Service Entrance verses Generator Feeder

[roger]

Since the generator/standby system is an impedance grounded neutral system, the neutral is required to be fully insulated with the same insulation as the phase conductors: ref. 250.186(B). I understand a neutral conductor is not run from the generator.

I see the last sentence negating the first and if we use 250.186(B) the engineer has another part of the design wrong

However, because the transfer switch is three-pole with solidly-connected neutral, the neutral (grounded) conductor from the service must be equivalently insulated because the code does not relinquish the requirement when the source neutral is not run to the system disconnecting means.

And per the drawing the insulation level would be the same using the THWN.

As for the grounding conductor, I speculate this must be the engineers' concept of a Multigrounded Neutral System: ref. 250.184(C), and being it establishes a parallel neutral current path, they want it insulated from the conduit embedding concrete.

Maybe, but that is not what he has here and if that was his thinking, why didn't he include that in his message?


I need to be fair and point out that by his own admission he and his firm do very little MV design.

Roger

 

[Smart $]

I see the last sentence negating the first and if we use 250.186(B) the engineer has another part of the design wrong

The last sentence does not negate the first in the vernacular of the code. The service neutral, while on emergency power, is connected to the "emergency" system, is it not?. Where in 250.186(B) does it except and permit any neutral to not have insulation the same as the phase conductors? ...and what other part of the design are you referring to?

And per the drawing the insulation level would be the same using the THWN.

That's not how I read it. Are not the phase conductors spec'd as MV CABLES (5KV)?

Maybe, but that is not what he has here and if that was his thinking, why didn't he include that in his message?

I need to be fair and point out that by his own admission he and his firm do very little MV design.

Well then... perhaps we should chalk it up to a classic case of an engineer not willing to be outclassed by an experienced electrician

 

[skeshesh]

Since the generator/standby system is an impedance grounded neutral system, the neutral is required to be fully insulated with the same insulation as the phase conductors: ref. 250.186(B). I understand a neutral conductor is not run from the generator. However, because the transfer switch is three-pole with solidly-connected neutral, the neutral (grounded) conductor from the service must be equivalently insulated because the code does not relinquish the requirement when the source neutral is not run to the system disconnecting means.

Maybe I'm being dense... why is there no neutral ran from the genset?

As for the grounding conductor, I speculate this must be the engineers' concept of a Multigrounded Neutral System: ref. 250.184(C), and being it establishes a parallel neutral current path, they want it insulated from the conduit embedding concrete.

I guess this could fly, but (a) as Roger said they definitly didn't bring this up as the reason (b) why not just ground the neutral at the transformer with a rod?

 

[Smart $]

Maybe I'm being dense... why is there no neutral ran from the genset?

I gather it is because there are no line-to-neutral loads.

I guess this could fly, but (a) as Roger said they definitly didn't bring this up as the reason (b) why not just ground the neutral at the transformer with a rod?

I don't know where the service point is, but that would be required if the transformer is under NEC purview. Even being POCO/NESC, it will likely have a rod bonded to the neutral anyway. However, doing so does not preclude a grounding conductor between bond points. I do not believe this method to be the intention behind a multigrounded neutral system, but it is not prohibited.

 

[skeshesh]

I gather it is because there are no line-to-neutral loads.

Maybe so, but the way the single line is drawn, with the power coming in from service transformer being a 4wire system I think chances favor the contrary.

I don't know where the service point is, but that would be required if the transformer is under NEC purview. Even being POCO/NESC, it will likely have a rod bonded to the neutral anyway. However, doing so does not preclude a grounding conductor between bond points. I do not believe this method to be the intention behind a multigrounded neutral system, but it is not prohibited.

I guess 250.184(C)(3)&(4) require the rods to be provided every 1300ft, meaning that in between that distance grounding of the multigrounded neutral may be accomplished by connection to the grounding wire. I do agree that is not the intention.

Having said all this, here's some other stuff I've been looking at though, lemme know what you guys think:

250.36(C)"...system neutral conductor shall not be connected to ground except through the grounding impedance."

250.36(D)Neutral Conductor Routing: "The conductor connecting the neutral point of the transformer or generator to the grounding impedance shall be permitted to be installed in a seperate raceway. It shall not be required to run this conductor with the phase conductors to the first system disconnecting means or overcurrent device."

250.180"where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with..."

250.186 Impedance Grounded Neutral Systems: "Impedance grounded... shall be permitted where all the following conditions are met...(3) Line-to-neutral loads are not served..."

 

[Smart $]

Maybe so, but the way the single line is drawn, with the power coming in from service transformer being a 4wire system I think chances favor the contrary.

I disagree. The grounded service conductor is required to be brought to the service disconnecting means, whether it is used or not.

I guess 250.184(C)(3)&(4) require the rods to be provided every 1300ft, meaning that in between that distance grounding of the multigrounded neutral may be accomplished by connection to the grounding wire. I do agree that is not the intention.

The 1300ft requirement is a maximum. The minimum would be 6ft.

I believe the intention is to earth ground without bonding between earth grounds, where the only intentional parallel neutral current path is through earth.

Having said all this, here's some other stuff I've been looking at though, lemme know what you guys think:

250.36(C)"...system neutral conductor shall not be connected to ground except through the grounding impedance."

That makes this a "hybrid" system. I've never seen one source solidly grounded while the other source is impedance grounded.

250.36(D)Neutral Conductor Routing: "The conductor connecting the neutral point of the transformer or generator to the grounding impedance shall be permitted to be installed in a seperate raceway. It shall not be required to run this conductor with the phase conductors to the first system disconnecting means or overcurrent device."

Yes, permitted... but I don't see how this relates to the installation in question???

250.180"where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with..."

Point?

250.186 Impedance Grounded Neutral Systems: "Impedance grounded... shall be permitted where all the following conditions are met...(3) Line-to-neutral loads are not served..."

And as stated previously, there are no L-N loads.

 

[skeshesh]

The 1300ft requirement is a maximum. The minimum would be 6ft.

Yea that's what I said, it's required (maximum distance is) to be every 1300ft.

I believe the intention is to earth ground without bonding between earth grounds, where the only intentional parallel neutral current path is through earth.

Agreed.

That makes this a "hybrid" system. I've never seen one source solidly grounded while the other source is impedance grounded.

It doesnt make it a hybrid system - these are the requirements for an impedance grounding like we have here.

Yes, permitted... but I don't see how this relates to the installation in question???

The first system OCPD after the generator is the transfer switch, which is also the disconnect means. This article allows the neutral not be ran between the said points.

Point?

Point is articles above which are under 480-1000V also apply to MV which is continued below.

And as stated previously, there are no L-N loads.

The point is not that there are no L-N loads, but that they are now allowed.

 

[Smart $]

Yea that's what I said, it's required (maximum distance is) to be every 1300ft.

Well that may be what your intention was, but it did not read that way to me.

...and the max distance only comes into play if it a MGNS. If it is a Single-point Neutral System, you could run distances longer than 1300ft without another grounding point.

It doesnt make it a hybrid system - these are the requirements for an impedance grounding like we have here.

But in my interpretation the system is not impedance grounded when it is operating on POCO power... thus making it a hybrid (and I'm using the term very loosely, as it is not NEC terminology).

Granted, the drawing image is cropped... and the full grounding scheme is not shown. But I have to assume the electrolytic GES shown, only in part in the upper right hand corner of the image, is likely solidly connected to the system ground bus at the switch gear.

The first system OCPD after the generator is the transfer switch, which is also the disconnect means. This article allows the neutral not be ran between the said points.

Actually, the disconnecting means is at the generator per Roger in Post #11: "...The OCPD at the generator is through CT's and relays."

I still don't see how this relates to the grounding wires or the service neutral being 5KV insulated.

Point is articles above which are under 480-1000V also apply to MV which is continued below.

I understand that... so I suppose my question should have said, which requirements apply that make this a controversial or non-compliant installation?

The point is not that there are no L-N loads, but that they are now allowed.

I assume you meant "not" allowed. No L-N loads vs. no L-N loads allowed... what's the difference if there are none?

 

[skeshesh]

Well that may be what your intention was, but it did not read that way to me.

...and the max distance only comes into play if it a MGNS. If it is a Single-point Neutral System, you could run distances longer than 1300ft without another grounding point.

yea I should've used better language.

But in my interpretation the system is not impedance grounded when it is operating on POCO power... thus making it a hybrid (and I'm using the term very loosely, as it is not NEC terminology).

thats true

Granted, the drawing image is cropped... and the full grounding scheme is not shown. But I have to assume the electrolytic GES shown, only in part in the upper right hand corner of the image, is likely solidly connected to the system ground bus at the switch gear.

Yea there's other possibilities out there, we cant speculate accurately with just that snapshot of the system...

Actually, the disconnecting means is at the generator per Roger in Post #11: "...The OCPD at the generator is through CT's and relays."

I still don't see how this relates to the grounding wires or the service neutral being 5KV insulated.

I missed that sorry... it doesnt relate to the service neutral, I was referring to it relating to the generator to xfer switch not having a neutral run.

I understand that... so I suppose my question should have said, which requirements apply that make this a controversial or non-compliant installation?

Just clarifyin, obviously you're experience, but others reading the post may want to know why I'm quoting from the LV section.

I assume you meant "not" allowed. No L-N loads vs. no L-N loads allowed... what's the difference if there are none?

yes no = not, sorry about that. The difference though is we were trying to see if there's a mistake and there should be a neutral ran from the generator or not. There's no indication about what kind of load is being served, so "not allowed" explains it, whereas having a 3wire run and assuming there are only L-N loads allows for the possibility of an error during drafting (3w instead of 4w).

 

[ceb58]

Just fix the thing Roger, we gotta keep the slot machine's going

 

[roger]

I have been pretty busy lately (and with out power for about 30 hours over the weekend) but to clarify, the GES is solidly grounded and is common to all the gear in the building, this includes the 480 V gear.

The load is a Chiller, there is no neutral in either system.

The fully insulated neutral (grounded conductor) would only be required to the generator side of the resistor. I can not see a requirement or reason for a fully insulated "Equipment Ground" (which is all the fourth conductor from the solidly grounded service is) on the utility (solidly grounded) side of the switch.

IMO, the only way we would see an MGN coming into play is if there were no padmount transformer and the gear was fed directly from a utility or privately owned grid. (for lack of better term)

What the engineer is calling "supplementary ground" is not a "supplementary ground" it is a simply a parallel EGC, there are numerous supplementary GE's and jumpers in the GES that is not shown, and I apologize for not showing a total snap shot.

Anyways, let me say thanks to everyone for the discussion.

Roger