2014 change to 250.122(B)

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iwire

Moderator
Staff member
Location
Massachusetts
Fully acknowledging I have not seen it all I have to say you guys seem to be struggling to find a solution to a problem that does not exist.

From my perspective it is more about flexing your brains then fixing a true problem.

Carry on. :cool:
 

Smart $

Esteemed Member
Location
Ohio
Fully acknowledging I have not seen it all I have to say you guys seem to be struggling to find a solution to a problem that does not exist.

From my perspective it is more about flexing your brains then fixing a true problem.

Carry on. :cool:
In truth, I think the biggest problem complying with 250.122(B) is when wanting to use cable. The proportionate upsizing of the EGC don't match up with standard cable EGC sizes... so custom sized cable has to be ordered.

I think the proposed revision I made early on would help alleviate that problem. I only say think because I haven't checked the sizing at the thresholds of Table 250.122 vs. highest possible ocpd rating vs. standard cable EGC sizes. Anyone up to doing that "for the cause". My spare time is quite limited at present.
 

iwire

Moderator
Staff member
Location
Massachusetts
In truth, I think the biggest problem complying with 250.122(B) is when wanting to use cable. The proportionate upsizing of the EGC don't match up with standard cable EGC sizes...

True, and that also is an issue with 250.122(F) but IMHO those are both design issues more than a change the code issues.

Unless you are running 3 phase 4 wire you can normally get an off the shelf cable with an extra full size conductor to use as the EGC. So the guy that wants to run 8 AWG UF cable out to a far away post light could choose 8/3 over 8/2, abandon the bare and use the red as the EGC.
 

Smart $

Esteemed Member
Location
Ohio
True, and that also is an issue with 250.122(F) but IMHO those are both design issues more than a change the code issues.

Unless you are running 3 phase 4 wire you can normally get an off the shelf cable with an extra full size conductor to use as the EGC. So the guy that wants to run 8 AWG UF cable out to a far away post light could choose 8/3 over 8/2, abandon the bare and use the red as the EGC.
Well that saves a custom ordered cable but it still costs more than just using an 8/2 for for voltage drop compensation.

I think back in the '75 when this upsizing requirement was first implemented, you could still buy cable without the EGC. If so, going from 8/2 with ground to 8/3 without wasn't as much of an issue.
 

kwired

Electron manager
Location
NE Nebraska
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.:sleep:

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.
First isn't the "fault path" only the portion of the circuit not normally intended to carry current, but the "fault current" will flow through a complete circuit including the source and portions normally intended to carry current? This was my reason for saying it like I did.

Otherwise I agree with about everything you said and have had similar experiences with long runs as you described. Even when NEC sized EGC is supplied long distances still have an impact on how much current will flow and will effect response time of an overcurrent device. I agree the NEC has taken a fairly conservative approach to create somewhat of a one size fits all rule, but in reality it is overkill in some instances and not enough in others, but I'm guessing to get enough precision the process may be a little too confusing for the average installer, so what is there is deemed sufficient (and some installers still find it confusing).

Fully acknowledging I have not seen it all I have to say you guys seem to be struggling to find a solution to a problem that does not exist.

From my perspective it is more about flexing your brains then fixing a true problem.

Carry on. :cool:
:)
 

Carultch

Senior Member
Location
Massachusetts
Just change the left column heading...

Conductor Ampacity*
15
20
...
...
5000
6000
*After application of adjustment and correction factors.


I like your idea, but what about rounding errors?

For instance, if I use 250 kcmil AL for a 200A circuit, per the present rules I can use a #6 Cu ground.

Now that my conductor ampacity exceeds 200A (even though the OCPD doesn't), can I still use a #6 Cu ground per your proposed rework?
 

Smart $

Esteemed Member
Location
Ohio
I like your idea, but what about rounding errors?

For instance, if I use 250 kcmil AL for a 200A circuit, per the present rules I can use a #6 Cu ground.

Now that my conductor ampacity exceeds 200A (even though the OCPD doesn't), can I still use a #6 Cu ground per your proposed rework?
Well, no.... and definitely a nuance that would have to be dealt with.

I was battling that prospect for my 2017 proposal, which goes by the maximum standard ocpd rating that could be used to protect the circuit conductor. Upsizing under 240.4(B) is permitted, not required.... so the maximum ocpd rating that could be required is 200A rather than 225A, though 225A is permitted. Perhaps I'll have to clarify that in the comment stage.
 

Smart $

Esteemed Member
Location
Ohio
To answer the specific question, I would agree with other comments that also indicated "no".
I would also like to direct to my response to the other post on this subject; 250.122(B) increased size.
I realize you haven't posted here much (yet? :)) so I'll try to be polite. :eek:

Your answer refers to a specific question, but we have to assume what question you are referring to. Please quote or otherwise indicate which question is being answered. In short threads, it's not much of a problem. But in a thread like this that has 127 previous posts, the probability of an assumption being in error can be staggering. :happyyes:

Also helps to provide a link when referring to a post in another thread:
http://forums.mikeholt.com/showthread.php?t=167986&p=1633107#post1633107
The preceding link was made by copying the link address associated with the thread number displayed at the top right of the post then pasted here.
 
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