourteen persons who were killed in fires. Three of those persons were killed in fires of electrical origin and all three of those were pre school aged children. It strikes me as strange that so many people want to forbid the fire service from talking about the task of dealing with the burned bodies of fire victims by accusing us of fear mongering or of being melodramatic. I'm not dysfunctional from post traumatic stress or anything even remotely like that but when I talk or write about fire prevention those people are never far from my mind. I read the dismissive comments of some posters about the low rate of fire deaths caused by arcing faults or some other cause and the claims that the remedy is more expensive then the number of deaths would warrant. Would you like to make book on whether the parents of those three kids would agree. I frankly don't care if I subject you to excessive drama as I believe that is preferable to subjecting anyone to death by fire.
-- Tom H
It's called Darwinism... Those who do stupid things, like buying cheap/faulty appliances or taking wiring shortcuts, or overloading outlets, etc. are taken out of the gene pool.
Sociecty is so busy protecting its "stupid" members that it will (if not already) become so stupid that it will fail.
Let stupid people die by their own actions.
Try telling that to Jeff Wiznia!
I would not think it that hard to ask UL about the research I have attributed to them. My problem is that they pay their bills by getting paid for their research efforts. Even if I bought a copy of their report the copyright would still prevent me from posting it on line. Is it your position that if I can't spoon feed you the supporting research that I should not make reference to it?
I didn't see the calculations you referred to would you help me out and point me to the postings that contain those calculations that I missed. Did I miss someones posting that indicated a source for some research that invalidates my willingness to depend on the testing done by UL?
Is it somehow illogical to assert that the length of the spiral armor measured along the spiral coupled with the lower conductivity of steel makes it a poor Equipment Grounding Conductor (EGC) unless the turns are shorted to each other so that the larger cross sectional area of the sheath can then make up for those factors? Why does corrugated armored cable not have a bonding strip while the spiral tape armored cable does have one? I've installed thousands of feet of the spiral armor type and several hundred feet of the corrugated armor type. The post installation testing of each type yields very similar results in terms of the Equipment Grounding Conductor loop voltage drop.
UL's tests of armored cable that is armored with spiral wound interlocking metal tape show that the armor alone is an inadequate EGC. I may indeed have misunderstood why that is true but that does not mean it is not true. Your position would appear to be that the spiral tape must be an adequate EGC. On what are you basing that assertion? What testing have you done or can you point the rest of us to that supports your conclusion? If the spiral tape armor is adequate as an EGC then why do the manufacturers install the bonding strip? The addition of the bonding strip to the sheath is not without cost. Do you believe that the manufacturers install it for some other purpose than bonding the turns of spiral interlocking metal tape armor to each other? If so what purpose do you allege the bonding strip serves.
What I do believe in this matter is that UL is acting in good faith. I cannot see what UL would have to gain by declining listing to a simpler cable construction that passes the listing standard. They are not the only laboratory that conducts electrical product testing so if they decline listing to a product that does meet the examination standard they would simply loose that manufacturers listing fees to another laboratory such as Southwest Research or ETL. I depend on the results of laboratory testing. I am willing to depend on the laboratory listing mark to determine whether an electrical component of a buildings wiring system is suitable for it's intended use. You and a few others here keep insisting that the reliance of myself and others here on the laboratory listing make us somehow wrong. Fact is I've offered more evidence than you have for our respective positions.
-- Tom H
According to SpamFree :
Let's not start all this again, and look at something else - Canadian CEC hasn't permitted cable sheath to be a ground for at least 30 years, has _never_ approved armored cable with that "ridiculously small gauge [bonding] wire", and has insisted for at least 30 years on a full size copper ground conductor.
Cable armor is physical protection and little more.
The facts are simple: the cross-sectional area of cable armor is small, perhaps not even as much as a copper wire. It's steel, not copper. Steel rusts. Even when galvanized, the edges rust. Even if aluminum, the manufacturing process will not produce long-term high conductance joints.
It's small cross-sectional area to begin with. Rust makes it smaller. It's really long. It's brittle. An impact can destroy the electrical conductivity if it cracks the strip. One must NOT rely on the conductivity of cable sheath or box clamps for ground continuity.
There's a reason that current carrying conductors are copper, not steel.
IIRC, US "MC" cable is armored with a full size copper ground. Use _that_.
In Canada, that's all we're allowed to use.
Tom is right. Cable armor is a lousy ground. It's just that reactive impedance is _not_ the reason why it's lousy.
[If I recall my calculations right, several hundred feed of cable armor will have an equivalent impedance of a few microhenries. At 60hz, the reactive impedance is insignificant - nowhere near enough to affect breaker trip. It's like saying "skin effect" matters at 60hz. Skin effect exists. But at 60hz it can be totally ignored.]
I realize that you're Canadian and have a patriotic reason for asserting the superiority of the Canadian Electrical Code however I have an equally patriotic reason for asserting the opposite. In fact, what Canada does wiring-wise has IMO as much relevance as the electrical code of ... oh, say Bulgaria . Canada doesn't allow AC? Well that's nice to know. They probably do have special problems wiring igloos .
So you say. However NYC, hardly a slouch in the imposition of onerous regulations, has always insisted on the use of AC (nowadays with the follower wire) and in using the armor as the EGC. Only in the last few years have there been the minutest changes to allow NM in some very suburban style non-rental houses. Commercial and rental buildings are still conduit or EMT or AC. I believe many of the large older US cities are in the same situation.
If the problem is as severe as you and Horne assert it would seem logical that, needing the rodent-protecting capabilities of armored cable, NYC would have changed to MC long ago. But they haven't.
Further, do you realize that there are hundreds of thousands (perhaps millions) of feet of old style (non-follower-wire) AC installed in thousands of buildings much of it dating to the forties and earlier? And that old-style AC is being used as a ground by its very nature. The metal box with the switch or the device is connected to the armor of the AC by a screw (not a clamp), the armor of the AC is connected to the load center metal (probably your denigrated steel) by another screw, and the load center is connected to the ground (likely the water main). Whether it actually forms an E(quipment) ground depends on the connection between the device and the box but in the case of direct wired lamps (isn't this Horne's dramatic case) it's almost certainly electrically bonded.
So applying your horror scenario, if a fault develops in the lamp or the box or in part of the cable the weak and ineffective armor will not be able to carry enough current to blow the circuit breaker and the home owner (or tenant), presuming a defective bulb, will be electrocuted when he investigates. Alternatively the AC will heat to incandescence (your earlier assertion and presumably Horne's dramatic case) and burn the building down.
Wow! I guess the NYC electricians guild hasn't thought of this. Just think of the work replacing all that old-style AC with more-expensive-than-new-style-AC MC. Just think of the mega bucks rolling in.
OK, I'm being sarcastic but the point stands. If old-style or new-style AC represented any significant danger then there'd be moves afoot to replace it (mandatorily) and there certainly wouldn't be any more installed. Isn't this what's happening with Knob & Tube?
Aw come on! The current carrying conductors have to be wound around screws and bent in tight arcs. I'd be the first to agree that steel is not suitable for this use.
That's a stupid comment! About equivalent to saying "Yeah they use lighted palm frond torches in Florida or Hawaii"! A reason for quoting the Canadian code was probably merely to point out "Here is an authority not unknown for good safety practices in many technical fields that regulates to the following standard."
The problem with using the BX cable armor as a ground is probably due to the difficulty of making and maintaining a good electrical connection to the armor at the ends of the cable. The clamps at the boxes are not the most reliable.
And how do you deal with plastic boxes? The dedicated copper ground wire is much easier to connnect to.
Mark
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