NEC Service Entrance Ground and Neutral

timm333

Senior Member
Location
Minneapolis, MN
I am trying to figure out the service entrance neutral and ground conductors as per NEC. We have a delta-wye connected utility transformer, wye connected local standby generator, and a service entrance rated auto transfer switch (ATS), as shown in attached picture.

We don’t have neutral application, so we can remove the jumper between neutral and ground in ATS, and the cable-3 (ATS to MCC) would be a 4-conductor cable (L1, L2, L3, EGC). I have following questions.

Should cable-1 (utility transformer to ATS) be a 5-conductor cable (L1, L2, L3, N, GEC), or will a 4-condctor cable (L1, L2, L3, N) be enough?
Also, should cable-2 (Generator to ATS) be a 5-conductor cable (L1, L2, L3, N, EGC), or will a 4-condctor cable (L1, L2, L3, N) be enough?

Thanks
 

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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
... wye connected local standby generator...
How are you connecting to the wye point of of the generator? Are you relying on the Neutral to Ground bonding at the utility service?
Where is your service entrance point/meter, at the transformer or the service entrance rated ATS?
 

timm333

Senior Member
Location
Minneapolis, MN
The Y point of the generator would be ungrounded. Or maybe it would be better to wire the generator as delta.

Metering would be by the utility. I don't know the exact location of the metering, but it would be somewhere in between transformer and ATS.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Metering would be by the utility. I don't know the exact location of the metering, but it would be somewhere in between transformer and ATS.
It appears the ATS will be the service entrance equipment, so you should plan on keeping the N-G bond at that location.

I would not have the generator in a configuration different than the utility supply.
 

timm333

Senior Member
Location
Minneapolis, MN
Yes ATS will be the service entrance equipment. Should both cable-1 and cable-2 have 5 conductors (L1, L2, L3, N, G), and the star point of the generator be grounded?

In that case there would be two grounding electrodes: one near generator which connects the star point of generator to ground, and the other near transformer which connects the star point of the transformer to ground. Should these two grounding electrodes be connected together?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes ATS will be the service entrance equipment. Should both cable-1 and cable-2 have 5 conductors (L1, L2, L3, N, G), and the star point of the generator be grounded?
Cable #1 would be 4 conductors (L1,L2,L3,N) not 5 as it's on the line side of the service disconnect. Cable #2 would have 5 conductors because it is a feeder.
 

Rock86

Member
Location
new york
Occupation
Electrical Engineer / Electrician
Where do you plan to drive your ground? Like Jim said, because you're ATS is your service entrance equipment, you would be keeping the neutral bond to a grounding electrode conductor, there for you would only need an equipment ground at the generator. There Infinity would be correct with his statement.
 

timm333

Senior Member
Location
Minneapolis, MN
The ground electrode would be at the utility transformer, which would connect the star point of the utility transformer to ground. Our loads don't need neutral, so I was thinking that Cable- 2 (Gen to ATS) can be 4 -conductors (L1, L2, L3, EGC).

Just to confirm, would the Cable-2 (Gen to ATS) be 5 conductors (L1, L2, L3, N, EGC) even though our loads at MCC don't need neutral?
 

timm333

Senior Member
Location
Minneapolis, MN
Still could not figure out the difference between the 4-conductor and 5-conductor service entrance cables. The attached image shows both 4-conductor (250-250-250-3/O) and 5-conductor (250-250-250-250-3/O). In which situation we should use the 5-conductor cable?

If we uses the 4-conductor cable (250-250-250-3/O), then will the 3/O conductor act as both neutral and ground conductor?
 

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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The ground electrode would be at the utility transformer, which would connect the star point of the utility transformer to ground. Our loads don't need neutral, so I was thinking that Cable- 2 (Gen to ATS) can be 4 -conductors (L1, L2, L3, EGC).

Just to confirm, would the Cable-2 (Gen to ATS) be 5 conductors (L1, L2, L3, N, EGC) even though our loads at MCC don't need neutral?
No cable #2 does not need an EGC in fact in using one it would create objectionable current and violate 250.6. It does require that the neutral be brought to the service disconnect. The service disconnect would require it's own connection to the GES. {250.24(A)(1)}
 

tom baker

First Chief Moderator
Staff member
The ground electrode would be at the utility transformer, which would connect the star point of the utility transformer to ground. Our loads don't need neutral, so I was thinking that Cable- 2 (Gen to ATS) can be 4 -conductors (L1, L2, L3, EGC).

Just to confirm, would the Cable-2 (Gen to ATS) be 5 conductors (L1, L2, L3, N, EGC) even though our loads at MCC don't need neutral?
A neutral is required from the utility transformer to the service point, while not needed for neutral current, its needed for fault current return in the event of a line to case fault.
 

timm333

Senior Member
Location
Minneapolis, MN
No cable #2 does not need an EGC in fact in using one it would create objectionable current and violate 250.6. It does require that the neutral be brought to the service disconnect. The service disconnect would require it's own connection to the GES. {250.24(A)(1)}
How shall we perform the equipment grounding of the body of generator enclosure, will we need a separate ground-rod for the equipment grounding of the generator enclosure?
 

timm333

Senior Member
Location
Minneapolis, MN
I think it should be similar to the new image (attached here). It is similar to exhibit 250.13 of NEC handbook.

The breaker of transfer-switch has 4-pole breaker (L1, L2, L3, N). Cable-1 from transformer to transfer-switch has 4-conductors (L1, L2, L3, N). Cable-2 from transfer-switch to generator has 5 conductors (L1, L2, L3, N, EGC). And cable 3 from transfer-switch to MCC has 4-conductors (L1, L2, L3, EGC). The grounding electrodes are installed at transformer, transfer-switch, and generator.

What do you guys think about it?
 

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