Can someone explain the logic behind the code?

[zappy]

If you have a 40amp breaker protecting 8awg, then you need a 10awg egc.
If you have a 40amp breaker protecting 6awg, then you need a 8awg egc

Why do you need to up size the egc? It's still a 40amp breaker.:-? Thank you for your help.

 

[cpal]

not sure where your going with this a article reference might help?

have you consulted 250.122 T??

 

[Dennis Alwon]

Charlie, I think zappy wants to know why 250.122(B) is there if the breaker size is the same. You can have a 20 amp breaker with #2 wire for VD and you would need #2 as an EGC. He wants to know why if a #12 works fine under normal conditions for 20 amp cir.

 

[mcclary's electrical]

Charlie, I think zappy wants to know why 250.122(B) is there if the breaker size is the same. You can have a 20 amp breaker with #2 wire for VD and you would need #2 as an EGC. He wants to know why if a #12 works fine under normal conditions for 20 amp cir.

If you increase the size of the ungrounded wire, you increase the available fault current, even if attached to the same breaker

 

[hardworkingstiff]

Can you explain your question a little more, maybe state it a little differently?

 

[cpal]

Charlie, I think zappy wants to know why 250.122(B) is there if the breaker size is the same. You can have a 20 amp breaker with #2 wire for VD and you would need #2 as an EGC. He wants to know why if a #12 works fine under normal conditions for 20 amp cir.

now that I have digested a 3ed or 4th cup of coffee I can see the issue with your help.

In addition to mcclary's explanation, if the circuit conductors are increase as a design issue (usually for voltage drop) it is resonable to require a propotional increase in the size of the egc. After all if it is called upon to provide a path to the point where it attaches to the grounded circuit conductor it can not tolerate a voltage drop that might impede the operation of the ocpd. The HB also has a helpfull explanation.

 

[Dennis Alwon]

If you increase the size of the ungrounded wire, you increase the available fault current, even if attached to the same breaker

I was just explaining what Zappy wanted to know-- I didn't have time to answer his question.

I also see it as the impedence on the circuit would be increased if the length was significantly long so the EGC should also be increased.

Now if for some reason you used a piece of #4 wire to a 30 amp breaker and it was just a few feet away, the I would think a #10 egc would work fine. I must stress that the code does not give permission here to use #10 as the EGC.

 

[charlie b]

This article never made sense to me, and I think it should be dropped. I don?t believe that the physics of the situation supports the requirement.

Consider the following example (NOTE: This is a discussion of physics, not code, so please don?t cite 250.122 back at me):
? 20 amp breaker, #12 wire, ?fairly long run,? total voltage drop at 3.2%.
? Nothing requires me to upsize the conductors (i.e., VD is not a code requirement).
? I infer that there is a low enough impedance in the #12 conductors, including the #12 EGC, to result in a high enough fault current to trip the breaker, should there be a fault at the end of the run.
? Let me emphasize: This is a safe installation, because I have confidence that the OCPD would trip, on a fault. The fault current is high enough already!
? Now I choose to replace the phase conductors with #10. I have not yet biggie-sized the EGC.
? That just caused the overall circuit impedance, in the event of a fault, to be lower than it was before.
? Therefore, the fault current will be higher than it was before.
? Therefore, the OCPD will trip even faster.
? Now, tell me, from a physics point of view, why I need to increase the fault current even more, by using a #10 EGC?

 

[mivey]

That just caused the overall circuit impedance, in the event of a fault, to be lower than it was before...
Now, tell me, from a physics point of view, why I need to increase the fault current even more, by using a #10 EGC?

A guess: For some cases, is there an assumption of a longer circuit given that you are using the same breaker size?

 

[mcclary's electrical]

This article never made sense to me, and I think it should be dropped. I don?t believe that the physics of the situation supports the requirement.

Consider the following example (NOTE: This is a discussion of physics, not code, so please don?t cite 250.122 back at me):
? 20 amp breaker, #12 wire, ?fairly long run,? total voltage drop at 3.2%.
? Nothing requires me to upsize the conductors (i.e., VD is not a code requirement).
? I infer that there is a low enough impedance in the #12 conductors, including the #12 EGC, to result in a high enough fault current to trip the breaker, should there be a fault at the end of the run.
? Let me emphasize: This is a safe installation, because I have confidence that the OCPD would trip, on a fault. The fault current is high enough already!
? Now I choose to replace the phase conductors with #10. I have not yet biggie-sized the EGC.
? That just caused the overall circuit impedance, in the event of a fault, to be lower than it was before.
? Therefore, the fault current will be higher than it was before.
? Therefore, the OCPD will trip even faster.
? Now, tell me, from a physics point of view, why I need to increase the fault current even more, by using a #10 EGC?

If you use examples from short runs,,,3.2%vd,,,,,,then you're right, it makes no sense. But stretch your wire out 1000 ft, attach to 15 amp breaker, and leave the original 14 guage EGC in place, you very well could have a fault that is not capable of clearing the OCPD witout an upsized ground.

 

[dcspector]

This article never made sense to me, and I think it should be dropped. I don?t believe that the physics of the situation supports the requirement.

Consider the following example (NOTE: This is a discussion of physics, not code, so please don?t cite 250.122 back at me):
? 20 amp breaker, #12 wire, ?fairly long run,? total voltage drop at 3.2%.
? Nothing requires me to upsize the conductors (i.e., VD is not a code requirement).
? I infer that there is a low enough impedance in the #12 conductors, including the #12 EGC, to result in a high enough fault current to trip the breaker, should there be a fault at the end of the run.
? Let me emphasize: This is a safe installation, because I have confidence that the OCPD would trip, on a fault. The fault current is high enough already!
? Now I choose to replace the phase conductors with #10. I have not yet biggie-sized the EGC.
? That just caused the overall circuit impedance, in the event of a fault, to be lower than it was before.
? Therefore, the fault current will be higher than it was before.
? Therefore, the OCPD will trip even faster.
? Now, tell me, from a physics point of view, why I need to increase the fault current even more, by using a #10 EGC?

Yes I will mention 250.122 (B) Your opinion, thoughts and theory are great. But what is written shall be enforced regardless of opinions. Submit a proposal to change that section. The problem is (Charlies Rule) you put out information like this and the electrical world on this forum installs per your post. I simply enforce what is written.

 

[ActionDave]

If you use examples from short runs,,,3.2%vd,,,,,,then you're right, it makes no sense. But stretch your wire out 1000 ft, attach to 15 amp breaker, and leave the original 14 guage EGC in place, you very well could have a fault that is not capable of clearing the OCPD witout an upsized ground.

This is exactly what makes the rule so silly though. Move up the wire size to 8 AWG with a 10 EGC and you can use a 40 amp breaker but you can't use a 15 amp for the same 1000' run.

 

[infinity]

Yes I will mention 250.122 (B) Your opinion, thoughts and theory are great. But what is written shall be enforced regardless of opinions. Submit a proposal to change that section. The problem is (Charlies Rule) you put out information like this and the electrical world on this forum installs per your post. I simply enforce what is written.

For now that's all we can do. I agree that this sections needs work because it many instances it makes no sense. Writing a singular code section to cover every scenario is where the problem is.

 

[Jerry the Phone Guy]

Now, tell me, from a physics point of view, why I need to increase the fault current even more, by using a #10 EGC?

Charlie, what you're saying is correct only if you consider hot-to-neutral faults. However, hot-to-ground faults are also required to be cleared promptly. And I believe that is why the EGC must be at least as big as the hot and neutral conductors.

 

[charlie b]

The problem is (Charlies Rule) you put out information like this and the electrical world on this forum installs per your post.

I give the members of this forum credit for more intelligence than that.

I simply enforce what is written.

And I simply design per what is written. I was explaining my reason for disliking this code article, and I was asking for a physics-related response.

I have submitted proposals to get it changed, and I believe one of my proposals is going to be implemented, at least in principle, in 2011. That won't get us where I think we belong (i.e., deletion of the requirement), but I have hopes that it will make the requirement more clear.

 

[charlie b]

Charlie, what you're saying is correct only if you consider hot-to-neutral faults. However, hot-to-ground faults are also required to be cleared promptly.

What I described was a hot-to-ground fault. If you have a hot-to-neutral fault, then the EGC is not a part of the fault current path.

And I believe that is why the EGC must be at least as big as the hot and neutral conductors.

You are welcome to your beliefs. But please allow me to point out that you did not contradict, or otherwise comment upon, my description of the physics of the situation.

 

[dcspector]

Charlie I agree with you Iam having a serious issue in DC with this. I really would like alot of articles rewritten. Enjoy the holiday all.

 

[dcspector]

For now that's all we can do. I agree that this sections needs work because it many instances it makes no sense. Writing a singular code section to cover every scenario is where the problem is.

Agreed Rob

 

[don_resqcapt19]

I think the easiest fix is to make T250.122 work like T250.66. The EGC should be matched up with the ungrounded conductor size and not the OCPD. That would cover the intent of 250.122(B) as when you increase the size of the ungrounded conductors, you would also increase the size of the EGC, but eliminate the issue of where the EGC for a circuit with an OCPD of 30 amps or less would have to be larger than a circuit with an OCPD of greater than 30 amps.

 

[kwired]

If you use examples from short runs,,,3.2%vd,,,,,,then you're right, it makes no sense. But stretch your wire out 1000 ft, attach to 15 amp breaker, and leave the original 14 guage EGC in place, you very well could have a fault that is not capable of clearing the OCPD witout an upsized ground.

Not capable or just would take longer to open? You certainly would still have well above 15 amps of current flowing in your scenario, if not you have a heater and not a conductor.

There are other times where a higher resistance fault occurs and you have the same issue here - overcurrent device will take a little longer to resopond but you still have a high level of current so it will eventually open. Motor with a ground fault someplace in middle of a winding would be a good example.