250.24(A)(2)

[I-learns]

250.24 (A) (2) requires a second neutral to grounding electrode connection where the service is supplied by an outdoor transformer. It says it shall be made at the transformer, or anywhere else outside the building. The handbook shows an example of the transformer and main service disconnect in two different locations, and shows a grounding electrode connection to two separate grounding electrodes, one being at the service, and the other at the transformer. I saw a situation with an outdoor transformer, and outdoor service disconnect, both located less than 10 feet apart. In this case, I think 250.50 would require the two grounding electrodes to be connected together, and this would cause objectionable current. How would one follow the requirements of 250.24A2 here?

 

[ActionDave]

You can't properly ground and bond a service and not get objectionable current.

 

[GoldDigger]

You can't properly ground and bond a service and not get objectionable current.

Which kind of means that in that environment it is not objectionable!

Sent from my XT1585 using Tapatalk

 

[ActionDave]

I call it the unavoidable objectionable.

 

[kwired]

The thing that doesn't quite make sense to me about this section is that the transformer it is talking about is usually utility owned and controlled - on the utility side of the service point and NEC doesn't apply to it.

If it would happen to be on the customer side of the service point - then the service conductors in that case are going to be medium voltage and the secondary of the transformer is not service conductors but rather a separately derived system.

 

[I-learns]

The thing that doesn't quite make sense to me about this section is that the transformer it is talking about is usually utility owned and controlled - on the utility side of the service point and NEC doesn't apply to it.

If it would happen to be on the customer side of the service point - then the service conductors in that case are going to be medium voltage and the secondary of the transformer is not service conductors but rather a separately derived system.

This is exactly the point. It is all customer owned. Also, it is hard to say it is a separately derived system either, when there is a common XO/HO bushing. Neither can it really be considered a service if the service point is on the high side.

 

[kwired]

This is exactly the point. It is all customer owned. Also, it is hard to say it is a separately derived system either, when there is a common XO/HO bushing. Neither can it really be considered a service if the service point is on the high side.

Having grounded conductor on both primary and secondary bonded together does not disqualify it as a separately derived system, doesn't matter if the grounded conductor on either side is a neutral or other conductor either.

If a circuit utilizes both primary and secondary winding as a part of the complete circuit like an autotransformer does, that is when it is not a separately derived system. Otherwise you would never have a separately derived system if both sides have a grounded conductor.

If service point is on the high side then the low voltage system is separately derived, and the service conductors are higher voltage conductors and the service disconnecting means does open the higher voltage conductors.

 

[Smart $]

Having grounded conductor on both primary and secondary bonded together does not disqualify it as a separately derived system, ...

Yes, it does. Please read the Separately Derived System definition.

 

[Smart $]

... both located less than 10 feet apart. In this case, I think 250.50 would require the two grounding electrodes to be connected together, and this would cause objectionable current. How would one follow the requirements of 250.24A2 here?

There is no requirement the two electrodes be bonded together by other than the grounded conductor. But if you believe so, then you have to look to 250.6 for the remedy... which will make you draw the same conclusion.

 

[big john]

I've always thought it was a technical violation, but seems like every private padmount I've seen is wired just like it was utility owned:

X0 bonding strap in place and then a MBJ bonding in the gear.

 

[kwired]

Originally Posted by kwired
Having grounded conductor on both primary and secondary bonded together does not disqualify it as a separately derived system, ...

Yes, it does. Please read the Separately Derived System definition.

I read it and it still doesn't change what I would have said.

Separately Derived System.
An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.

You have two separate systems, both have a grounded conductor that are bonded together because they are grounded. But one system doesn't depend on the grounded conductor of the other system to function.

Even if they only bring one "grounded lead" out of such a transformer there is still two separate primary and secondary sets of windings that are independent of one another, they just both have one point tied together and it gets grounded somewhere.

 

[Smart $]

...
Even if they only bring one "grounded lead" out of such a transformer there is still two separate primary and secondary sets of windings that are independent of one another, they just both have one point tied together and it gets grounded somewhere.

Grounded conductors connected together is not established by grounding and bonding. You emphasized the wrong part of the definition...

Separately Derived System. An electrical source, other
than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those
established by grounding and bonding connections.

 

[electrofelon]

I've always thought it was a technical violation, but seems like every private padmount I've seen is wired just like it was utility owned:

X0 bonding strap in place and then a MBJ bonding in the gear.

Which is ok under 250.30(A)(1) exception #2. What is a little dicey is bonding the HO to the XO.

 

[kwired]

Grounded conductors connected together is not established by grounding and bonding. You emphasized the wrong part of the definition...

Unless there is exceptions, the general rule is that the primary must have an EGC run separate from any current carrying grounded conductor. That would make any interconnection between primary and secondary a result of grounding and/or bonding conductors.

Typical 7200 x 120/240 transformer like we see POCO's use certainly isn't ordinarily configured as an autotransformer.

 

[Smart $]

Unless there is exceptions, the general rule is that the primary must have an EGC run separate from any current carrying grounded conductor. That would make any interconnection between primary and secondary a result of grounding and/or bonding conductors.
...

If H0 and X0 are bonded to the EGC is one thing... but a primary grounded conductor is a circuit conductor, which is what the definition specifically states cannot be directly interconnected.

 

[kwired]

If H0 and X0 are bonded to the EGC is one thing... but a primary grounded conductor is a circuit conductor, which is what the definition specifically states cannot be directly interconnected.

We might be more in agreement here then it may seem

I agree for a NEC defined SDS the primary grounded conductor can not be grounded or bonded beyond the service disconnect, first disconnect if fed from an SDS, if you do use the grounded conductor as a circuit conductor a separate equipment grounding conductor must be run with supply conductors.

This can make a transformer that utilities often use unacceptable for use in a NEC application if the secondary has a conductor permanently connected to a primary circuit conductor in any way (and usually bonded to the case of the transformer as well.) But I seem to recall there are some exceptions where you still could use such transformers in an NEC application. Seems like it did have to be for outdoor feeders at the very least. I don't do medium voltage work so this is not something I really have to remember...