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I am replacing an undersized 35 amp main lug, so the panel, conductors and breaker in the main panel are all being replaced.

The new panel will be 125 amp rated, the new conductors will be #3 THHN, with an ampacity rating of 115, which will be protected by a 100 amp 2 pole breaker in the main panel.

What size does the grounding conductor leading back to the main panel need to be?

This calculator seems to indicate I need a #8 copper conductor, but I'm not sure this is the correct table for me to use.

This project must be done to code for Arapahoe county Colorado (2017 NEC)

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  • Try to avoid using copper for large gauge hot+neutral, it's rather expensive and is a dissimilar metal from the lugs in the panel, which are aluminum. You may have heard the horror stories in house wiring where aluminum and copper were carelessly mixed. – Harper Nov 10 '17 at 1:18
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It actually depends on whether the ground is going to be installed ahead of the main breaker or if it is installed after the main. Most of the contractors I know will install a #6 simply because it is less expensive to stock one size than two and it covers everything up to a 200A main. So I would go with a #6. 2014 NEC reference Tables 250.102(C)(1) and Table 250.122.

Use extreme caution when working around panelboards. I recommend DIYers make sure the panel is disconnected from all power sources before performing any work.

Hope this helps.

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  • For residential Rod pipe or plate electrodes for grounding #6 copper. Concrete encased electrode or uffer ground #4 is required. – Ed Beal Nov 8 '17 at 18:48
  • @EdBeal -- the requirement for #4 in a concrete encase electrode is only if the wire itself is being encased in concrete and used as an electrode -- the GEC connecting to the electrode can still be #6 (and mostly is, as the actual electrode conductors are typically rebars) – ThreePhaseEel Nov 8 '17 at 23:25
  • My county requires 4 because the picture in the NEC handbook shows 4 I have argued this one on several occasions. Look at the exhibit 250.30 and you will see the attachment from building steel is listed as not larger than 4. Makes no sense to me but that same photo has been in the book for several cycles. And in the 2014 is labeled that way (just after 250.66) the labels are gone in 2017 but the wording is connected to a concrete encased electrode not required to be larger than #4 250.66.b not that the wire is encased, I know I lost this one both times. – Ed Beal Nov 9 '17 at 0:28

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