250.24(A)(2) Outdoor transformers

[tryinghard]

True, and I believe we're both suggesting to Zane that they need to treat their HV as service, but currently I understand he is inspecting it as a distribution feeder. Your drawing in post #18 is normal for utility service to a service disconnect.

If the system remains as a feeder the bonding jumper can be at the source and source disconnect but only through exception 2 of 250.30(A)(1): "...bonding jumper at both the source and first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor", using PVC is the most common way to install this.

 

[tryinghard]

RUwired,
Do you know what conditions my result from have the systems bonding jumpers installed at both the outdoor transformer and the indoor service disconnect means and have the grounding conductors and the grounded conductors installed between both locations?

Watch Neutral Current Where It Doesn?t Belong to see reasons.

 

[RUWired]

True, and I believe we're both suggesting to Zane that they need to treat their HV as service, but currently I understand he is inspecting it as a distribution feeder.

By NEC standards, either way is permissible. To maintain a standard amungst all feeders and services. It would be recommended to install the bonding jumper at both locations and eliminate the grounding conductor.

By definitions the privately owned transformers are feeders to buildings and could follow article 250.32 and article 225 part 2.

Rick

 

[tryinghard]

By NEC standards, either way is permissible. To maintain a standard amungst all feeders and services. It would be recommended to install the bonding jumper at both locations and eliminate the grounding conductor.

By definitions the privately owned transformers are feeders to buildings and could follow article 250.32 and article 225 part 2.

Rick

Rick,

This will be a violation of 250.32, in 2005, unless it meets ALL the criteria of 250.32(B)(2) - which is not possible with metallic conduit.

It will be in violation of 250.32, in 2008, unless it's pre-existing - which is not Zanes case.

It will be in violation of 250.30(A)(1) unless it meets the criteria of its Exception No. 2.

To be NEC compliant as a feeder system Zane can eliminate the EGC if he wants but he will need to use PVC to eliminate a parallel neutral path and comply with exception 2 of 250.30(A)(1), otherwise he must install an EGC of 250.118 if its a 277/480v circuit.

 

[Smart $]

True, and I believe we're both suggesting to Zane that they need to treat their HV as service, but currently I understand he is inspecting it as a distribution feeder. Your drawing in post #18 is normal for utility service to a service disconnect.

If the system remains as a feeder the bonding jumper can be at the source and source disconnect but only through exception 2 of 250.30(A)(1): "...bonding jumper at both the source and first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor", using PVC is the most common way to install this.

I understand what you mean... but if he treats the HV as the service, and xfmr secondary as feeders, where is the service disconnect?

 

[tryinghard]

I understand what you mean... but if he treats the HV as the service, and xfmr secondary as feeders, where is the service disconnect?

It would be at the end of the secondary I do understand the main bonding jumper is required in this case (250.28). I just think it's more hazardous than needed because the grounded conductor bonds at the source XFMR, again it's my own peeve :roll: to avoid a wrongfull parallel

 

[ZZZ]

The transformer and the service disconnect are required to be bonded. There is no reason to run a grounding conducter between them and create parallel current paths. We have dozens of these installations at the University and none of it is owned by the utility co.

 

[ZZZ]

The utility companies don't run grounding condutors between their transformers and the services and for several reasons. The AHJ is the only person who has to be o.k. with this and Universities have their own AHJ as local inspectors are usually not allowed to inspect state property.

 

[tryinghard]

The transformer and the service disconnect are required to be bonded. There is no reason to run a grounding conducter between them and create parallel current paths. We have dozens of these installations at the University and none of it is owned by the utility co.

How are you meeting the criteria of 250.30?

 

[RUWired]

Rick,

This will be a violation of 250.32, in 2005, unless it meets ALL the criteria of 250.32(B)(2) - which is not possible with metallic conduit.

It will be in violation of 250.32, in 2008, unless it's pre-existing - which is not Zanes case.

It will be in violation of 250.30(A)(1) unless it meets the criteria of its Exception No. 2.
.

Zane has'nt mentioned any metal raceways, but even with a pad mount and underground metal raceways, exception # 2 does'nt consider it a parallel path.What if this was a service with metal raceways,bonding would be the only different requirement.

 

[tryinghard]

Zane has'nt mentioned any metal raceways, but even with a pad mount and underground metal raceways, exception # 2 does'nt consider it a parallel path.What if this was a service with metal raceways,bonding would be the only different requirement.

Exception #2 doesn't have to consider it a parallel path, if the conduit system is metallic it WILL be a parallel path and neutral current will now travle ALL common paths to source!

Utilities want these in non-metalic like PVC.

 

[RUWired]

Exception #2 doesn't have to consider it a parallel path, if the conduit system is metallic it WILL be a parallel path and neutral current will now travle ALL common paths to source!

Utilities want these in non-metalic like PVC.

I agree it is a strech, but i always understood as the connection through earth (in the ground)was excluded, such as a common ground grid which will also carry current, and also as a strech the metal raceway.Theres not much difference.

 

[tryinghard]

I agree it is a strech, but i always understood as the connection through earth (in the ground)was excluded...and also as a strech the metal raceway.Theres not much difference.

Rick,

There's a major difference, its not a stretch if metal raceway is used in this fashion, it's a violation because it allows returning current to go everywhere a common path to source exists whether upstream or downstream from the distribution. Earth has little to do with this type of objectionable current the metal pipe has allowed. This picture is a good graphic of what happens.

Also notice how Mike Holt explains this in Mike Holt's Newsletter & Article for EC&M Magazine "Grounding and Bonding of Separately Derived Systems", "Dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment, metal piping, and structural steel if you install more than one system bonding jumper. Because of this, you can?t have a neutral-to-case connection on the load side of the system bonding jumper, except as permitted in 250.142(B).

You also get objectionable current if the system bonding jumper is located somewhere other than where the grounding electrode conductor (GEC) terminates to the neutral conductor. You can terminate the GEC to the neutral conductor either at the SDS system or at the system disconnecting means. But pick one?you can?t have it in both locations [250.30(A)(3)]".

I hope I'm understanding you correctly with your replies?

 

[Smart $]

Exception #2 doesn't have to consider it a parallel path, if the conduit system is metallic it WILL be a parallel path and neutral current will now travle ALL common paths to source!

Utilities want these in non-metalic like PVC.

Use of underground metallic conduit doesn't automatically create a parallel path. More often than not, the conduit is not connected to the xfmr case... instead it stubs up into the open bottom. However, I believe many mistakenly bond the conduit with grounding bushings on the stubs. This is only a problem if the entire run is metallic. If the main run is PVC and only the stub-ups are metallic, then the stubs should be bonded.

Nonetheless, when the entire run is metallic and properly not bonded at the xfmr, it still minimizes the distance of using earth as a path (i.e. reduced resistance). By code it is not a violation.

 

[RUWired]

I agree with you and the picture, that has always been the case.But that is overhead and is not addressing the exception with the earth. I need clarification on that. Are they talking about the earth itself or the ground wire and metal underground raceways through the earth.

"For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path."

But, if you look at the scenerio with metal underground or overhead raceway,as a service, the the utility will only put in the neutral and bond at the service disconnect.With it being a feeder, what makes the difference.If you take away the exception and and apply the rule,pvc will be a requirement, because with a service, the bondinding jumper is a must at the service disconnect.

I would'nt have a problem with seperating the neutral and ground at the building disconnect if it was a feeder, but that would be taking away a the "standard" bonding at the first OCPD.
Rick

 

[tryinghard]

Use of underground metallic conduit doesn't automatically create a parallel path. More often than not, the conduit is not connected to the xfmr case... instead it stubs up into the open bottom. However, I believe many mistakenly bond the conduit with grounding bushings on the stubs. This is only a problem if the entire run is metallic. If the main run is PVC and only the stub-ups are metallic, then the stubs should be bonded.

How is not bonding a metallic conduit in a 12kv/480v transformer code compliant?
If you install this in PVC it will be code compliant as per 250.30(A)(1) Exception #2

Nonetheless, when the entire run is metallic and properly not bonded at the xfmr, it still minimizes the distance of using earth as a path (i.e. reduced resistance). By code it is not a violation.

I disagree; again if you simply install it in PVC there is no problem. Again is there direction in the NEC on how to "properly not bond" a metallic conduit system in a 480V environment?

 

[tryinghard]

But, if you look at the scenerio with metal underground or overhead raceway,as a service, the the utility will only put in the neutral and bond at the service disconnect.With it being a feeder, what makes the difference.If you take away the exception and and apply the rule,pvc will be a requirement, because with a service, the bondinding jumper is a must at the service disconnect.

This is why utilities want PVC, if it is metallic it will be a parallel and it will have current (objectionable current) where it does not belong, this is a violation.

This scenario can be either service or feeder, Zane and his client need to pick one and accept safe consistent installation practices. I still recommend they consult an electrical engineer for this decision.

• Service = bonding jumper at both source and source disconnect
• Feeder = bonding jumper at either source or source disconnect and both only if a parallel path does not exist other than earth, i.e. PVC should be used.

I would'nt have a problem with seperating the neutral and ground at the building disconnect if it was a feeder, but that would be taking away a the "standard" bonding at the first OCPD.Rick

This wouldn't be the "standard" bonding for a feeder only a service, (250.24(5), 250.30(A)(1), 250.32(B)(2), and 250.142(B)). One can bond up to the first disconnect from a SDS, it is most commonly done at the SDS and must be where the GEC is located.

In light of the main bonding jumpers purpose one can ask, is the equipment and non-current-carrying items connected to the source "XO" to enable the operation of overcurrent protection? The answer in both scenarios is yes but one scenario allows objectionable current and the other doesn't.

 

[Smart $]

How is not bonding a metallic conduit in a 12kv/480v transformer code compliant?
...

I disagree; again if you simply install it in PVC there is no problem. Again is there direction in the NEC on how to "properly not bond" a metallic conduit system in a 480V environment?

I think you mistook my statement. I didn't mean the conduit is not bonded at all... only not bonded at the xfmr end. It could just as easily be done correctly by bonding at the xfmr end and not at the service disconnect end, if the run simply stubbed up into open-bottom switchgear. The point is: bonding at both ends creates a parallel path.

 

[tryinghard]

I think you mistook my statement. I didn't mean the conduit is not bonded at all... only not bonded at the xfmr end. It could just as easily be done correctly by bonding at the xfmr end and not at the service disconnect end, if the run simply stubbed up into open-bottom switchgear. The point is: bonding at both ends creates a parallel path.

I believe PVC is the only real safe way to install this. If you do not bond the metallic conduit in the transformer pad what resistance exists between the conduit and the transformer electrode? This cannot be known at any given moment and current WILL exist on the conduit allowing dangerous touch voltage.

The practice of "not bonding" a metallic conduit in a 12KV/480V environment is too bizarre!

 

[RUWired]

This is why utilities want PVC, if it is metallic it will be a parallel and it will have current (objectionable current) where it does not belong, this is a violation.

I checked with the poco today and they require rigid metal with direct buried conduits and pvc with concrete encased.

Service = bonding jumper at both source and source disconnect

This would still create a parallel path and allowed.

• Feeder = bonding jumper at either source or source disconnect and both only if a parallel path does not exist other than earth, i.e. PVC should be used.

Yes i agree that pvc should be used and i would gather that Zane is using pvc.

If metal is used, he could leave out the jumper in the main gear, but like i said before, it's hard for inspectors to agree with that. They all want the jumper in at the first ocpd.

I still don't have a clear understanding of what " through the earth " is referring to. I think that it means if there is a common ground grid or wire connecting the two.If that is what they mean, then a metal raceway should be ok as long as it's in the earth.

Rick