Can I use the old flexible conduit?

[Deguza]

Hello all:

I have a home built in 1950. I?m planning to rewire the home. I loo ked under the crawlspace and saw that the whole house is wired with wires in flexible metal conduits.

If I can use these conduits to care the new wiring, I don?t have t o tear up any sheet rock, Except for replacing some of the metal boxesthat may be on the small side.

My electrician friend says that I should be able to run Romex cable through them.

Another friend says that I should just run individual insulated wires.It mi ght be cheaper and easier to work with, he says.

From what I understand both are acceptable in terms of code.

What do you folks advise?

Thanks

Al

[dpb]

That's BX armored cable. Why pull wire in the same run; in all likelihood there's nothing wrong at all with the cable unless it's been flooded or some other reason.

With BX, the conduit generally served as the ground path so there's likely not a ground conductor if this is original wiring and there probably aren't grounded outlets unless some have been added since.

NEC limits the amount of "fill" such that going from 2-wire to 3-wire the extra ground conductor may exceed allowable but if doesn't NM will be much harder to pull and there's no point in doing so.

A more definitive answer would be to know what is the purpose behind the work envisioned.

[gfretwell]

If it is really FMC (at least 7/8" OD for 1/2") then you can pull in new wire but I would not try Romex. You probably won't make it through. Use THHN/THWN. If this stuff is more like 1/2"-5/8" OD it is BX or AC cable and it is an assembly. You probably can't get the old wire out and it is very unlikely you will get new wire in. It is hard enough pulling wire in FMC. You can't use the vacuum trick to pull in a string and pushing it in is almost impossible if there are any bends at all. Be sure you pull in a string when you pull out the old wire. I would use 1/8" nylon if you don't have "jet line" (the stuff the sparkies use. You don't want it to break.

[trader_4]

I guess because he's upgrading all the wiring, and per your later comments, what he has doesn't have a proper ground, can't be used as part of new wiring, etc.

If it's very old BX, there was no ground back then. Today BX has a separate ground wire in it, which provides the ground. In between those two, I guess BX might have been used to provide a ground where the metal jacket alone was used, but that doesn't provide an adequate ground, which is why it now has the additional wire.

How would you ever pull anything in BX cable? Even if you theoretically could get what's in there out, I don't think NM would ever fit inside.

Agree. IDK what he has, because he says an electrician friend suggested he could run Romex inside, which doesn't make sense to me if he has BX. But BX would seem to be the most likely thing for a house, not actual conduit.

[Mark]

You need to check YOUR codes what's good in one place may be bad in another. If your in Chicago both romex and BX are a no go , greenfield is ok to a point - in some cases. Everything must be in pipe in Chicago , Go 20 miles north romex may be permitted.

[Iggy]

replying to Deguza, Iggy wrote: I agree with dpb. It may be flexible conduit, but that practice is very rare. Armored cable is the best stuff and actually keeps the unexposed wires brand new. Every armored cable I've re-cut for longer ends that had been cut-down over the decades were always like new in everyway, my oldest so far were 80-years old. Unless you need a larger gauge wire, I'd either leave what's there or cut the ends back to re-locate the boxes. However, your friend is wrong about the Romex replacement idea, you don't know if you now have or will have cable chewing rodents in the future. Always upgrade and never downgrade.

[Clare Snyder]

New York and Chicago require arboured cable for ratproofing. If Chicago rerquires EMT it's just to feed the mob.

[trader_4]

What he has, as he's described it, very likely can't be used as part of his upgrade because if it's old BX, it does not have a ground wire in it. Homes in the 50s had ungrounded receptacles.

[Iggy]

replying to trader_4, Iggy wrote: You're absolutely right IF it's actually flexible conduit for just in the crawlspace. However, If the entire place is actually BX, then the jacket can be very successfully used as a ground conductor between a grounding outlet and a grounding rod. I've done it and have had it done, the ground will be fully confirmed by any and all Wiring Testers, GFCI Affirmers and other testing methods.

Sure, a separate ground wire MAY be better, but only due to matched or slightly better impedance though at much less capacity. 1st the system's ability needs to be confirmed, then each outlet needs confirmation and necessary correction. But, he needs to understand that new wire isn't better nor a guarantee, replacing sound 14-gauge with new 14-gauge is no upgrade and completely wasted effort for no benefit.

[Iggy]

replying to Clare Snyder, Iggy wrote: You've got that right! But, an EMT job is real purty.

[trader_4]

If you're doing it to code and making it safe it can't be used. That old BX has no bonding wire inside and the jacket is not reliable and effective as a ground conductor without it, because it cannot guarantee the necessary low impedance path. It's a clear NEC violation. Also the primary ground fault current path is not through a ground round, it's back to the neutral.

I've done it and have had it done, the ground will be fully

[Iggy]

replying to trader_4, Iggy wrote: Correct, only if you're doing a re-wire is it NEC-illegal, though I wouldn't agree at all that it's inadequate. There are plenty of buildings in every town proving the code wrong daily and who converted very easily to fully grounded systems with nothing but new outlets and keeping their plumbing grounds.

Just be careful about the "what's currently legal" stuff, everything out there was legal at some point and even the remaining knob & tube stuff is still perfectly fine. BX just happened to be one of those lucky strikes with the current code just slightly improving upon those situations and opened the door for aluminum armor.

[Clare Snyder]

There are several versions of "bx" cable. The original was just the spiral jacket - and is no longer considered a ground. The next version had a "ribbon" or "tape" conductor running the length inside.. The third version got a bare copper wire ground.

I believe the current versions are called MC and AC. AC is "armored cable" - steel? armor and aluminum ribbon bonding strip that allows the armor to be used as a ground (250.118(9)).

MC is "Metal Clad" and has no bonding ribbon, and a full circuit ground conductor. (article 330) It is an aluminum sheath IIRC.

[Clare Snyder]

EMT, properly done, IS a far sight better looking then "BX", but for residential wiring up here we hide it inside the walls, so even that awfull looking "romex" looks just fine when the job is finished.(particularly that new-fangled color-coded plastic covered stuff!!)

[trader_4]

But of course that is exactly the context of the question asked and the whole thread. He is asking about a re-wire.

though I wouldn't

Sure, who should we believe? You with opinions or the nec based on real data and facts?

There are plenty of buildings in every town

Which of course, again is not code compliant and would not pass inspection. But keep digging your hole deeper.

Sure, what's there is grandfathered, but that wasn't the question. The question was about doing an upgrade.

BX just happened to be one of those lucky strikes with the

Just stop already.

[micky]

Is there any relevance to "matched" impedance when dealing with house current? If it matched, what would it match?

Are you thinking about electronics, not electricity? Like matching output transformer impedance to speaker impedance.

In fact, what is the point of using the word impedance? Aren't we talking about plain old resistance, with no capacitative or inductive component?

[Iggy]

replying to trader_4, Iggy wrote: Sorry, it just looks like he's asking if he can re-use what he currently thinks is flexible conduit for a possibly completely useless non-upgrade. My "opinion" is what the NEC said and yes using BX's armor as a ground in existing wiring is completely legal and compliant. You or anyone else saying 60-years of perfect function and reliability is garbage is outright absurd.

I can plug a GFCI or Circuit Tester into a 3-prong adapter in contact with the cover plate's screw and always get a correct read and have a GFCI outlet test perfectly all day long with a BX's armor grounding the box and the ground actually works correctly and as expected. Again, latest code is nothing but here's our newest tweaks. At no time does the code say its old codes are garbage or dangerous. They simply say this is only how we want the new stuff.

[Iggy]

replying to Clare Snyder, Iggy wrote: Yep, I'm just talking about this guy's old and wonderful stuff good for another

50-years...if that's what it is and not actually empty BX that literally is flexible conduit.

[Iggy]

replying to micky, Iggy wrote: No, there's really no relevance to "matched", I was just giving some credit to the NEC of how a copper or aluminum wire might be a slight improvement over steel. Thus, Impedance is all of those other words and why it's so vital to electronics and speakers. Although, that's also why a slight improvement really doesn't matter when we're talking big dirty power and not transformed filtered small power. Therefore, why electronics and speakers don't care 1-bit if there's no ground nor how "approved" a ground is.

[trader_4]

No

You're right, there is nothing to match. You just want the impedance of the ground path to be as low as practically possible.

There is always some capacitance and inductance involved. You can model a cable by a number of inductors in series and capacitors between them that go across the conductors. At 60hz, it's not much of a factor. But the ground path on occasion has to serve as the path for transients, eg a lightning surge. And to that the impedance of the path matters greatly. Even for a sudden direct short, the impedance matters, because again, it's not a 60hz event, there are high frequency components. With more impedance you'll see a higher voltage on the whole end of the ground path where the fault occure ed.

I suspect those kind of considerations went into the newer BX type cables because the ones that still rely on the outer metal jacket for the ground have included a thin wire inside. I suspect part or maybe all of it's purpose is to provide a straight, short path for the current. Because otherwise you already have a long coiled path for the current in the spiral jacket. It's one continuous piece of metal from end to end, right? It's just that it's spiraled onto itself. If the spiral edges make perfect contact with each other, then it would be like a pipe, a direct path. If they were perfectly insulated where they contact each other on every turn, it would look like a very long inductor. I suspect in practice, it's some of both. Putting that additional bare wire inside makes it less of an inductor, as well as shortening the effective length, which lowers both the resistance and impedance. I suspect that's why they did it.