Article 250.122(F)

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don_resqcapt19

don_resqcapt19

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I seem to recall a couple of case studies with larger cable tray and high current circuits where the separation distance between the circuit conductors and the EGC was enough that the increased impedance of the EGC reduced the fault current from the instantaneous part of the OCPD to the short time part increasing the damage at the point of the fault as a result of the increased fault clearing time.

I don't have my green book handy but as I recall a 3' separation between the circuit conductors and the EGC doubles the impedance of the fault clearing path.
 
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Dale001289

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don_resqcapt19 said:
I seem to recall a couple of case studies with larger cable tray and high current circuits where the separation distance between the circuit conductors and the EGC was enough that the increased impedance of the EGC reduced the fault current from the instantaneous part of the OCPD to the short time part increasing the damage at the point of the fault as a result of the increased fault clearing time.

I don't have my green book handy but as I recall a 3' separation between the circuit conductors and the EGC doubles the impedance of the fault clearing path.
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Yes, and don't forget earlier tests involving an external EGC conducted in the 50's - impedance goes up with further separation
 
iwire

iwire

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Dale001289 said:
Yes, and don't forget earlier tests involving an external EGC conducted in the 50's - impedance goes up with further separation
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I think I can safely say all of us in this thread understand impedance goes up with separation.

do I need an individual EGC, (sized per Table 250.122),in each set of the four parallel circuits, or can use 1 EGC to suffice for all four sets?
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The NEC only requires 1 EGC in that installation. You are free to exceed the NEC.

That is where this comes in

90.1 Purpose.

(B) Adequacy. This Code contains provisions that are considered
necessary for safety. Compliance therewith and
proper maintenance results in an installation that is essentially
free from hazard but not necessarily efficient, convenient,
or adequate for good service or future expansion of
electrical use.
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david luchini

david luchini

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Dale001289 said:
What you should be looking for is something that PERMITS it; not the opposite. I believe we've beat this horse to death.
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In the 1999 NEC, 300-3(b) required conductors of the same circuit, including EGCs to be run in the same cable tray.

318-8(d) required paralleled conductors in cable trays to be installed in groups consisting of not more than one conductor per PHASE OR NEUTRAL (no requirement for EGC to be grouped.)

If I ran all of the paralleled conductors and a single EGC sized per 250.122 in the same cable tray, I have met the requirements of the NEC and have not done anything prohibited by the NEC.
That would be a compliant installation.
 
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Dale001289

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david luchini said:
In the 1999 NEC, 300-3(b) required conductors of the same circuit, including EGCs to be run in the same cable tray.

318-8(d) required paralleled conductors in cable trays to be installed in groups consisting of not more than one conductor per PHASE OR NEUTRAL (no requirement for EGC to be grouped.)

If I ran all of the paralleled conductors and a single EGC sized per 250.122 in the same cable tray, I have met the requirements of the NEC and have not done anything prohibited by the NEC.
That would be a compliant installation.
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I plan to submit this issue for CMP review. I don't see how you can alter the physics of each bundle without impacting fault path and short-time trip @ the upstream breaker or 50/51. Its obvious this needs to be changed. Until then I will continue to utilize Article 90.1 and do it the right way, the coordinated way and ultimately the safe way.
 
don_resqcapt19

don_resqcapt19

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Dale001289 said:
I plan to submit this issue for CMP review. I don't see how you can alter the physics of each bundle without impacting fault path and short-time trip @ the upstream breaker or 50/51. Its obvious this needs to be changed. Until then I will continue to utilize Article 90.1 and do it the right way, the coordinated way and ultimately the safe way.
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I think it is a rare case where this is a real world issue, and not sure it warrants a code change. It takes a very high current circuit, with a large separation between the circuit conductors and the EGC. Maybe an Informational Note or an Annex would do the job. I only recall reading of a couple of cases where it has been a problem.

There was a similar issue with underground duct banks and not derating because of the increased heat from the multiple ducts, but that was never addressed in the actual code language. It is addressed in Annex B.

As far as your PI (public input) for a 2020 code change, they are due 11/3/2017.
 
al hildenbrand

al hildenbrand

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Dale001289 said:
I see an issue with the 'one EGC for all four sets' philosophy. Art 300.3 (B) describes how all conductors, including the EGC, must be contained within the same raceway or cable or heat will increase as well as impedance. Each set of 'parallels' represents a cable; thus if a single EGC is routed externally to 3 of the 4 sets, the low-impedance/low heat benefit is reduced.
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While, in your mind, you want each set of parallels to represent a "cable", that is not the definition of the word in the 2014 NEC citations you are referencing.
2016 NFPA Glossary of Terms

Cable: A factory assembly of two or more conductors having an overall covering.
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As far as the NEC is concerned, what you are describing is a field assembled grouping of conductors.

For your edification, the NFPA Glossary is available to download from the NFPA here.
 
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Dale001289

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I have instructed the team to add an EGC per bundle, based on sound engineering evidence, that a low-impedance path is needed for each bundle and in accordance with Art 90.1.
Ask yourself this: if they were 4 multiconductor cables versus 4 bundles of 3-single conductors, would you purchase 3 of the multiconductor cables WITHOUT a grounding conductor except for one set? It think I just heard a rhetorical "NO"
 
david luchini

david luchini

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Dale001289 said:
and in accordance with Art 90.1.
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I'm not sure what 90.1 has do to with it...

Not only is an EGC not required for each "bundle" of paralleled conductors in the cable tray, you could have multiple circuits (properly bundled) sharing a single EGC in the cable tray.
 
al hildenbrand

al hildenbrand

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Minnesota
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Electrical Contractor, Electrical Consultant, Electrical Engineer
Dale001289 said:
I see an issue with the 'one EGC for all four sets' philosophy. Art 300.3 (B) describes how all conductors, including the EGC, must be contained within the same raceway or cable or heat will increase as well as impedance. Each set of 'parallels' represents a cable; thus if a single EGC is routed externally to 3 of the 4 sets, the low-impedance/low heat benefit is reduced.
Click to expand...

Dale001289 said:
Code Panels are obviously confused on this topic since earlier versions (2002, e.g.) made no mention whatsoever of the one EGC per tray scenario.
As far as links as to why each bundle should have its own EGC, see IEEE Green book and do a search.

"one of the marks of true manhood is admitting you are wrong" - Dale
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Dale001289 said:
I plan to submit this issue for CMP review. I don't see how you can alter the physics of each bundle without impacting fault path and short-time trip @ the upstream breaker or 50/51. Its obvious this needs to be changed. Until then I will continue to utilize Article 90.1 and do it the right way, the coordinated way and ultimately the safe way.
Click to expand...

[IRONY] Wow! I now see the error of countless thousands of electrical professionals, steeped in Physics, collaborating over many decades, to wordsmith and adopt into legal standard a consensus document of safe assembly rules!

Dale has the marks of true manhood because he says its obvious. [/IRONY]
 
tkb

tkb

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Dale001289 said:
I have instructed the team to add an EGC per bundle, based on sound engineering evidence, that a low-impedance path is needed for each bundle and in accordance with Art 90.1.
Ask yourself this: if they were 4 multiconductor cables versus 4 bundles of 3-single conductors, would you purchase 3 of the multiconductor cables WITHOUT a grounding conductor except for one set? It think I just heard a rhetorical "NO"
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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.
 
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