100-Amp Equipment Ground Conductor

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Bruce Turner

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We have been having a discussion with an electrical inspector that has said that the proper equipment grounding conductor for a 100-ampere panelboard feeder with #1 AWG conductors is #6 not #8 per NEC Table T250.122. His reasoning is that the NEC allows the use of #2 AWG for the conductors so we have in effect increased the conductor size, therefore we need to increase the ground size. We do not believe his reasoning to be accurate and would appriciate the comments of others.
 
The NEC also allows a #3 to serve a 100 amp panel. I did the math, and using the Inspector's reasoning you would need a #4, not a #6. :confused:

Actually, at first glance I cannot find fault with the Inspector's reasoning. That said, my company has a standard schedule for sizing feeders and branch circuits. I have a copy on display on my cubicle wall. For a 100 amp feeder, it shows #1 phase conductors and #8 EGC. That is consistent with your point of view.

I'll have to think this one through a bit.
 
roger said:
See 250.122(B)
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I believe Bruce was aware of that section, given that he mentioned increasing the conductors in size. The problem is that the concept of "increased from what" is not clear. If I choose to supply the 100 amp panel with a #1, does that or does that not constitute "increasing the conductor size"?
 
That doesn't make any sense> If I use a 150 amp breaker w/ 1/0 copper I would only need #8. So why would you need a #6 with #1 and a 100 amp breaker
 
charlie b said:
You can't feed a 100 amp rated panel with a 150 amp upstream breaker.
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Ok suppose the panel was rated 200 and you ran a 150 amp feeder to it. What size EGC would you need? Then if you fed it with a 100 amp feeder upsizing the conductor one size then you have to use a larger egc than the 150 amp feeder. What am I missing?
 
Charlie, what I read (interpreted) Bruce as saying was that he was increasing the required #3 conductors by 2 sizes, (using a 75 deg conductor) so the Table would only be a reference starting point for the minimum size EGC and increasing the EGC proportionatley would be covered in 250.122(B).

Roger
 
Roger, that seems to be the point of view of the Inspector, and Bruce appears to disagree with that point of view. I infer that you are agreeing with the Inspector.

For my part, I have not yet decided how I feel about this question. If 250.122(B) had clearly said, ". . . increased in size above the minimum size conductor that has the required ampacity," then I think we would have no discussion here. But the code does not explain what "increased in size" means, in terms of "increased from what."

OK, I have a 100 amp panel. It is to be fed by a 100 amp breaker. Looking at 310.16, I see that I can use a #3 THHN. But I choose to use #1 instead. Does that count as "increasing in size"? It may seem so, but I am not yet certain.
 
To me, I'd say the starting point for determining upsizing would be the 60C column of table 310.15. How does the circuit breaker know how many H's there are in your conductor. You'd have to use #1 cu for a full 100A feeder if it was 60C (could get by with #2 if the load was really 95A). See if you can persuade him to use the 60C column as the baseline for upsizing. If not, he's the judge of nebulous code language.
 
One of my cronies tells me that when our standard table of feeders was developed, our specs required the use of the 60C column for sizing conductors, because 75C rated terminations were not commonly available. I am not sure if that is true today, but we do deal with older buildings, and must be ready to work with whatever they have installed.
 
Let's assume if you are using thhn then the 75C column comes into play. With that a #3 is possible but instead a #1 is used.

#3= 52620 cir mil
#1= 83690 cir mil

83690/52620= 1.6

#8= 16510
16510 * 1.6= 26416
26416 is greater than the #6 (26240) thus you would need a #4 wire not a #6


If you use the 60C column # 2 is possible but you use #1
#2= 66360
#1= 83690

83690/66360= 1.3

#8= 16510

16510*1.3=21463
#6 = 26240 thus you can use #6 as the egc

Take your pick--- which one does one use
 
Dennis Alwon said:
26416 is greater than the #6 (26240) thus you would need a #4 wire not a #6
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I have to agree, since that is the same calculation I alluded to earlier.

Dennis Alwon said:
If you use the 60C column # 2 is possible but you use #1
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Not unless you have a load calculation that shows the total load to be 95 amps or lower. That is why our standard table shows a #1 phase conductor and a #8 EGC, for a 100 amp feeder.
 
charlie b said:

Not unless you have a load calculation that shows the total load to be 95 amps or lower. That is why our standard table shows a #1 phase conductor and a #8 EGC, for a 100 amp feeder.
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Yes but it is a possible scenario.

Scenario #2
Same panel 150? from feeder panel.
If I run a #1 wire 100 feet and switch over to #2 for the last 50 feet, then what size EGC do I use? Do I base it on the larger conductor or the smaller conductor. Or do I upsize the EGC for the first 100 feet and drop down the last 50 feet?
 
Corerect me if I am wrong but It sounds like the inspector is sizing the EGC based on the conductor size and not the Breakers size.
 
Jomaul said:
Correct me if I am wrong, but it sounds like the inspector is sizing the EGC based on the conductor size and not the Breakers size.
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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.
 
PROBLEM SOLVED!

I have just been reminded of 110.14(C)(1)(a)(2). The circuit is rated at 100 amps or less, and the wire size is within the range of 14 AWG through 1 AWG. Therefore, the conductor?s ampacity must be based on the 60C table. Since we have not been given a load calculation that proves the load to be 95 amps or lower (i.e., the ampacity of a #2 at 60C), we must assume that the load is 100 amps. Therefore, we need to use a #1 as the minimum conductor size. As a result, the use of a #1 does not constitute ?increasing in size,? and we do not have to increase the EGC beyond the #8 given in Table 250.122.

QED.
 
I've commented on this in the past:
If you look at the _definition_ of AWG, rather than the tabulated values for cross sectional area, then the same _numeric_ change in AWG value will give the same _proportional_ change in cross sectional area.

If you calculate your areas using the definition of AWG, then the cross section of a #6 as compared to a #8 has exactly the same proportion as a #1 to a #3.

See the AWG formula here:
http://www.unc.edu/~rowlett/units/dictG.html#gauge

And here is a table that I generated using the formula, compared to values from Table 8:
Code: 
Gauge  Diameter  Area(form) Area(Tab 8) 
    0  0.324861  105534.5   105600 
    1  0.289297   83692.66   83690 
    2  0.257626   66371.3    66360 
    3  0.229423   52634.83   52620 
    4  0.204307   41741.32   41740 
    5  0.181941   33102.37 
    6  0.162023   26251.37   26240 
    7  0.144285   20818.29 
    8  0.128490   16509.65   16510 
    9  0.114424   13092.75 
   10  0.101897   10383.02   10380 
   11  0.090742    8234.11 
   12  0.080808    6529.95    6530 
   13  0.071962    5178.48 
   14  0.064084    4106.72    4110 
   15  0.057068    3256.78 
   16  0.050821    2582.74    2580 
   17  0.045257    2048.21 
   18  0.040303    1624.3     1620 
   19  0.035891    1288.13 

Ratio #1 to #3    1.590062  1.590460 
Ratio #6 to #8    1.590062  1.589340

By the formula the ratios match; by the Table 8 values the #6 EGC is too small. IMHO the #6 EGC is the correct answer if you consider the allowable tolerances for wire diameter in actual production.

-Jon
 
charlie b said:
Looking at 310.16, I see that I can use a #3 THHN. But I choose to use #1 instead. Does that count as "increasing in size"?
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Charlie you have me confused.

My answer to that question would be "Of course it is increased in size from the NEC minimum."
 
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