Sizing GEC

Z

zander

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W238N1087 Sherwood Dr. Waukesha, WI 53188
Hello Everyone,

I'm installing a 320 amp service, 2x 200 amp breakers in the pedestal, 2x separate 4/0, 4/0, 4/0, 2/0 SER cables run to 2x separate 200 amp panels in the basement, 2x ground rods tied together with #4 bare (I know I could have gotten away with #6) run back to both panels.

Here is the question.
Table 250.66 says that the GEC is based on the size of the largest ungrounded conductor or equivalent area for PARALLEL CONUCTORS. Taking that word for word, I came to the conclusion that a #4 bare copper wire would meet those requirements. The inspector, however, is saying that the size of the GEC should be based on the 320 amp service rating. I don't agree with this because the 2x 200 amp feeds going to the panels are by definition individual, and not parallel. What is the general consensus on this? Thank you in advance.
 
GEC is not sized based on feeders (your 4-wire SE-R to the basement are feeders Article 215) it would be sized based on your service conductors (the conductors upstream of the service disconnect Article 230), and since you say the pedestal is attached it should be run to the service disconnect(s).
 
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It would appear neither of you are correct, but more than likely the inspector was paraphrasing and just misspoke. The GEC is based on the size of the service entrance conductors, which should be, the two parallel 4/0 or similar. It can't be based on the "320" amps because there are not any ground tables based on ampacity, only other wire sizes, 250.66 or breaker size, 250.122.
 
Strathead said:
It would appear neither of you are correct, but more than likely the inspector was paraphrasing and just misspoke. The GEC is based on the size of the service entrance conductors, which should be, the two parallel 4/0 or similar. It can't be based on the "320" amps because there are not any ground tables based on ampacity, only other wire sizes, 250.66 or breaker size, 250.122.
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What the OP described are feeders. He has not given us the size of the service entrance conductors.
 
This type of installation is why Note 2 to Table 250.66 exists. In this case, I would pick a conductor that can carry 320 amps and size the GEC to that conductor.
 
Actually nothing should be run to the basement panels other than the Equipment Grounding Conductor (EGC) in the SE-R cable. Grounds and neutrals should be separated in those sub-panels.
 
Assuming Don is correct that there are no service entrance conductors, the GEC should be a 1/0 copper. (It happens to make no difference if 310.12 applies.) It should go directly to the pedestal, nothing should be run to the subpanels.
 
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