2014 change to 250.122(B)

Merry Christmas
Status
Not open for further replies.

Smart $

Esteemed Member
Location
Ohio
...
absolutely - no question - excellent method. Possibly the best yet. (assuming you are discussing post 63, 67?)
Actually I was referring to post #42... but even that could be further refined with 63, 67

But that only covers the case of conductive raceway**. And it puts the NEC even farther into the design guide. And isn't a redundant EGC a design issue? And the issue isn't personnel safety but is rather a save the structure issue
I don't see it that way. The wiring method is usually determined prior to the wire size... and its still optional as to what the wiring method is. Not saying anyone has to use a particular wiring method for this situation.

** Which maybe is the only case you wish to cover.
No. I'm all ears for any revision (with substantiation) which the CMP will either accept outright or accept in principle. I realize there's no guarantee... but I'd at least like the submission to have an outside chance of going through.
 

Smart $

Esteemed Member
Location
Ohio
Okay here is another one.
Where a wire-type EGC supplements a rigid, IMC, or EMT wiring method (any others? perhaps cable tray, wireway, ...???), the wire-type EGC size is a design issue - art 250.122 does not apply.

Informational Note:
Example: For a 1200A CB, feeding a 1000 foot feeder, the NEC equally allows a #14 as well as a 250kcmil.​

ice
JMO, but I believe that's a prime example of a submission that'll get an immediate rejection by CMP... :happyyes:
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
JMO, but I believe that's a prime example of a submission that'll get an immediate rejection by CMP...

I'm sure you are correct. However, I am curious
Do you think the code panel will object to the idea that a supplimental EGC in a metalic wireway is anything other than a design decision?

Do you think the code panel will object to the idea that a wire type EGC in a metalic raceway does not effectively reduce the short circuit impedance?

If the answers are No, and No, then why would the EGC size matter to the NEC? The minimum spec has been met. That all the NEC does - mandate a job that is dead marginal.

Any additional EGC is being installed for a different reason - a design issue

So, why is it you would expect immediate rejection?

And this is yet another prime example of why I don't get close to the process. As soon as I start questioning the panel members about the physics behind a particular code section - they invariabily get red faced and close up - go figure. Hard to believe it could be me and my attitude :angel:

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... but I'd at least like the submission to have an outside chance of going through.
I bow to the South and applaude you. (Seriously)

A couple of years ago I was doing some research and had an occasion to read the proposals to Art 250. It was readly apparent that most were submitted just to say:
  • The submitter had made a proposal.
  • The submitter was in a competition for the most proposals
  • The submitter had a financial stake to drive.

There is no question in my mind why the committies buy REJECTED stamp ink by the tankcar load.

ice
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
Do you think the code panel will object to the idea that a wire type EGC in a metalic raceway does not effectively reduce the short circuit impedance?
...

It is a parallel path, how could it not reduce the short circuit impedance?

Actually I think CMP 5 would be more likely to accept a proposal that would require an EGC of the wire type for all circuits, than it would be to accept a proposal that said a wire type EGC installed in a raceway type EGC did not have to be sized per 250.122.
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
...
Do you think the code panel will object to the idea that a supplimental EGC in a metalic wireway is anything other than a design decision?
...
And here is where the problem lies. Supplementing a qualifying EGC metal raceway is a design decision in most cases. Yet the Code currently imposes 250.122(B) regardless of whether it be supplemental or not.

Do you think the code panel will object to the idea that a wire type EGC in a metalic raceway does not effectively reduce the short circuit impedance?
Covered by Don... :D

If the answers are No, and No, then why would the EGC size matter to the NEC? The minimum spec has been met. That all the NEC does - mandate a job that is dead marginal.
In the case of 250.122(B) it seems readily apparent the CMP (not necessarily the current members) feel the marginal aspect is changed by an increase in size of the ungrounded conductors. I can understand the likelihood when compensating for voltage drop AND the wire-type EGC is not supplemental. Suggesting it be revised is for other-than scenarios.

So, why is it you would expect immediate rejection?
In a way, you answered your own question...

And this is yet another prime example of why I don't get close to the process. As soon as I start questioning the panel members about the physics behind a particular code section - they invariabily get red faced and close up - go figure. Hard to believe it could be me and my attitude :angel:
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
It is a parallel path, how could it not reduce the short circuit impedance? ....
Recommend a little research. The information is available. Check on the difference in impedance of the conduit and the conduit with an internal EGC. Do that and the physics will be clear. Also note I didn't say it wouldn't reduce the impedance. I said it wouldn't effectively reduce the impedance.

... Supplementing a qualifying EGC metal raceway is a design decision in most cases. Yet the Code currently imposes 250.122(B) regardless of whether it be supplemental or not.

... Covered by Don...

... In a way, you answered your own question...
... Actually I think CMP 5 would be more likely to accept a proposal that would require an EGC of the wire type for all circuits, than it would be to accept a proposal that said a wire type EGC installed in a raceway type EGC did not have to be sized per 250.122.
All sounds political to me. No physics involved.

I'm certain I won't be any help on these issues.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
And yet another reason... :happyyes:

You asked. I gave you two and alluded to a third.

I'm okay if you don't like them.

I'm really okay if you don't like them for political reasons.

If you want to tell me my physics is bad - well, you have not shown that - - - yet.

icr
 

bob

Senior Member
Location
Alabama
I have not downloaded/installed the GEMI application because I wouldn't use it on my personal computers for anything other than posting here on the forum... which amounts to never installing it on my personal computers. That said, does anyone know if the program provides a grounding analysis for where both raceway and wire-type EGC are used in combination.

GEMI give you both. eg#1 #10 egc max ckt 290 ft. eg#2 #10 egc and 1/2 emt max ckt 340 ft.
 

Smart $

Esteemed Member
Location
Ohio
You asked. I gave you two and alluded to a third.

I'm okay if you don't like them.

I'm really okay if you don't like them for political reasons.

If you want to tell me my physics is bad - well, you have not shown that - - - yet.

icr
I am not excluding your reasons.

As to your physics, when you actually show me yours (other than what appears to be idle speculation, that is), I'll consider whether I can challenge it.

So far, my ideas for revision do not rely on any calculation other than what is already in the Code and perhaps using GEMI software (it does the calculations; already pointed out that I will not be installing it).
 

Smart $

Esteemed Member
Location
Ohio
GEMI give you both. eg#1 #10 egc max ckt 290 ft. eg#2 #10 egc and 1/2 emt max ckt 340 ft.
Satisfactory to me. I also believe it would be acceptable to most (especially since the software is free ;)). Of course what really matters is whether it is acceptacle' to the CMP, at least initially... as there's always the additional approval stages. Just seems that CMP acceptance influences the rest of the process.

So propose as an exception?

Suggestions on wording?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... As to your physics, when you actually show me yours (other than what appears to be idle speculation, that is), I'll consider whether I can challenge it. ...
Idle speculation? Really?:(

edit to add: I figured this out 35+ years ago. And if a dumb university educated engineer can do it, surely you can. So: As for the calculations - nope, if you want to use it - you will have to figure it out. However, I suspect you have no inclination to do so, because:

... So far, my ideas for revision do not rely on any calculation other than what is already in the Code and perhaps using GEMI software (it does the calculations; already pointed out that I will not be installing it).

ice
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
Idle speculation? Really?:(
I said "appears". And your post on the Corwin Theory delegated me to do the calculation. That's analogous to me submitting it as a proposal and for substantiation telling the CMP to do the calculation... point?

edit to add: I figured this out 35+ years ago. And if a dumb university educated engineer can do it, surely you can. So: As for the calculations - nope, if you want to use it - you will have to figure it out. However, I suspect you have no inclination to do so, because:
Not saying I can't do it. I would even consider doing it at some other point in time. Just happens that I'm leaving home tomorrow to go work out-of-state. Currently have higher priorities. Even taking time to post here is a deterrence in fulfilling those priorities... :happyyes:
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Not saying I can't do it. I would even consider doing it at some other point in time. Just happens that I'm leaving home tomorrow to go work out-of-state. Currently have higher priorities. Even taking time to post here is a deterrence in fulfilling those priorities... :happyyes:

Yes, I an sure you can (do the calcs) And I did/do appreciate reading about what you are looking to do. And I am serious about bowing to the south (post 84)

ice
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Recommend a little research. The information is available. Check on the difference in impedance of the conduit and the conduit with an internal EGC. Do that and the physics will be clear. Also note I didn't say it wouldn't reduce the impedance. I said it wouldn't effectively reduce the impedance. ...
I ran a few different circuits on the GEMI software with and without an EGC of the wire type in rigid steel conduit. That software shows that maximum effective length (effective fault clearing path) of the circuit increases 20 to 35% when you add a code sized EGC of the wire type to the conduit.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I ran a few different circuits on the GEMI software with and without an EGC of the wire type in rigid steel conduit. That software shows that maximum effective length (effective fault clearing path) of the circuit increases 20 to 35% when you add a code sized EGC of the wire type to the conduit.

I did not know it was that much. Q: Does the change in max effective length = change in impedance?

Apparently Nema may not know that either. Although they do know the overall impedance is reduced.
From Nema Bulletin 97, 2009


Nema Bulletin 97 said:


  • [*=1]Where lengths do not exceed the maximum allowable computed by the GEMI software, supplemental grounding conductors in secondary power systems enclosed in steel EMT, IMC or Rigid Conduit do not add to safety in a phase to neutral fault. The use of supplementary equipment grounding conductors, when participating in the fault circuit, reduce the overall impedance and may or may not increase the allowable length of the run, depending on the size and system design.
    [*=1]The maximum allowable length for a specific system depends on conductor size, steel conduit size and fault type. In many cases, the maximum allowable length for a phase to neutral fault is shorter than the maximum allowable length for a phase to steel conduit fault. Thus, in most cases, the steel conduit is not the limiting factor in a conductor to neutral fault
You wouldn't cherry pick your data would you? - Nah

In any case I think you and $S are on the right track. The trend is going toward "use an internal EGC regardless of raceway".

I say we go back to the broad brush of previous editions:
"Any ccc up-size, gets an equal EGC up-size. Exception: Calculate under engineering supervision (TCC diagram showing available fault current)"​

Easy to figure.
Gets charley off the hook if the EC uses a different route.
Harder to lawyer out of it.

ice
 

Smart $

Esteemed Member
Location
Ohio
I did not know it was that much. Q: Does the change in max effective length = change in impedance?

Apparently Nema may not know that either. Although they do know the overall impedance is reduced.
From Nema Bulletin 97, 2009



You wouldn't cherry pick your data would you? - Nah
That bulletin doesn't say the phase to ground fault impedance is reduced. It also says, that a supplemental wire EGC mat or may not reduce impedance for a phase to ground fault. Seems quite inconclusive to me.

So what if supplementing with a wire EGC actually impedes more than without... what say you then?

What it does say as if conclusive, is a supplemental wire EGC has no effect on a phase to neutral fault. That seems quite a logical deduction to me, without having to use any software. :happyyes:

In any case I think you and $S are on the right track. The trend is going toward "use an internal EGC regardless of raceway".
That's the 'trend' in practice... and it may possibly be detrimental in some cases, going by NEMA's inconclusive statement.

I say we go back to the broad brush of previous editions:
"Any ccc up-size, gets an equal EGC up-size. Exception: Calculate under engineering supervision (TCC diagram showing available fault current)"​
Again going back to NEMA's statement, what if upsizing actually impedes trip action???

And to provide exception to calculate under engineering supervision could result in the use of something similar to the Corwin Theory.... :?

Perhaps two proposals: 1) as 'suggested' in post #42, and 2) an exception for use of GEMI analysis.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I did not know it was that much. Q: Does the change in max effective length = change in impedance? ...
I expect that it does mean that, but I don't know exactly how the software calculates the maximum effective lengths.

Also I ran some more and found cases were the increase was only 12 to 15%.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I expect that it does mean that, but I don't know exactly how the software calculates the maximum effective lengths. ....
Yes, that is always a problem when using free software from the internet.

...Perhaps two proposals: 1) as 'suggested' in post #42, and 2) an exception for use of GEMI analysis.

Usng an unknown free internet software with no defined algolrithm does not sound like a good idea to me. I can only hope the CP would not pick a specific software.

However, if the algolrithm were published in a peer reviewed IEEE paper, then the proposal could read like 310.15.C.

ice
 
Status
Not open for further replies.
Top