Sizing the GEC and Ground Wire For Sub Panels

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  #1  
Old 03-12-06, 09:28 AM
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Sizing the GEC and Ground Wire For Sub Panels

I知 trying to size my GEC and my ground wire to my sub panels. I知 somewhat confused with my interpretation of the two different ground tables in the NEC. This is what I致e interpreted it as, am I correct? Or am I looking at this all wrong. By the way I知 using NEC 1999 edition (My County has not gone to a different edition yet).
  1. I have an outdoor load center (200A) with feed thru lugs, its being used as the main panel.
  2. I have an indoor sub panel (200A) fed thru the feed thru lugs on the outdoor load center the disconnect for it is the main breaker on the outdoor load center.
  3. I have an indoor sub panel (100A) in the garage, being fed off a 100A DP breaker in the outdoor load center.
Using table 250-66 GEC for Alternating Current System, I致e come up with that the conductor from my outdoor load center (200A) to the two ground rods would have to be #4 copper or #2 aluminum because my largest service conductor is 2/0 copper? Also is this table only used to calculate the GEC going to the ground rod, this would not calculate the ground wire gauge going to my sub panels?

Using table 250-122 Minimum Size Equipment Ground Conductors for Grounding Raceway and Equipment. This table would be used to calculate the gauge I would need for both sub panels? If this were true then my 200A sub panel would need a #6 copper or #4 aluminum ground wire, and my 100A sub panel would need a #8 copper or #6 aluminum ground wire?

Thanks in advance, these two tables somewhat confused me even after reading the sub sections after and before them.

Roy
 
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Old 03-12-06, 10:54 AM
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I’m trying to size my GEC and my ground wire to my sub panels. I’m somewhat confused with my interpretation of the two different ground tables in the NEC. This is what I’ve interpreted it as, am I correct? Or am I looking at this all wrong. By the way I’m using NEC 1999 edition (My County has not gone to a different edition yet).

I have an outdoor load center (200A) with feed thru lugs, its being used as the main panel.
I have an indoor sub panel (200A) fed thru the feed thru lugs on the outdoor load center the disconnect for it is the main breaker on the outdoor load center.
I have an indoor sub panel (100A) in the garage, being fed off a 100A DP breaker in the outdoor load center.

Using table 250-66 GEC for Alternating Current System, I’ve come up with that the conductor from my outdoor load center (200A) to the two ground rods would have to be #4 copper or #2 aluminum because my largest service conductor is 2/0 copper? Also is this table only used to calculate the GEC going to the ground rod, this would not calculate the ground wire gauge going to my sub panels?

Using table 250-122 Minimum Size Equipment Ground Conductors for Grounding Raceway and Equipment. This table would be used to calculate the gauge I would need for both sub panels? If this were true then my 200A sub panel would need a #6 copper or #4 aluminum ground wire, and my 100A sub panel would need a #8 copper or #6 aluminum ground wire?

Thanks in advance, these two tables somewhat confused me even after reading the sub sections after and before them.


Roy, Table 250-66 is for sizing your grounding electrode conductor which is specifically for service entrance. Table 250-122 is for sizing equipment grounding conductors. As you will notice 250-66 is based on largest service entrance conductor or equivalent area for parallel conductors. 250-122 is based on the size of the overcurrent device protecting the circuit. In your case the 200 amp feeder would require a #6 copper grounding conductor and the 100 amp would require a #8 copper grounding conductor. Both of the sub-panels will require a seperate ground bar that is attached directly to the panel. All of the equipment grounding conductors coming in your branch circuit cables will go to this. The nuetral bar in each feeder panel is to be isolated. Do not connect the bonding jumper that comes with these panels. Discard it.
From your 200 amp feeder panel to the main you will install a neutral and equipment ground wire and they will connect to the neutral bar in the main. Now you will install the bonding jumper that came with your panel.
As per table 250-66 you will need to install a #4 copper conductor to your grounding electrode system. I do not know what you mean by two ground rods. I do not have a copy of the 99 NEC here at my house it is at work. I know in the 2003 there were some changes made to the grounding electrode system section. The latest copy I have here at my house is a 1981 copy and in 250-81 for grounding electrode system there are four different means of obtaining a system and if they are "available" they are to be used. Made and other electrodes are only to be used where none of the four specifed are not available. It is very common in this area to not see any of the four used in residential wiring and only a single rod is normally used.
Article 250 is very complex and it takes a lot of reading and discussion to understand it. I could go on and on about it as it applies to your installation.
I hope this helps you out.
 
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Old 03-12-06, 11:48 AM
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So what I had planned on doing seems right. In my county I have to provide two ground rods unless I can show the ohm reading of the soil. Its easier to sink two ground rods rather than find someone with a meter that can do this for less than two rods cost.
 
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Old 03-12-06, 05:08 PM
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Originally Posted by gabe24
I’m trying to size my GEC and my ground wire to my sub panels.
Why do you have GECs to your subpanels?


> to the two ground rods would have to be #4 copper

NEC 250.66(A).


> this would not calculate the ground wire gauge going to my sub panels?

Correct. Such is an EGC not a GEC.



> table 250-122 ... would be used to calculate the gauge I would need for both sub panels?

Yes.

> If this be true then my 200A sub panel needs a #6 copper or #4 aluminum ground wire,
> and my 100A sub panel would need a #8 copper or #6 aluminum ground wire?

Yes.
 
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