GEC size for 400 amp service

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jjhoward

jjhoward

Senior Member
Location
Northern NJ
Occupation
Owner TJ Electric
Hello all;
Some clarification on the size of the GEC in a service to a residence with 2 150 amp main breaker panels.
Is it determined by the size of the Service Entrance Conductor? (yes..?):
Is that the cable from the point of attchment to the top of the meter?
Or is it the cable(s) from the bottom of the meter to the 2 main breaker panels.
This residence has 400 kcm Cu conductors from the POA down to the top of the meter.
Then 2 sets of 2/0 Cu from the bottom of the meter to the 2 seperate main breaker panels for the house.

Are the cables from the POA down to the meter the ones that determine the ground conductor size or is the smaller cables from the bottom of the meter that go to the 2 150 amp panels?
GEC size for 400 Kcm Cu is 1/0.
GEC size for 2/0 Cu is 4 awg.
Which size is the correct GEC size?

Thank you.
 
jjhoward said:
Hello all;
Some clarification on the size of the GEC in a service to a residence with 2 150 amp main breaker panels.
Is it determined by the size of the Service Entrance Conductor? (yes..?):
Is that the cable from the point of attchment to the top of the meter?
Or is it the cable(s) from the bottom of the meter to the 2 main breaker panels.
This residence has 400 kcm Cu conductors from the POA down to the top of the meter.
Then 2 sets of 2/0 Cu from the bottom of the meter to the 2 seperate main breaker panels for the house.

Are the cables from the POA down to the meter the ones that determine the ground conductor size or is the smaller cables from the bottom of the meter that go to the 2 150 amp panels?
GEC size for 400 Kcm Cu is 1/0.
GEC size for 2/0 Cu is 4 awg.
Which size is the correct GEC size?

Thank you.
Click to expand...
It would be the ampacity of the common riser conductors. Without the 83% rule for dwellings, this would be 335A. For dwellings, it would be 400 (because we cant seem to go above 400 otherwise it would be 404).

Because you have two service enclosures, see 250.64(D) - depending on what method you use, there may be taps that can be smaller than the main GEC.
 
You can either run one GEC sized for the entire service size, or separate GECs, each sied for each panel.
 
I don't know why there are different sized wires on the LINE side versus the LOAD side of the meter in this overhead service. Both are service entrance conductors are are typically sized the same. But because one set is oversized, its my opinion that you size the GEC based on the largest set of service entrance conductors (which is 400 KCMil in your case). So the water bond/ground needs to be 1/0 CU.
 
suemarkp said:
I don't know why there are different sized wires on the LINE side versus the LOAD side of the meter in this overhead service. Both are service entrance conductors are are typically sized the same. But because one set is oversized, its my opinion that you size the GEC based on the largest set of service entrance conductors (which is 400 KCMil in your case). So the water bond/ground needs to be 1/0 CU.
Click to expand...
Where 230.40 exception #2 is employed and there is a common set of service conductors, the common set is almost always smaller. They would have more diversity and article 220 allows them to be smaller. Also they are allowed to be sized by the load not the OCPD which is another reason they are typically smaller.
 
If you have 2 sets of 2/0 then the grounding electrode conductor is not #4. You have to add the cm twice. 133,100 x 2 = 266,200 so you are looking at 300kcm. The grounding electrode conductor is #2 not #4 per table 250.66

IMO, you use the largest service conductors so 1/0 would be the grounding electrode conductor needed
 
I'm not understanding why 400 MCM cu. was used here when you have a 300 amp service ( 2 - 150 amp main disconnects)
If using 2017, 83% of 300 = 249 minimum ampacity of conductors. 2020 NEC went back to the old way and T.310.12 and shows a 300 A
residential service using 250 MCM cu. T. 310.16 @ 75 degrees C shows a 250 MCM having an ampacity of 255 A. (works out both ways)
T. 310.16 also shows that a #1 cu. at 75 degrees could be paralleled to achieve 260 amps. (above your required 249 ampacity) but the minimum size conductor that can be paralleled is 1/0. (T. 8 1/0 is 105,600 cmil x 2 = 211,200 cmil) The 250 MCM would be my largest ungrounded conductor. My GEC would be a #2 cu. That's beside the point here but that is way I would have built this service.

I agree with the guys above. Since you installed the 400 MCM in the riser, it is the largest ungrounded conductor (compared to your paralleled 2/0 cu. conductors totaling 266,200 cmil) Using T. 250.66, 1/0 would be the proper GEC.

Food for thought, If you had no metal water pipe and there was no UFER ground also, you just had two ground rods at least 6' apart,
Could you be code compliant by using a #6 bare cu. as your GEC in the above stated scenario? see Art. 250.66 (A)
 
Rick 0920 said:
I'm not understanding why 400 MCM cu. was used here when you have a 300 amp service ( 2 - 150 amp main disconnects)
If using 2017, 83% of 300 = 249 minimum ampacity of conductors. 2020 NEC went back to the old way and T.310.12 and shows a 300 A
residential service using 250 MCM cu. T. 310.16 @ 75 degrees C shows a 250 MCM having an ampacity of 255 A. (works out both ways)
T. 310.16 also shows that a #1 cu. at 75 degrees could be paralleled to achieve 260 amps. (above your required 249 ampacity) but the minimum size conductor that can be paralleled is 1/0. (T. 8 1/0 is 105,600 cmil x 2 = 211,200 cmil) The 250 MCM would be my largest ungrounded conductor. My GEC would be a #2 cu. That's beside the point here but that is way I would have built this service.

I agree with the guys above. Since you installed the 400 MCM in the riser, it is the largest ungrounded conductor (compared to your paralleled 2/0 cu. conductors totaling 266,200 cmil) Using T. 250.66, 1/0 would be the proper GEC.

Food for thought, If you had no metal water pipe and there was no UFER ground also, you just had two ground rods at least 6' apart,
Could you be code compliant by using a #6 bare cu. as your GEC in the above stated scenario? see Art. 250.66 (A)
Click to expand...
You never need to run larger than #6 to a ground rod or #4 to a CEE, unless you also make a bonding jumper to other electrodes in same run.
 
You still need to answer the question if you have interior water piping that is metal. Water bond (whether its a GEC or just interior piping) is sized the same as GEC and there is no max limit like there is with rods and CEE.

I tried to edit my post above, but I waited too long. The 400 MCM in the riser should have been 300 or maybe even 250 based on the 2/0 copper conductors going to 150A panels. They just wasted money for all that heavy copper. I did my own 400A service with 400/400/350 copper down the mast and carrying and controlling all of that up the ladder was a challenge (each was 18' long).
 
suemarkp said:
You still need to answer the question if you have interior water piping that is metal. Water bond (whether its a GEC or just interior piping) is sized the same as GEC and there is no max limit like there is with rods and CEE.

I tried to edit my post above, but I waited too long. The 400 MCM in the riser should have been 300 or maybe even 250 based on the 2/0 copper conductors going to 150A panels. They just wasted money for all that heavy copper. I did my own 400A service with 400/400/350 copper down the mast and carrying and controlling all of that up the ladder was a challenge (each was 18' long).
Click to expand...
Yes there is an interior water line that the GEC will be bonded to via a pipe clamp.
 
Rick 0920 said:
I'm not understanding why 400 MCM cu. was used here when you have a 300 amp service ( 2 - 150 amp main disconnects)
If using 2017, 83% of 300 = 249 minimum ampacity of conductors. 2020 NEC went back to the old way and T.310.12 and shows a 300 A
residential service using 250 MCM cu. T. 310.16 @ 75 degrees C shows a 250 MCM having an ampacity of 255 A. (works out both ways)
T. 310.16 also shows that a #1 cu. at 75 degrees could be paralleled to achieve 260 amps. (above your required 249 ampacity) but the minimum size conductor that can be paralleled is 1/0. (T. 8 1/0 is 105,600 cmil x 2 = 211,200 cmil) The 250 MCM would be my largest ungrounded conductor. My GEC would be a #2 cu. That's beside the point here but that is way I would have built this service.

I agree with the guys above. Since you installed the 400 MCM in the riser, it is the largest ungrounded conductor (compared to your paralleled 2/0 cu. conductors totaling 266,200 cmil) Using T. 250.66, 1/0 would be the proper GEC.

Food for thought, If you had no metal water pipe and there was no UFER ground also, you just had two ground rods at least 6' apart,
Could you be code compliant by using a #6 bare cu. as your GEC in the above stated scenario? see Art. 250.66 (A)
Click to expand...
Hi Rick;
We did not install the conductors for the service. There service was pre-existing. We are installing a 24Kw standby generator at the site. We have installed 2 200 amp transfer switches near the meter pan & running the necessary conductors for the gen feed. Fortunately, there is a run of 1 1/2 PVC (empty) from the site near the meter pan, across the property to the basement where the 2 150 amp breaker panels are (~100'). We have re-confgured the two 150 amp panels in the basement as sub panels, the 2 200 amp transfer switches are getting their feeds from the double lugs in the 400 amp meter pan. We simply re-routed the original service feeds that were from the meter pan to the basement panels to be now feed from the transfer switches. We are looking for a solid answer on how to install a properly sized GEC.
 
To clarify: We did not install the conductors for the service. There service was pre-existing. We are installing a 24Kw standby generator at the site. We have installed 2 200 amp transfer switches near the meter pan & running the necessary conductors for the gen feed. Fortunately, there is a run of 1 1/2 PVC (empty) from the site near the meter pan, the PVC runs across the property to the basement where the 2 150 amp breaker panels are (~100'). We have re-confgured the two 150 amp panels in the basement as sub panels, the 2 200 amp transfer switches (mounted by the meter pan) are getting their feeds from the double lugs in the 400 amp meter pan. We simply re-routed the original service feeds that were from the meter pan to the basement panels to be now fed from the transfer switches. We are looking for a solid answer on how to install a properly sized GEC.
 
Are your main disconnects now 200A each? If so, i dont think 2/0 copper is sufficient because each set it not carrying the full dwelling load. Therefore, the conductors to each transfer switch need to be rated for the rating of that breaker (or calculated load if within roundup limits). You cant use the residential derating factor if the conductor set doesnt feed the entire dwelling. I think you need 3/0 there and that probably puts you to a 1/0 GEC without any questions.
 
If not mistaken, you should be able to run 1- #1/0 from within 5’ where the waterline enters the home out to the transfer switch locations and make irreversible splices of #4 from each transfer switch to the 1/0 GEC.
At least that’s the way I would do it and I believe Mike Holt also has illustrations of this method also, just involving plain old disconnects. Should not be any different for service rated transfer switches
 
suemarkp said:
Are your main disconnects now 200A each? If so, i dont think 2/0 copper is sufficient because each set it not carrying the full dwelling load. Therefore, the conductors to each transfer switch need to be rated for the rating of that breaker (or calculated load if within roundup limits). You cant use the residential derating factor if the conductor set doesnt feed the entire dwelling. I think you need 3/0 there and that probably puts you to a 1/0 GEC without any questions.
Click to expand...
I was just considering bringing that up myself.
 
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