Upsizing EGC with conductor size?
- Thread starter JoeNorm
- Start date
- Location
- Bremerton, Washington
Section 250.122 has changed about teason but the latest version makes it clear if you increase the ungrounded conductors for any reason, the egc is to be increased in size
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
But the 2020 NEC also added:
250.122(B) said:
So if you're qualified, and you know that using the minimum size ungrounded conductors and EGC would work fine, but choose to upsize the ungrounded conductors for some other reason, you can invoke the Exception to skip upsizing the EGC.
Cheers, Wayne
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What's the NEC definition of a qualified person when applying 250.122(B)?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
First of all, in my view, only a licensed professional electrical engineer would be "qualified," in the sense intended by this exception. Secondly, as such a PE EE, I would never try to take advantage of this exception.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The same definition whenever the term is used in the NEC, as found in Article 100:
Qualified Person. One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved.
Cheers, Wayne
- Location
- New Jersey
- Occupation
- Journeyman Electrician
An electrician is qualified especially if the wiring method already qualifies as an EGC because then the point is moot. If I'm running EMT I do not need to pull an EGC but if I choose to install one it never is required to be upsized because I already know that the raceway is sufficient as the EGC.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
I will disagree, as there is an Article 100 definition to refer to. So there's no particular "sense" to this usage in 250.122(B) Exception, there's just the definition.
Secondly, I would think the logic of "this installation will be code compliant and work fine, including for clearing faults, with 2% VD if I size everything to the minimum, so the minimum size EGC will provide an "effective ground fault current path", and upsizing the ungrounded conductors isn't going to change that" is unassailable.
Cheers, Wayne
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
This article changed at least twice over the past several cycles (for which I am grateful). The 2023 edition makes it clear that choosing a "different" (at this point, I won't use the word "increase") size conductor because you have a high ambient temperature or you have many current-carrying conductors in the same conduit does NOT constitute an "increase in size" (OK, I used that word this time) and does not require a larger EGC.
I can't claim any credit for this revision, though I did submit several code revision recommendations over a decade and more. I tried to make the point that the phrase, "increased in size" makes no sense whatsoever, if you don't clarify the starting point "i.e., increased from what."
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
The only definitive starting point is 'code minimum.'
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Tom and Wayne, I will disagree right back at you. The exception cited in post #3 requires the "qualified" person to ascertain, to determine, to verify that the chosen EGC will suffice to clear any postulated fault. If I am performing a design review of the proposed installation, I will insist on seeing the associated calculation. I would not accept a simple, "well, I think it will work just fine."
Ask yourself this: if I can use #10 ungrounded conductors in a 20 amp circuit and stick with a #12 EGC, why eas the "upsiae the EGC rule" ever included in the NEC?
Ask yourself this: if I can use #10 ungrounded conductors in a 20 amp circuit and stick with a #12 EGC, why eas the "upsiae the EGC rule" ever included in the NEC?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
True.
Let's take an example. Five 20 amp circuit breakers, 5 single-phase circuits in a conduit (i.e., 5 pairs of phase and neutral conductors), for a total of 10 current-carrying conductors. That requires the #12's to be derated by 50%, so that they could not serve a 20 amp circuit. You then choose #10 conductors. In this example, the #10's represent the "code minimum," and do not represent an "increase in size." Therefore, the EGCs can be #12.
That was the clarification that I believed was needed.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
My response would be:
1) Here's the VD calculation using the NEC minimum sized conductors. It is 2%, well within industry standards. Therefore an installation using minimum sized ungrounded conductors and minimum sized EGCs would be provide an "effective ground fault current path" and be NEC compliant. I don't even need a definition or standard of "effective ground fault current path," I just need this one reference point that I know is an "effective ground fault current path."
2) I choose to then upsize the ungrounded conductors. Maybe because I think 1% VD will save me money in the long run, maybe because I have a lot of one size on my truck, or maybe because I like numbers to be multiples of 4. Regardless, the ground fault impedance has simply decreased slightly from case 1 above, and so my EGC is still an "effective ground fault current path" without upsizing the EGC.
For the situation where case 1 above doesn't hold, that minimum sized ungrounded conductors would have unreasonable voltage drop, and hence there is reason to be concerned about excessive ground fault impedance.
Cheers, Wayne
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
In this instance, the "equipment" consists of wires, conduit, the voltage source and its internal impedance, and inductive loads (e.g., motors) installed in the facility. Wires have resistive and inductive reactance, the later of which depends in part on the material and size of the conduit, and both of which depend on the point along the wire that the fault takes place.
Under fault current conditions, certain conduit types (and not certain others) develop high inductive reactance in association with the wire carrying thr fault current. This can tend to choke current flow, in essence to provide a higher impedance as seen at the fault point, so that the wires might not be able to carry what would otherwise be the total available fault current. Thus, the EGC is not as effective, it may take longer for the OCPD to respond, and the event can result in greater damage.
The higher the fault current, the greater the choking effect. The amount of available fault current depends on the "strength" of the voltage source (essentially its internal impedance), the impedance of all the conductors leading from the voltage source to the fault point, and any contribution to the fault current from any induction motors that happen to be running at the time of the event.
You might just need a larger EGC to overcome this situation. Or you might not. How does one tell the difference? By performing a calculation. That is where the PE EE comes into the process.
On the other hand, one could take the easy way out, by declining to make use of the exception to 250.122(B) and upsizing the EGC. Speaking as a PE EE, I prefer this approach.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
What if someone decides later to utilize that larger conductor at its full capacity?
Could that possibly render the original EGC size too small now?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Sure, but that's a future problem on them, to check that the EGC size is adequtae.
Cheers, Wayne
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Sorry, Wayne, but that reasoning is not valid. The phenomenon of voltage being dropped along a wire carrying normal load current, a situation in which the EGC is not a player, is nothing like that of extremely high current under fault conditions, for which the job of the EGC is to ensure that the OCPD is able to quickly clear the fault.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Charlie, you obviously know a lot about the details of that calculation. So please tell me this: If for a certain set of circumstances, ungrounded conductor size X and EGC size Y calcs out to be OK, and then the only change I make is to increase the ungrounded conductor size, does it automatically still calc out to be OK? I'm under the impression that it does.
Cheers, Wayne
- Location
- Bremerton, Washington
The steel tube institute has a free software program “Gemi” that is very usefull for section 260.122 applications
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I will concede that this is the one thing that always confused me. It made me wonder why 250.122(B) was ever put into the code. Changing from #12 to #10 phase conductors will reduce the overall circuit impedance, increase the available fault current, and cause the OCPD to respond more rapidly. Why then, do we need to reduce the fault circuit impedance even further by using a #10 EGC?