Cable shielding grounding

[Isaiah]

On 13.8kV incoming feeder they tied the cable shielding together with #6 AWG and took it (back through the ZCT) to the ground bus within the switchgear. The inspector says it needs to be #2/0 instead of #6 because that’s the size of the EGC routed with the phase conductors. Not sure why this needs to be so big since the shield is not designed to carry a large fault in the first place.


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[JoeStillman]

I agree with you. The shield is not intended to carry any ground fault at all. It can, but only for a moment before it burns up. It is exactly because it DOESN"T have capacity to carry a ground fault that you have to run an equipment ground with the MV cables in the first place.

 

[Isaiah]

I agree with you. The shield is not intended to carry any ground fault at all. It can, but only for a moment before it burns up. It is exactly because it DOESN"T have capacity to carry a ground fault that you have to run an equipment ground with the MV cables in the first place.

Are there any guidelines for sizing this conductor relative to the shields? One of the guys said it could be as small as #10AWG?


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[infinity]

Are there any guidelines for sizing this conductor relative to the shields? One of the guys said it could be as small as #10AWG?


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I thought that the cable manufacturer determines the conductor size?

 

[Hv&Lv]

https://www.okonite.com/media//catalog/product/files/Tech_30.pdf

 

[Isaiah]

I thought that the cable manufacturer determines the conductor size?

Collectively the 3 shields tied together equal #2AWG in terms of circular mil.
If the intent is to carry the fault for only a very short period, (? cycles per protective relaying) then I don’t see why it couldn’t be a #10AWG that ties the shields to ground


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[Hv&Lv]

Doesn’t the footnote in the ampacity tables cover ALL conductors in a raceway?

 

[Isaiah]

Doesn’t the footnote in the ampacity tables cover ALL conductors in a raceway?

There doesn’t seem to any NEC guidelines on the shield ampacities etc


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[Hv&Lv]

There doesn’t seem to any NEC guidelines on the shield ampacities etc


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I’m thinking back about 5-6 code cycles when there was a 250.4
I have no clue if it was moved or deleted.

 

[Julius Right]

NEC 250.190 use the shield as equipment grounding.
(2) Shielded Cables. The metallic insulation shield encircling
the current-carrying conductors shall be permitted to be used
as an equipment grounding conductor, if it is rated for clearing
time of ground-fault current protective device operation without
damaging the metallic shield.
(3) Sizing. Equipment grounding conductors shall be sized in
accordance with Table 250.122 based on the current rating of
the fuse or the overcurrent setting of the protective relay.
If the shield is grounded both ends-at the source side and at the receiver side in order to mitigate the induced voltage- build up. In this case a circulating current will flow through the shield
This current may be calculated if we know the cable data
For instance, for 3 single core cables of 15 kV in flat arrangement 7.5" apart ,500 kcmils copper conductor XLPE insulated, copper, tape shielded, 3000 ft length, the currents
are 22.4,16,39 A. Then 10 awg it is o.k.

 

[infinity]

Are these conductors in a metallic raceway? If so why would the shield need to be considered part of the EGC?

 

[Isaiah]

Are these conductors in a metallic raceway? If so why would the shield need to be considered part of the EGC?

They are in fiberglass conduit underground.


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[Isaiah]

NEC 250.190 use the shield as equipment grounding.
(2) Shielded Cables. The metallic insulation shield encircling
the current-carrying conductors shall be permitted to be used
as an equipment grounding conductor, if it is rated for clearing
time of ground-fault current protective device operation without
damaging the metallic shield.
(3) Sizing. Equipment grounding conductors shall be sized in
accordance with Table 250.122 based on the current rating of
the fuse or the overcurrent setting of the protective relay.
If the shield is grounded both ends-at the source side and at the receiver side in order to mitigate the induced voltage- build up. In this case a circulating current will flow through the shield
This current may be calculated if we know the cable data
For instance, for 3 single core cables of 15 kV in flat arrangement 7.5" apart ,500 kcmils copper conductor XLPE insulated, copper, tape shielded, 3000 ft length, the currents
are 22.4,16,39 A. Then 10 awg it is o.k.

Yes you’re right. I have the cable shield data from okonite, I just have to verify the relay is set to trip before the shield is damaged -
thank you!


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