250.24 (A)(2) Outdoor Transformer

[unsaint34]

250.24
(A)(1) General. "The grounding electrode conductor connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means."

(A)(2) Outdoor Transformer. "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."
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My question #1: Let's say I have an outside transformer servicing the building. Regarding 250.24 (A)(2), I am thinking, the service point, the load end of the service lateral, is on the primary side of the outside transformer. Am I not correct?

My question #2: According to 250.24 (A)(1) GEC connection can be made any where between the service point and disconnect, INCLUDING the outdoor transformer's secondary side. Right?

My question #3: So, if I decide to make the GEC connection at the transformer secondary, as permitted by 250.24 (A)(1), rather than at the disconnect, do I need to install another GEC connection either outside or at the transformer per 250.24 (A)(2)?

 

[augie47]

There are variations between a utility system and a facility SDS.
In your scenario where is the point of demarcation for the utility ? Is the transformer primary supplied by utility or customer wiring ? Where is the connection between POCO and the customer ?

 

[don_resqcapt19]

It is a good question because I have no idea what 250.24(A)(2) is trying to tell us.

 

[unsaint34]

There are variations between a utility system and a facility SDS.
In your scenario where is the point of demarcation for the utility ? Is the transformer primary supplied by utility or customer wiring ? Where is the connection between POCO and the customer ?

Well, let's say the end of the lateral was at the primary wiring terminals of the transformer.

 

[unsaint34]

I know the most common grounding connection point is at the disconnect, but some people believe it is better to have the grounding connection outside the building so lightning caused surge does not enter the building.

I found the following picture. It explained my question regarding 250.24(A)(2)

 

[Smart $]

My question #1: Let's say I have an outside transformer servicing the building. Regarding 250.24 (A)(2), I am thinking, the service point, the load end of the service lateral, is on the primary side of the outside transformer. Am I not correct?

As Gus mentioned, it depends on service point location.

My question #2: According to 250.24 (A)(1) GEC connection can be made any where between the service point and disconnect, INCLUDING the outdoor transformer's secondary side. Right?

The service point would have to be the secondary terminals.

My question #3: So, if I decide to make the GEC connection at the transformer secondary, as permitted by 250.24 (A)(1), rather than at the disconnect, do I need to install another GEC connection either outside or at the transformer per 250.24 (A)(2)?

When you make a GEC connection at a secondary terminal of an outside service transformer, it will generally be the GE of 250.24(A)(2). Why would you run the service GES GEC all the way out to the transformer when it can be connected at the service equipment?

Also, many POCO's require a grounding electrode (typically a ground rod) connected at their (outside) meter. This satisifies 250.24(A)(2) even when the service disconnecting means is inside.

 

[augie47]

The diagram works well for a Utility transformer, but, I have a bit of a problem when that transformer is a customer SDS.

In the '08 Code it seems to me that 250.30(A)(1) exception 2 allowed the SDS to be treated like a Utility transformer IF there was no parallel path. Now along comes '11 and they added 250.30(A)(2) requiring the addition of a supply side bonding jumper between the transformer and the disconnect means.

 

[unsaint34]

Why would you run the service GES GEC all the way out to the transformer when it can be connected at the service equipment?

I know it is common to make the grounding connection at the disconnect, but for the sake of my education I would like to ask this... Since there has to be a grounding connection at the outside transformer anyway (whether it is done by POCO or not), and 250.24(A)(1) allows me to have the system grounding connection at the transformer, why can't I just use that grounding connection as my system grounding connection?

I'm sure I am asking this question out of ignorance, but I want to clarification. Thanks.

 

[augie47]

I think 250.24(A)(1) dictates the GEC be connected at the endof the service drop or lateral

 

[Smart $]

I know it is common to make the grounding connection at the disconnect, but for the sake of my education I would like to ask this... Since there has to be a grounding connection at the outside transformer anyway (whether it is done by POCO or not), and 250.24(A)(1) allows me to have the system grounding connection at the transformer, why can't I just use that grounding connection as my system grounding connection?

...

You can... but a GES is required at each building or structure served (except those served by a branch circuit under specific conditions) and connection at the disconnect thereto. Logic dictates an outside service transformer and the building or structure served cannot be the same structure, for typical installations.

 

[Smart $]

I think 250.24(A)(1) dictates the GEC be connected at the endof the service drop or lateral

As does 250.24(A)(2).

The only time the additional grounding electrode can be connected at the transformer is when the secondary terminals are the service point and the secondary conductors are service-entrance conductors.

 

[unsaint34]

As does 250.24(A)(2).

The only time the additional grounding electrode can be connected at the transformer is when the secondary terminals are the service point and the secondary conductors are service-entrance conductors.

250.24 (A)(2) Outdoor Transformer. "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."

This article seems to contradict your comment. So, you are saying, if the service point is the primary side of a outside transformer, I don't need to follow the rule of 250.24 (A)(2)?

 

[david luchini]

250.24 (A)(2) Outdoor Transformer. "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."

This article seems to contradict your comment. So, you are saying, if the service point is the primary side of a outside transformer, I don't need to follow the rule of 250.24 (A)(2)?

There is no contradiction. Read the part in red. If the service point is on the primary side, then the transformer is not "supplying the service." It is being supplied by the service.

If the service point is on the primary side, then the secondary side of the transformer would be a separately derived system (assuming its not a y-y transformer). In that case, 250.30 would apply on the secondary, rather than 250.24. (See Gus' post #7)

 

[unsaint34]

You can... but a GES is required at each building or structure served (except those served by a branch circuit under specific conditions) and connection at the disconnect thereto. Logic dictates an outside service transformer and the building or structure served cannot be the same structure, for typical installations.

Now I seem to understand this. 250.32 requires each building to have electrode, generally. Also, 250.24 (B) requires the main bonding jumper within enclosure for each service disconnect. That is why I need to make the MBJ at the disconnect, even when I have to make the same connection at the outdoor transformer. By "same connection" I mean the connection of the electrode and transformer enclosure and the service grounded conductor.
Thanks.

 

[unsaint34]

There is no contradiction. Read the part in red. If the service point is on the primary side, then the transformer is not "supplying the service." It is being supplied by the service.

If the service point is on the primary side, then the secondary side of the transformer would be a separately derived system (assuming its not a y-y transformer). In that case, 250.30 would apply on the secondary, rather than 250.24. (See Gus' post #7)

thanks for the clarification. That makes sense. But that got me wonder... then, what if the transformer was customer owned? Would that make the secondary side of the transformer a separately derived system? Or is there some exception that the utility company can make so that a customer can own the transformer yet the service point is beyond or at the secondary of the transformer?

 

[david luchini]

Now I seem to understand this. 250.32 requires each building to have electrode, generally. Also, 250.24 (B) requires the main bonding jumper within enclosure for each service disconnect.

I think you're still a little off here. 250.32 would apply to a structure supplied by a feeder. 250.24(B) wouldn't apply to the disconnect in the structure (it would only apply if the structure was supplied by a service, not a feeder.) So the two sections don't work in conjunction with each other.

thanks for the clarification. That makes sense. But that got me wonder... then, what if the transformer was customer owned? Would that make the secondary side of the transformer a separately derived system? Or is there some exception that the utility company can make so that a customer can own the transformer yet the service point is beyond or at the secondary of the transformer?

Yes, the secondary side of a customer owned transformer would be a separately derived system. There is no exception (that I'm aware of) to have the service point be on both the primary and secondary of a customer owned transformer.

 

[unsaint34]

I think you're still a little off here. 250.32 would apply to a structure supplied by a feeder. 250.24(B) wouldn't apply to the disconnect in the structure (it would only apply if the structure was supplied by a service, not a feeder.) So the two sections don't work in conjunction with each other.

Yes, the secondary side of a customer owned transformer would be a separately derived system. There is no exception (that I'm aware of) to have the service point be on both the primary and secondary of a customer owned transformer.

I forgot 250.32 applies to separate buildings. So, 250.50 is a better choice then? "250.50 All ... electrodes.. at each building served shall be... installed and used."

 

[augie47]

It's a bit of an oddity in the Code (IMHO), that we don't treat the two situations exactly the same. On a utility owned/maintained transformer the secondary is treated as a service and 250.24 applies

If the customer/POCO demarcation point is ahead of the transformer (the customer owns/maintains the primary also), the the transformer is a SDS and 250.30 applies.
Grounding pretty much the same , however, it must now comply with 250.30(B) & (C)

 

[unsaint34]

This was a highly learning experience. I thank you for your replies. I would like to summarize what I learned so that maybe I can be corrected if some parts are not correct.

* When the service point is the 2ndary side of a utility xfrmr (transformer), it is considered service-supplied. In that case, the MBJ (main bonding jumper) HAS to be in the service disconnect, per 250.24 (B) "...within the enclosure for each service disconnect..."
But an outdoor xfrmr has to have its own grounding connection, per per 250.24 (A)(2). I am understanding the grounding connection it is talking about fits the definition of a MBJ since 250.24 (A)(2) reads "one additional grounding connection shall be made from the grounded service conductor to a grounding electrode."
I believe having two MBJ (one at the utility outside xfrmr and at the disconnect) is allowed as long as doing so does not establish a parallel path for the grounded conductor. But I cannot find the code article for this.

* When the service point is on the primary side of a customer owned xfrmr, it is considered SDS (separately derived system). Per 250.30(A)(1), the SBJ (system bonding jumper) shall be made between the source and the first OCPD. So, if the xfrmr has 2ndry OCPD, then the SBJ has to be made in the transformer. But additional SBJ can be made in the service disconnect per 250.30(A)(1) Exception No. 2 "A system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor."

Any confirmation or correction would be appreciated.

 

[Smart $]

This was a highly learning experience. I thank you for your replies. I would like to summarize what I learned so that maybe I can be corrected if some parts are not correct.

* When the service point is the 2ndary side of a utility xfrmr (transformer), it is considered service-supplied. In that case, the MBJ (main bonding jumper) HAS to be in the service disconnect, per 250.24 (B) "...within the enclosure for each service disconnect..."
But an outdoor xfrmr has to have its own grounding connection, per per 250.24 (A)(2). I am understanding the grounding connection it is talking about fits the definition of a MBJ since 250.24 (A)(2) reads "one additional grounding connection shall be made from the grounded service conductor to a grounding electrode."
I believe having two MBJ (one at the utility outside xfrmr and at the disconnect) is allowed as long as doing so does not establish a parallel path for the grounded conductor. But I cannot find the code article for this.

The additional grounding connection in 250.24(A)(2) is not an MBJ. It is an additional grounding electrode connection to the grounded service conductor. The grounding electrode must also be outside. It can be made at the transformer (under specific conditions), or more likely at the service disconnecting means (SDM) or the meter, either of which must also be outside. If made at the SDM or meter, the GE is also considered part of the building GES, so it can also qualify say, for example, as the required supplementary electrode for the water pipe electrode. It is not meant to bond the GC and EGC, as the MBJ does.

* When the service point is on the primary side of a customer owned xfrmr, it is considered SDS (separately derived system). Per 250.30(A)(1), the SBJ (system bonding jumper) shall be made between the source and the first OCPD. So, if the xfrmr has 2ndry OCPD, then the SBJ has to be made in the transformer. But additional SBJ can be made in the service disconnect per 250.30(A)(1) Exception No. 2 "A system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor."

Not the service disconnect. An SBJ, or two, is on an SDS only. Very seldom, if ever, will a system bonding jumper at both the source and the first disconnecting means not establish a parallel path for the grounded conductor, because an EGC (or supply-side bonding jumper {SSBJ}) is required. In fact, since requiring a SSBJ, I have no idea why the exception is still in there.