Effectiveness of 1920's armored cable ground

big john

Senior Member
Location
Portland, ME
I would think that trying to measure ohmic resistance could be misleading.

I'm not at all sure there's a relationship between the resistance measured by a 9V multimeter versus the cable impedance when it's passing 1kA during a short circuit.
 

user 100

Senior Member
Location
texas
Yep, and I imagine TX and VA have similar conditions, tho humidity and freezing here might affect outside fixtures more. I admit Im one who doesnt like the stuff.

Al and all, wouldnt be a simple enough proposition to take each case of old AC on its own? How hard would it be to measure resistance to panel? If I got 0.7 ohms (say j-box AC 20' from the panel), it would be an acceptable EGC, but if you get 4 or 13.72 ohms, it's not? (based on 1.5 ohm standard). Is this too much work, or liability for the EC? or am I missing the point entirely; Old AC; acceptable ground or not?

eta: there has to be someone with a definitive answer to this question; does MH have any videos r: this topic?

The short quick answer is that for a lot of ecs testing is simply not seen as cost effective/ preventative vs. something like gfci the circuit.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
What they may mean by that phrase "grounding means exists" is wiring that already has an egc and the second part that specifically mentions the term egc is referencing an egc that has been added per 250.130. Basically covering their bases to make it clear that when one replaces a receptacle on wiring that already has an egc or wiring that has had an egc added via 250 exception for nongrounded bc ext's, it must be connected to an egc per 406.4(C) or 250.130(C).

All those examples (in bolded red) are EGCs. Plain and simple, because they come from EGCs.

But what exactly is "grounding means"?

From studying the Codes written before the Article 100 first definition of Equipment Grounding Conductor in, if I recall correctly, the late Sixties, the Code used different language to mean the same thing. "Grounding means" is deliberately used to extend 406.4(D)(1) into the older language, in my opinion.
 

user 100

Senior Member
Location
texas
All those examples (in bolded red) are EGCs. Plain and simple, because they come from EGCs.



From studying the Codes written before the Article 100 first definition of Equipment Grounding Conductor in, if I recall correctly, the late Sixties, the Code used different language to mean the same thing. "Grounding means" is deliberately used to extend 406.4(D)(1) into the older language, in my opinion.

Idk- they could just say "currently listed egc's" and just dump the suggestive, open to interpretation stuff.:)
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Al and all, wouldnt be a simple enough proposition to take each case of old AC on its own? How hard would it be to measure resistance to panel? If I got 0.7 ohms (say j-box AC 20' from the panel), it would be an acceptable EGC, but if you get 4 or 13.72 ohms, it's not? (based on 1.5 ohm standard). Is this too much work, or liability for the EC? or am I missing the point entirely; Old AC; acceptable ground or not?

eta: there has to be someone with a definitive answer to this question; does MH have any videos r: this topic?
Until 320.108's "adequate path for fault current as required by 250.4(A)(5) or (B)(4) to act as an equipment grounding conductor" is given a procedural definition that is adopted into local ordinance, we only have each other to use as guidance. The local AHJ and the property owner will have the greatest influence. The property owner, by accepting to pay for upgrades, and / or, the AHJ for guidelines on local enforcement / interpretation. My ability as the negotiator, electrical contractor, and counselor are the third influence.

And remember, 320.108 applies to BONDING STRIP armored cable bought brand new Monday morning, as well.
 

JFletcher

Senior Member
Location
Williamsburg, VA
The short quick answer is that for a lot of ecs testing is simply not seen as cost effective/ preventative vs. something like gfci the circuit.

True, I understand the cost factor. A GFCI will not protect against anything upstream (line side), so a breaker would be far better than a receptacle in that respect (less line side to protect).
 

JFletcher

Senior Member
Location
Williamsburg, VA
Until 320.108's "adequate path for fault current as required by 250.4(A)(5) or (B)(4) to act as an equipment grounding conductor" is given a procedural definition that is adopted into local ordinance, we only have each other to use as guidance. The local AHJ and the property owner will have the greatest influence. The property owner, by accepting to pay for upgrades, and / or, the AHJ for guidelines on local enforcement / interpretation. My ability as the negotiator, electrical contractor, and counselor are the third influence.

One thing I noticed about the studies wayne mentioned earlier is they seem to have taken lengths of old AC from houses and tested the resistance. I'd surmise that still intact AC may have less resistance because of multiple ground paths to source. That is still making the enormous assumption that the original was:

a) installed correctly and
b) not modified to circumvent the ground path

which, regardless of whether or not old AC is an acceptable EGC, are assumptions I will not make.
 

user 100

Senior Member
Location
texas
True, I understand the cost factor. A GFCI will not protect against anything upstream (line side), so a breaker would be far better than a receptacle in that respect (less line side to protect).

A problem w/ a lot of bx installs are panels that are so obsolete you can't get gfci breakers for them or they still have fuses or the receptacle solution is hard because rec's are often fed thru the lights-really time to talk to customer about panel upgrade- however, sometimes you can just refeed bc's via gfci blanks or recs underneath/ near ancient panel- not an ideal or pretty solution but workable- that pesky 210.12 ​could bite you though.
 
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al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
That is still making the enormous assumption that the original was:

a) installed correctly and
b) not modified to circumvent the ground path

which, regardless of whether or not old AC is an acceptable EGC, are assumptions I will not make.

That is, without a doubt, your choice.

In my choice, I draw that line at telling my property owner that their old armored cable Type AC is only replaceable unless I can show the damage, corrosion, DIY mess, lack of EGC continuity, etc. My local AHJs support this approach as well.

Your point about the mesh like parallel paths in real world dwellings reducing EGC impedance is an excellent one.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Again, where does the code make that distinction or even a similar distinction?

320.108 DOESN'T make a distinction between bonding strip and non-bonding strip armored cable Type AC. That's the whole point. Because 320.108 doesn't make the distinction, it applies to both.

I especially applies to new construction AC cable systems that I install next Monday.
 

JFletcher

Senior Member
Location
Williamsburg, VA
That is, without a doubt, your choice.

In my choice, I draw that line at telling my property owner that their old armored cable Type AC is only replaceable unless I can show the damage, corrosion, DIY mess, lack of EGC continuity, etc. My local AHJs support this approach as well.

Your point about the mesh like parallel paths in real world dwellings reducing EGC impedance is an excellent one.

Thank you. . The flip side of user100's last post(#149) is that the panel has been replaced, and it's full of nm-not a piece of AC in sight, until you start opening switches, receptacles, walls, like in a remodel. Get in the crawlspace and you see old AC run into plastic boxes. Its painfully obvious from what I've seen that existing AC is not properly grounded anymore. I've never seen an install like you described earlier, where everything is essentially untouched from 60+years ago.
 

big john

Senior Member
Location
Portland, ME
320.108 DOESN'T make a distinction between bonding strip and non-bonding strip armored cable Type AC. That's the whole point. Because 320.108 doesn't make the distinction, it applies to both.
I meant where is the distinction made that a wiring method can be used when it does not match the description in the applicable article?

Small gauge aluminum NM was once perfectly legal. By this logic I should be able to extend an existing circuit using #14AL despite the fact that 334.104 says that #14 NM shall be copper.

I can't pick and chose what modern code sections I want to apply.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
I meant where is the distinction made that a wiring method can be used when it does not match the description in the applicable article?.

How do you know you can turn on a switch on a.K&T existing branch circuit and use the obsolete K&T? The Code doesn't describe grandfathering.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Its painfully obvious from what I've seen that existing AC is not properly grounded anymore. I've never seen an install like you described earlier, where everything is essentially untouched from 60+years ago.

Those stable old installs are a dime a dozen here.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
How do you know you can turn on a switch on a.K&T existing branch circuit and use the obsolete K&T? The Code doesn't describe grandfathering.
Article 394 is one big grandfather. The only uses permitted for K&T are "(1) For extensions of existing installations (2) Elsewhere by special permission". Various other places in the code provide for such explicit grandfathering; otherwise there is none.

Cheers, Wayne
 

big john

Senior Member
Location
Portland, ME
How do you know you can turn on a switch on a.K&T existing branch circuit and use the obsolete K&T? The Code doesn't describe grandfathering.
I don't follow. Hasn't the entire discussion here been about modifying existing BX circuits?

We're not talking about allowing previously grandfathered systems to remain in use when unaltered. Of course that's permitted.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Those stable old installs are a dime a dozen here.
Honest questions here, I'd like to understand your experience:

Do you have any in situ data for the armor resistance on these stable old installs? When you've run into a ground fault where the only grounding means is cable armor, how often do you see them trip the OCPD in a timely fashion versus seeing evidence of excess heating of surrounding materials?

Cheers, Wayne
 

JFletcher

Senior Member
Location
Williamsburg, VA
.

.

Those stable old installs are a dime a dozen here.

Then, while I agree with your base premise, based on experiences, and if our situations were reversed, would you at least agree that I might hold your position, and you mine?

I honestly dont care who's right or wrong here; imho, no debate on grounding would go a quarter the replies this thread, and others like it, have garnered if the NEC were more clear. By replies, you appear to hold the minority opinion, which is by no means incorrect, it's just, as before, I ask for substantiation from someone or someones from 'higher up'; has MH made a video about this? If not, would it be possible for the right folk to ask him to address it? If so, link and case (maybe) closed.
 
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