Proportional Increase of EGC for VD

[wwhitney]

Where we seem to be in disagreement is whether or not this particular note and the above logic applies to the EGC.

I think your logic is this: sometimes we can avoid upsizing an ungrounded conductor by using a higher insulation temperature rating. So can we avoid upsizing an EGC by using a higher insulation temperature rating on the EGC? And isn't that what the sentence in the commentary to 250.122(B) is talking about?

The consensus answer to both questions is no, as everyone has explained. And if you think about it, insulation temperature rating isn't a thing for the EGC. The EGC could, after all, be bare. And under normal conditions, the EGC is not carrying current, so you don't have to worry about the heat it generates damaging its insulation. A higher temperature insulation on the EGC in no way improves its ability to do its job, unlike a circuit conductor.

{I'm ignoring the obvious limitations likes "don't use 60C only insulation on the EGC alongside conductors sized based on 75C or 90C ampacities.]

Cheers, Wayne

 

[Jerramundi]

The consensus answer to both questions is no...

I'm not in disagreement that "no" is the consensus answer... and given both (1) your point that this note and those like them are not explicitly part of the actual code and (2) that my interpretation of the note doesn't help my current circumstances anyway as a #10 EGC sized according to the 90C column doesn't equal the ampacity of a #8 sized at 75C... I'm moving forward accepting of the consensus answer, for now, lol.

I do still however have a tick in the back of my brain that thinks this note could be said to apply to the EGC and that there exist circumstances in which it could be applicable, even if that's not the consensus answer. But I yield to the majority of people's interpretations of the code that I both respect and value.

 

[david luchini]

that my interpretation of the note doesn't help my current circumstances anyway as a #10 EGC sized according to the 90C column doesn't equal the ampacity of a #8 sized at 75C... I'm moving forward accepting of the consensus answer, for now, lol.

Just another note to maybe help you along: "Ampacity" is not relevant to EGC's.

 

[charlie b]

NOTE: David got his comment in while I was typing mine. But I will keep typing anyway.

Let me offer this notion: We size the ungrounded conductors on the basis of the required ampacity. That ampacity value is influenced by the wire's insulation system. But that is not how we size the EGC. We size it on the basis of the upstream breaker. It's insulation system is not relevant to the table that gives us EGC sizes. Indeed, as has already been pointed out, the EGC and be bare copper.

 

[Jerramundi]

Just another note to maybe help you along: "Ampacity" is not relevant to EGC's.

Not to be argumentative, but purely out of intellectual curiosity, how can we definitively say that " 'ampacity' is not relevant to the EGC?" True, as Charlie just pointed out, the NEC requires us to size the EGC based on the size of the OCPD, but I would suspect that those NEC EGC values were chosen because of some mathematical relationship to ampacity...

Maybe I'm taking us off topic and it's time we wrap this up, but I had to ask...

 

[Jerramundi]

NOTE: David got his comment in while I was typing mine. But I will keep typing anyway.

Let me offer this notion: We size the ungrounded conductors on the basis of the required ampacity. That ampacity value is influenced by the wire's insulation system. But that is not how we size the EGC. We size it on the basis of the upstream breaker. It's insulation system is not relevant to the table that gives us EGC sizes. Indeed, as has already been pointed out, the EGC and be bare copper.

Again, just out of intellectual curiosity, in more of a physics based argument than an NEC one, does a wire that can more easily dissipate heat not have a lower resistance than the same sized wire with a lower ability to dissipate heat?

Maybe that's where I'm going wrong with all of this. I'm thinking more "physics" than I am bowing to the almighty NEC, lol.

I'm thinking that an EGC of a higher insulation temperature rating would have a lower resistance and thus a lower voltage drop, which would make sense of my interpretation of the note that one could choose an EGC of a higher insulation temperature rating (as opposed to the proportionate increase) when upsizing the EGC for voltage drop purposes.

Granted this is not what the NEC explicitly allows and I am agreement with the consensus opinion.
Feel free to close the thread after this. I was just curious...

I tend to piss a lot people off with my curiosity so apologies, lol. Cheers to civil discourse and debate!

 

[charlie b]

. . . , but I would suspect that those NEC EGC values were chosen because of some mathematical relationship to ampacity...

I think not. Ampacity is all about the ability of a wire to carry current for long periods of time, without overheating its insulation system and without risking causing a fire. An EGC will sit idle, doing nothing, carrying no current, for decades on end. Then, for one brief shining moment it will carry the very high current associated with a fault. At the end of that moment, the breaker will have tripped, and the EGC will go back to its boring role of doing nothing.

We size the EGC not on its ability to carry current without overheating its insulation. Rather, we size the EGC on the basis of its resistance (or "impedance" might be a more accurate term). Impedance has nothing to do with the insulation. It only depends on the wire's material (copper versus aluminum) and its cross-sectional area (AWG size).

 

[charlie b]

Again, just out of intellectual curiosity, in more of a physics based argument than an NEC one, does a wire that can more easily dissipate heat not have a lower resistance than the same sized wire with a lower ability to dissipate heat?

It's a good question , but your description of the physical conditions is not quite accurate. Two identical wires with different temperatures (due to ambient temperature and due to the amount of current flowing) will have different resistances. The higher the temperature, the higher the resistance, and therefore the lower ability to carry current. However,

1. This effect is very small for the types of circuits we commonly deal with,
2. This effect is based on the actual temperature, not the ability of the insulation system to withstand high temperatures, and
3. This effect is completely insignificant for the amount of time that an EGC carries current during a fault.

 

[Jerramundi]

An EGC will sit idle, doing nothing, carrying no current, for decades on end. Then, for one brief shining moment it will carry the very high current associated with a fault. At the end of that moment, the breaker will have tripped, and the EGC will go back to its boring role of doing nothing.

Poor EGC So under appreciated, lol.

 

[ActionDave]

Again, just out of intellectual curiosity, in more of a physics based argument than an NEC one, does a wire that can more easily dissipate heat not have a lower resistance than the same sized wire with a lower ability to dissipate heat?

Maybe that's where I'm going wrong with all of this. I'm thinking more "physics" than I am bowing to the almighty NEC, lol.

I'm thinking that an EGC of a higher insulation temperature rating would have a lower resistance and thus a lower voltage drop, which would make sense of the note that one could choose an EGC of a higher insulation temperature rating (as opposed to the proportionate increase) when upsizing the EGC for voltage drop purposes.

Feel free to close the thread after this. I was just curious...

You haven't answered the question that has been asked at three times at least,... What do you use to size an EGC that has no insulation?

The physics of it is temp ajustments are based on insulation ability to withstand heat. A 12awg wire can handle 50 amps it's the insulation that can only handle 30 amps. That's the physics of it

 

[LarryFine]

Again, just out of intellectual curiosity, in more of a physics based argument than an NEC one, does a wire that can more easily dissipate heat not have a lower resistance than the same sized wire with a lower ability to dissipate heat?

I'd say no, the resistance has nothing to do with heat dissipation. Only area and length (in an everything-else-being-equal discussion.)

I'm thinking that an EGC of a higher insulation temperature rating would have a lower resistance and thus a lower voltage drop, which would make sense of my interpretation of the note that one could choose an EGC of a higher insulation temperature rating (as opposed to the proportionate increase) when upsizing the EGC for voltage drop purposes.

No again. The only thing insulation temperature has to do with is how hot the conductors can get without causing insulation damage.

 

[jaggedben]

Other examples:
1. Equipment has a minimum lug capacity that requires you to use a larger wire than otherwise necessary for NEC ampacity.
2. You sized for the 60C rating, unaware that the equipment on both sides was listed otherwise for 75C.
3. You didn't bother to take credit for the next size up rule, 240.4(B). Maybe your conductors are in-series with transformer secondaries, where 240.4(B) isn't allowed to apply, and it makes more sense to standardize.
4. You've sized with future use in mind (e.g. full 300A, but are only using 225A at the time of construction). #4 Cu would be good as the EGC for when you do come back later and use the full 300A. But you are currently building with 300A of wire on a 225A circuit, so it appears to be an upsizing.

All these reasons are why this is the dumbest rule in the code book. It should at least go back to stating that it is only required if the reason were purely for voltage drop. Let AHJs adjudicate whether the installer had another reason or not.

At least the 2020 NEC has the new exception. Now I just have to become a 'qualified person'.

 

[kwired]

All these reasons are why this is the dumbest rule in the code book. It should at least go back to stating that it is only required if the reason were purely for voltage drop. Let AHJs adjudicate whether the installer had another reason or not.

At least the 2020 NEC has the new exception. Now I just have to become a 'qualified person'.

IMO "increased in size from the minimum size that has sufficient ampacity for the intended installation" mostly only leaves you with voltage drop and just plain old "I want to run larger conductors" being what this applies to. Increases for number of conductors in raceway or ambient temp are a part of what determines minimum ampacity needed.

 

[jaggedben]

IMO "increased in size from the minimum size that has sufficient ampacity for the intended installation" mostly only leaves you with voltage drop and just plain old "I want to run larger conductors" being what this applies to. Increases for number of conductors in raceway or ambient temp are a part of what determines minimum ampacity needed.

You're responding to a post that was a response to a post that listed several other reasons.

 

[winnie]

Again, just out of intellectual curiosity, in more of a physics based argument than an NEC one, does a wire that can more easily dissipate heat not have a lower resistance than the same sized wire with a lower ability to dissipate heat?

If a particular wire had better ability to dissipate heat, it would run cooler and thus have slightly lower resistance. However higher temperature rating has nothing to do with heat dissipation capacity. It simply means that the insulation would survive the higher temperature.

Maybe that's where I'm going wrong with all of this. I'm thinking more "physics" than I am bowing to the almighty NEC, lol.

Only intended as friendly ribbing: you need to get the physics right before invoking it to trump the NEC

I'm thinking that an EGC of a higher insulation temperature rating would have a lower resistance and thus a lower voltage drop, which would make sense of my interpretation of the note that one could choose an EGC of a higher insulation temperature rating (as opposed to the proportionate increase) when upsizing the EGC for voltage drop purposes.

As others have noted, this is not correct. The resistance of a wire has little to do with the temperature rating of its insulation. However imagine it were true: you could solve voltage drop problems by using higher temperature rated wire.

Enjoy!

Jon

 

[kwired]

You're responding to a post that was a response to a post that listed several other reasons.

OK, second two of those I can agree you should increase EGC accordingly, first two I think the reasons are a part of determining minimum ampacity needed for that application and increase of EGC should not be necessary - JMO.

 

[kwired]

If a particular wire had better ability to dissipate heat, it would run cooler and thus have slightly lower resistance. However higher temperature rating has nothing to do with heat dissipation capacity. It simply means that the insulation would survive the higher temperature.



Only intended as friendly ribbing: you need to get the physics right before invoking it to trump the NEC



As others have noted, this is not correct. The resistance of a wire has little to do with the temperature rating of its insulation. However imagine it were true: you could solve voltage drop problems by using higher temperature rated wire.

Enjoy!

Jon

You solve VD problems by using a conductor that has higher conductivity characteristics - that usually means more CSA if still same material, or if a different material that conducts better is likely more expensive.

 

[LarryFine]

Think about the purpose of an EGC: to assure operation of short-circuit protection equipment, not over-current protection.

Insulation rating is not relevant because a short circuit is (normally) a short-duration event; temperature-rise is no concern.

On the other hand, instantaneous impedance matters greatly. Up-sizing assures the event duration is short (pun intended.)

 

[kwired]

Think about the purpose of an EGC: to assure operation of short-circuit protection equipment, not over-current protection.

Insulation rating is not relevant because a short circuit is (normally) a short-duration event; temperature-rise is no concern.

On the other hand, instantaneous impedance matters greatly. Up-sizing assures the event duration is short (pun intended.)