Objectionable Current on Rigid Conduit

[don_resqcapt19]

I don't follow where the code allows it. 250.6 says grounding of (ALL) electrical systems shall be installed in a manner arranged to prohibit objectionable current. It would seem that the hazards associated with this exist wherever it is in a service.

In the event one bonded on the transformer per 250.24(2) (additional bonding jumper required), then installed a main service bonding jumper, and used rigid metal conduit between the transformer and service disconnect, there would be be objectionable neutral current which could present a fire or other hazard.

The transformer bond for a transformer that supplies a service is on the line side of the service point and not subject to the rules in the NEC. Services are required by the application of a number of code rules to have parallel paths for neutral current.

 

[Smart $]

The transformer bond for a transformer that supplies a service is on the line side of the service point and not subject to the rules in the NEC. Services are required by the application of a number of code rules to have parallel paths for neutral current.

The service point can be and in many instances is at the transformer secondary terminals. For this scenario, per 250.24(A)(2), where "the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building."

 

[electrofelon]

The transformer bond for a transformer that supplies a service is on the line side of the service point and not subject to the rules in the NEC. Services are required by the application of a number of code rules to have parallel paths for neutral current.

can you give an example of a situation that cant be remediated by applying the solutions in 250.6(B)? IMO an inspector is within is rights to require, say, a non conductive raceway instead of a conductive one, or a change to a bonding jumper instead of a "direct" bond to the neutral. I know the code is never enforced that way, but that is what the language states.

 

[don_resqcapt19]

can you give an example of a situation that cant be remediated by applying the solutions in 250.6(B)? IMO an inspector is within is rights to require, say, a non conductive raceway instead of a conductive one, or a change to a bonding jumper instead of a "direct" bond to the neutral. I know the code is never enforced that way, but that is what the language states.

250.6 does not really require you to take any actions, but it permits actions. I don't see the service raceway in parallel with the grounded conductor as a real world hazard, however, you are correct that it could eliminated.

If you down that route what do you do about the other code required parallel paths? One example is the TV cable coax that is required to be connected to the electrical grounding system at each building. To me this is a much more dangerous path than metallic conduit.

Another code required parallel path is a common metal underground water system that is required to be connected to the electrical grounding system at each building. This has resulted in sever and sometimes fatal shocks to plumbers when they are working on an outside under ground water line that serves a building with compromised grounded conductor.

 

[electrofelon]

Another code required parallel path is a common metal underground water system that is required to be connected to the electrical grounding system at each building. This has resulted in sever and sometimes fatal shocks to plumbers when they are working on an outside under ground water line that serves a building with compromised grounded conductor.

That is a good one. That one is actually a huge pet peeve of mine, although I'm not sure what can be done about it without cooperation from other trades/standards.

 

[yesterlectric]

Some more details about the imaginary application in question. It would be a customer owned pad mount transformer that is NOT a separately derived system. it seems strange to me that if this single transformer were a separately derived system supplying premises wiring, one would only be allowed to bond at both the source and the disconnecting means if it didn't create a parallel path (250.30A1exc#2). However, at a service, we can and may be required to bond twice because 250.24 requires it and the code doesn't define objectionable current?

If it is dangerous on a separately derived system, why would it be safe on a main system/service just because the letter of the code may allow it?

 

[iwire]

Some more details about the imaginary application in question. It would be a customer owned pad mount transformer that is NOT a separately derived system. it seems strange to me that if this single transformer were a separately derived system supplying premises wiring, one would only be allowed to bond at both the source and the disconnecting means if it didn't create a parallel path (250.30A1exc#2). However, at a service, we can and may be required to bond twice because 250.24 requires it and the code doesn't define objectionable current?

If it is dangerous on a separately derived system, why would it be safe on a main system/service just because the letter of the code may allow it?

Because the code can't count on what the utility will do.

 

[yesterlectric]

Because the code can't count on what the utility will do.

But this is a customer owned transformer, not utility owned.

 

[iwire]

But this is a customer owned transformer, not utility owned.

Where is the service disconnecting means?

 

[Smart $]

...
If it is dangerous on a separately derived system, why would it be safe on a main system/service just because the letter of the code may allow it?

Because the code can't count on what the utility will do.

But this is a customer owned transformer, not utility owned.

You asked about a service. Your question got answered as asked.

Nonetheless, where the customer owns a service transformer is quite uncommon. And this is all...

...about the imaginary application in question. ...

Please remind us why we're even discussing this quite rare imaginary application...
:slaphead:

 

[yesterlectric]

This scenario is not as uncommon as it may be made out to be. Happens all the time in utility scale solar applications. The change of ownership is at a customer owned recloser on a customer owned pole, often 12, 22, or even 35KV. Therefore, the MV side including the MV transformers are covered by the NEC code.

What I haven't seen a lot of is requirements for rigid metal conduit. However, it is conceivable that some developers or investors may want this.

 

[electrofelon]

This scenario is not as uncommon as it may be made out to be. Happens all the time in utility scale solar applications. The change of ownership is at a customer owned recloser on a customer owned pole, often 12, 22, or even 35KV. Therefore, the MV side including the MV transformers are covered by the NEC code.

What I haven't seen a lot of is requirements for rigid metal conduit. However, it is conceivable that some developers or investors may want this.

Sure, customer owned mv systems are not uncommon, but parallel paths on sds's are not allowed. What is the question?

 

[Smart $]

This scenario is not as uncommon as it may be made out to be. Happens all the time in utility scale solar applications. The change of ownership is at a customer owned recloser on a customer owned pole, often 12, 22, or even 35KV. Therefore, the MV side including the MV transformers are covered by the NEC code.

What I haven't seen a lot of is requirements for rigid metal conduit. However, it is conceivable that some developers or investors may want this.

And thus not a service transformer.

As I stated, customer-owned service transformers are quite rare. More often than not, it is someone misinterpreting who owns what and/or where the service point is located.