Upsizing EGC with conductor size?

[charlie b]

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.

I agree, given that that was the only change. The difficulty is that the code needs to be written in such a way that it covers as many situations as possible without being unruly and impossible to enforce.

 

[wwhitney]

I agree, given that that was the only change. The difficulty is that the code needs to be written in such a way that it covers as many situations as possible without being unruly and impossible to enforce.

Right, so the default language is upsize the EGC if you upsize the ungrounded conductor. Yet the Exception provides the flexibility for more nuanced application of the rule, which I am embracing.

Cheers, Wayne

 

[wwhitney]

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?

My take: 250.122(B) is using "upsizing the ungrounded conductors" as a proxy for "circuit is long enough that the minimum size EGC may have too high an impedance." Since the typical reason to upsize the ungrounded conductors is for voltage drop, which is an issue when the circuit becomes long. And impedance is proportional to conductor length.

Then when you know you've upsized the ungrounded conductors for a reason other than length, the exception lets you skip upsizing the EGC.

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.

To be clear, I wasn't saying that the voltage drop calculation proves physics-wise that the fault behavior will be OK. I was saying that it demonstrates that the reason we upsized the ungrounded conductors wasn't due to length. I'm trusting that the minimum wire size specified by the NEC will provide an "an effective ground fault current path" for "normal" circuit lengths.

Cheers, Wayne

 

[wwhitney]

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.

I was wondering if you could expand on the above a bit:

Say we have an actual installation and I go to some point and simulate a fault by applying a resistance across two conductors (can be one ungrounded and the EGC if you like). I start off at a high resistance and decrease it, monitoring the current and voltage across the resistor in both magnitude and phase, but I do this fast enough that nothing starts to fail. (Maybe this has become a thought experiment.)

Then for each value of the current, I can compare the voltage magnitude and phase to the starting (infinite resistance) voltage magnitude and phase, and infer an upstream impedance from delta V = I Z. Your statement above implies the inferred impedance will differ at say 10,000A from 10A or 100A. At what current level does the upstream impedance typically start to significantly deviate from the low current case, and by what factor can it change once we hit the AFC? Just looking for an order of magnitude type answer.

Thanks,
Wayne

 

[LarryFine]

I would ask what the difference in OCPD operation time would be.

 

[jaggedben]

... 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.

This right here is why it's the dumbest rule in the book.

I'll get off my soapbox now.

 

[wwhitney]

This right here is why it's the dumbest rule in the book.

Yet clearly there is a circuit length at which the standard size EGC has too high an impedance to do its job.

It would be nice if a reasonable rule of the form "the EGC from the MBJ/SBJ to the outlet may have a maximum impedance of X ohms" could be formulated. But I don't that format provides sufficient flexibility to cover all cases.

Cheers, Wayne

 

[charlie b]

Just looking for an order of magnitude type answer.

Sorry, but I can't give you one. It's not that simple. For starters, even as a thought experiment, you could not possibly turn your test circuit on and off fast enough.

Any coil of wire (e.g., motors) has the characteristic of inductance. There is also inductance between the conduit and the wires within it, but it's value is normally negligible. It's value depends on the material and size of the conduit. It makes its presence known when the fault current shows up.

The key effect an inductor brings to the table is that it hates when you try to change the current flowing through it. When you try, it creates across itself a voltage oriented to oppose your attempt to change the current. The faster you try to change the current, the higher the opposing voltage. That is why your thought experiment would not work.

If you create a fault condition by shorting two wires, that instantly establishes a very large current, and the inductive reactance of the conduit/wire won't like it. That is where the choking effect I mentioned earlier comes into play. There are so many factors that can influence the result.

 

[wwhitney]

Sorry, but I can't give you one.

Ah, I think what you are saying is that V = I * Z (complex) holds for the steady state, but for high fault currents the steady state will never be reached, as OCPD starts opening and things start failing. So a dynamic or time dependent analysis is required, starting at the instant a fault happens.

So let me rephrase my question and make it a practical experiment to boot: same idea, but the resistance is slowly varying. Up to approximately what currents can I expect the steady state analysis to hold? Barring an installation problem, I would certainly expect it to hold up to the conductor ampacity, and then presumably to some multiple thereof.

And in this regime I can also expect that the inferred upstream impedance will be constant, independent of current?

Thanks,
Wayne

 

[jaggedben]

Yet clearly there is a circuit length at which the standard size EGC has too high an impedance to do its job.

It would be nice if a reasonable rule of the form "the EGC from the MBJ/SBJ to the outlet may have a maximum impedance of X ohms" could be formulated. But I don't that format provides sufficient flexibility to cover all cases.

Then the rule should be based on length, or at least not kick in until circuit length is long enough to where the effect is not practically negligible. (100ft? More?)

As I understand it, the history of this is more or less as follows:

The potential problem was recognized. It was then assumed that at any length where it was an issue, the conductors would be upsized for voltage drop. This is an erroneous assumption of competence and diligence in the first place. Then (because people were claiming, truthfully or not, that they had other reasons, to get out of the rule?) it was further assumed that the *converse* was true, that all upsizing must be enforced upon as if it were for voltage drop. Never mind that there are plenty of other legit reasons to upsize conductors. So piling erroneous assumption on top of erroneous assumption. The result is a rule that illogically applies in numerous situations where it shouldn't, perhaps more than where it should.

Thank goodness most resi inspectors are either ignorant of this rule or choose not to enforce it.

 

[charlie b]

Ah, I think what you are saying is that V = I * Z (complex) holds for the steady state. . . .

That formula holds true at all times, steady state or not.

The upstream impedance, specifically the value of inductance (which is given the symbol "L"), is based on materials (i.e., IMC will have inductive impedance but PVC will not) and geometry (e.g., the conduit diameter and the location of the affected wire within the conduit - does it sit on the bottom or are there other wires below it). Thus, "L" is a constant value for any given installation.

What changes during a fault is the impact the inductor's self-imposed voltage has on the circuit. The formula for that voltage is V = L(dI/dT). Put into words rather than calculus, voltage equals inductance times the amount the current changes (dI) divided by the time it takes for that change to take place (dT). Thus, if you have a large change in current divided by a very short time interval, you get a high self-imposed voltage across the conduit/wire combination. That is where the choking effect can come into play.

 

[wwhitney]

That formula holds true at all times, steady state or not.

I'm not following what you mean by this (for the formula V = I * Z (complex)).

Certainly I agree that for a resistance R, with current I(t) through it, and voltage drop V(t) across it, we have V(t) = R * I(t) for all points in time t, and any shape of the waveforms I(t) and V(t).

For an inductance L, as you say the equation is V(t) = L * dI/dt. Then if over a time period we know that I(t) will be a sinewave of a fixed frequency f, and view that as a complex number I, it follows that dI/dt = 2*pi*f*j I. Which gives V = I * Z where Z = 2*pi*f*L j.

But this required assuming that I(t) is a sinewave of frequency f. For the general case where V(t) and I(t) may not be sinewaves, we'd have to solve the differential equation V(t) = L * dI/dt. And I would think that the very short term behavior of a fault or a switch being thrown would involve transients that are not of the fixed base frequency. So V = I * Z with Z fixed would only apply if a steady state is achieved due to those transients eventually decaying to zero.

Cheers, Wayne

 

[Strathead]

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.

I disagree. Qualified is used all the time in various industries and is generally accepted as one who has been trained in the discipline. Any licensed Journeyman has been trained in the discipline. Lett me give you a scenario where this badly written code was applied before the exception. 200A breaker feeds a non fused disconnect feeding an AC unit spec'd out at 200A MOCP. The provided unit had a 150A MOCP. The Engineer chose to have the breaker dialed down to 150A in lieu of changing the disconnect out to a fused disconnect. The inspector, being a super stickler for the code failed the installation. 3-3/0 and #6 EGC is acceptable on a 200 amp breaker, but when the breaker is dialed down to 150A the EGC must be changed to a #4 to comply with the code. I am qualified enough to know that is ridiculous. Take another situation. I have a 400A feedthrough panel with a calculated load of 300A. If I choose to put 600 KCMIL conductors to it, do I have to upsize my ground, I could have used the "next larger wire size" rule, so technically a #600 is oversized. Take it a step further, I have some 750 laying around the shop, so I choose to put that in rather than buy some 500. I again, think I am qualified enough to know I don't have to upsize my ground. I strongly suspect you are too Charlie. (yes this last is sarcasm because I know you are significantly more qualified than me to do it.)

 

[jaggedben]

... I am qualified enough to know that is ridiculous. ...

Hear hear.

I would add more examples, but really there are too many.

 

[charlie b]

Strathead, I welcome all opinions and cherish everyone's right to disagree. I also honor and respect the knowledge and experience it takes a person to become a Journeyman or Master Electrician. I would never question such a person's qualifications to design and install electrical equipment. But those qualifications are not the same as would be needed to ascertain that an effective ground fault return path exists even if you don't upsize the EGC. See my post #14 for my reasoning.

The science of logical reasoning (OK, I recall this from a college course I took 50 years ago) includes several "classical" invalid arguments. One goes something like this:
Person 1 - "What needs to be done here is XXX."
Person 2 - "Can you prove that?"
Person 1 - "This is my area of expertise. I am fully qualified to make this call. Trust me."

It may turn out that Person 1 is right. But the proof totally failed to prove anything. This type of invalid reasoning is called, "begging the question." This is what comes to my mind when I read statements along the lines of, "I am qualified enough to know I don't have to upsize my ground." How would a reviewer know who made the call not to upsize? How would a reviewer know that person's qualifications? A PE has the privilege (and indeed the duty) to declare to the world (by sealing and signing design documents) that, "This work was done by me or under my supervision." Is that needed, in order to take advantage of the exception under discussion here? I think so. You and others have disagreed.

Bottom line: If I am performing a plan review, or perhaps an on-site inspection of an installation in progress, and I see a 20 amp circuit with #10 phase and neutral conductors and a #12 EGC, and if there is no written justification for not using a #10 EGC, then I will cite this as a code violation.

 

[jaggedben]

...

Bottom line: If I am performing a plan review, or perhaps an on-site inspection of an installation in progress, and I see a 20 amp circuit with #10 phase and neutral conductors and a #12 EGC, and if there is no written justification for not using a #10 EGC, then I will cite this as a code violation.

Okay here's a written justification I might jot down for you by hand on a piece of paper on the spot.

"The specs called for conductors to be 12awg. Unfortunately I only had 10awg for all circuit conductors on the truck (sorry). I upsized all circuit conductors to match. In doing so, I could only have reduced the impedance of a faulted circuit, therefore the EGC that is to original spec and complies with Table 250.122 will still be as effective a ground fault current path as before. This is all the more obvious because the circuit wiring is only 10ft long and is protected by a conventional molded case circuit breaker.

Signed,
Not my first rodeo"

 

[charlie b]

Okay here's a written justification I might jot down for you by hand on a piece of paper on the spot.

That wouldn't help, as it is not an official document that could be included with the record drawings. Absent a sealed and signed calculation, I would still cite this as a code violation.

I do understand your reasoning. As I said earlier in this thread, I always had the same problem with this requirement. You change to #10's, thus making the available fault current higher, so why do you need to make it higher still by using a #10 EGC? I never found a reasonable answer to that question.

Nevertheless, getting back to Charlie's Rule, "The words are what the words are." The words call for upsizing the EGC. I have already given my opinion on the qualifications needed to take advantage of the exception.

 

[Mr. Serious]

Do I understand this correctly? If a licensed journeyman or contractor electrician does a calculation and writes it down, and signs it with their name and license number, it can't be an "official document" simply because it's not signed by a licensed PE?

As a licensed PE, could you simply add your signature and seal to the document, and then it becomes an official document? I understand you probably don't want to do that without charging for the service. It's the same situation we all run into - people ask us to sign off on things, but we don't want to do that for free because it's our reputation and license at stake, which incorporates a lot of cost.

 

[Npstewart]

What's the NEC definition of a qualified person when applying 250.122(B)?

Qualified = Saying that you're qualified on your website.

 

[jaggedben]

That wouldn't help, as it is not an official document ...

The code doesn't require an 'official' document.