Sizing GEC

Z

zander

Member
Location
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).
 
Last edited:
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.
Click to expand...
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.
 
I think there is an exception somewhere in 250 where if the GEC is just for ground rods it need not be larger than #6, but the GEC to UFER should be full size.
 
pipe_bender said:
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).
Click to expand...
My bad, both #4s are run back to the pedestal, not the panels. I misremembered.
 
pipe_bender said:
I think there is an exception somewhere in 250 where if the GEC is just for ground rods it need not be larger than #6, but the GEC to UFER should be full size.
Click to expand...
That applies to a single rod. So for example 1/0 GEC to the first rod, then a 6awg jumper to the second rod. Or, probably cheaper and easier, separate 6awg GECs to each rod is allowed.

zander said:
My bad, both #4s are run back to the pedestal, not the panels. I misremembered.
Click to expand...
So one GEC to each rod? Then that is fine, you are compliant.
 
They mentioned the meter/main was attached to the house so it's actually Service Conductors that are run to the structure.

Jap>
 
jaggedben said:
That applies to a single rod. So for example 1/0 GEC to the first rod, then a 6awg jumper to the second rod. Or, probably cheaper and easier, separate 6awg GECs to each rod is allowed.


So one GEC to each rod? Then that is fine, you are compliant.
Click to expand...
Unless I am misreading you, you are incorrect. #6 to all ground rods is OK: here is the code

(A) Connections to a Rod, Pipe, or Plate Electrode(s). If the
grounding electrode conductor or bonding jumper connected
to a single or multiple rod, pipe, or plate electrode(s), or any
combination thereof, as described in 250.52(A)(5) or (A)(7),
does not extend on to other types of electrodes that require a
larger size conductor, the grounding electrode conductor shall
not be required to be larger than 6 AWG copper wire or 4 AWG
 
You must log in or register to reply here.
Top