Medium Voltage feeder and Ground conductor

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mshields

mshields

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Location
Boston, MA
Am reviewing a project with medium voltage feeders (4160V), multiple conductors per phase, each set in it's own conduit and each conduit containing 3 conductors only. i.e. no equipment ground. I gather they are relying on the shield for grounding. Is that permissible OR should I add a ground conductor to each conduit. If the latter, and given that this is a service, my inclination would be to go with a 3/0 bare copper ground conductor in accordance with Table 310.66. What is the correct approach. Obviously it can't hurt to add additional grounding but is it a waste? and what is the letter of the code where this is concerned?

Thanks,

Mike
 
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paulengr

Senior Member
Is the 4160 high resistance grounded, low resistance grounded, ungrounded, or solidly grounded? High resistance is most common and doesn't require much ampacity since 5-25 A is a typical maximum ground fault and shielding has plenty of ampacity. Solidly grounded is where heavy grounds are needed. IEEE Red Book among other sources is pretty clear that the most economical and best performance ground for 4160 is high resistance. Also you can get 3 conductor MV-105 with grounds. If it's ungrounded they have to put fillers in the voids anyways.

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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
mshields said:
Am reviewing a project with medium voltage feeders (4160V), multiple conductors per phase, each set in it's own conduit and each conduit containing 3 conductors only. i.e. no equipment ground. I gather they are relying on the shield for grounding. Is that permissible OR should I add a ground conductor to each conduit. If the latter, and given that this is a service, my inclination would be to go with a 3/0 bare copper ground conductor in accordance with Table 310.66. What is the correct approach. Obviously it can't hurt to add additional grounding but is it a waste? and what is the letter of the code where this is concerned?

Thanks,

Mike
Click to expand...

There seems to be some confusion here in the responses given. You indicate that these are service conductors. There is no EGC in that case. Review Part X of Article 250, especially 250.186.
 
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paulengr

Senior Member
texie said:
There seems to be some confusion here in the responses given. You indicate that these are service conductors. There is no EGC in that case. Review Part X of Article 250, especially 250.186.
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I'm confused. I see feeders and the word service. Are they service conductors or feeders?


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mshields

mshields

Senior Member
Location
Boston, MA
clarification

clarification

It is actually a service. And it is solidly grounded off a 24.9 to 4160 delta wye transformer. So ok, it's service neutral conductor and not an EGC. Do I still need a dedicated conductor to that end. And would be it therefore be based on table 250.102?
 
Wire-Smith

Wire-Smith

Senior Member
Location
United States
electrofelon said:
Perhaps it's the mythical "service feeder" ;)

On a serious note, not that I do much MV, but I have also always been unclear on if or when a shield can be used as an EGC. Hoping we get some responses on this.
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Given that NEC 250.118 does not specifically list a cable shield, and Table 250.112 lists the minimum sizes of Equipment Grounding Conductors(with which your shield probably does not comply), along with common sense and a knowledge of how an inspector would interpret the NEC, I would say that a MV cable shield could not be used.
Do not design a system based on what the NEC doesn't directly prohibit, rather, it is up to you to find an article allowing what you want to design (especially if it isn't in common usage).
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Cable shields do NOT have sufficient ampacity or low enough impedance to be considered an equipment grounding conductor.
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https://www.eng-tips.com/viewthread.cfm?qid=102131


I have seen installations where they don't pull a ground(whether intentionally or unintentionally the shield(s) was the equipment grounding conductor), I don't believe it is usually NEC compliant, the concentric stranding has to plenty big and there is still a concern about damage if there is ever a ground fault on the system. I have seen installations like this with foil tape and concentric wire shield.


https://iaeimagazine.org/magazine/2001/05/16/the-nec-and-installations-over-600-volts/
It should be noted that the metal shield of most medium voltage cables is not suitable to be used as an equipment grounding conductor.
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it appears some may be able to be used
 
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paulengr

Senior Member
Wire-Smith said:
https://www.eng-tips.com/viewthread.cfm?qid=102131


I have seen installations where they don't pull a ground(whether intentionally or unintentionally the shield(s) was the equipment grounding conductor), I don't believe it is usually NEC compliant, the concentric stranding has to plenty big and there is still a concern about damage if there is ever a ground fault on the system. I have seen installations like this with foil tape and concentric wire shield.


https://iaeimagazine.org/magazine/2001/05/16/the-nec-and-installations-over-600-volts/

it appears some may be able to be used
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A couple cases. First example is SHD-GC which is a mining duty cable. In this cable each conductor has a shield, and there is another overall shield, and 2 or 3 grounds in it. Obviously qualifies with the integral grounds. Lots of others have similar designs with a ground in contact with the shields.

Second is concentric wound neutrals. It has kind of fallen out of favor because it’s fairly expensive. But you essentially have a fully rated neutral where the strands are spread out and a power conductor run inside the neutral. It’s stiffer and more work both pulling and terminating but the “shield” has a much higher ampacity. This is a classic Okonite product and they are very helpful about working with it.


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jtinge

jtinge

Senior Member
Location
Hampton, VA
Occupation
Sr. Elec. Engr
mshields said:
It is actually a service. And it is solidly grounded off a 24.9 to 4160 delta wye transformer. So ok, it's service neutral conductor and not an EGC. Do I still need a dedicated conductor to that end. And would be it therefore be based on table 250.102?
Click to expand...

Is the xfmr customer owned or utility owned? Where is the service point? If the xfmr is customer owned, it would be a separately derived system and the cable would be a feeder not a service. The location of the service point is critical to answering whether it is a service or feeder.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
I think this is the cable:
https://www.okonite.com/media/catalog/product/files/2-8.pdf
According to ICEA P-45-482 the maximum permissible short-circuit current for 1 sec it is 3000 A
[conductor maximum temperature 90oC] -in my opinion the minimum length has to be 2200 ft.
for 0.1 sec fault clearing time the shield could withstand 9500 A- that means minimum 675 fts.

.
 
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