15KV MV cable shield ground conductor size

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marksemail

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I am needing to size the shield grounding conductor for some 15KV 500MCM feeders I am installing. This conductor will be attached to the cable shield and go to ground. I will not be using this as an EGC. What NEC guideline will this fall under? I cant seem to find any standard directly related to shield grounding size in medium voltage applications.
 

Medium Voltage underground feeders

For a 5KV or 15kV (anything utilizing shielded cable) underground distribution system, given that the cable is shielded, is a ground conductor required. If the answer is yes, table 250-122 is the applicable reference is it not. i.e. it is independent of voltage correct?
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Something I came across recently...


Apparently the high impedance of the foil shields is intentional. If you make it too conductive the shield has more losses than the cable itself. This totally makes me rethink concentric neutral cable vs a separate neutral. Also more or less proves it can’t be a ground.

There is a minimum cable size for power conductors in NEC but it’s rather obscure. It is in the ampacity table that shows a minimum along with a footnote. This applies to any wire in the power cable raceway and certainly applies to shield drains.

There is also a debated issue about grounding one or both ends. If you ground both ends it forms a complete circuit with the bonding so it can have currents induced on it. With only one end grounded this can’t happen. There is a minimum length where grounding one end only is the best way then after that point both ends since impedance is high enough it’s better to have the extra ground for voltage control.
 
marksemail said:
I am needing to size the shield grounding conductor for some 15KV 500MCM feeders I am installing. This conductor will be attached to the cable shield and go to ground. I will not be using this as an EGC. What NEC guideline will this fall under? I cant seem to find any standard directly related to shield grounding size in medium voltage applications.
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I curious as to what your calling “shield”.
Is it tape shield, or the 14awg (10awg?) wires concentrically laid around the cable?

If it’s concentric neutral cable is it full neutral or one-third neutral?
 
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