Grounding electrode conductor sizing and ground bus sizing

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Tainted

Tainted

Senior Member
Location
New York
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Engineer (PE)
I have 3 service switches:

3-Pole 600A, conductors entering are 2 sets of 4#350KCMILS copper

3-Pole 1200A, conductors entering are 3 sets of 4#600KCMILS copper

3-Pole 800A, conductors entering are 2 sets of 4#600KCMILS copper

I want to connect all service switch neutrals to the copper ground bus. The ground bus will also connect to building steel, water main and ground rods.

I sized all my grounding electrode conductors per table 250.66 (see picture).
Basically #2/0 coming out of the 600A switch,
#3/0 coming out of the 1200A switch and
#3/0 coming out of the 800A switch.

Please correct me if I made errors sizing these grounding electrode conductors, not sure if they are oversized or undersized.

What I am not sure about is how to use table 250.66 to size the grounding electrode conductor for the water main, building steel, and ground rod. I am also not sure how to size a ground bus such as thickness and such. Hope someone can explain. Thank you.

IMG_7729.jpg
 
The GECs are of the correct size. The GECs to the water and steel will be 3/0. The bus needs to be 1/4" x 2" as long as necessary to fit your connections.
 
don_resqcapt19 said:
The GECs are of the correct size. The GECs to the water and steel will be 3/0. The bus needs to be 1/4" x 2" as long as necessary to fit your connections.
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How did you get 3/0 for water main and building steel? Is that usually sized based on the largest grounding electrode conductor? And I also want to know how you came up with 1/4"x2" for the ground bus. Appreciate your help
 
Tainted said:
How did you get 3/0 for water main and building steel? Is that usually sized based on the largest grounding electrode conductor? And I also want to know how you came up with 1/4"x2" for the ground bus. Appreciate your help
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250.66 table over 1100 = 3/0
250.64 (F) (3)1/4" x 2"
 
I do not see any metering equipment in your drawing.
Is this a new utility service to a building with three service disconnects.
 
Tulsa Electrician said:
I do not see any metering equipment in your drawing.
Is this a new utility service to a building with three service disconnects.
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800A disconenct has a meter before it, I just didn't drew it. The two other disconnects feed meter banks. Yes it will be a new service.
LarryFine said:
And rods never need larger than #6 cu, Ufer grounds #4 cu.
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why must the building steel and water main be 3/0 but the ground rod is #6?
 
Unless prohibited by POCO or some authority, it sure would be a lot less work to connect your GECs in the "Service end box rather than each disconnect,. One 3/0 to steel or water with sp[lit bolt connectors for taps top ground rod, etc.
 
Tainted said:
how did you get 3/0 for building steel and water main using 250.66. Why is the ground rod #6 and not 3/0?
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Look at table 250.66, over 1100= 3/0.
This is where250. 66 and 250.102 C 1 vary.
250.66= 3/0 max.
250.102 C 1 see notes for over 1100. Note 1 usually applies.

As for the ground rod #6
Look at 250.66 (A) connection to Rod. This will tell you as per your drawing #6 cu is all you need.
 
Tulsa Electrician said:
Look at table 250.66, over 1100= 3/0.
This is where250. 66 and 250.102 C 1 vary.
250.66= 3/0 max.
250.102 C 1 see notes for over 1100. Note 1 usually applies.

As for the ground rod #6
Look at 250.66 (A) connection to Rod. This will tell you as per your drawing #6 cu is all you need.
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over 1100 compared to which conductors? Still don't understand how you sized 3/0 for building steel and water main. I know how to do it with the disconnect switch neutrals because the incoming conductors are known.

so basically you're saying 250.66 (A) tells you ground rods, pipe, and plate electrodes are not required to be larger than 6 awg. Isn't the water main considered a pipe and therefore shouldn't that be #6 too?

Tulsa Electrician said:
For some good read on what you had drawn.
Review 250.58 (D)
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It doesn't have subsection (D) what code year are you using?
 
Tainted said:
Isn't the water main considered a pipe and therefore shouldn't that be #6 too?
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No, because the concern is that a contact between a conductor and building steel or water piping be able to trip a main breaker, rather than energize the metal. A contact to a ground rod is not such a concern.
 
LarryFine said:
No, because the concern is that a contact between a conductor and building steel or water piping be able to trip a main breaker, rather than energize the metal. A contact to a ground rod is not such a concern.
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Makes sense but why does code mention pipes in 250.66 (A)? It made it seem like water main pipes can be #6
 
Tainted said:
Makes sense but why does code mention pipes in 250.66 (A)? It made it seem like water main pipes can be #6
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I haven't read it, but I think it refers to other piping, not the water service.

Things were different before non-metallic water services came into being.
 
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