Grounding Ring Sizing

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safigh

Member
Hello,

I have a building of 8 appartments with 100A Panelboard for each appartment. Each panel is fed by 50 mm2 cable from a main distribution board which is around 50 meter away from the building. So 8 cables are coming into the building.

What is the enough size for a grounding ring cable that will go around the building? Following 250.66 will 25 mm2 of cable ring around the building be enough?

Is following 250.52(A)(4) minimum conductor size of 2 AWF is a must in this case?

Same question for another building with 59 appartments + 59 panels + 59 Feeders of 50 mm2 for each appartment? what would be the minimum size here?

Many thanks
 

safigh

Member
I did not think the ground ing needed to be any larger than a #2AWG. 250.66(C) and 250.52(A)(4)
My question is that can I go lower than # 2? and on what basis I can reduce the size of the cable? Shall I consider in this case largest feeder (here all are equal) or I will use sum of feeders?

Thank you
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Well I read it as you cannot use smaller than #2 no matter what size service as well as it does not need to be larger than the ground ring size (#2 or larger based on 250.66). It appears the gec to the ring can be sized to 250.66 so it can be smaller. GEC is based on service size not feeder unless it is a detached structure.
 
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safigh

Member
Let me state it in another way and sorry for repitition but still I see it not logical ( I am new to NEC):

CASE 1:

1 BUILDING with 59 Appartments (6 Stories)
Each appartment Fed with a feeder from the Main Site Distribution Board with a 4x #0.
Main Breaker for each appartment 100Amps

CASE 2:

1 Building with 8 Appartments
Each appartment Fed with a feeder from the Main Site Distribution Board with a 4x #0.
Main Breaker for each appartment 100Amps

How can I design the Grounding Ring? What is the size of the cable?

Referring to table 250.66 I can use # 6 Cable. Am I referring to correct table? Is this size for the ground ring itself or only the cable to connect the panel to the ring?

Is there a minimum size for the grounding rod to be burried on the 4 corners of the building? I am using 4'. is this complying with NEC?

Thank you
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Okay the ground ring needs to be size from T250.66 based on the sum of the parallel feeds of the service entrance conductors. From there just pull the same size gec to the ring from the main distribution box. It will never be larger than 3/0.

No need to size each apartment to the ground ring.
 
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