Grounding of pole line 12.47kV wye circuit

J

joshtrevino

Member
Location
Beaumont, TX
I have a customer who has an incoming three phase 12.47kV circuit (3 bare phase conductors on crossarms with a static/neutral below the phases) coming into the facility from the utility. The circuit passes through the utility metering point and becomes customer owned (premises wiring) at a customer owned recloser. The circuit (now customer owned) continues into the facility in the same configuration described above feeding many customer owned three phase transformers (with both delta and wye primary connections). There are also single phase transformers for lighting, recloser control power, etc fed by the 12.47kV circuit. It seems to me that the neutral/static is also functioning as the EGC (equipment ground conductor) back to the source. For example, there is no separate EGC for the recloser housing. I am working on a project for which I will be modifying this system and want to understand how this configuration fits into the NEC, especially article 250. I know outside the facility the utility is governed by NESC, but inside the facility NEC is the governing standard. What article in NEC (2023 please) allows for this grounding configuration for a medium voltage pole line circuit?

Please let me know if I need to provide further details or clarity.
 
joshtrevino said:
I have a customer who has an incoming three phase 12.47kV circuit (3 bare phase conductors on crossarms with a static/neutral below the phases) coming into the facility from the utility. The circuit passes through the utility metering point and becomes customer owned (premises wiring) at a customer owned recloser. The circuit (now customer owned) continues into the facility in the same configuration described above feeding many customer owned three phase transformers (with both delta and wye primary connections). There are also single phase transformers for lighting, recloser control power, etc fed by the 12.47kV circuit. It seems to me that the neutral/static is also functioning as the EGC (equipment ground conductor) back to the source. For example, there is no separate EGC for the recloser housing. I am working on a project for which I will be modifying this system and want to understand how this configuration fits into the NEC, especially article 250. I know outside the facility the utility is governed by NESC, but inside the facility NEC is the governing standard. What article in NEC (2023 please) allows for this grounding configuration for a medium voltage pole line circuit?

Please let me know if I need to provide further details or clarity.
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Where is the service disconnect?
 
electrofelon said:
You can use the neutral for grounding per the NEC if you meet the requirements, see 250.184. otherwise you would need a separate neutral and EGC just like you would for low voltage.
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electronfelon, it seems that we have a "multigrounded neutral system" per 250.184.C, but I do not see that the language allows using the neutral as the low impendence path to ground instead of an EGC. Is this implied? Is it explicitly stated elsewhere? Not challenging you but honestly asking.
 
joshtrevino said:
electronfelon, it seems that we have a "multigrounded neutral system" per 250.184.C, but I do not see that the language allows using the neutral as the low impendence path to ground instead of an EGC. Is this implied? Is it explicitly stated elsewhere? Not challenging you but honestly asking.
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I'll have to look over the wording when I get more time, but my initial thought it is just implied. The wording in part X is pretty sloppy and vague with the term "grounding". Note the title of part X says "grounding of systems and circuits".

joshtrevino said:
Per the single line diagram the utility has a fuse upstream of the metering point. Utility fuse -> Utility Meter ->Customer Recloser.
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So where is the service disconnect?😉
 
joshtrevino said:
electronfelon, it seems that we have a "multigrounded neutral system" per 250.184.C, but I do not see that the language allows using the neutral as the low impendence path to ground instead of an EGC. Is this implied? Is it explicitly stated elsewhere? Not challenging you but honestly asking.
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There have been some changes recently with the over 1000 volts stuff what NEC version are you on? In the 2023 NEC see 235.106, 250.190 and 495.37.
The NEC wording describing the relationship of electrical systems and ground is wordy and out of date, I wish they adopted the IEC terminology for earthing systems in the over 1000V stuff, its pretty intuitive;

The first letter indicates the relationship between the power-supply (transformer, generator, SDS, battery .. etc) and ground (dirt):
"T" = Direct connection of a point on the power-supply to a Grounding Electrode system (GES). Commonly seen on a center tap split phase, wye, corner grounded delta transformer.
"I" = Non grounded All live parts isolated from Earth or impedance grounded.

The second letter indicates which wire serves as the equipment ground (bonding the exposed-conductive-parts of the installation, and Earth):

"N" = Direct connection between a point on the power-supply and a GES. Typically the neutral point of a wye transformer, or center tap of split phase.

"T" = GES is totally independent of a power-supply connection to Earth. (Common in Japan not here, other than a perhaps a ungrounded DC PV system)

Subsequent letter(s) indicate:
"S" = Neutral and equipment ground functions provided by separate conductors.
"C" = Neutral and equipment ground functions provided by the same single conductor.

Some common examples are;
A typical utility system here is TN-C
A NEC service - main breaker - feeder system is TN-C-S
A NEC SDS is TN-S
 
joshtrevino said:
electronfelon, it seems that we have a "multigrounded neutral system" per 250.184.C, but I do not see that the language allows using the neutral as the low impendence path to ground instead of an EGC. Is this implied? Is it explicitly stated elsewhere? Not challenging you but honestly asking.
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You are not an electric utility, don't try to act one.
We are governed by the NEC. For the vast majority of circuits the NEC does not allow you to use the neutral instead of the EGC.
 
Over 1000V my understanding is you pick whatever you want, but I have not gone thru all the 2023 changes;
250.184(B) & 250.186(A) covers TN-C-S systems
250.184(C) & 250.186(A) Covers TN-C systems.
250.187 & 250.186(A)(4) Cover IT Systems
 
electrofelon said:
But the discussion is about MGN systems. For the most part, if the system is outside, then it can be an MGN. That probably covers the majority of medium voltage systems.
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What specific NEC sections allow for an MGN.
I don't think I have ever seen one in +47 years. But I only licensed in North Dakota, South Dakota, Minnesota, Wisconsin and Illinois.

I know the rules for bonding neutrals and grounds on feeders to out buildings have changed over the years, but theses are not typically medium voltage. The few MV wye-wye customer transformers i was involved with were usually Wye primary to Ground Wye secondary, so no internal jumper.
 
jim dungar said:
What specific NEC sections allow for an MGN.
I don't think I have ever seen one in +47 years. But I only licensed in North Dakota, South Dakota, Minnesota, Wisconsin and Illinois.

I know the rules for bonding neutrals and grounds on feeders to out buildings have changed over the years, but theses are not typically medium voltage. The few MV wye-wye customer transformers i was involved with were usually Wye primary to Ground Wye secondary, so no internal jumper.
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250.184(C). I have used MGN's for the MV portion of a step up step down. I don't think have ever seen a non utility MGN in the field, but I have only come across a couple Customer MV systems in general so very low sample size.
 
Doesn't that section say the neutral can be a bare conductor, such as found in Concentric Neutral cables. Of course these are the common cable used in underground MV installation. Maybe my experience with customer owned MV systems the conductor primarily serves the purpose of being a ground and not a true MGN as done in POCO installations.
 
Is there any "neutral load" on this system? OP mentioned transformers with wye primary but do they have the neutral connected or is it floating? If no actual neutral load then all you need beyond the service equipment is EGC's. Also been asked where is the service disconnect? I question whether the fuse upstream the metering or the recloser qualify as a service disconnect per NEC. More details could change that but the typical fuse holder or recloser you commonly see located at pole top on utility systems I wouldn't think would qualify. If they have the right accessories to make the operating device readily accessible they might be acceptable.
 
jim dungar said:
Doesn't that section say the neutral can be a bare conductor, such as found in Concentric Neutral cables. Of course these are the common cable used in underground MV installation. Maybe my experience with customer owned MV systems the conductor primarily serves the purpose of being a ground and not a true MGN as done in POCO installations.
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It specifically permits a MGN system.
(C) Multigrounded Neutral Systems.
If a multigrounded neutral system is used, the following shall apply
(1) The neutral conductor of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations: ...
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jim dungar said:
Doesn't that section say the neutral can be a bare conductor, such as found in Concentric Neutral cables. Of course these are the common cable used in underground MV installation. Maybe my experience with customer owned MV systems the conductor primarily serves the purpose of being a ground and not a true MGN as done in POCO installations.
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The wording in part X of 250 is horrible, but I'm pretty sure that the intention is that essentially with an MGN you can use the neutral as the fault clearing conductor and bonding conductor.
 
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