Ground Conductor

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Electriman

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Dear Friends,

I am working on a project for which I have to size the ground conductor from the service point to MCC.
The service point is 12.47 kv that I step it down to 4.16kv via delta-wye transformer. The neutral point of the transformer secondary is grounded via high resistance ground (HRG). The secondary of the transformer feeds a MV MCC.

My question is based on NEC 250 what size of ground conductor do I have to select between transformer secondary and MV MCC? Do I need to size it based on 250.122 or can I select a smaller size of the ground conductor since my ground current will never be greater than 10 A? The secondary breaker is 1200 A. What is the minimum ground conductor size I can select that fulfill NEC?

Thanks,
 
Electriman said:
Dear Friends,

I am working on a project for which I have to size the ground conductor from the service point to MCC.
The service point is 12.47 kv that I step it down to 4.16kv via delta-wye transformer. The neutral point of the transformer secondary is grounded via high resistance ground (HRG). The secondary of the transformer feeds a MV MCC.

My question is based on NEC 250 what size of ground conductor do I have to select between transformer secondary and MV MCC? Do I need to size it based on 250.122 or can I select a smaller size of the ground conductor since my ground current will never be greater than 10 A? The secondary breaker is 1200 A. What is the minimum ground conductor size I can select that fulfill NEC?

Thanks,
Click to expand...

250.122 is for selecting the size of the equipment grounding conductor (EGC). The EGC (if it is a wire) is always sized based on the rating of the upstream OCPD.

I don't see any exceptions that would allow you to size it any smaller. No wire type EGC is required at all if you use some kind of metallic raceway.

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122 ...
Click to expand...

I am not entirely sure though that the conductor you are asking about is actually an EGC.
 
If the service point you talk about is not also the service disconnect. it is still service wiring and there should be a grounded conductor but not an EGC or GEC.
 
petersonra said:
250.122 is for selecting the size of the equipment grounding conductor (EGC). The EGC (if it is a wire) is always sized based on the rating of the upstream OCPD.

I don't see any exceptions that would allow you to size it any smaller. No wire type EGC is required at all if you use some kind of metallic raceway.



I am not entirely sure though that the conductor you are asking about is actually an EGC.
Click to expand...

Let me correct myself. The wire that I am talking about is a ground conductor coming from transformer ground pad (not neutral point) to ground bus of the MV MCC.

My system is three wire but based on the NEC 250.120 I have to run another wire for grounding (EGC).
 
Electriman said:
Let me correct myself. The wire that I am talking about is a ground conductor coming from transformer ground pad (not neutral point) to ground bus of the MV MCC.

My system is three wire but based on the NEC 250.120 I have to run another wire for grounding (EGC).
Click to expand...

this might actually be the SSBJ.
 
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