Article 250.122(F)

[don_resqcapt19]

There are cases, where Dale has a very valid point. They are not commonly encountered but this has been an issue. Like I said before I don't thing the code needs to address it with new rules, but it is something for the design community to look at when this type of installation in being made.

 

[Dale001289]

You just don't want to listen to sound logic when it comes to the NEC and it seems that you have your mind made up before you even ask a question.

You can certainly add more EGC cables, but if I were the contractor and this wasn't designed or specified this way at bid time, I would be submitting a change order for this additional work above the NEC.

I don't see where 90.1 talks about a low impedance path. You are making up your own code to suit your delusions.

Why did you come here if you won't listen to the most informed people on the NEC anywhere.

Listen, this issue came up a few years ago when I was overseas, naturally its a big deal when cost is involved. So I called the folks at the NFPA (NEC) up in Massachusetts and asked them to show the calculations used to determine ONE equipment grounding conductor provide sufficient fault path for all bundles...wanna know what they said, and I quote "We don't have any"....see what I mean?
Just because a bunch of smart guys sit in a think tank agree on a technical solution doesn't mean its absolute without the math - i.e. proof.
If it were multiconductors instead of singles, you'd need an EGC for each. Its good judgment and therefore good practice to install an EGC for each set of parallel circuits.

 

[Dale001289]

There are cases, where Dale has a very valid point. They are not commonly encountered but this has been an issue. Like I said before I don't thing the code needs to address it with new rules, but it is something for the design community to look at when this type of installation in being made.

Thanks Don, I see why you were chosen as a Moderator.

 

[tkb]

Listen, this issue came up a few years ago when I was overseas, naturally its a big deal when cost is involved. So I called the folks at the NFPA (NEC) up in Massachusetts and asked them to show the calculations used to determine ONE equipment grounding conductor provide sufficient fault path for all bundles...wanna know what they said, and I quote "We don't have any"....see what I mean?
Just because a bunch of smart guys sit in a think tank agree on a technical solution doesn't mean its absolute without the math - i.e. proof.
If it were multiconductors instead of singles, you'd need an EGC for each. Its good judgment and therefore good practice to install an EGC for each set of parallel circuits.

You came here to discuss a NEC code article and was given what the article requires.
Because you don't agree with the NEC, which is a minimum safe installation, you can specify a more stringent specification.

But to say the contractor is incorrect because he made his installation as per the NEC is wrong.
You should have specified what was required and the contractor would have priced it accordingly.

 

[Carultch]

If you have multiple bundles in parallel in the cable tray, that does not make them separate cables. It is necessary to bundle all CCCs into each bundle, but no great harm in separating the EGC which carries only fault current.
If you want to include a fractional size EGC in each bundle I see no reason not to do that either.

Case and point, see this graphic:
http://ecmweb.com/site-files/ecmweb.com/files/uploads/2012/12/250s122F003.jpg

 

[al hildenbrand]

Listen, this issue came up a few years ago when I was overseas, naturally its a big deal when cost is involved. So I called the folks at the NFPA (NEC) up in Massachusetts and asked them to show the calculations used to determine ONE equipment grounding conductor provide sufficient fault path for all bundles...

And YOUR calculations that demonstrate the INSUFFICIENCY. . . .? They are . . . where?

Let's get down to "for instance". Just because you can imagine a "low-impedance/low heat benefit is reduced" (your words) doesn't mean there is an un-safe condition. I believe you have to dispassionately calculate the manner that an Unsafe condition occurs and share it, to catch the attention of the CMPs.

(Added) And remember. . . we're talking about an Equipment Grounding Conductor on cable tray which most commonly is in an environment where the single EGC laying in the cable tray is only ONE of MANY multiple parallel fault paths. . . Start off with a model of a theoretical set of parallel feeders and a single ISOLATED from-any-other-conductive-path EGC and then do more real world modeling of a factory, etc., with lots of steel.

 

[Dale001289]

And YOUR calculations that demonstrate the INSUFFICIENCY. . . .? They are . . . where?

Let's get down to "for instance". Just because you can imagine a "low-impedance/low heat benefit is reduced" (your words) doesn't mean there is an un-safe condition. I believe you have to dispassionately calculate the manner that an Unsafe condition occurs and share it, to catch the attention of the CMPs.

(Added) And remember. . . we're talking about an Equipment Grounding Conductor on cable tray which most commonly is in an environment where the single EGC laying in the cable tray is only ONE of MANY multiple parallel fault paths. . . Start off with a model of a theoretical set of parallel feeders and a single ISOLATED from-any-other-conductive-path EGC and then do more real world modeling of a factory, etc., with lots of steel.

I want to thank everyone on this thread for excellent feedback. I've accepted the 'one EGC' concept as sufficient and code compliant (cable tray scenarios only) however I remain unconvinced its best way to go. It may not be 'Unsafe' for personnel or equipment, but without seeing the math, I remain pessimistic insofar as fault-path efficiency is concerned, under certain fault conditions.

 

[jtinge]

Assuming we are talking about metallic cable trays, isn't the argument regarding the sufficiency of a single EGC in a cable tray moot since cable trays are required to be grounded and bonded per 250.96 and 392.60(A)? Unless I'm missing something, the single EGC would not be the sole ground fault path if the cable tray is properly grounded and bonded.

 

[iwire]

Assuming we are talking about metallic cable trays, isn't the argument regarding the sufficiency of a single EGC in a cable tray moot since cable trays are required to be grounded and bonded per 250.96 and 392.60(A)? Unless I'm missing something, the single EGC would not be the sole ground fault path if the cable tray is properly grounded and bonded.

I don't believe we are allowing facts to get in the way of opinion.

 

[Dale001289]

Assuming we are talking about metallic cable trays, isn't the argument regarding the sufficiency of a single EGC in a cable tray moot since cable trays are required to be grounded and bonded per 250.96 and 392.60(A)? Unless I'm missing something, the single EGC would not be the sole ground fault path if the cable tray is properly grounded and bonded.

Cable tray bonded/grounded to earth is primarily an extension of the equipotential plane. But yes, it can also serve as a fault path... if done properly. Usually, an EGC is considered a more effective fault path due to its proximity to the phase conductors.

 

[Dale001289]

I don't believe we are allowing facts to get in the way of opinion.

facts are supported by math/physics; when not available, 'facts' simply become opinion

 

[al hildenbrand]

facts are supported by math/physics; when not available, 'facts' simply become opinion

With the notable exception of "opinion," such as the words of the NEC, that are adopted as statute of an Authority Having Jurisdiction.

 

[iwire]

Usually, an EGC is considered a more effective fault path due to its proximity to the phase conductors.

The phase conductors would be sitting directly on the cable tray, a wire EGC will be no closer.

 

[Dale001289]

The phase conductors would be sitting directly on the cable tray, a wire EGC will be no closer.

Magnetic flux interaction with a steel or aluminum tray would not be the same as with EGC. Here again we are arguing opinions. Until the math is produced, that's all it is....but it IS fun and that's the best part of this Forum

 

[Dale001289]

With the notable exception of "opinion," such as the words of the NEC, that are adopted as statute of an Authority Having Jurisdiction.

I've never met an AHJ that had his Ch*t together.

 

[al hildenbrand]

I've never met an AHJ that had his Ch*t together.

That response deflects. . .

With the notable exception of "opinion," such as the words of the NEC, that are adopted as statute of an Authority Having Jurisdiction.

The published words of the NEC that are adopted into local ordinance are not accompanied by "physics / math" yet they are more than "opinion", they are law.

 

[iwire]

Magnetic flux interaction with a steel or aluminum tray would not be the same as with EGC. Here again we are arguing opinions. Until the math is produced, that's all it is....but it IS fun and that's the best part of this Forum

So produce 'the math' it is you that is disputing the safety of the exsiting rules.

 

[Dale001289]

So produce 'the math' it is you that is disputing the safety of the exsiting rules.

I'm not disputing 'safety'; I'm disputing functionality, effectiveness and efficiency of fault path. Once I've figured out a viable way to prove it (either through math/hand calc's or Etap) I'll let you know the result. But...I wont publish the numbers; someone on this thread would publish them and get all the credit

 

[don_resqcapt19]

The phase conductors would be sitting directly on the cable tray, a wire EGC will be no closer.

There are times where the tray is not suitable for use as the EGC because of the ampacity of the circuits, and there are cases with high ampacity circuits, tray 3' or wider, and a single EGC where this has been an issue. These cases are rare, and as I said before, I don't think there needs to be a code change...maybe an Informational Note, but it is something that the design engineer should think about if he/she is designing a circuit like this.

This is not something that you should be able to make a contractor change on his dime....A single EGC is code compliant, but in rare cases, is not an effective fault clearing path. This determination needs to be made by the PE who stamps the drawings and not by the EC who is installing the project.

This is very similar to the fact that some multi-duct duct banks with highly loaded circuits fail from excessive heat when installed per the rules in Chapter 3. The issue is addressed in Informative Annex B, but that is not an enforceable rule. It is not a real common issue, but it does happen, much like the issue being discussed in this thread.

 

[iwire]

Don,

Who is running a feeder so large in a cable tray that it's associated EGC is 3' away?

I get it distance adds imedance yet other than someone's vague recollection of lab tests we have no real life examples of a problem.

Likely the same kind of lab that tells us NM is overheating. :roll: