I am building a plant where we will be buying power at 69kv and reducing to 4160v for distribution and then down to 480v. I have (4) 4000amp 480v services (2 buildings with 2 services each). Normally I would use the service grounding table and install a 3/O for each, in this case is the new code saying that these should be considered equipment when grounding, in which case I would be installing 500mcm to steel, gnd rod, water ect.?
Multiple Service Grounding
- Thread starter acerock
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I am not sure I am following but if you are talking about grounding electrodes then you use 250.66 not 250.122. Connection to an electrode never needs to be larger than 3/0 copper.
welcome to the Forum.
I'm unsure which Code you are referencing when you say "new code", but I find nothing in '08 or '11 that would require you to do anything other than address each 4000 amp service on it's own.
All grounding electrodes at the building must be connected together in some fashion.
(Note: '11 is not currently in use here so my knowledge of it is limited and there may be a change that effects this that I didn't see in my review)
I'm unsure which Code you are referencing when you say "new code", but I find nothing in '08 or '11 that would require you to do anything other than address each 4000 amp service on it's own.
All grounding electrodes at the building must be connected together in some fashion.
(Note: '11 is not currently in use here so my knowledge of it is limited and there may be a change that effects this that I didn't see in my review)
- Location
- Connecticut
- Occupation
- Engineer
It appears to me that these would be separately derived systems and not services. Grounding should be per 250.30.
I think you'd need a 750mcm system bonding jumper (assuming 10 sets of 600mcm phase conductors) per 250.30(A)(1), and as Dennis pointed out, a 3/0 grounding electrode conductor per 250.30(A)(3).
Code Reference
Code Reference
Thank you for the responses.
What I am being told is that my service entrance is at 69kv therefore the 480v services should be grounded according to equipment ground rather than service ground. I have (11) 500mcm feeding services therefore my ground should be a 500mcm. I don't understand why there is a difference in where the utility stops and where I start. The 2008 code reference being refered to is 250.32 (B) "The equipment grounding conductor shall be sized in accordance with 250.122."
Code Reference
Thank you for the responses.
What I am being told is that my service entrance is at 69kv therefore the 480v services should be grounded according to equipment ground rather than service ground. I have (11) 500mcm feeding services therefore my ground should be a 500mcm. I don't understand why there is a difference in where the utility stops and where I start. The 2008 code reference being refered to is 250.32 (B) "The equipment grounding conductor shall be sized in accordance with 250.122."
- Location
- Connecticut
- Occupation
- Engineer
It is a little unclear as to whether you are feeding from one building to the other building at 4160V or at 480V.
I will assume that your feeds from building to building are at 480V. You have two issues then. It is correct that your service entrance is at 69kV. The "service" is the connection to the utility. Where you step down to 480V, you have "separately derived systems." These "systems" most be grounded per 250.30.
If the SDS to step down to 480V occur in the first building, and then 480V is feed to the second building, then the second building would fall under 250.32 "Buildings or structures Supplied by Feeders..." In this case, an equipment grounding conductor would be included with the feeder (sized per 250.122) and would be connected to the disconnecting means and to the grounding electrode at the second building.
I will assume that your feeds from building to building are at 480V. You have two issues then. It is correct that your service entrance is at 69kV. The "service" is the connection to the utility. Where you step down to 480V, you have "separately derived systems." These "systems" most be grounded per 250.30.
If the SDS to step down to 480V occur in the first building, and then 480V is feed to the second building, then the second building would fall under 250.32 "Buildings or structures Supplied by Feeders..." In this case, an equipment grounding conductor would be included with the feeder (sized per 250.122) and would be connected to the disconnecting means and to the grounding electrode at the second building.
- Location
- Massachusetts
If you are buying at 69KV I think it is a safe bet that your service stops somewhere between the 69KV and the 4160 equipment making the 480 volt 'services' not services but feeders.
- Location
- Connecticut
- Occupation
- Engineer
With what you have described, "equipment grounding conductors" do NOT come into play for your 480V system grounding. The 480V systems are "separately derived systems" and should be grounded per 250.30(A).
You should have a system bonding jumper size of 700mcm (based on your 11 sets of 500mcm) and a grounding electrode conductors size of 3/0.
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