Parallel Grounds

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tkb

tkb

Senior Member
Location
MA
I have a job that we are designing that will consist of upgrading an existing 1000 amp feeder to a 1200 amp feeder.
The existing feeder is 3 sets of 4-500kcmil & 1-2/0g.
The 2/0 ground is too small for a parallel ground per table 250.122. The ground size needs to be 3/0.
The neutral is not being used on the existing feeder and is not needed for the new feeder.

We want to disconnect the existing 2/0 grounds and disconnect the unused 500kcmil neutral.
We can either use the conduit as a ground or the now unused 500kcmil as the ground.
We will be adding another set of 3-500kciml and a 3/0g.

My question is, does the additional ground need to be a 3/0 or does it need to be sized the same as the other grounds at 500kcmil?
NEC 2008 250.122(F) does not specify.
 
That is what I was looking for.

So we will have to have the added ground at 500kcmil.

Thanks Bob.
 
We want to disconnect the existing 2/0 grounds and disconnect the unused 500kcmil neutral.
We can either use the conduit as a ground or the now unused 500kcmil as the ground.
We will be adding another set of 3-500kciml and a 3/0g.
Click to expand...

iwire said:
All the parallel runs must have the same physical characteristics.

310.4
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(B) Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner
(C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
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I don't see where 310.4 requires the EGC to be the same in all conduits. In fact 310.4.C appears to say specifically the EGC need not be the same.

bob - If you see something here that I am missing, please let me know.

tbk - I don't see any regulatory issue about having the EGCs to be the same size. Nothing says you can't oversize an EGC.
 
Cold Fusion said:
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I don't see where 310.4 requires the EGC to be the same in all conduits. In fact 310.4.C appears to say specifically the EGC need not be the same.

bob - If you see something here that I am missing, please let me know.

tbk - I don't see any regulatory issue about having the EGCs to be the same size. Nothing says you can't oversize an EGC.
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(B)Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner
(C)Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.


I believe the way (C) reads is, that all of Phase A needs to have the same charactistics, Phase B, Phase C, the neutral and EGC all need to be the same also. Phase a and Phase B don't need to be the same.

For example Phase A is THHN and Phase B is XXHW thats OK as long as all of the parallels for the same phase is the same.
You cannot mix charactistics on the same phase.
 
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Cold F. it is right there in what you posted.

(B)Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:

(1) Be the same length

(2) Have the same conductor material

(3) Be the same size in circular mil area

(4) Have the same insulation type

(5) Be terminated in the same manner

(C)Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
Click to expand...
 
tkb said:
(B)Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner
(C)Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
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iwire, tkb -
Yes, I did read the section you both highlighted. I promise I actually did read the whole of (2008) 310.4, sections A through E. I even checked 2002, there were some differences. 2002 doesn't list the EGC for consideration.

However, as I said earlier, refer to (2008) 310.4.C. That section appears to apply directly to the described installation.

edit to respond to tbk edit: I suggest 310.4.C clearly says that for separate raceways, which you have, the ECG do not need to be the same in each conduit. This section is about balance. There is no science/physics to support the EGCs have any contribution to balance. The NEC code panel got that right.
 
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Cold Fusion said:
However, as I said earlier, refer to (2008) 310.4.C. That section appears to apply directly to the described installation.
Click to expand...


That section 'C' does not say what you think it says. :smile:

That section allows the conductors of phase A to be different from the conductors of phase B and different from the conductors used for the EGC.

It does not allow the conductors of any phase, grounded or grounding conductor to be different from the same phase, grounded or grounding conductor.
 
iwire said:
That section 'C' does not say what you think it says. :smile: ---
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Or it does. At least I've got the science on my side.

iwire said:
---That section allows the conductors of phase A to be different from the conductors of phase B and different from the conductors used for the EGC.---
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This statement is partially correct. I agree the A phase conductors can be different than the B phase conductors. There is nothing in 310.4.C discussing the difference or similarity of phase conductors to EGCs.

iwire said:
---It does not allow the conductors of any phase, grounded or grounding conductor to be different from the same phase, grounded or grounding conductor.
Click to expand...
And I think 310.4.C says something quite different. As I noted earlier, from 2008, 310.4.C:
Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance
Click to expand...
The "to achieve balance" part is important. There is nothing about the EGCs that contributes or detracts from "balance".

But, I'm getting repetative - enough.

cf
 
cf all I can say now is your simply mistaken.

All of TKB's EGCs are NEC required to be the same size.

He can certainly up size the EGC if he wants but if he does it to one he has to do it to all.

It is a fact and I am not going to agree to disagree on this one. :smile:
 
CF,
I have to agree with Bob here. The EGCs in each raceway must match those in each of the other raceways.
I don't see any physics to support this, but that is what the code rule says. In codes prior to the 2008 code 310.4 did not apply to EGCs and they were added to the 2008 code without any substatiation. It was proposal 6-7a and was generated by CMP6.
 
Guys -
310.4 is about phase conductor current balance. It never discusses EGC current balance under fault conditions.

As Don said, there is no physics to support EGC size parity has anything to do with phase conductor current balance.

So, we have a code section that:
1. Never discuses ECG current balance
2. No physics to support that EGC size parity has anything to do with phase conductor current balance
3. As Don said, the change was adopted with out substantiation - so we have no idea what the CMP had in mind.

With those three points in mind, I don't think it is reasonable to think that 310.4 mandates EGC size parity.

Now, after having carefully read all of your post, doing my best to understand, I think you are telling me, "Under the 2008 NEC, a majority of AHJs will require the ECGs, in raceways with parallel conductors, to be the same size." If so, I'll agree with that. Doesn't mean they are right. Doesn't mean they are safer. Just means they are the ones with the badge and the gun.

What would I do:
Pull in the bigger wire for the EGC to match. Most of the stuff I work on, the extra dollars wouldn't matter.

Having a conversation with the AHJ, explaining why it was okay to to save the few dollars really would not be worth it.

cf all I can say now is your simply mistaken. ---
---It is a fact and I am not going to agree to disagree on this one. :smile:
Click to expand...
I really don't have much interest is a conversation that deteriorates to, "You're wrong! I'm right!" There is nothing there for me to learn.

cf
 
Cold Fusion said:
Guys -
310.4 is about phase conductor current balance. It never discusses EGC current balance under fault conditions.
Click to expand...

Where does it discuss phase conductor current balance?

As Don said, there is no physics to support EGC size parity has anything to do with phase conductor current balance.

So, we have a code section that:
1. Never discuses ECG current balance
2. No physics to support that EGC size parity has anything to do with phase conductor current balance
3. As Don said, the change was adopted with out substantiation - so we have no idea what the CMP had in mind.

With those three points in mind, I don't think it is reasonable to think that 310.4 mandates EGC size parity
Click to expand...
.

I have never said there is any reason beyond the code requirement, that fact that you and I may see no reason for it (and there may not be) does not change the rule as it is written

Now, after having carefully read all of your post, doing my best to understand, I think you are telling me, "Under the 2008 NEC, a majority of AHJs will require the ECGs, in raceways with parallel conductors, to be the same size." If so, I'll agree with that. Doesn't mean they are right. Doesn't mean they are safer. Just means they are the ones with the badge and the gun.
Click to expand...

They would be correct, they would be enforcing the code section as written.


Cold Fusion said:
I really don't have much interest is a conversation that deteriorates to, "You're wrong! I'm right!" There is nothing there for me to learn.
Click to expand...

It appeared to me you where not interested in learning about the NEC. It appeared you had already made up your mind based on theory and science neither of which has anything to do with interpreting the words written on the page.

I provided the code section that requires the EGCs to be the same size. You choose not accept it, I did not want to bother repeating the same information.
 
tkb said:
I have a job that we are designing that will consist of upgrading an existing 1000 amp feeder to a 1200 amp feeder.
The existing feeder is 3 sets of 4-500kcmil & 1-2/0g.
The 2/0 ground is too small for a parallel ground per table 250.122. The ground size needs to be 3/0.
The neutral is not being used on the existing feeder and is not needed for the new feeder.

We want to disconnect the existing 2/0 grounds and disconnect the unused 500kcmil neutral.
We can either use the conduit as a ground or the now unused 500kcmil as the ground.
We will be adding another set of 3-500kciml and a 3/0g.

My question is, does the additional ground need to be a 3/0 or does it need to be sized the same as the other grounds at 500kcmil?
NEC 2008 250.122(F) does not specify.
Click to expand...

oops...misread the OP...never mind
 
iwire said:
Where does it discuss phase conductor current balance?---.
Click to expand...
Well, other than the general flavor of the section, one could try for understanding what the code panel had in mind with the phrase, "---to achieve balance", at the end of 310.4.C

iwire said:
---It appeared you had already made up your mind based on theory and science ---
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Thank you for the compliment. But you did not get it quite right. It would be, "based on peer reviewed testing and science".

I would have thought when I said:
Now, after having carefully read all of your post, doing my best to understand, ---
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That would be a clue that I ,ahhhhh....., did infact read all of the posts and do my best to understand your position.

But enough of this. You are right. One should pull in the larger EGC. For several reasons:
It will look funny if you don't
Most AHJ will erroneously think that's what the code requires.
There is no reason to not spend the extra $1.42 to walk around the first two.

bummer - got interupted with work. Got to stay with that. Can't even take time to proof - later

cf
 
Bob, do you have room on your soapbox where I can join you ? :smile:
NEC says what it says !
The -08 Code went so far as to add "equipment grounding conductor" to the wording to aid in stressing that point.
Time permittig, I will look at the ROP/ROC to see if there is help to substanuiate the fact.
 
Cold Fusion said:
It will look funny if you don't
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Best reason. :smile:

Most AHJ will erroneously think that's what the code requires.
Click to expand...

It will not be an erroneous thought, they will be doing what the NEC now requires.

You can choose to ignore that fact and hope you can convince the AHJ to ignore the NEC.

If you accept that 310.4 requires all the phase conductors to be the same size you must also accept that the EGCs must be the same size because the wording in 310.4 includes 'equipment grounding conductor'.
 
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