100-Amp Equipment Ground Conductor

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EDITED AFTER I READ IT AGAIN;

Are you saying that 110.14(C)(1)(a)(3) allows you to use the 75C column, if you can show that the equipment is rated for 75C terminations?
 
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charlie b said:
EDITED AFTER I READ IT AGAIN;

Are you saying that 110.14(C)(1)(a)(3) allows you to use the 75C column, if you can show that the equipment is rated for 75C terminations?
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That is correct and is the reason I assumed 75 deg.

Roger
 
:confused: This messes with a good spreadsheet. Ignorance really is bliss.

The inspector is saying that we have lowered the impedance of the feeder so we must lower the impedance of the fault return path.

Just considering the increase in size from 2 to 1 the ratio is 1.26.
(8awg) 16510*1.26=20802
Next size up is #6 @ 26240
Looks like he is correct.

Next time I guess I don't use up those short pieces left over from the last job.
 
charlie b said:

Not quite. The Inspector is starting by sizing the EGC based on the breaker size. But then the Inspector is suggesting that the use of a #1 phase conductor is more than is needed, based on a required ampacity of 100 amps. The Inspector is then using article 250.122(B) (the article, not the table) to say that because the phase conductor has been increased in size, the EGC must also be increased in size.
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Let me ask this question if the engineer, having done all the calaculations, called for a specific conductor size,EGC size etc for a feed to a sub panel and the EC decides on his own to increase the wire size, why wouldn't you increase all the wire sizes. Why would you go to all the trouble to increase all the conductors except the EGC. Later down the road the owner decides to change out this 100 amp panel to lets say 200 amp and the phase conductors were the correct size that they would not have to be changed then the EGC would be to small and you would have to repull the wires just to bring the EGC up to code, so what would you have accomplished.
 
ptonsparky said:
The inspector is saying that we have lowered the impedance of the feeder so we must lower the impedance of the fault return path.
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That is exactly my take on it.

This is about a bolted short at the sub-panel to ground, the phase and ground conductors have to hold the full short circuit current until some upstream protection device opens. The table sizes are based on an assumption of an impedance (and hence heating) "balance", and if you change the relative sizes of the phase and ground conductors you are changing the balance of the heating effects, which may be deleterious to the ground wiring or it's surrounds should the bolted short ever happen.
 
charlie b said:
EDITED AFTER I READ IT AGAIN;

Are you saying that 110.14(C)(1)(a)(3) allows you to use the 75C column, if you can show that the equipment is rated for 75C terminations?
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But are we required to use the 75 degree C temperature rating?
 
infinity said:
But are we required to use the 75 degree C temperature rating?
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Good question, Trevor. It brings me back to the basic question, "increased from WHAT."
 
charlie b said:

Good question, Trevor. It brings me back to the basic question, "increased from WHAT."
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Increased from the minimum size allowed by the NEC for the particular installation.

No we are not required to use the 75 rating but if the equipment and conductors are rated 75 than IMO the minimum size would be based on that.

Thats my story and I think it is a good one. :D

I don't think its a perfect section of code and in the past I have asked the same type of questions.
 
iwire said:
Increased from the minimum size allowed by the NEC for the particular installation.
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It's not a bad answer. But nothing in the NEC forces us to apply that answer.

Engineers have a tendency to think in terms of "a little extra just to be sure," as opposed to "minimum required for compliance." So I might specify a conductor size based on the possibility that the contractor (who has not been named yet because the design has not yet been sent out for bids) might find a good bargan on 60C terminations. That would lead me to pick a #1 THHN as the feeder to a 100 amp panel. In that case, the feeder will not have been "increased in size," since the design is based on 60C terminations.

So what would happen if the contractor used 75C terminations? Would that make it incumbent on the contractor to either, (1) Use a #2 for phase conductors, or (2) Increase the EGC to #8, or (3) Take the responsibility for the ensuing citation for a code violation?
 
charlie b said:
It's not a bad answer. But nothing in the NEC forces us to apply that answer.
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Something can force us to, the inspector.

I do agree that what I stated is not spelled out in the NEC but neither is any other method of determine what the NEC minimum conductor size.......Now wait, as I typed that I have a problem with it, the NEC does tell us how to find the minimum conductor size for any particular installation.

Step 1

Find the absolute NEC minimum size conductor. (Including the use of Table 310.15(B)(6)

Step 2

Any increase in size for whatever voluntary reason above that minimum size is in fact an increase in size above the minimum size.

I see no reason why an inspector could not enforce it that way.

Do I think they always should (or do)?

No.

But IMO the sections are in the NEC to support that conclusion.

Your specific question sounds to me a little like 'If it's inconvenient and it does not quite make sense we can ignore it.' :) Not that I should be throwing stones, I am sure I have a one or two code sections that I might feel that way about. :grin:
 
Lets not forget the note at the bottom of Table 250.122

Note:Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table.

250.(A)(5) is to me one of the most important sections in the entire code book and real world scenarios we come across.
The first word in the title to the table is MINIMUM, and that is what the sizes in the table are.
For an equipment ground conductor to be effective, it must open the circuit as quickly as possible. Less impedance in the fault circuit will help to create a situation where opening the circuit to protect people and equipment can actually occur without damage. In most instances a proper sized EGC helps tremendously in part to solving this situation. One thing to remember about a fault current path is not just the size of the conductors, but also the different types of terminations that may be part of that path. A larger conductor (of course to a point) will help in reducing the impedance of the path.
 
It must be presumed, and it is rational, that the EGC requirements of Table 250.122 must accommodate and be adequate for the condition of conductors selected on the basis of 60 C insulation.

Therefore, it is appropriate to permit the use of conductors based on 60 C ampacity as the basis for complying with 250.122(B), notwithstanding the actual ratings of the terminals.
 
Bob NH said:
It must be presumed, and it is rational, that the EGC requirements of Table 250.122 must accommodate and be adequate for the condition of conductors selected on the basis of 60 C insulation.

Therefore, it is appropriate to permit the use of conductors based on 60 C ampacity as the basis for complying with 250.122(B), notwithstanding the actual ratings of the terminals.
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Bob
How did you come to this conclusion?
 
Since 60 C conductors are not prohibited, and ampacities for various sizes at 60 C are provided in Table 310.16, it must be presumed that the wisdon of the panel members who created 250.122(B) would ensure that the EGC would be adequate if the 60 C rated conductors were actually used.

Therefore, it is reasonable to use the 60 C conductor sizes as the base from which "increased in size" is calculated.
 
If you take a look back at the '71 - '78 NEC, you will see that a change was made in the EGC table to the current or at least very close to the current table we use today. I would think that is where we may find some info to help us interpret what is really what the CMPs were thinking. Also, Soares book has some info I will study to see what I can see. I am curious as to when the note at the bottom of the table was installed.
 
Bob NH said:
It must be presumed, and it is rational, that the EGC requirements of Table 250.122 must accommodate and be adequate for the condition of conductors selected on the basis of 60 C insulation.
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I can't agree with your presuming. ;)



Bob NH said:
Since 60 C conductors are not prohibited, and ampacities for various sizes at 60 C are provided in Table 310.16, it must be presumed that the wisdon of the panel members who created 250.122(B) would ensure that the EGC would be adequate if the 60 C rated conductors were actually used.

Therefore, it is reasonable to use the 60 C conductor sizes as the base from which "increased in size" is calculated.
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I disagree, IMO we are looking for the minimum size permitted, if the equipment can handle 75 we would have to figure the minimum size based on that.
 
If the equipment connections are labeled 60/75 where is it written that we must use the higher rating?
 
infinity said:
If the equipment connections are labeled 60/75 where is it written that we must use the higher rating?
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Not that you 'must' per se, but that you can; since the higher temperature rating afffords a smaller conductor to start with, that defines the 'minimum' starting point.

That's my take on it, anyway.
 
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