2014 change to 250.122(B)
- Thread starter charlie b
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Looking for direction on where to find example C.
Smart $, would you be willing to offer some insight?
Only a problem with some... yes, perhaps most.
It is not "unknown" software. If you researched it, you would discover the details. IIRC, it is based on the empirical data from a study sponsored by IAEI. It is compiled by a team of students under the tutelage of the same professor that supervised the study, I think... too many hours on the road to have total recall at this point... so I'll leave it to you to uncover any error...
iceworm
Curmudgeon still using printed IEEE Color Books
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And I'm certain they did a fantastic job. However, until the algolrithm is published and peer reviewed - it is unknown. Quien Sabe. Maybe the iaei journal is considered peer reviewed. And the article they published is considered sufficient - I wouldn't think so, but maybe it is.
The other issue is the NEC never picks a specific software to use. Rather they will pick an algorithm. Article 310.15.C is an example. What ever software might be chosed is a design decision.
I would never ascribe to me sufficient understanding to even offer a guess as to an NEC code panel actions. However, I can only hope they would not pick a specific software.
All this aside (so seriously): You care. You're making a good faith effort to help out and clear up a nebulous section. I am absolutely in favor of your efforts - and impressed.:thumbsup: It's way more than I could do.
ice
"One of the top experts in the field of grounding, Dr. Sakis Meliopoulos, professor of Electrical & Computer Engineering at Georgia Tech, headed the
grounding research which was completed in 1994. This research represented the first update on the impedance and permeability of steel conduit in over forty years." Copied from the links provided by Smart$
I had several conversations with Dr. Meliopoulos, while the study was in progress. He sent a copy of his finding to me. The math involved
was extremely complex. I decided not to check it for errors. Another source on this subject is the Soares Book on Grounding.
grounding research which was completed in 1994. This research represented the first update on the impedance and permeability of steel conduit in over forty years." Copied from the links provided by Smart$
I had several conversations with Dr. Meliopoulos, while the study was in progress. He sent a copy of his finding to me. The math involved
was extremely complex. I decided not to check it for errors. Another source on this subject is the Soares Book on Grounding.
You make a good point. So the question remains how to present the proposal so the CMP will accept or accept in principle. As I noted earlier, if a simple exception for engineering supervision, we have no guarantee of the algorithm used, not to mention it precludes non-engineers. What if we state something on the order of software developed for the purpose and approved by the authority having jurisdiction... then mention the GEMI software by way of an informational note. AFAIK, it is the only software which is designed to perform this analysis. Are there any others?
iceworm
Curmudgeon still using printed IEEE Color Books
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Right on both counts. Generally speaking, if one wishes to do engineering for hire - get a PE ticket.
Repeated from previous posts:
Why is it mandatory to use software? Why can't one use paper, pencil, and an abacus? No other similar code sections post anything except the algolrithm. The method of compliance is a design decision. The code never mentions specific name brand software. The closest they get is to mention other engineering standards, API, NFPA, ANSI, IEEE, even UL - but never a specific brand
ice
First, that's why I said mention GEMI in an Informational Note. IMO, GEMI isn't a specific brand. Something has to be commercialized in some manner before it is a brand. In fact, IEEE, ANSI, NFPA, UL, etc. are closer to being brands than is GEMI. And like I said, I don't know of any other software designed for the purpose. Do you?
As far as published and peer reviewed, or even practicing what may appear as an engineering task, I believe you are a bit biased being an engineer yourself. Ultimately, it comes down to, if it works, why knock it. Seems to me you are just looking for such a reason. As far as i'm concerned, it is published in the form of software. Perhaps if it were Code approved for use, more details would be available on the 'net.
As to why use software instead of paper and pencil... it's time to get your time frame into the 21st century. We are not going to go backwards. And as Bob mentioned, it is a complex calculation. I'm guessing it's on par with Neher-McGrath calc's. So tell me what percentage of engineers performing those calc's resort to using paper and pencil vs. software...???
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How do we know that it works? Any single source of information that was paid for by an organization with a vested economic interest in the outcome is always a bit suspect.
But the information on how the Neher-McGraph calculations is out there and has been reviewed. As far as how you do the actual calculation is not as important as how or if the calculation actually works.
I agree to a degree. But I ask you, having made that statement, who stands to benefit economically? I also see what appears to be much less substantiation went into the current Code requirement. Granted, I have not been made privy to discussions that CMP's may have had on the matter. All I see is what started it, and what the end result is.
My entire reason for even pursuing this issue is that over and over, again and again, I read someone posting here about this issue as it is dealt with as a matter of Code requirement. I am not looking to to instigate separation between parties. Quite the opposite actually. I'm just trying to appease the masses... but it is appearing more and more like there's too much distance between those that have interest to bridge the gap. Perhaps that's the reason 250.122(B) is what it is to begin with... :blink:
BTW, I discovered there is other software available that does the calc's. I believe ETAP is one, but do not know for certain. Just do a search using "short circuit analysis" to see there is at least one other besides GEMI.
iceworm
Curmudgeon still using printed IEEE Color Books
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I'm certain I am biased. When I was getting started, maybe 40 years ago, state law discussed "product of engineering" and "engineering for hire". If one wishes to do those things, then the state requires getting a PE ticket. The law has not changed much in the last 40 years. And what you are advocating certainly appears to be "product of engineering" - the same as any other similar section (allowing calculaton using an accepted algolrithm)
Hummm... So you think I should give up my steam powered sliderule - NEVER
Yes there are reasons that one might not use software. For example: I have ETAP available on my computer. I get asked regularly about some specific arcflash. Now, I can set up an ETAP model and let the computer calculate. Takes me a few hours. Or I can take the equations right out of IEEE 1584, use pen and calculator and be done in 1/2 hour.
Another example: Supose one needs a single phase Short Circuit Current. I don't even know how to setup an ETAP model for this. However, I can use the Busmann point to point, and be done in an hour.
Of course, my coal fired calculator uses RPN which automatically makes me significantly faster than any of the parentheses machines.
And another - not exactly on point, but still: Show me a spreadsheet that can do complex math (3ph circuit analysis). Program it for a polar- rectangular swap. One can do it but it is a mess. Now add complex matrix math. (transmission lines) Spreadsheet-yuck. My 20 year old calculator - yep, it will do these and relatively cleanly.
All this required one understands the physics. No way to do this if the algolrithm is not published.
IMO:
Understanding the physics is not going backwards.
Depending on canned software with out knowing the physics, is worse than going backwards.
Well, you hit the key word, you're guessing. And that is because ..... (drum roll)
It is not published.
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The Steel Tube Institute sponsored the work that led to the GEMI software. I think that the main focus of the project was not on the fault clearing path, the more on the ability of steel conduit to provide shielding to sensitive circuits.
As I understand on review, it was NEMA that sponsored the study....
"[IAEI Executive Director J. Phillip] Simmons? request was presented to the NEMA 5RN Technical Committee and they agreed to sponsor the testing."
...and it was to verify maximum lengths published in Soares grounding book. It appears the only association with the steel conduit industry was one or more members of the NEMA committee were an Allied Tubing representative. I believe shielding ability was just a secondary objective that was easily measured in the testing process.
Nevertheless, I don't see how the study economically benefits the steel conduit industry. I feel the wiring method is determined well before any consideration of effective fault clearing becomes an issue. The only effect I see is less copper or aluminum wire being used... not more steel conduit. If no steel conduit is used, it only affects the copper and aluminum wire industry. How much? Unless implemented, I think it would be difficult to determine without an extensive database of past usage.
Is there something I'm missing?
Well let's get down to the nuts and bolts of the issue. Using any wire-type EGC size between Table 250.122 size and upsized proportionately with upsize in ungrounded conductor (i.e. an excepted size) to be determinable by the lay person. Short of that, I will not be submitting a proposal. Feel free to submit an proposal for exception under engineering supervision yourself.... :happyyes:
iceworm
Curmudgeon still using printed IEEE Color Books
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Nope. Wish I could.
You're the man for this. I have the faith you will pick a good one. Of all the posts on NEC proposals discussed here, you are by far trying to do the right thing. The others are alo trying to do the right thing, but you have them way out classed. Stay the course.
ice
hurk27
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The problem with leaving the sizing of the EGC on a long run up to the lay person is that there are very few who would have the ability to even know how to calculate it down to the point of knowing that how many bolted fault amps flowing on the EGC for how long that will cause conductor insulation damage.
You pointed this out back in post 29 of my calculation in post 25 as being wrong, as you may be correct as I have no idea what 343 amps on a #8 EGC flowing for up to 9 seconds before the OCPD would clear the fault would heat the conductor to what temp, from published guides that say a common 90?c rated THHN/THWN conductor if exposed to 150?c temperature for 5 seconds or more will cause insulation damage which is what I was basing my time on which I can see is in error because we don't know exactly how hot the 343 amps for 9 seconds would cause the conductor to reach? I calculated the above using 30?c rating used in chapter 9's ohmic values.
What we do know is that most common breakers we use has a trip curve that has three basic trip ranges, long term, mid term and instantaneous, with instantaneous being the fastest from .005 seconds to about 1.5 seconds @ 10+ times the handle rating with the mid trip point of the curve being from around the 2 second point to the 500 second point from about 10 times down to 1.25 times the handle rating, above this is long term or overload protection.
It is in the mid range part where we can get into trouble with wire damage from a fault and in this case a fault to ground that involves the EGC as the return path on long circuit runs, when I found that the bolted fault current would keep the OCPD from opening any longer then 5 seconds I would assume that the conductor was in danger of being damaged, this as I now know is in error as it was assumed that the conductor would reach a temp of 150?c or over for 5 seconds or more, but in reality with out doing the physics calculations I would have no idea as to how hot the conductors would really reach, all I knew was they would be seeing this current for more then 5 seconds.
Here's a PDF from Square D but it is about Circuit Breaker Characteristic Trip Curves and Coordination and is not really the point of discussion but you can see in the tripping curves where the reduction of available fault current cause by long circuit runs can put a conductor in jeopardy where it causes the breaker to take to long to open.
As for the OP discussion the change in the 2014 basically puts 250.122(B) back to saying what the 1999 NEC already said and the 2011 points out in the commentary that this should only apply to when the wires are up sized for voltage drop not when they are adjusted for CCC in a raceway as pointed out in post #24.
I agree that in many cases it over does the upsizing of the EGC to an unnecessary size, but as I have seen in many other threads posted on this subject no one has come up with a better way of doing it that would be accepted by the code panel, the idea of a chart is a good one but it would be unbarring large to cover all the lengths up to lets say 1'000 feet runs for every size conductor, since it is not based directly upon the circuit size and more based upon the resistance of the fault path.
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kwired
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Not just resistance of the fault path, but resistance/impedance in the entire circuit (including source impedance) during the fault is what determines how much current will flow
hurk27
Senior Member
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- Portage, Indiana NEC: 2008
Is that not part of the fault path? I also pointed it out in post 25 when I made the statement that I was not even including the impedances ahead of the circuit that would lower the current even more that would cause even longer clearing times of the OCPD.
I am by no means an expert on the math it takes to calculate all that is needed to determine the exact size EGC that will safely clear the OCPD in time to prevent conductor damage, but over the years I have seen the results of faults happening on long runs that would not clear a fault but instead cause the wire to burn or at least damage the insulation if the OCPD did finely open, the problem was many of us myself included would just attributed it to a bad or stubborn breaker that would not open or just took too long to do so, I had always known that resistance is current limiting but back then I didn't have a clue about breaker tripping curves and how current affected the clearing times making it dangerous when it took to long to clear the fault in the mid part of the tripping curves.
From the above I wanted the truth behind it and sought out the correct answers, what I found was many different answers with some being correct and some being just ol wives tales, but I learned enough to keep the EGC large enough that there was enough head room that in most cases following 250.122(B) would keep my wires safe and I could rest at night knowing that the breaker would have a low enough impedance path on a fault that I wouldn't have to worry about my work causing a fire down the road, sure it can be an over kill in many installations and end up having larger EGC's than is needed but I had to also accept that I'm not a math master and adding another long and very hard calculation to my work was not in my interest, guess in a way you could say I was being lazy and just let 250.122(B) be the guide I would use even if it was an over kill, if someone could post a better way and get it past the code making panel I would be all behind it in a heart beat, but it better be correct to the point that it will keep the wire protected as I would not want to be behind a method that may work for some but be so hard to calculate that many would not do it.
When I myself tried to find an easier method, I found like the weather there are so many variables that can't always be counted for that can cause a problem in finding the right size EGC, here are just some of these:
Quality of the insulation used on todays conductors coming from third world manufactures that we do not always have the information about what we are getting other then they are 90?c rated, it may pass UL's requirements for a 90?c rating but what if the insulation chemical formula rapidly fails at 120?c and above, I have seen 150?c rated conductors in UL listed light fixtures have the insulation melt just from the heat of the lamps that were manufactured in a third world country but they were UL listed and I did send UL a sample and they did officially list them.
Ambient temperature of the conductors at the time of the fault, which can be a big variable depending upon where a person is doing the work and the temperature changes the conductor may face in their intended installation.
Damage to the insulation at a later time that could make it easier to cause insulation failure when the fault event takes place.
I'm sure we could add to this list but after working two doubles in a row it's a little hard to get the ol mind to work any harder right now.
but if we look at all the variables that can affect the fault handling capabilities of a conductor we can kind of see why the CMP's took a very conservative approach to the sizing of EGC's on long runs, mainly because there is no one size fit's all.
So until a better way is found that is as safe I will keep up sizing my EGC's proportionately to the up sizing of my ungrounded conductors on voltage drop issues.
hurk27
Senior Member
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- Portage, Indiana NEC: 2008
I would like to point out that 250.4(A)(5) in a very indirect way even requires us to provide a safe low impedance fault current path that is capable of facilitating the operation of the over current device, this is not an FPN it is a requirement, but it is not even thought about when we look at long runs and or voltage drop because voltage drop is only an FPN, long runs can effectively cause OCPD's to fail in protecting the conductors of a circuit, maybe this needs to be better looked at by the CMP's and or UL, because 250.4(A)(5) already requires us to provide a low impedance fault path for grounding, and if a long run does not accomplish this then we should be required to make sure it is done, I think 250.4(A)(5) should trump the FPN on voltage drop, it's just not something we think about when it comes to voltage drop issues.
Can a long run EGC safely carry the maximum ground-fault current likely to
be imposed on it?
Not always, and can be a fire hazard as I have seen over the years.
Can a long run EGC safely carry the maximum ground-fault current likely to
be imposed on it?
Not always, and can be a fire hazard as I have seen over the years.
I can relate to this. I'm currently working 6/12's on nights. One advantage is I can squeeze off a post here and there while I'm at work (but obviously not working...
I'm sure we could add to this list but after working two doubles in a row it's a little hard to get the ol mind to work any harder right now.
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