Increased EGC per 250.122(B)

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beanland

Senior Member
Location
Vancouver, WA
I just want a reality check on the absurdity when increasing the size of EGC per 250.122(B).

If I use 250kcmil copper phase conductors, fed by a 200A OCPD, I can use a #6 copper EGC. 250.122(A)

If I use a 30A OCPD, because #10 copper is rated for 30A and the EGC for 30A is #10 copper, 250.122(B) requires I use a 250kcmil copper EGC with the 250kcmil phase conductor.

Correct?
 

iwire

Moderator
Staff member
Location
Massachusetts
I just want a reality check on the absurdity when increasing the size of EGC per 250.122(B).

If I use 250kcmil copper phase conductors, fed by a 200A OCPD, I can use a #6 copper EGC. 250.122(A)

Actually in the first example you might need 4 AWG as 3/0 copper would normally be large enough for 200 amps so the 250s are copper are oversized.

If I use a 30A OCPD, because #10 copper is rated for 30A and the EGC for 30A is #10 copper, 250.122(B) requires I use a 250kcmil copper EGC with the 250kcmil phase conductor.

Correct?

Yes.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
If I use a 30A OCPD, because #10 copper is rated for 30A and the EGC for 30A is #10 copper, 250.122(B) requires I use a 250kcmil copper EGC with the 250kcmil phase conductor.

Correct?

Or, if you are using EMT for example, you could be reasonable and just omit the ground wire altogether :)

So its 250KCM or 0. There's nothing absurd about that is there? :)
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I agree with Bob, in scenario #1 your EGC would need to be increased in size for the 250's. The problem with this section is that it's almost impossible to write it with having some absurdities.
 

beanland

Senior Member
Location
Vancouver, WA
EMT as EGC

EMT as EGC

Does 250.122(B) require me to increase the area of the EGC when the EGC is EMT?

If I use 1" EMT and install #12 copper on a 20A OCPD but then decide to increase the size of the conductors to #3 copper, still using the 20A OCPD, how do I increase the area of the EGC to comply?
 

iwire

Moderator
Staff member
Location
Massachusetts
Does 250.122(B) require me to increase the area of the EGC when the EGC is EMT?

No.

If I use 1" EMT and install #12 copper on a 20A OCPD but then decide to increase the size of the conductors to #3 copper, still using the 20A OCPD, how do I increase the area of the EGC to comply?

Luckily you are not required to.
I suspect this is because conduit EGCs are already oversized for the circuits that they can contain.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Another player in this conversation is the insulation rating of the conductors. In the original question, if the wire used was 250MCM type TW, then you would not need to upsize the EGC to #4.
 

iwire

Moderator
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Location
Massachusetts
Another player in this conversation is the insulation rating of the conductors. In the original question, if the wire used was 250MCM type TW, then you would not need to upsize the EGC to #4.

I agree.

And that is always the tricky part, what is the 'normal' wire size for a particular OCPD?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Does 250.122(B) require me to increase the area of the EGC when the EGC is EMT?

If I use 1" EMT and install #12 copper on a 20A OCPD but then decide to increase the size of the conductors to #3 copper, still using the 20A OCPD, how do I increase the area of the EGC to comply?

If you're installing a conductor type EGC within the EMT then yes, you would need to use a #3 AWG EGC.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
The worst are multi-conductor cables oversized for voltage drop. It's almost a guarantee that you're special ordering cable to get the upsized ground wire you need.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The worst are multi-conductor cables oversized for voltage drop. It's almost a guarantee that you're special ordering cable to get the upsized ground wire you need.

The really odd thing here is that we could cheat (this is all hypethetical, of course).

We could run our 250KCM cable from a 250 A breaker. Now the cables aren't up-sized, and a #4 ground is good. That should be available as a standard cable.

Then, at the load end, we add a single 30A breaker to feed our load. Since we are close to the load, now we can probably run #10 wire from the breaker to the load.

When you think about it, the really odd thing here is that reducing the breaker size means we have to increase the ground wire size. And increasing the breaker size lets us reduce the ground size, even though the ground will have to carry more current to trip the circuit breaker.

Steve
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
That is not cheating at all, Steve. You just turned your 250 MCM run into a feeder, and installed a 30 amp branch circuit at the other end. But I agree that the whole business is odd.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
This problem would go away if Table 250.122 was changed to be like Table 250.66. With this change the size of the EGC would be based on the size of the circuit conductors and not on the size of the OCPD. This would increase the size of the EGC as the size of the circuit conductors are increased even when the OCPD stayed the same. It would eliminate the calculations and other issues caused by the existing method.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
That is not cheating at all, Steve. You just turned your 250 MCM run into a feeder, and installed a 30 amp branch circuit at the other end. But I agree that the whole business is odd.

It sure feels like cheating. Increasing the breaker size by a factor of 8 has allowed us to reduce the EGC size by a factor of 6. Common sense would tell us we need to increase the EGC size when we increase the breaker rating.

And I think we've managed to bypass the intent of the code rule.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
This problem would go away if Table 250.122 was changed to be like Table 250.66. With this change the size of the EGC would be based on the size of the circuit conductors and not on the size of the OCPD. This would increase the size of the EGC as the size of the circuit conductors are increased even when the OCPD stayed the same. It would eliminate the calculations and other issues caused by the existing method.

Very good point. I think some of the "oddness" of this example is that we are starting at 30 amps, where the EGC size is the same as the phase conductor size. We are not getting credit for the fact that #10 is good for the EGC up to 60 amps.
 
I remember asking about this 250.122 before, as I recall the upsizing of the egc is because of the impendence that would increase in a long run. You want low inpedence to be sure you can open the ocpd. So the 'original' though behind this section is good, even though over the years it seems to have gotten convoluted. :)
 

Finite10

Senior Member
Location
Great NW
I just want a reality check on the absurdity when increasing the size of EGC per 250.122(B).

If I use 250kcmil copper phase conductors, fed by a 200A OCPD, I can use a #6 copper EGC. 250.122(A)

If I use a 30A OCPD, because #10 copper is rated for 30A and the EGC for 30A is #10 copper, 250.122(B) requires I use a 250kcmil copper EGC with the 250kcmil phase conductor.

Correct?
A couple thoughts;
1) How are you terminating 250 MCM to 30A OCPD lugs?
2) Seems like a proposal just waiting to be made.
 

Finite10

Senior Member
Location
Great NW
I remember asking about this 250.122 before, as I recall the upsizing of the egc is because of the impendence that would increase in a long run. You want low inpedence to be sure you can open the ocpd. So the 'original' though behind this section is good, even though over the years it seems to have gotten convoluted. :)

Seems like the biggest element of impedence would be Resistance -per Ch.9 Table 8:
#4 is .308 ohms/kFT
250 MCM is .0515 ohms/kFT

The #4 is 6 times the resistance and tripping of the OCPD would be delayed. Is this the intent here or am I looking at it wrong?
 
Seems like the biggest element of impedence would be Resistance -per Ch.9 Table 8:
#4 is .308 ohms/kFT
250 MCM is .0515 ohms/kFT

The #4 is 6 times the resistance and tripping of the OCPD would be delayed. Is this the intent here or am I looking at it wrong?
In short answer, Yes. I believe that is the intent.
remember that the formula for voltage drop is VD=2kIL/CM single phase.
and 1.73KIL/CM three phase.
And looking at Ch.9 table 8 you would see the resistance go down as the wire size increase.
Apparently, (Im going to speculate here), the Code panel figured since one is 'upsizing' the conductors for vd that the distance had to be long enough to also upsize the ground to be sure the ocpd would open.

This upsizing of the egc in 'theory' is also for the upsizing for ampacity. If you have more than 9 ccc, lets say 10 #12s in a 3/4" emt you would either have to lower the breaker to a 15 amp or upsize the wire to 10. Even though the egc is not generally a ccc, it would carry the maximum current under a fault condition. So its believed that since the ccc had to be upsized to handle the current safely , so in porportionally the egc has to be up sized to handle it safely under fault/sc conditions.

:)
 
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